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2024Al matrix composites synthesized by flake powder thixoforming
Journal Pre-proof Hierarchical microstructure architecture: A roadmap towards strengthening and toughening reduced graphene oxide/2024Al matrix composites synthesized by flake powder thixoforming Pubo Li, Luyao Chen, Bo Cao, Keren Shi PII:
S0925-8388(20)30178-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.153815
Reference:
JALCOM 153815
To appear in:
Journal of Alloys and Compounds
Received Date: 8 November 2019 Revised Date:
9 January 2020
Accepted Date: 11 January 2020
Please cite this article as: P. Li, L. Chen, B. Cao, K. Shi, Hierarchical microstructure architecture: A roadmap towards strengthening and toughening reduced graphene oxide/2024Al matrix composites synthesized by flake powder thixoforming, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153815. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Pubo Li: Data curation, Writing- Original draft preparation, Writing- Reviewing and Editing.
Luyao
Chen: Conceptualization,
Methodology. Bo
Validation. Keren Shi: Visualization, Investigation, Supervision.
Cao:
Software,
RGO-free zones
RGO/Al powders
RGO-rich zones
Al powders
Strengthening and toughening
Hierarchical microstructure RGO-free zones
RGO RGO RGO-rich zones 30 µm
1 µm Al2O3 (104) 0.263 nm
RGO (100) 0.214 nm
Al Al (200) 0.202 nm
10 nm
Hierarchical microstructure architecture: A roadmap towards strengthening and toughening reduced graphene oxide/2024Al matrix composites synthesized by flake powder thixoforming Pubo Li a,*, Luyao Chen a, Bo Cao a, Keren Shi b a
Ningxia Key Laboratory of Photovoltaic Materials, Ningxia University, Yinchuan,
750021, P. R. China b
State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical
Engineering, Ningxia University, Yinchuan 750021, P. R. China * Corresponding author. Tel: +86 951 2062414 E-mail address: [email protected] (Pubo Li). Abstract Graphene as one of the best reinforcements for improving the comprehensive properties of the composites has attracted extensive attention. However, the severe aggregation of graphene in the matrix obviously weakened the strengthening efficiency. A novel method, flake powder thixoforming (FPT) that combines both the advantages of flake powder metallurgy and thixoforming, was proposed to construct the FPT-0.4wt.%RGO/Al composite with hierarchical microstructure that reduced graphene oxide (RGO) only uniformly distributed into the local regions of secondary solidified structures (SSSs). The ultimate tensile strength (UTS, 439 MPa), yield strength (YS, 294 MPa) and elongation (8.5 %) of FPT-0.4wt.%RGO/Al composite fabricated by FPT were increased by 45.8 % and 44.8 % and decreased by 44.1 % compared with the 2024Al alloy, respectively. Moreover, a 34.9 % enhancement in YS and a 54.5 % improvement in elongation were achieved when compared with the HEM-0.4wt.%RGO/Al composite prepared by high energy milling (HEM). The RGO-rich zones in the FPT-0.4wt.%RGO/Al composite behaved as reinforcing units that can sustain the tensile stress and thus strengthened the matrix. Simultaneously, crack deflection and bridging contributed by the RGO-free zones effectively toughened the composite. FPT is promising for strengthening and toughening the RGO/Al composites by tailoring the spatial arrangement of RGO to form hierarchical microstructure.
Keywords Reduced graphene oxide; Aluminum matrix composites; Hierarchical microstructure; Flake powder thixoforming; Mechanical properties. 1. Introduction Owing to the two-dimensional single-atom thick-layer structure and its superior properties, such as higher strength (130 GPa) and elasticity modulus (1.1 TPa), graphene (Gr) was regarded as the most promising candidate for reinforcing the metal matrix composites [1, 2]. Gr/Al matrix composites have gained considerable attention among aerospace and automotive industry due to their higher specific strength and specific stiffness, engineering composites with unattainable combinations of mechanical and physical properties to meet the ever increasing performance requirements of advanced technologies [3, 4]. It was reported that the introduction of 2 wt.% Gr resulted in 1.25 times increase in the ultimate tensile strength (UTS) compared to the Al matrix material [5]. Li [4] et al. reported that the elastic modulus and hardness of the 0.3 wt.% reduced graphene oxide (RGO)/Al composites increased by 18 % and 17 %, respectively, compared with the Al matrix. The main strengthening contribution was attributed to the load transfer mechanism resulting from the strong RGO/Al interfacial bonding as well as the large specific surface area of RGO. Although the significant progresses were achieved in the Gr reinforced metal matrix composites, the improvement in the comprehensive properties was always not commensurate with the extraordinary properties of Gr. One of the reasons was the serious aggregation of Gr due to the Van der Walls interactions. The majority of investigations were focused on the dispersion of Gr within the matrix and the obtained results indicated that high-energy ball milling (HEM) and flake powder metallurgy (FPM) were the two mainstream methods [4, 6]. FPM not only homogeneously dispersed Gr but also remained its structural integrity as compared with HEM [7]. Moreover, the strengthening effect and fracture ductility of the nano-laminated Gr/Al composites prepared by FPM were simultaneously improved. The fabricated processes of FPM are essentially similar to the traditional powder metallurgy (PM) except the mixing of Gr with the flake powders. PM is the most attractive method due
to the controlled interface reaction and uniformly dispersed reinforcements. However, the wide application of this method is retarded by the difficulty in fabricating components with compact microstructures and complex shapes. Thixoforming is suitable for near-net-shape forming of components with complicated shapes [8]. Especially, the performance of the thixoformed components can be enhanced through decreasing or even eliminating the pores. Combining with PM and thixoforming technology, a powder thixoforming approach for particle reinforced composites was proposed in our previous works [9]. The tensile strength of the SiC/Al composite prepared by powder thixoforming was improved compared with the composite prepared by other traditional methods, revealing the superiority for strengthening the matrix alloy. However, the mismatch between the strength and ductility of the composites hindered their wide applications in the engineering industries. The strengthening efficiency of the Al matrix composites reinforced by homogeneously distributed Gr was higher than the composite reinforced by traditional reinforcements, such as SiC and Al2O3 particles, a key problem was that the increased strength also accompanied by the decreased ductility. Tailoring the distribution of reinforcements at the meso-scale to produce unique microstructural architectures, such as tri-modal, layered and hierarchical architectures, was demonstrated as an effective method to overcome property tradeoffs [10-12]. A hierarchical composite consists of matrix and reinforcing phase comprising the matrix in which is embedded into the reinforcement [12]. That is to say, the matrix in the composite with hierarchical microstructure is reinforced by a composite in itself. These structures depart from the traditional thought that the homogeneous distribution of reinforcement is a prerequisite to enhance the mechanical properties of the composites. For example, a superior combination of strength and fracture ductility of 5083Al alloy by arrays of fiber-like B4Cp-rich zones that contain B4Cp embedded in the Al matrix was achieved [11]. There were only a few investigations involved with the synthesis of Gr reinforced hierarchical composites due to the difficulty in controlled Gr dispersion [2, 13]. The Gr-rich zones reinforced Cu matrix composites prepared by molecular-level mixing process exhibited obvious improvement in both
of strength and ductility compared with the pure Cu matrix [2]. However, disadvantages such as procedure complexity and impurity introduction, limited its practical application. In this work, a novel method for RGO reinforced composites with hierarchical microstructure, flake powder thixoforming (FPT), was proposed by combining the advantages of both FPM and powder thixoforming technology. A pre-compacted mixture of powders of the metal matrix and the reinforcement was prepared by introducing a certain amout of the flake matrix powders that were regarded as the efficient carrier for uniformly distributing RGO into the surrounding of spherical powders and then hot pressing. Subsequently, the powder compacts were formed through the partial remelting and forming procedures of thixoforming, synthesizing the hierarchical RGO/Al composite that RGO only homogeneously dispersed within the local regions of the secondary solidified structures (SSSs). The effects of arrays of RGO-rich zones on the thixoformed microstructure, mechanical properties, fracture behaviour and corresponding strengthening mechanisms were studied. This study offered a promising technology for effectively improving the strength as well as ductility of the RGO/Al composites through tailoring the spatial arrangement of RGO to form a hierarchical microstructure. 2. Experimental 2.1. Raw materials The matrix material was spherical 2024Al powders with an average particle size of 20 µm. Its chemical composition was Al-4.22Cu-1.35Mg-0.5Si-0.5Fe (in wt.%) and the solid and liquid temperatures were 498.04 °C and 663.71 °C, respectively, which was confirmed by the differential thermal analysis reported in our previous work [14]. Glacial acetic acid, anhydrous ethanol (AR) and other reagents were purchased from China Pharmaceutical Group Co. Ltd. 2.2. Preparation of GO GO was prepared from squamous graphite by modified Hummers method [15]. First, flake graphite powders (2 g) and concentrated HNO3 (50 ml) and H2SO4 (50 ml) were mixed slowly and then KMnO4 (5 g) was added after 15min of stirring.
Subsequently, the obtained suspension was heated at 40 °C for 2 h through the water bath. After 10 min of stirring, 10 ml hydrogen peroxide water was added and placed for 12 h and then the supernatant was centrifuged at the speeds of 8000 r/min and 10000 r/min in sequence. According to the above steps, GO with smooth surface and structural integrity can be obtained. 2.3. Preparation of RGO/Al matrix composites The typical fabrication of the hierarchical RGO/Al composites prepared by FPT mainly included six processes, as illustrated in Fig. 1. First, to increase the surface area of 2024Al powders, the spherical powders were milled into flakes by QM-QX2 omnidirectional planetary ball mill with a ball-to-powder ratio of 20:1, ball milling time of 8 h, rotation speed of 300 rpm and the interval between positive and negative rotation of 10 min (Fig. 1a and b). AR was added as dispersant. The prepared Al flakes were filtered and vacuum dried at 65 °C for 10 h. To eliminate the harmful effect of alumina inevitably existed on the surface of Al powders and thus further facilitate the uniform dispersion of GO through direct electrostatic adsorption between the fresh Al powder surface and GO, the as-prepared Al flakes were firstly acidified by 2 vol.% glacial acetic acid (Fig. 1c). After 5 min of ultrasonication with magnetic stirring of 350 rpm, the gray-black liquid in the upper suspension was separated from the precipitated Al flakes. The Al flakes were rinsed with deionized water for several times until the upper solution became grey and a neutral aqueous dispersion was achieved and then a slurry of acid-treated Al flakes was prepared. Second, GO was dispersed in deionized water by ultrasonication for 1 h in the process of adsorption. The obtained GO aqueous solution (1 mg/ml) was slowly added (5 ml/min) into the slurry of acid-treated 2024Al flakes with the assistance of continuous stirring until the upper solution became transparent, synthesizing the GO/Al composite powders that 2.4 wt.% GO uniformly distributed on the surface of Al flakes after being filtered and vacuum dried at 65 °C for 10 h (Fig. 1d). Third, an appropriate amount of the spherical 2024Al powders (80 wt.%) were then mixed with 2.4 wt.% GO/Al flakes through a V-blender with a ball-to-powder ratio of 5:1, mixing time of 40 min, rotation speed of 120 rpm, preparing the 20wt.%(2.4wt.%GO/Al)/80wt.%Al
composite powders that 2.4 wt.% GO/Al flakes homogeneously dispersed surrounding the spherical Al powders (Fig. 1e). The above mentioned 20 wt.% and 80 wt.% represent the mass fraction of flake Al powders and spherical Al powders in the composite powders, respectively. Further, the GO/Al composite powders formed by electrostatically adsorbing GO with the acidified Al flakes were thermally annealed at 400 °C for 6 h in a 9:1 argon-hydrogen mixed atmosphere. After annealing, the GO was reduced to RGO, obtaining the RGO/Al hybrid powders. Fourth, the powder samples were consolidated by vacuum hot-pressing sintering furnace at 300 °C and 445 MPa for 45 min, obtaining a cylindrical specimen with dimensions of Ø45 mm×16 mm (Fig. 1f). Fifth, the hot-pressed compact was remelted at 625 °C for 1 h (Fig. 1g). Finally, the semisolid ingot was immediately poured into the die attached to the press for thixoforming (Fig. 1h). The forming parameters are as follows: the semisolid ingot was quickly placed in a mold preheated to 300 °C in advance and held at 192 MPa for 20 s to obtain a cylindrical 20wt.%(2.4wt.%RGO/Al)/80wt.%Al composite (denoted as FPT-0.4wt.%RGO/Al) with dimensions of Ø50 mm×12 mm. For comparison, another group of 0.4 wt.% RGO/Al matrix composite was prepared by HEM (HEM-0.4wt.%RGO/Al) that 0.4 wt.% RGO was directly blended with the spherical 2024Al powders without introduction of Al flakes. A ball-to-powder ratio of 10:1, ball milling time of 240 min, and the rotation speed of 300 rpm were used. The 2024Al alloy was prepared as a matrix sample in contrast to the above composite.
Fig. 1. Schematic of the FPT technology.
2.4. Characterization The morphology of the composite and the distribution of RGO in the matrix were observed by field emission scanning electron microscopy (SEM, JSM-7500F, Japan) and transmission electron microscopy (TEM, FEI talos200s, Japan). To further study the RGO distribution in the thixoformed composite, the metallographic specimens were etched by an electrochemical dissolution technology in an electrolyte comprising 20 vol.% perchloric acid in ethanol at a voltage of 10 V D.C. and 2 A current. Raman spectrometer with a laser wavelength of 785 nm and equipped with an optical
microscope (DXR, USA) was used to study the damage of two-dimensional planar RGO structures in composites. The dumbbell-shaped tensile specimens were machined from the thixoformed RGO/Al composites parallel to the forming pressure. The gauge length, width and thickness of the tensile specimens were 10 mm, 8 mm and 1.5 mm, respectively, which was clearly shown in Fig. 2. Tensile tests were carried out by an universal testing machine (CMT5000, China) with crosshead speed of 0.5 mm/min. For each material, the tensile strength was measured from four tensile specimens.
Fig. 2. Dimensions of the tensile test specimen (unit: mm).
3. Results and discussion 3.1. Morphologies and structure of RGO/Al composite powders Fig. 3a shows the morphology of spherical 2024Al powders with an average particle size of 20 µm. The spherical powders were changed into Al flakes with an average particle diameter of 40 µm and thickness of 2 µm after being ball milled (Fig. 3b). The surface of the Al flakes was also extremely smooth and no fragmental Al powders on their surface were observed. It is known that cold welding, crushing and plastic deformation occurred during ball milling process. The dimensions as well as morphologies of the thin Al flakes could be well optimized by regulating the ball milling parameters. GO prepared by modified Hummers method exhibited a typically wrinkled morphology and a large specific surface area (Fig. 3c). It was observed that RGO could be directly adsorbed on the surface of the acidified 2024 aluminum flakes by electrostatic attraction force and homogeneously mixed with the prepared Al flakes (Fig. 3d). The supernatant liquid of the GO/2024Al suspension was colorless and transparent instead of light brown resulted from the aggregation of GO in the solution, which confirms the success of the GO dispersion (Fig. 3d). The high magnification micrograph indicated that RGO did not agglomerate into clusters and the surface of the Al flakes was entirely coated by RGO, implying the great superiority of RGO dispersion by using Al flakes as the efficient carrier (Fig. 3e). TEM micrograph further indicated that the completely unfolded RGO homogeneously distributed on the
Al flakes (Fig. 3f). However, the spherical Al powders changed into irregular shape and RGO still aggregated into clusters with large size in the RGO/Al composite powders prepared by HEM method (Fig. 3g and h). This results demonstrate that the aluminum flakes as an effective carrier have a much better effect on uniformly dispersing RGO in the matrix than HEM method.
Fig. 3. Micrographs of (a) spherical 2024Al powders, (b) 2024Al flakes, (c) GO, (d, e, f) 2.4wt.%RGO/Al hybrid powders prepared by FPT and (g, h) 0.4wt.%RGO/Al hybrid powders prepared by HEM.
Raman spectroscopy is crucial to detect composition and analyze carbon structure. Fig. 4a shows the Raman spectra of the corresponding composite powders. It is obvious that two vibration peaks appeared in the RGO powders, which are known as the D-band (1357.7cm-1) and G-band (1596.8cm-1), respectively. It is known that the ID/IG ratio exhibits the degree of graphitization as well as the structural defect of carbonaceous materials and a higher ID/IG ratio demonstrates that its structural integrity is significantly destroyed [16]. The ID/IG value of the FPT-0.4wt.%RGO/Al hybrid powders (1.03) was only 4.0 % higher than that of the RGO powders (0.99), indicating that little extra damage to the structural integrity of RGO was caused during the mixing process. However, the ID/IG ratio of the HEM-0.4wt.%RGO/Al hybrid powders prepared by HEM method at a rotation speed of 300 rpm for 4 h was increased to ~1.14 that was 15.2 % and 10.7 % higher than that of the RGO and FPT-0.4wt.%RGO/Al composite powders, respectively, implying significant damage of RGO structure. In addition, G-bands of the FPT-0.4wt.%RGO/Al and the HEM-0.4wt.%RGO/Al hybrid powders on the Raman spectra did not exhibit obvious blue shift characteristics which were related to the larger damage of RGO’s carbon structure [17, 18].
Fig. 4. Raman spectra of (a) RGO/Al hybrid powders and (b) RGO/Al composites.
The uniform distribution and well-maintained structural integrity of RGO were the most crucial factors for improving the performance of RGO reinforced Al matrix
composites. The dispersion of RGO can be only improved to some extent through using the traditional ball milling procedure with a longer milling time and a higher rotation speed, but the RGO structure would be seriously and simultaneously damaged (Fig. 4a). The ball milling effectively increased the surface area of 2024Al powders, which was believed to have even more contact area with RGO than spherical Al powders because the required size and morphology of the metal matrix powders were compatible with those of the RGO that possessed with larger specific surface [18], facilitating RGO dispersion. Fig. 3e shows that RGO was evenly distributed on the 2024Al matrix, this phenomenon fully demonstrated that the flake 2024Al powders meeted the requirements of morphological compatibility between RGO and matrix powders. However, the Al2O3 film inevitably existed on the Al powder surface, especially inhibiting a better RGO dispersion through direct electrostatic adsorption [4]. The dense Al2O3 film on the as-received Al flake powders was peeled off after being acidified to expose fresh Al surface. The ionization of Al powders within the GO/Al suspension during blending procedures can then be obviously accelerated and the continuously released Al3+ attached to the surface of acidified Al flakes. The positively charged Al flakes would be tightly combined with the negatively charged GO by electrostatic attraction force due to the interattraction of the opposite charges, which makes the unfolded GO to uniformly and firmly distribute on the surface of the matrix powders and prevents the re-aggregation of GO on the metal surface during the whole mixing procedures. On the contrary, the large shear stress resulted from the repeated ball-to-powder impact not only achieved relatively limited distribution of RGO but also caused serious damage to the RGO structure during the long duration HEM required to achieve the homogeneous RGO distribution. Therefore, it is better to homogeneously disperse RGO with well-maintained structure by introducing flaky Al powders as effective carrier to achieve the electrostatic interaction between GO and matrix powders. 3.2. Microstructure of the hierarchical RGO/2024Al composites Fig. 5 shows the SEM micrographs of the FPT-0.4wt.%RGO/Al green compacts heated at 625 °C for different time. It indicated that the Al powders within the
as-received green compact interconnected with each other and many irregular pores with an average size of 3 µm appeared between the powders (marked by arrows in Fig. 5a). The Al powders connected together due to the plastic deformation occurred during hot pressing, generating the indistinct boundary and many black pores among the powders. Simultaneously, the characteristics of flaky powders existed between the spherical Al powders were extremely obvious and no stacking and agglomeration of these flaky powders occurred (Fig. 5b). The FPT-0.4wt.%RGO/Al green compact was fabricated by pressing the composite powders consisted of spherical Al powders uniformly surrounded by the RGO/Al flakes. Thus, it can be concluded that the RGO/Al flaky powders uniformly distributed in the green compact after being pressed. The liquid phase appeared between the α-Al primary particles and the inter-granular pores were significantly decreased after being heated at a semisolid temperature of 625 °C for 30 min (compared with Fig. 5a and c). Moreover, it was observed that not only the spherical particles but also the fiber-like microstructure appeared in the semisolid ingot (Fig. 5d). Our previous investigations indicated that the liquid phase formed resulting from the melting of the edges of the powders during partial remelting and a single powders within the green compact evolved into a primary particle in the semisolid ingot [19]. The majority of the inter-granular pores between the α-Al powders could be filled by the generated liquid phase as the heating time increases because the gradually increased liquid amount accelerated the pore filling, resulting in the microstructure densification.
Fig. 5. SEM images of FPT-0.4wt.%RGO/Al green compact heated at 625 °C for (a, b) 0 min and (c, d) 30 min.
Fig. 6 shows the SEM micrographs of the thixoformed FPT-0.4wt.%RGO/Al composite with hierarchical microstructure. The microstructure was consisted of primary α-Al particles and inter-granular secondary solidified structures (SSSs) (Fig. 6a). SSSs mainly included secondary α-Al phase and eutectic α-Al and Al2Cu phases. Moreover, the fiber-like α-Al particles were still distributed among the spherical α-Al particles (Fig. 6b), which was similar to the results shown in Fig. 5c. Fig. 6c and d
show the EDS results from Fig. 6b, indicating that the SSSs close to the fiber-like Al particles were rich in carbon element, while the regions far away from these fiber-like microstructure were almost free of carbon element. Thus, RGO was embedded into some local regions of the SSSs in the thixoformed RGO/Al composite. In order to further confirm the RGO distribution, the microstructure obtained by electrolytic etching of the metallographic sample was also examined. The deep corrosion treatment causes the Al matrix to be seriously consumed, forming many corrosion pits with different dimensions, while the eutectic phases and RGO reinforcement were remained (Fig. 6e). Especially, it was observed that the RGO existed within the corrosion pits uniformly dispersed among the flaky Al particles (marked by arrows in Fig. 6e and f). RGO located in the local regions of liquid phase that was close to the fiber-like Al particles due to the partial remelting of the RGO/Al composite powders and then RGO distributed in some regions of the SSSs in the FPT-0.4wt.%RGO/Al composite after thixoforming process, forming RGO-rich regions. Therefore, it was easy to understand that the FPT-0.4wt.%RGO/Al composite with hierarchical microstructure that RGO only embedded in the local regions of SSSs was successfully constructed in this work by FPT technology. Raman spectra of the thixoformed RGO/Al composites were also investigated to clarify whether RGO structure was further affected by the following processing parameters besides the mixed parameters. As shown in Fig. 4b, ID/IG value of the FPT-0.4wt.%RGO/Al composite (1.01) was only 2.0 % higher than that of the as-received RGO (0.99). Moreover, its value was 1.9 % lower than that of the FPT-0.4wt.%RGO/Al hybrid powders (1.03) and thus no significant damage of RGO structure
was
further
induced
by
thixoforming
technology.
For
the
HEM-0.4wt.%RGO/Al composite, ID/IG value was increased to 1.15, which is 16 % and 14 % higher than that of the as-received RGO and FPT-0.4wt.%RGO/Al composite, respectively. It can be concluded that little influence of the processing parameters on the structural integrity of RGO in the FPT-0.4wt.%RGO/Al composite was induced, which directly contributes to the improvement in the mechanical properties of the composite.
Fig. 6. (a, b) SEM images of the FPT-0.4wt.%RGO/Al composite. (c, d) EDS spectra of Area 1 and Area 2 in Fig. 6b, respectively. (e, f) SEM images of the electrolytic polished cross section of FPT-0.4wt.%RGO/Al composite.
Fig. 7 shows the TEM micrographs of the FPT-0.4wt.%RGO/Al composite. Gr can be observed uniformly embedding in the Al matrix (Fig. 7a and b), consisting with the results shown in Fig. 6. There is evidence of Al2O3 and Al4C3 formation between the RGO with Al matrix and simultaneously the void free RGO/Al interface was formed (Fig. 7c-e). It was approved that formation of small amount of Al4C3 facilitated interfacial bonding and thus contributed to the enhanced strengthening effect [20]. Two kinds of oxygen atoms in the RGO/Al composite were existed: one is resulting from residual functional groups on the RGO and the other is the oxygen atoms in the Al matrix caused by the oxidation of Al alloy during processing procedures. Generally speaking, Al2O3 film is detrimental to the mechanical properties and thus extensive investigations were focused on reducing its concentration during processing procedures. Yuan et. al. synthesized Gr possessing about 12 at.% residual oxygen by a thermal reduction method [21]. The residual oxygen on the Gr surface enhances the Gr/Mg
interfacial
bonding
by
in
situ
forming
semi-coherent
Gr/MgO-nanoparticles/Mg bonds that efficiently accommodate the load transfer between the matrix and Gr, strengthening the Mg matrix. These findings demonstrated that the a small amount of the in situ oxide formed in the Gr/matrix interface has positive effect on improving the mechanical properties of the composites due to the enhanced interfacial bonding. Therefore, a hybrid Al/Al2O3/C and Al/Al4C3/C interfaces were formed in the FPT-0.4wt.%RGO/Al composite.
Fig. 7. TEM images of the FPT-0.4wt.%RGO/Al composite.
3.3. Tensile properties of the RGO/Al composites The tensile results of the RGO/Al composites were shown in Fig. 8. The values of the ultimate tensile strength (UTS), yield strength (YS) and elongation for the specimens were summarized in Table 1, indicating that the UTS, YS and elongation of
the 2024Al matrix alloy were measured to be 301 MPa, 203 MPa, and 15.2 %, respectively. The incorporation of only 0.4 wt.% RGO into the Al matrix resulted in 8.0 %, and 7.4 % improvements in the UTS and YS and 63.8 % decrease in the elongation of the HEM-0.4wt.%RGO/Al composite as compared with the 2024Al alloy. For the FPT-0.4wt.%RGO/Al composite, the UTS, YS as well as elongation were 35.1 %, 34.9 % and 54.5 % higher than those of the HEM-0.4wt.%RGO/Al composite, respectively. It should be noted that the same concentration of RGO was used for fabricating the RGO/Al composites in this work, while the strengthening and toughening effects of the FPT-0.4wt.%RGO/Al composite were more significant than those of the HEM-0.4wt.%RGO/Al composite. For comparison, the mechanical properties of the Gr/Al matrix composites prepared by different methods were also summarized in Table 1. The tensile strengths appeared to obviously increase along with the decrease of elongation. The (3Gr+10SiC)/2024Al composite prepared by PM technology possessed an UTS of 387 MPa, while the elongation (5.7 %) was 49 % lower than that of the FPT-0.4wt.%RGO/Al composite in this work [22]. Apparently, the RGO/Al composite with hierarchical microstructure was much more efficient in strengthening as well as toughening the matrix alloy as compared with the current composites reported so far. Therefore, this work proposed a promising technology that could improve the strength as well as elongation of the RGO/Al composites through tailoring RGO spatial arrangement.
Fig. 8. Stress-strain curves of the 2024Al alloy and RGO/Al composites. Table 1 Mechanical properties of RGO/Al composites. Material
Method
UTS (MPa)
YS (MPa)
Elongation (%)
Reference
2024Al alloy
FPT
301
203
15.2
This work
FPT-0.4wt.%RGO/Al
FPT
439
294
8.5
This work
HEM-0.4wt.%RGO/Al
FPT
325
218
5.5
This work
0.5vol.%RGO/Al (200-nm-thick Al flakes)
PM
256
224
6.4
[7]
0.1vol.%RGO/Al (1µm-thick Al flakes)
PM
145
90
16.3
[7]
2vol.% Gr/Al-Cu
PM
280
260
6.3
[23]
3vol.%Gr/10vol.%SiC/2024Al
PM
387
281
5.7
[22]
3wt.% Gr/2124Al
PM
421
180
4.8
[24]
To investigate the strengthening mechanism of the RGO/Al composites, the examination of the corresponding fracture surfaces after tensile failure was carried out (Fig. 9). A large number of dimple fracture morphology of 2024Al alloy such as small and deep dimples and tearing edges were appeared on the fracture surface (Fig. 9a and b), which is related to the ductile fracture characteristic during loading. Fig. 10 shows the side views of fracture surfaces of the RGO/Al composites and 2024Al alloy during tensile testing. The cracks almost propagated across the primary α-Al particles that were deformed plastically and elongated parallel to the tensile direction (marked by the arrows A in Fig. 10a), exhibiting a transgranular fracture mode of the 2024Al alloy. Moreover, many micro-pores and micro-cracks existed in the SSSs among the primary particles (marked by the arrows B in Fig. 10a). We further examined the regions away from the fracture surface, which was more obvious to exhibit the crack initiation and propagation progress. The primary particles were obviously elongated along with the tensile direction and many slip bands within these deformed particles were observed (Fig. 10b), verifing that the substantial plastic deformation of the Al matrix material prior to fracture occurred. During tensile process, the primary α-Al particles preferentially deformed, which can be demonstrated by the formed slip bands within the elongated primary particles. The SSSs between the primary particles mainly included the secondary α-Al phase and eutectic α-Al and Al2Cu phases. The existence of a hard and brittle eutectic Al2Cu phase weakened the plastic deformation ability of the SSSs compared the relatively soft primary α-Al particles. That is, the primary phase particles have stronger deformation ability and greater deformation degree. As the tensile testing continues, some micro-pores formed in the SSSs because of the incoordinated plastic deformation (Fig. 10a and b). The micropores grew into microcracks and then evolved into large cracks as the tensile testing
proceeds, resulting in material failure through the propagation of the interconnected microcracks across the deformed α-Al particles. It can be thus concluded that the fracture mechanism of the matrix is: firstly, the primary particles deform plastically during tensile process and then micropores and microcracks are generated in the SSSs. Subsequently, the number and size of the microcracks increase and finally the microcracks interconnect together with each other, causing the cracks to propagate across the deformed primary particles and resulting in the fracture of the material.
Fig. 9. SEM images of the fracture surface of (a, b) 2024Al alloy, (c, d) FPT-0.4wt.%RGO/Al composite and (e, f) HEM-0.4wt.%RGO/Al composite.
For the FPT-0.4wt.%RGO/Al composite, it is obvious that its fracture surface was similar to that of the 2024Al alloy and a typically ductile fracture mode characterized by deep dimples was also observed (compared with Fig. 9a and c). However, as indicated by the white dashed ellipses in Fig. 9c, some flaky microstructure features with the similar dimension of the flaky Al particles existed in the thixoformed microstructure appeared. Micro-cracks followed the interface between the flaky microstructure and surrounding matrix, as indicated by the arrows in Fig. 9c. The surface of the fiber-like structure was not only uniformly coated by many dimples, but also contained a higher concentration of the debonded and fractured RGO reinforcement (Fig. 9d). The side views of fracture surfaces indicated that the fracture mode still belonged to a transgranular fracture and a large size pits appeared (Fig. 10c). It is obvious that many densely distributed slip bands were distributed within the elongated α-Al particles and simultaneously micro-pores and micro-cracks among the primary particles were observed. Cracks firstly initiated in the regions close to the flaky Al particles but did not continuously propagated across the flaky particles (marked by the arrows in Fig. 10d), exhibiting an obvious crack deflection. It is well known that the debonding and fracture of reinforcements due to the poor interfacial bonding, stress concentration and aggregation of reinforcements always resulted in the premature fracture and even deteriorative mechanical properties of the composites [25]. During tensile testing, the matrix of the RGO-rich regions plastically deformed
and stress gradually concentrated at the RGO/Al interface. The dislocation density around the RGO should be higher because a smaller inter-particle distance can effectively hinder the dislocation movement through the cross slip and climb of dislocations [26]. The interaction between the RGO and dislocations resulted in the formation of dislocation punched zones around the RGO and thus contributed to enhance the strength of the composite. In addition, the different coefficient of thermal expansion between the RGO and Al matrix will generate thermal stress near the RGO/Al interface during cooling from the processing temperature to room temperature, increasing the dislocation density. On the other hand, the large specific area of RGO contributed to the effective load transfer from the softer Al matrix to the harder RGO through the strong RGO/Al interface, contributing to the strength improvement and thus resulting in the load transfer strengthening. It was reported that the individual dislocation punched zones will superposed together with each other if the volume fraction and size of the nanoparticles was reasonably tailored and then a transition of failure mechanisms occur from a reinforcement/matrix boundary dominated failure initiation to a dislocation punched zone/matrix interface dominated failure initiation. The flaky microstructure associated with the failed RGO on the fracture surface and the crack deflection of the RGO-rich zones indicated that most of the cracks possibly nucleated near the interface between the RGO-rich zones and the matrix (Fig. 9d and 10d). Thus, it is expected that the overlapping of the dislocation punched zones is more likely to occur within the RGO-rich zones and the RGO-rich regions can behave as reinforcing units to strengthen the matrix alloy. The strengthening effect of the fiber-like strengthening units was much more effective than the spherical strengthening units because the large aspect ratio of reinforcements obviously contributes the tensile strength. The dislocation can be gradually concentrated at the RGO-rich regions/matrix interface during tensile testing, which induced the pull-out of the RGO-rich zones through the microcrack initiation and propagation at those interfaces. The flaky pits associated with the RGO on the fracture surface should be corresponded to the RGO-rich regions that debonded and/or fractured during tensile process. On the other hand, the RGO-free zones can block the
crack tip and increase the propagated routes of cracks, toughening the composite.
Fig. 10. Side views of fracture surfaces of (a,b) 2024Al alloy, (c, d) FPT-0.4wt.%RGO/Al composite and (e, f) FPT-0.4wt.%RGO/Al composite.
The fracture surface of the HEM-0.4wt.%RGO/Al composite was obviously different from the ductile fracture characteristic of the 2024Al alloy and FPT-0.4wt.%RGO/Al composite (compared with Fig. 9a, c and e). The RGO severely agglomerated into clusters and was easily peeled off from the matrix (Fig. 9f), which further demonstrated the inferior effect of HEM procedure on RGO dispersion and a weak strengthening efficiency. The side views of fracture surfaces indicated that the RGO aggregation induced crack initiation and propagation and the fracture mode transferred into intergranular fracture (Fig. 10e and f). Due to the agglomeration and overlap of the reinforcing phase in the HEM-0.4wt.%RGO/Al composite, the interface bonding strength was weakened. In addition, the RGO structure of the HEM-0.4wt.%RGO/Al composite was obviously damaged during HEM process that can be confirmed by the variation of ID/IG ratio (Fig. 4b). RGO debonded and fractured prematurely as the external load increases, inducing the crack nucleation and propagation along the matrix and then causing the premature failure of the composite during tensile testing. Since the material structure determines the performance, the above phenomenon corresponds to the change in the mechanical properties shown in Table 1. Therefore, the strengthening effect of RGO/Al composites could be brought about by introduction of RGO, while an unattainable combination of ultrahigh strength and good elongation was achieved by tailoring the spatial array of RGO, forming a hierarchical microstructure that RGO only homogeneously distributed in the local regions of SSSs. 4. Conclusion This study successfully constructed the RGO/2024Al composites with hierarchical microstructure by FPT technology that combined FPM and thixoforming technology. An ideal semi-solid ingot suitable for thixoforming can be obtained when the green compact containing the RGO/Al flakes and spherical Al powders was partially
remelted at 625 °C. RGO can be uniformly dispersed in the local regions of the SSSs by intentionally introducing Al flakes as effective carrier, but HEM method was not conducive to disperse RGO without impairing its structural integrity, verifying that FPT technology is more superior to HEM for the efficient dispersion of RGO in the matrix. For the FPT-0.4wt.%RGO/Al composite, the UTS, YS and elongation were 35.1 %, 34.9 %, and 54.5 % higher than those of the HEM-0.4wt.%RGO/Al composite, respectively. The reasons should be attributed to the synthesized fiber-like RGO-rich zones that behave as reinforcing units and thus enhancing the strengthening efficiency. The fracture mode transferred from a transgranular fracture of the FPT-0.4wt.%RGO/Al
composite
into
an
intergranular
fracture
of
the
HEM-0.4wt.%RGO/Al composite. This work proposed a promising technology that strengthened and toughened the composites through tailoring the spatial arrangement of RGO. 5. Acknowledgments The authors acknowledge the financial support by the Ningxia Natural Science Foundation of China (Grant No. 2018AAC03031), the Basic Scientific Fund of Ningxia university (Grant No. NGY2018009), the Natural Science Foundation of Ningxia University (Grant No. ZR1702), the Key R & D Project of Ningxia (2018BEE03008) and the Science and Technology Project for Young Talents of Ningxia (Grant No. TJGC2019042). Reference [1] J.C. Meyer, A.K. Geim, M.I. Katsnelson, K.S. Novoselov, T.J. Booth, S. Roth. The structure of suspended graphene sheets, Nature 446 (2007), 60-63. [2] Z.Y. Yang, L.D. Wang, Z.D. Shi, M. Wang, Y. Cui, B. Wei, S.C. Xu, Y.P. Zhu, W.D. Fei. Preparation mechanism of hierarchical layered structure of graphene/copper composite with ultrahigh tensile strength, Carbon 127 (2018), 329-339. [3] X.N. Mu, H.M. Zhang, H.N. Cai, Q.B. Fan, Z.H. Zhang, Y. Wu, Z.J. Fu, D.H. Yu. Microstructure evolution and superior tensile properties of low content graphene nanoplatelets reinforced pure Ti matrix composites, Mater. Sci. Eng. A 687 (2017), 164-174. [4] Z. Li, G.L. Fan, Z.Q. Tan, Q. Guo, D.B. Xiong, Y.S. Su, Z.Q. Li, D. Zhang. Uniform dispersion
of graphene oxide in aluminum powder by direct electrostatic adsorption for fabrication of graphene/aluminum composites, Nanotechnology 25 (2014), 325601. [5] T. Lokesh, U.S. Mallik. Effect of ECAP process on the Microstructure and Mechanical Properties of Al6061-Gr Composites, Materials Today: Proceedings 5 (2018), 2453-2461. [6] A. Naseer, F. Ahmad, M. Aslam, B.H. Guan, W.S.W. Harun, N. Muhamad, M.R. Raza, R.M. German. A review of processing techniques for graphene-reinforced metal matrix composites, Mater. Manuf. Processes 34 (2019), 957-985. [7] L. Zhao, Q. Guo, Y. Shi, Y. Liu, S. Osovski, Z. Li, D.-B. Xiong, Y. Su, D. Zhang. Interfacial Effect on the Deformation Mechanism of Bulk Nanolaminated Graphene-Al Composites, Metall. Mater. Trans. A 50 (2019), 1113-1118. [8] Z. Fan. Semisolid metal processing, Int. Mater. Rev. 47 (2002), 49-85. [9] P.B. Li, T.J. Chen, H. Qin. Effects of mold temperature on the microstructure and tensile properties of SiCp/2024 Al-based composites fabricated via powder thixoforming, Mater. Des. 112 (2016), 34-45. [10] E. Ma, T. Zhu. Towards strength–ductility synergy through the design of heterogeneous nanostructures in metals, Mater. Today 20 (2017), 323-331. [11] L. Jiang, H. Yang, J.K. Yee, X. Mo, T. Topping, E.J. Lavernia, J.M. Schoenung. Toughening of aluminum matrix nanocomposites via spatial arrays of boron carbide spherical nanoparticles, Acta Mater. 103 (2016), 128-140. [12] L.J. Huang, L. Geng, H.X. Peng. Microstructurally inhomogeneous composites: Is a homogeneous reinforcement distribution optimal?, Prog. Mater. Sci. 71 (2015), 93-168. [13] S. Xiang, X. Wang, M. Gupta, K. Wu, X. Hu, M. Zheng. Graphene nanoplatelets induced heterogeneous bimodal structural magnesium matrix composites with enhanced mechanical properties, Sci. Rep. 6 (2016), 38824. [14] P.B. Li, T.J. Chen, S.Q. Zhang, R.G. Guan. Research on semisolid microstructural evolution of 2024 aluminum alloy prepared by powder thixoforming, Metals 5 (2015), 547-564. [15] H.J. Yu, Z.G. Zou, F. Long, C.Y. Xie, H. Ma. Preparation of graphene with ultrasound-assisted in the process of oxidation, Applied Mechanics and Materials 34-35 (2010), 1784-1787. [16] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D.
Jiang, K.S. Novoselov, S. Roth. Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006), 187401. [17] J.-M. Ju, G. Wang, K.-H. Sim. Facile synthesis of graphene reinforced Al matrix composites with improved dispersion of graphene and enhanced mechanical properties, J. Alloys Compd. 704 (2017), 585-592. [18] L. Jiang, G. Fan, Z. Li, X. Kai, D. Zhang, Z. Chen, S. Humphries, G. Heness, W.Y. Yeung. An approach to the uniform dispersion of a high volume fraction of carbon nanotubes in aluminum powder, Carbon 49 (2011), 1965-1971. [19] P.B. Li, T.J. Chen, S.Q. Zhang. Effects of reheating time on microstructure and tensile properties of SiCp/2024 Al-based composites fabricated using powder thixoforming, Met. Mater. Int. 23 (2017), 193-201. [20] B. Chen, L. Jia, S. Li, H. Imai, M. Takahashi, K. Kondoh. In Situ Synthesized Al4C3 Nanorods with Excellent Strengthening Effect in Aluminum Matrix Composites, Adv. Eng. Mater. 16 (2014). [21] Q.-h. Yuan, G.-h. Zhou, L. Liao, Y. Liu, L. Luo. Interfacial structure in AZ91 alloy composites reinforced by graphene nanosheets, Carbon 127 (2018), 177-186. [22] C.-j. Hu, H.-g. Yan, J.-h. Chen, B. Su. Microstructures and mechanical properties of 2024Al/Gr/SiC hybrid composites fabricated by vacuum hot pressing, Trans. Nonferrous Met. Soc. China 26 (2016), 1259-1268. [23] B. Pu, J. Sha, E. Liu, C. He, N. Zhao. Synergistic effect of Cu on laminated graphene nanosheets/AlCu composites with enhanced mechanical properties, Mater. Sci. Eng. A 742 (2019), 201-210. [24] A. El-Ghazaly, G. Anis, H.G. Salem. Effect of graphene addition on the mechanical and tribological behavior of nanostructured AA2124 self-lubricating metal matrix composite, Compos. Part A 95 (2017), 325-336. [25] M.K. Habibi, A.S. Hamouda, M. Gupta. Hybridizing boron carbide (B4C) particles with aluminum (Al) to enhance the mechanical response of magnesium based nano-composites, J. Alloys Compd. 550 (2013), 83-93. [26] A.F. Boostani, R.T. Mousavian, S. Tahamtan, S. Yazdani, R.A. Khosroshahi, D. Wei, J.Z. Xu, D. Gong, X.M. Zhang, Z.Y. Jiang. Graphene sheets encapsulating SiC nanoparticles: A roadmap
towards enhancing tensile ductility of metal matrix composites, Mater. Sci. Eng. A 648 (2015), 92-103.
Figure Captions: Fig. 1. Schematic of the FPT technology. Fig. 2. Dimensions of the tensile test specimen (unit: mm). Fig. 3. Micrographs of (a) spherical 2024Al powders, (b) 2024Al flakes, (c) GO, (d, e, f) 2.4wt.%RGO/Al hybrid powders prepared by FPT and (g, h) 0.4wt.%RGO/Al hybrid powders prepared by HEM. Fig. 4. Raman spectra of (a) RGO/Al hybrid powders and (b) RGO/Al composites. Fig. 5. SEM images of FPT-0.4wt.%RGO/Al green compact heated at 625 °C for (a, b) 0 min and (c, d) 30 min. Fig. 6. (a, b) SEM images of the FPT-0.4wt.%RGO/Al composite. (c, d) EDS spectra of Area 1 and Area 2 in Fig. 6b, respectively. (e, f) SEM images of the electrolytic polished cross section of FPT-0.4wt.%RGO/Al composite. Fig. 7. TEM images of the FPT-0.4wt.%RGO/Al composite. Fig. 8. Stress-strain curves of the 2024Al alloy and RGO/Al composites. Fig. 9. SEM images of the fracture surface of (a, b) 2024Al alloy, (c, d) FPT-0.4wt.%RGO/Al composite and (e, f) HEM-0.4wt.%RGO/Al composite. Fig. 10. Side views of fracture surfaces of (a,b) 2024Al alloy, (c, d) FPT-0.4wt.%RGO/Al composite and (e, f) FPT-0.4wt.%RGO/Al composite.
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Flake powder thixoforming technology was developed. A FPT-0.4wt.%RGO/Al composite with hierarchical microstructure was fabricated. The strengthening and toughening are achieved by tailoring the dispersion of RGO.
Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. Pubo Li wrote the paper and contributed to all activities. Luyao Chen performed the experiments under Pubo Li’s guidance. Bo Cao, Luyao Chen, Keren Shi contributed to analysis tools and the interpretation and discussion of experimental results.
S0925-8388(20)30178-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.153815
Reference:
JALCOM 153815
To appear in:
Journal of Alloys and Compounds
Received Date: 8 November 2019 Revised Date:
9 January 2020
Accepted Date: 11 January 2020
Please cite this article as: P. Li, L. Chen, B. Cao, K. Shi, Hierarchical microstructure architecture: A roadmap towards strengthening and toughening reduced graphene oxide/2024Al matrix composites synthesized by flake powder thixoforming, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153815. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Pubo Li: Data curation, Writing- Original draft preparation, Writing- Reviewing and Editing.
Luyao
Chen: Conceptualization,
Methodology. Bo
Validation. Keren Shi: Visualization, Investigation, Supervision.
Cao:
Software,
RGO-free zones
RGO/Al powders
RGO-rich zones
Al powders
Strengthening and toughening
Hierarchical microstructure RGO-free zones
RGO RGO RGO-rich zones 30 µm
1 µm Al2O3 (104) 0.263 nm
RGO (100) 0.214 nm
Al Al (200) 0.202 nm
10 nm
Hierarchical microstructure architecture: A roadmap towards strengthening and toughening reduced graphene oxide/2024Al matrix composites synthesized by flake powder thixoforming Pubo Li a,*, Luyao Chen a, Bo Cao a, Keren Shi b a
Ningxia Key Laboratory of Photovoltaic Materials, Ningxia University, Yinchuan,
750021, P. R. China b
State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical
Engineering, Ningxia University, Yinchuan 750021, P. R. China * Corresponding author. Tel: +86 951 2062414 E-mail address: [email protected] (Pubo Li). Abstract Graphene as one of the best reinforcements for improving the comprehensive properties of the composites has attracted extensive attention. However, the severe aggregation of graphene in the matrix obviously weakened the strengthening efficiency. A novel method, flake powder thixoforming (FPT) that combines both the advantages of flake powder metallurgy and thixoforming, was proposed to construct the FPT-0.4wt.%RGO/Al composite with hierarchical microstructure that reduced graphene oxide (RGO) only uniformly distributed into the local regions of secondary solidified structures (SSSs). The ultimate tensile strength (UTS, 439 MPa), yield strength (YS, 294 MPa) and elongation (8.5 %) of FPT-0.4wt.%RGO/Al composite fabricated by FPT were increased by 45.8 % and 44.8 % and decreased by 44.1 % compared with the 2024Al alloy, respectively. Moreover, a 34.9 % enhancement in YS and a 54.5 % improvement in elongation were achieved when compared with the HEM-0.4wt.%RGO/Al composite prepared by high energy milling (HEM). The RGO-rich zones in the FPT-0.4wt.%RGO/Al composite behaved as reinforcing units that can sustain the tensile stress and thus strengthened the matrix. Simultaneously, crack deflection and bridging contributed by the RGO-free zones effectively toughened the composite. FPT is promising for strengthening and toughening the RGO/Al composites by tailoring the spatial arrangement of RGO to form hierarchical microstructure.
Keywords Reduced graphene oxide; Aluminum matrix composites; Hierarchical microstructure; Flake powder thixoforming; Mechanical properties. 1. Introduction Owing to the two-dimensional single-atom thick-layer structure and its superior properties, such as higher strength (130 GPa) and elasticity modulus (1.1 TPa), graphene (Gr) was regarded as the most promising candidate for reinforcing the metal matrix composites [1, 2]. Gr/Al matrix composites have gained considerable attention among aerospace and automotive industry due to their higher specific strength and specific stiffness, engineering composites with unattainable combinations of mechanical and physical properties to meet the ever increasing performance requirements of advanced technologies [3, 4]. It was reported that the introduction of 2 wt.% Gr resulted in 1.25 times increase in the ultimate tensile strength (UTS) compared to the Al matrix material [5]. Li [4] et al. reported that the elastic modulus and hardness of the 0.3 wt.% reduced graphene oxide (RGO)/Al composites increased by 18 % and 17 %, respectively, compared with the Al matrix. The main strengthening contribution was attributed to the load transfer mechanism resulting from the strong RGO/Al interfacial bonding as well as the large specific surface area of RGO. Although the significant progresses were achieved in the Gr reinforced metal matrix composites, the improvement in the comprehensive properties was always not commensurate with the extraordinary properties of Gr. One of the reasons was the serious aggregation of Gr due to the Van der Walls interactions. The majority of investigations were focused on the dispersion of Gr within the matrix and the obtained results indicated that high-energy ball milling (HEM) and flake powder metallurgy (FPM) were the two mainstream methods [4, 6]. FPM not only homogeneously dispersed Gr but also remained its structural integrity as compared with HEM [7]. Moreover, the strengthening effect and fracture ductility of the nano-laminated Gr/Al composites prepared by FPM were simultaneously improved. The fabricated processes of FPM are essentially similar to the traditional powder metallurgy (PM) except the mixing of Gr with the flake powders. PM is the most attractive method due
to the controlled interface reaction and uniformly dispersed reinforcements. However, the wide application of this method is retarded by the difficulty in fabricating components with compact microstructures and complex shapes. Thixoforming is suitable for near-net-shape forming of components with complicated shapes [8]. Especially, the performance of the thixoformed components can be enhanced through decreasing or even eliminating the pores. Combining with PM and thixoforming technology, a powder thixoforming approach for particle reinforced composites was proposed in our previous works [9]. The tensile strength of the SiC/Al composite prepared by powder thixoforming was improved compared with the composite prepared by other traditional methods, revealing the superiority for strengthening the matrix alloy. However, the mismatch between the strength and ductility of the composites hindered their wide applications in the engineering industries. The strengthening efficiency of the Al matrix composites reinforced by homogeneously distributed Gr was higher than the composite reinforced by traditional reinforcements, such as SiC and Al2O3 particles, a key problem was that the increased strength also accompanied by the decreased ductility. Tailoring the distribution of reinforcements at the meso-scale to produce unique microstructural architectures, such as tri-modal, layered and hierarchical architectures, was demonstrated as an effective method to overcome property tradeoffs [10-12]. A hierarchical composite consists of matrix and reinforcing phase comprising the matrix in which is embedded into the reinforcement [12]. That is to say, the matrix in the composite with hierarchical microstructure is reinforced by a composite in itself. These structures depart from the traditional thought that the homogeneous distribution of reinforcement is a prerequisite to enhance the mechanical properties of the composites. For example, a superior combination of strength and fracture ductility of 5083Al alloy by arrays of fiber-like B4Cp-rich zones that contain B4Cp embedded in the Al matrix was achieved [11]. There were only a few investigations involved with the synthesis of Gr reinforced hierarchical composites due to the difficulty in controlled Gr dispersion [2, 13]. The Gr-rich zones reinforced Cu matrix composites prepared by molecular-level mixing process exhibited obvious improvement in both
of strength and ductility compared with the pure Cu matrix [2]. However, disadvantages such as procedure complexity and impurity introduction, limited its practical application. In this work, a novel method for RGO reinforced composites with hierarchical microstructure, flake powder thixoforming (FPT), was proposed by combining the advantages of both FPM and powder thixoforming technology. A pre-compacted mixture of powders of the metal matrix and the reinforcement was prepared by introducing a certain amout of the flake matrix powders that were regarded as the efficient carrier for uniformly distributing RGO into the surrounding of spherical powders and then hot pressing. Subsequently, the powder compacts were formed through the partial remelting and forming procedures of thixoforming, synthesizing the hierarchical RGO/Al composite that RGO only homogeneously dispersed within the local regions of the secondary solidified structures (SSSs). The effects of arrays of RGO-rich zones on the thixoformed microstructure, mechanical properties, fracture behaviour and corresponding strengthening mechanisms were studied. This study offered a promising technology for effectively improving the strength as well as ductility of the RGO/Al composites through tailoring the spatial arrangement of RGO to form a hierarchical microstructure. 2. Experimental 2.1. Raw materials The matrix material was spherical 2024Al powders with an average particle size of 20 µm. Its chemical composition was Al-4.22Cu-1.35Mg-0.5Si-0.5Fe (in wt.%) and the solid and liquid temperatures were 498.04 °C and 663.71 °C, respectively, which was confirmed by the differential thermal analysis reported in our previous work [14]. Glacial acetic acid, anhydrous ethanol (AR) and other reagents were purchased from China Pharmaceutical Group Co. Ltd. 2.2. Preparation of GO GO was prepared from squamous graphite by modified Hummers method [15]. First, flake graphite powders (2 g) and concentrated HNO3 (50 ml) and H2SO4 (50 ml) were mixed slowly and then KMnO4 (5 g) was added after 15min of stirring.
Subsequently, the obtained suspension was heated at 40 °C for 2 h through the water bath. After 10 min of stirring, 10 ml hydrogen peroxide water was added and placed for 12 h and then the supernatant was centrifuged at the speeds of 8000 r/min and 10000 r/min in sequence. According to the above steps, GO with smooth surface and structural integrity can be obtained. 2.3. Preparation of RGO/Al matrix composites The typical fabrication of the hierarchical RGO/Al composites prepared by FPT mainly included six processes, as illustrated in Fig. 1. First, to increase the surface area of 2024Al powders, the spherical powders were milled into flakes by QM-QX2 omnidirectional planetary ball mill with a ball-to-powder ratio of 20:1, ball milling time of 8 h, rotation speed of 300 rpm and the interval between positive and negative rotation of 10 min (Fig. 1a and b). AR was added as dispersant. The prepared Al flakes were filtered and vacuum dried at 65 °C for 10 h. To eliminate the harmful effect of alumina inevitably existed on the surface of Al powders and thus further facilitate the uniform dispersion of GO through direct electrostatic adsorption between the fresh Al powder surface and GO, the as-prepared Al flakes were firstly acidified by 2 vol.% glacial acetic acid (Fig. 1c). After 5 min of ultrasonication with magnetic stirring of 350 rpm, the gray-black liquid in the upper suspension was separated from the precipitated Al flakes. The Al flakes were rinsed with deionized water for several times until the upper solution became grey and a neutral aqueous dispersion was achieved and then a slurry of acid-treated Al flakes was prepared. Second, GO was dispersed in deionized water by ultrasonication for 1 h in the process of adsorption. The obtained GO aqueous solution (1 mg/ml) was slowly added (5 ml/min) into the slurry of acid-treated 2024Al flakes with the assistance of continuous stirring until the upper solution became transparent, synthesizing the GO/Al composite powders that 2.4 wt.% GO uniformly distributed on the surface of Al flakes after being filtered and vacuum dried at 65 °C for 10 h (Fig. 1d). Third, an appropriate amount of the spherical 2024Al powders (80 wt.%) were then mixed with 2.4 wt.% GO/Al flakes through a V-blender with a ball-to-powder ratio of 5:1, mixing time of 40 min, rotation speed of 120 rpm, preparing the 20wt.%(2.4wt.%GO/Al)/80wt.%Al
composite powders that 2.4 wt.% GO/Al flakes homogeneously dispersed surrounding the spherical Al powders (Fig. 1e). The above mentioned 20 wt.% and 80 wt.% represent the mass fraction of flake Al powders and spherical Al powders in the composite powders, respectively. Further, the GO/Al composite powders formed by electrostatically adsorbing GO with the acidified Al flakes were thermally annealed at 400 °C for 6 h in a 9:1 argon-hydrogen mixed atmosphere. After annealing, the GO was reduced to RGO, obtaining the RGO/Al hybrid powders. Fourth, the powder samples were consolidated by vacuum hot-pressing sintering furnace at 300 °C and 445 MPa for 45 min, obtaining a cylindrical specimen with dimensions of Ø45 mm×16 mm (Fig. 1f). Fifth, the hot-pressed compact was remelted at 625 °C for 1 h (Fig. 1g). Finally, the semisolid ingot was immediately poured into the die attached to the press for thixoforming (Fig. 1h). The forming parameters are as follows: the semisolid ingot was quickly placed in a mold preheated to 300 °C in advance and held at 192 MPa for 20 s to obtain a cylindrical 20wt.%(2.4wt.%RGO/Al)/80wt.%Al composite (denoted as FPT-0.4wt.%RGO/Al) with dimensions of Ø50 mm×12 mm. For comparison, another group of 0.4 wt.% RGO/Al matrix composite was prepared by HEM (HEM-0.4wt.%RGO/Al) that 0.4 wt.% RGO was directly blended with the spherical 2024Al powders without introduction of Al flakes. A ball-to-powder ratio of 10:1, ball milling time of 240 min, and the rotation speed of 300 rpm were used. The 2024Al alloy was prepared as a matrix sample in contrast to the above composite.
Fig. 1. Schematic of the FPT technology.
2.4. Characterization The morphology of the composite and the distribution of RGO in the matrix were observed by field emission scanning electron microscopy (SEM, JSM-7500F, Japan) and transmission electron microscopy (TEM, FEI talos200s, Japan). To further study the RGO distribution in the thixoformed composite, the metallographic specimens were etched by an electrochemical dissolution technology in an electrolyte comprising 20 vol.% perchloric acid in ethanol at a voltage of 10 V D.C. and 2 A current. Raman spectrometer with a laser wavelength of 785 nm and equipped with an optical
microscope (DXR, USA) was used to study the damage of two-dimensional planar RGO structures in composites. The dumbbell-shaped tensile specimens were machined from the thixoformed RGO/Al composites parallel to the forming pressure. The gauge length, width and thickness of the tensile specimens were 10 mm, 8 mm and 1.5 mm, respectively, which was clearly shown in Fig. 2. Tensile tests were carried out by an universal testing machine (CMT5000, China) with crosshead speed of 0.5 mm/min. For each material, the tensile strength was measured from four tensile specimens.
Fig. 2. Dimensions of the tensile test specimen (unit: mm).
3. Results and discussion 3.1. Morphologies and structure of RGO/Al composite powders Fig. 3a shows the morphology of spherical 2024Al powders with an average particle size of 20 µm. The spherical powders were changed into Al flakes with an average particle diameter of 40 µm and thickness of 2 µm after being ball milled (Fig. 3b). The surface of the Al flakes was also extremely smooth and no fragmental Al powders on their surface were observed. It is known that cold welding, crushing and plastic deformation occurred during ball milling process. The dimensions as well as morphologies of the thin Al flakes could be well optimized by regulating the ball milling parameters. GO prepared by modified Hummers method exhibited a typically wrinkled morphology and a large specific surface area (Fig. 3c). It was observed that RGO could be directly adsorbed on the surface of the acidified 2024 aluminum flakes by electrostatic attraction force and homogeneously mixed with the prepared Al flakes (Fig. 3d). The supernatant liquid of the GO/2024Al suspension was colorless and transparent instead of light brown resulted from the aggregation of GO in the solution, which confirms the success of the GO dispersion (Fig. 3d). The high magnification micrograph indicated that RGO did not agglomerate into clusters and the surface of the Al flakes was entirely coated by RGO, implying the great superiority of RGO dispersion by using Al flakes as the efficient carrier (Fig. 3e). TEM micrograph further indicated that the completely unfolded RGO homogeneously distributed on the
Al flakes (Fig. 3f). However, the spherical Al powders changed into irregular shape and RGO still aggregated into clusters with large size in the RGO/Al composite powders prepared by HEM method (Fig. 3g and h). This results demonstrate that the aluminum flakes as an effective carrier have a much better effect on uniformly dispersing RGO in the matrix than HEM method.
Fig. 3. Micrographs of (a) spherical 2024Al powders, (b) 2024Al flakes, (c) GO, (d, e, f) 2.4wt.%RGO/Al hybrid powders prepared by FPT and (g, h) 0.4wt.%RGO/Al hybrid powders prepared by HEM.
Raman spectroscopy is crucial to detect composition and analyze carbon structure. Fig. 4a shows the Raman spectra of the corresponding composite powders. It is obvious that two vibration peaks appeared in the RGO powders, which are known as the D-band (1357.7cm-1) and G-band (1596.8cm-1), respectively. It is known that the ID/IG ratio exhibits the degree of graphitization as well as the structural defect of carbonaceous materials and a higher ID/IG ratio demonstrates that its structural integrity is significantly destroyed [16]. The ID/IG value of the FPT-0.4wt.%RGO/Al hybrid powders (1.03) was only 4.0 % higher than that of the RGO powders (0.99), indicating that little extra damage to the structural integrity of RGO was caused during the mixing process. However, the ID/IG ratio of the HEM-0.4wt.%RGO/Al hybrid powders prepared by HEM method at a rotation speed of 300 rpm for 4 h was increased to ~1.14 that was 15.2 % and 10.7 % higher than that of the RGO and FPT-0.4wt.%RGO/Al composite powders, respectively, implying significant damage of RGO structure. In addition, G-bands of the FPT-0.4wt.%RGO/Al and the HEM-0.4wt.%RGO/Al hybrid powders on the Raman spectra did not exhibit obvious blue shift characteristics which were related to the larger damage of RGO’s carbon structure [17, 18].
Fig. 4. Raman spectra of (a) RGO/Al hybrid powders and (b) RGO/Al composites.
The uniform distribution and well-maintained structural integrity of RGO were the most crucial factors for improving the performance of RGO reinforced Al matrix
composites. The dispersion of RGO can be only improved to some extent through using the traditional ball milling procedure with a longer milling time and a higher rotation speed, but the RGO structure would be seriously and simultaneously damaged (Fig. 4a). The ball milling effectively increased the surface area of 2024Al powders, which was believed to have even more contact area with RGO than spherical Al powders because the required size and morphology of the metal matrix powders were compatible with those of the RGO that possessed with larger specific surface [18], facilitating RGO dispersion. Fig. 3e shows that RGO was evenly distributed on the 2024Al matrix, this phenomenon fully demonstrated that the flake 2024Al powders meeted the requirements of morphological compatibility between RGO and matrix powders. However, the Al2O3 film inevitably existed on the Al powder surface, especially inhibiting a better RGO dispersion through direct electrostatic adsorption [4]. The dense Al2O3 film on the as-received Al flake powders was peeled off after being acidified to expose fresh Al surface. The ionization of Al powders within the GO/Al suspension during blending procedures can then be obviously accelerated and the continuously released Al3+ attached to the surface of acidified Al flakes. The positively charged Al flakes would be tightly combined with the negatively charged GO by electrostatic attraction force due to the interattraction of the opposite charges, which makes the unfolded GO to uniformly and firmly distribute on the surface of the matrix powders and prevents the re-aggregation of GO on the metal surface during the whole mixing procedures. On the contrary, the large shear stress resulted from the repeated ball-to-powder impact not only achieved relatively limited distribution of RGO but also caused serious damage to the RGO structure during the long duration HEM required to achieve the homogeneous RGO distribution. Therefore, it is better to homogeneously disperse RGO with well-maintained structure by introducing flaky Al powders as effective carrier to achieve the electrostatic interaction between GO and matrix powders. 3.2. Microstructure of the hierarchical RGO/2024Al composites Fig. 5 shows the SEM micrographs of the FPT-0.4wt.%RGO/Al green compacts heated at 625 °C for different time. It indicated that the Al powders within the
as-received green compact interconnected with each other and many irregular pores with an average size of 3 µm appeared between the powders (marked by arrows in Fig. 5a). The Al powders connected together due to the plastic deformation occurred during hot pressing, generating the indistinct boundary and many black pores among the powders. Simultaneously, the characteristics of flaky powders existed between the spherical Al powders were extremely obvious and no stacking and agglomeration of these flaky powders occurred (Fig. 5b). The FPT-0.4wt.%RGO/Al green compact was fabricated by pressing the composite powders consisted of spherical Al powders uniformly surrounded by the RGO/Al flakes. Thus, it can be concluded that the RGO/Al flaky powders uniformly distributed in the green compact after being pressed. The liquid phase appeared between the α-Al primary particles and the inter-granular pores were significantly decreased after being heated at a semisolid temperature of 625 °C for 30 min (compared with Fig. 5a and c). Moreover, it was observed that not only the spherical particles but also the fiber-like microstructure appeared in the semisolid ingot (Fig. 5d). Our previous investigations indicated that the liquid phase formed resulting from the melting of the edges of the powders during partial remelting and a single powders within the green compact evolved into a primary particle in the semisolid ingot [19]. The majority of the inter-granular pores between the α-Al powders could be filled by the generated liquid phase as the heating time increases because the gradually increased liquid amount accelerated the pore filling, resulting in the microstructure densification.
Fig. 5. SEM images of FPT-0.4wt.%RGO/Al green compact heated at 625 °C for (a, b) 0 min and (c, d) 30 min.
Fig. 6 shows the SEM micrographs of the thixoformed FPT-0.4wt.%RGO/Al composite with hierarchical microstructure. The microstructure was consisted of primary α-Al particles and inter-granular secondary solidified structures (SSSs) (Fig. 6a). SSSs mainly included secondary α-Al phase and eutectic α-Al and Al2Cu phases. Moreover, the fiber-like α-Al particles were still distributed among the spherical α-Al particles (Fig. 6b), which was similar to the results shown in Fig. 5c. Fig. 6c and d
show the EDS results from Fig. 6b, indicating that the SSSs close to the fiber-like Al particles were rich in carbon element, while the regions far away from these fiber-like microstructure were almost free of carbon element. Thus, RGO was embedded into some local regions of the SSSs in the thixoformed RGO/Al composite. In order to further confirm the RGO distribution, the microstructure obtained by electrolytic etching of the metallographic sample was also examined. The deep corrosion treatment causes the Al matrix to be seriously consumed, forming many corrosion pits with different dimensions, while the eutectic phases and RGO reinforcement were remained (Fig. 6e). Especially, it was observed that the RGO existed within the corrosion pits uniformly dispersed among the flaky Al particles (marked by arrows in Fig. 6e and f). RGO located in the local regions of liquid phase that was close to the fiber-like Al particles due to the partial remelting of the RGO/Al composite powders and then RGO distributed in some regions of the SSSs in the FPT-0.4wt.%RGO/Al composite after thixoforming process, forming RGO-rich regions. Therefore, it was easy to understand that the FPT-0.4wt.%RGO/Al composite with hierarchical microstructure that RGO only embedded in the local regions of SSSs was successfully constructed in this work by FPT technology. Raman spectra of the thixoformed RGO/Al composites were also investigated to clarify whether RGO structure was further affected by the following processing parameters besides the mixed parameters. As shown in Fig. 4b, ID/IG value of the FPT-0.4wt.%RGO/Al composite (1.01) was only 2.0 % higher than that of the as-received RGO (0.99). Moreover, its value was 1.9 % lower than that of the FPT-0.4wt.%RGO/Al hybrid powders (1.03) and thus no significant damage of RGO structure
was
further
induced
by
thixoforming
technology.
For
the
HEM-0.4wt.%RGO/Al composite, ID/IG value was increased to 1.15, which is 16 % and 14 % higher than that of the as-received RGO and FPT-0.4wt.%RGO/Al composite, respectively. It can be concluded that little influence of the processing parameters on the structural integrity of RGO in the FPT-0.4wt.%RGO/Al composite was induced, which directly contributes to the improvement in the mechanical properties of the composite.
Fig. 6. (a, b) SEM images of the FPT-0.4wt.%RGO/Al composite. (c, d) EDS spectra of Area 1 and Area 2 in Fig. 6b, respectively. (e, f) SEM images of the electrolytic polished cross section of FPT-0.4wt.%RGO/Al composite.
Fig. 7 shows the TEM micrographs of the FPT-0.4wt.%RGO/Al composite. Gr can be observed uniformly embedding in the Al matrix (Fig. 7a and b), consisting with the results shown in Fig. 6. There is evidence of Al2O3 and Al4C3 formation between the RGO with Al matrix and simultaneously the void free RGO/Al interface was formed (Fig. 7c-e). It was approved that formation of small amount of Al4C3 facilitated interfacial bonding and thus contributed to the enhanced strengthening effect [20]. Two kinds of oxygen atoms in the RGO/Al composite were existed: one is resulting from residual functional groups on the RGO and the other is the oxygen atoms in the Al matrix caused by the oxidation of Al alloy during processing procedures. Generally speaking, Al2O3 film is detrimental to the mechanical properties and thus extensive investigations were focused on reducing its concentration during processing procedures. Yuan et. al. synthesized Gr possessing about 12 at.% residual oxygen by a thermal reduction method [21]. The residual oxygen on the Gr surface enhances the Gr/Mg
interfacial
bonding
by
in
situ
forming
semi-coherent
Gr/MgO-nanoparticles/Mg bonds that efficiently accommodate the load transfer between the matrix and Gr, strengthening the Mg matrix. These findings demonstrated that the a small amount of the in situ oxide formed in the Gr/matrix interface has positive effect on improving the mechanical properties of the composites due to the enhanced interfacial bonding. Therefore, a hybrid Al/Al2O3/C and Al/Al4C3/C interfaces were formed in the FPT-0.4wt.%RGO/Al composite.
Fig. 7. TEM images of the FPT-0.4wt.%RGO/Al composite.
3.3. Tensile properties of the RGO/Al composites The tensile results of the RGO/Al composites were shown in Fig. 8. The values of the ultimate tensile strength (UTS), yield strength (YS) and elongation for the specimens were summarized in Table 1, indicating that the UTS, YS and elongation of
the 2024Al matrix alloy were measured to be 301 MPa, 203 MPa, and 15.2 %, respectively. The incorporation of only 0.4 wt.% RGO into the Al matrix resulted in 8.0 %, and 7.4 % improvements in the UTS and YS and 63.8 % decrease in the elongation of the HEM-0.4wt.%RGO/Al composite as compared with the 2024Al alloy. For the FPT-0.4wt.%RGO/Al composite, the UTS, YS as well as elongation were 35.1 %, 34.9 % and 54.5 % higher than those of the HEM-0.4wt.%RGO/Al composite, respectively. It should be noted that the same concentration of RGO was used for fabricating the RGO/Al composites in this work, while the strengthening and toughening effects of the FPT-0.4wt.%RGO/Al composite were more significant than those of the HEM-0.4wt.%RGO/Al composite. For comparison, the mechanical properties of the Gr/Al matrix composites prepared by different methods were also summarized in Table 1. The tensile strengths appeared to obviously increase along with the decrease of elongation. The (3Gr+10SiC)/2024Al composite prepared by PM technology possessed an UTS of 387 MPa, while the elongation (5.7 %) was 49 % lower than that of the FPT-0.4wt.%RGO/Al composite in this work [22]. Apparently, the RGO/Al composite with hierarchical microstructure was much more efficient in strengthening as well as toughening the matrix alloy as compared with the current composites reported so far. Therefore, this work proposed a promising technology that could improve the strength as well as elongation of the RGO/Al composites through tailoring RGO spatial arrangement.
Fig. 8. Stress-strain curves of the 2024Al alloy and RGO/Al composites. Table 1 Mechanical properties of RGO/Al composites. Material
Method
UTS (MPa)
YS (MPa)
Elongation (%)
Reference
2024Al alloy
FPT
301
203
15.2
This work
FPT-0.4wt.%RGO/Al
FPT
439
294
8.5
This work
HEM-0.4wt.%RGO/Al
FPT
325
218
5.5
This work
0.5vol.%RGO/Al (200-nm-thick Al flakes)
PM
256
224
6.4
[7]
0.1vol.%RGO/Al (1µm-thick Al flakes)
PM
145
90
16.3
[7]
2vol.% Gr/Al-Cu
PM
280
260
6.3
[23]
3vol.%Gr/10vol.%SiC/2024Al
PM
387
281
5.7
[22]
3wt.% Gr/2124Al
PM
421
180
4.8
[24]
To investigate the strengthening mechanism of the RGO/Al composites, the examination of the corresponding fracture surfaces after tensile failure was carried out (Fig. 9). A large number of dimple fracture morphology of 2024Al alloy such as small and deep dimples and tearing edges were appeared on the fracture surface (Fig. 9a and b), which is related to the ductile fracture characteristic during loading. Fig. 10 shows the side views of fracture surfaces of the RGO/Al composites and 2024Al alloy during tensile testing. The cracks almost propagated across the primary α-Al particles that were deformed plastically and elongated parallel to the tensile direction (marked by the arrows A in Fig. 10a), exhibiting a transgranular fracture mode of the 2024Al alloy. Moreover, many micro-pores and micro-cracks existed in the SSSs among the primary particles (marked by the arrows B in Fig. 10a). We further examined the regions away from the fracture surface, which was more obvious to exhibit the crack initiation and propagation progress. The primary particles were obviously elongated along with the tensile direction and many slip bands within these deformed particles were observed (Fig. 10b), verifing that the substantial plastic deformation of the Al matrix material prior to fracture occurred. During tensile process, the primary α-Al particles preferentially deformed, which can be demonstrated by the formed slip bands within the elongated primary particles. The SSSs between the primary particles mainly included the secondary α-Al phase and eutectic α-Al and Al2Cu phases. The existence of a hard and brittle eutectic Al2Cu phase weakened the plastic deformation ability of the SSSs compared the relatively soft primary α-Al particles. That is, the primary phase particles have stronger deformation ability and greater deformation degree. As the tensile testing continues, some micro-pores formed in the SSSs because of the incoordinated plastic deformation (Fig. 10a and b). The micropores grew into microcracks and then evolved into large cracks as the tensile testing
proceeds, resulting in material failure through the propagation of the interconnected microcracks across the deformed α-Al particles. It can be thus concluded that the fracture mechanism of the matrix is: firstly, the primary particles deform plastically during tensile process and then micropores and microcracks are generated in the SSSs. Subsequently, the number and size of the microcracks increase and finally the microcracks interconnect together with each other, causing the cracks to propagate across the deformed primary particles and resulting in the fracture of the material.
Fig. 9. SEM images of the fracture surface of (a, b) 2024Al alloy, (c, d) FPT-0.4wt.%RGO/Al composite and (e, f) HEM-0.4wt.%RGO/Al composite.
For the FPT-0.4wt.%RGO/Al composite, it is obvious that its fracture surface was similar to that of the 2024Al alloy and a typically ductile fracture mode characterized by deep dimples was also observed (compared with Fig. 9a and c). However, as indicated by the white dashed ellipses in Fig. 9c, some flaky microstructure features with the similar dimension of the flaky Al particles existed in the thixoformed microstructure appeared. Micro-cracks followed the interface between the flaky microstructure and surrounding matrix, as indicated by the arrows in Fig. 9c. The surface of the fiber-like structure was not only uniformly coated by many dimples, but also contained a higher concentration of the debonded and fractured RGO reinforcement (Fig. 9d). The side views of fracture surfaces indicated that the fracture mode still belonged to a transgranular fracture and a large size pits appeared (Fig. 10c). It is obvious that many densely distributed slip bands were distributed within the elongated α-Al particles and simultaneously micro-pores and micro-cracks among the primary particles were observed. Cracks firstly initiated in the regions close to the flaky Al particles but did not continuously propagated across the flaky particles (marked by the arrows in Fig. 10d), exhibiting an obvious crack deflection. It is well known that the debonding and fracture of reinforcements due to the poor interfacial bonding, stress concentration and aggregation of reinforcements always resulted in the premature fracture and even deteriorative mechanical properties of the composites [25]. During tensile testing, the matrix of the RGO-rich regions plastically deformed
and stress gradually concentrated at the RGO/Al interface. The dislocation density around the RGO should be higher because a smaller inter-particle distance can effectively hinder the dislocation movement through the cross slip and climb of dislocations [26]. The interaction between the RGO and dislocations resulted in the formation of dislocation punched zones around the RGO and thus contributed to enhance the strength of the composite. In addition, the different coefficient of thermal expansion between the RGO and Al matrix will generate thermal stress near the RGO/Al interface during cooling from the processing temperature to room temperature, increasing the dislocation density. On the other hand, the large specific area of RGO contributed to the effective load transfer from the softer Al matrix to the harder RGO through the strong RGO/Al interface, contributing to the strength improvement and thus resulting in the load transfer strengthening. It was reported that the individual dislocation punched zones will superposed together with each other if the volume fraction and size of the nanoparticles was reasonably tailored and then a transition of failure mechanisms occur from a reinforcement/matrix boundary dominated failure initiation to a dislocation punched zone/matrix interface dominated failure initiation. The flaky microstructure associated with the failed RGO on the fracture surface and the crack deflection of the RGO-rich zones indicated that most of the cracks possibly nucleated near the interface between the RGO-rich zones and the matrix (Fig. 9d and 10d). Thus, it is expected that the overlapping of the dislocation punched zones is more likely to occur within the RGO-rich zones and the RGO-rich regions can behave as reinforcing units to strengthen the matrix alloy. The strengthening effect of the fiber-like strengthening units was much more effective than the spherical strengthening units because the large aspect ratio of reinforcements obviously contributes the tensile strength. The dislocation can be gradually concentrated at the RGO-rich regions/matrix interface during tensile testing, which induced the pull-out of the RGO-rich zones through the microcrack initiation and propagation at those interfaces. The flaky pits associated with the RGO on the fracture surface should be corresponded to the RGO-rich regions that debonded and/or fractured during tensile process. On the other hand, the RGO-free zones can block the
crack tip and increase the propagated routes of cracks, toughening the composite.
Fig. 10. Side views of fracture surfaces of (a,b) 2024Al alloy, (c, d) FPT-0.4wt.%RGO/Al composite and (e, f) FPT-0.4wt.%RGO/Al composite.
The fracture surface of the HEM-0.4wt.%RGO/Al composite was obviously different from the ductile fracture characteristic of the 2024Al alloy and FPT-0.4wt.%RGO/Al composite (compared with Fig. 9a, c and e). The RGO severely agglomerated into clusters and was easily peeled off from the matrix (Fig. 9f), which further demonstrated the inferior effect of HEM procedure on RGO dispersion and a weak strengthening efficiency. The side views of fracture surfaces indicated that the RGO aggregation induced crack initiation and propagation and the fracture mode transferred into intergranular fracture (Fig. 10e and f). Due to the agglomeration and overlap of the reinforcing phase in the HEM-0.4wt.%RGO/Al composite, the interface bonding strength was weakened. In addition, the RGO structure of the HEM-0.4wt.%RGO/Al composite was obviously damaged during HEM process that can be confirmed by the variation of ID/IG ratio (Fig. 4b). RGO debonded and fractured prematurely as the external load increases, inducing the crack nucleation and propagation along the matrix and then causing the premature failure of the composite during tensile testing. Since the material structure determines the performance, the above phenomenon corresponds to the change in the mechanical properties shown in Table 1. Therefore, the strengthening effect of RGO/Al composites could be brought about by introduction of RGO, while an unattainable combination of ultrahigh strength and good elongation was achieved by tailoring the spatial array of RGO, forming a hierarchical microstructure that RGO only homogeneously distributed in the local regions of SSSs. 4. Conclusion This study successfully constructed the RGO/2024Al composites with hierarchical microstructure by FPT technology that combined FPM and thixoforming technology. An ideal semi-solid ingot suitable for thixoforming can be obtained when the green compact containing the RGO/Al flakes and spherical Al powders was partially
remelted at 625 °C. RGO can be uniformly dispersed in the local regions of the SSSs by intentionally introducing Al flakes as effective carrier, but HEM method was not conducive to disperse RGO without impairing its structural integrity, verifying that FPT technology is more superior to HEM for the efficient dispersion of RGO in the matrix. For the FPT-0.4wt.%RGO/Al composite, the UTS, YS and elongation were 35.1 %, 34.9 %, and 54.5 % higher than those of the HEM-0.4wt.%RGO/Al composite, respectively. The reasons should be attributed to the synthesized fiber-like RGO-rich zones that behave as reinforcing units and thus enhancing the strengthening efficiency. The fracture mode transferred from a transgranular fracture of the FPT-0.4wt.%RGO/Al
composite
into
an
intergranular
fracture
of
the
HEM-0.4wt.%RGO/Al composite. This work proposed a promising technology that strengthened and toughened the composites through tailoring the spatial arrangement of RGO. 5. Acknowledgments The authors acknowledge the financial support by the Ningxia Natural Science Foundation of China (Grant No. 2018AAC03031), the Basic Scientific Fund of Ningxia university (Grant No. NGY2018009), the Natural Science Foundation of Ningxia University (Grant No. ZR1702), the Key R & D Project of Ningxia (2018BEE03008) and the Science and Technology Project for Young Talents of Ningxia (Grant No. TJGC2019042). Reference [1] J.C. Meyer, A.K. Geim, M.I. Katsnelson, K.S. Novoselov, T.J. Booth, S. Roth. The structure of suspended graphene sheets, Nature 446 (2007), 60-63. [2] Z.Y. Yang, L.D. Wang, Z.D. Shi, M. Wang, Y. Cui, B. Wei, S.C. Xu, Y.P. Zhu, W.D. Fei. Preparation mechanism of hierarchical layered structure of graphene/copper composite with ultrahigh tensile strength, Carbon 127 (2018), 329-339. [3] X.N. Mu, H.M. Zhang, H.N. Cai, Q.B. Fan, Z.H. Zhang, Y. Wu, Z.J. Fu, D.H. Yu. Microstructure evolution and superior tensile properties of low content graphene nanoplatelets reinforced pure Ti matrix composites, Mater. Sci. Eng. A 687 (2017), 164-174. [4] Z. Li, G.L. Fan, Z.Q. Tan, Q. Guo, D.B. Xiong, Y.S. Su, Z.Q. Li, D. Zhang. Uniform dispersion
of graphene oxide in aluminum powder by direct electrostatic adsorption for fabrication of graphene/aluminum composites, Nanotechnology 25 (2014), 325601. [5] T. Lokesh, U.S. Mallik. Effect of ECAP process on the Microstructure and Mechanical Properties of Al6061-Gr Composites, Materials Today: Proceedings 5 (2018), 2453-2461. [6] A. Naseer, F. Ahmad, M. Aslam, B.H. Guan, W.S.W. Harun, N. Muhamad, M.R. Raza, R.M. German. A review of processing techniques for graphene-reinforced metal matrix composites, Mater. Manuf. Processes 34 (2019), 957-985. [7] L. Zhao, Q. Guo, Y. Shi, Y. Liu, S. Osovski, Z. Li, D.-B. Xiong, Y. Su, D. Zhang. Interfacial Effect on the Deformation Mechanism of Bulk Nanolaminated Graphene-Al Composites, Metall. Mater. Trans. A 50 (2019), 1113-1118. [8] Z. Fan. Semisolid metal processing, Int. Mater. Rev. 47 (2002), 49-85. [9] P.B. Li, T.J. Chen, H. Qin. Effects of mold temperature on the microstructure and tensile properties of SiCp/2024 Al-based composites fabricated via powder thixoforming, Mater. Des. 112 (2016), 34-45. [10] E. Ma, T. Zhu. Towards strength–ductility synergy through the design of heterogeneous nanostructures in metals, Mater. Today 20 (2017), 323-331. [11] L. Jiang, H. Yang, J.K. Yee, X. Mo, T. Topping, E.J. Lavernia, J.M. Schoenung. Toughening of aluminum matrix nanocomposites via spatial arrays of boron carbide spherical nanoparticles, Acta Mater. 103 (2016), 128-140. [12] L.J. Huang, L. Geng, H.X. Peng. Microstructurally inhomogeneous composites: Is a homogeneous reinforcement distribution optimal?, Prog. Mater. Sci. 71 (2015), 93-168. [13] S. Xiang, X. Wang, M. Gupta, K. Wu, X. Hu, M. Zheng. Graphene nanoplatelets induced heterogeneous bimodal structural magnesium matrix composites with enhanced mechanical properties, Sci. Rep. 6 (2016), 38824. [14] P.B. Li, T.J. Chen, S.Q. Zhang, R.G. Guan. Research on semisolid microstructural evolution of 2024 aluminum alloy prepared by powder thixoforming, Metals 5 (2015), 547-564. [15] H.J. Yu, Z.G. Zou, F. Long, C.Y. Xie, H. Ma. Preparation of graphene with ultrasound-assisted in the process of oxidation, Applied Mechanics and Materials 34-35 (2010), 1784-1787. [16] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D.
Jiang, K.S. Novoselov, S. Roth. Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006), 187401. [17] J.-M. Ju, G. Wang, K.-H. Sim. Facile synthesis of graphene reinforced Al matrix composites with improved dispersion of graphene and enhanced mechanical properties, J. Alloys Compd. 704 (2017), 585-592. [18] L. Jiang, G. Fan, Z. Li, X. Kai, D. Zhang, Z. Chen, S. Humphries, G. Heness, W.Y. Yeung. An approach to the uniform dispersion of a high volume fraction of carbon nanotubes in aluminum powder, Carbon 49 (2011), 1965-1971. [19] P.B. Li, T.J. Chen, S.Q. Zhang. Effects of reheating time on microstructure and tensile properties of SiCp/2024 Al-based composites fabricated using powder thixoforming, Met. Mater. Int. 23 (2017), 193-201. [20] B. Chen, L. Jia, S. Li, H. Imai, M. Takahashi, K. Kondoh. In Situ Synthesized Al4C3 Nanorods with Excellent Strengthening Effect in Aluminum Matrix Composites, Adv. Eng. Mater. 16 (2014). [21] Q.-h. Yuan, G.-h. Zhou, L. Liao, Y. Liu, L. Luo. Interfacial structure in AZ91 alloy composites reinforced by graphene nanosheets, Carbon 127 (2018), 177-186. [22] C.-j. Hu, H.-g. Yan, J.-h. Chen, B. Su. Microstructures and mechanical properties of 2024Al/Gr/SiC hybrid composites fabricated by vacuum hot pressing, Trans. Nonferrous Met. Soc. China 26 (2016), 1259-1268. [23] B. Pu, J. Sha, E. Liu, C. He, N. Zhao. Synergistic effect of Cu on laminated graphene nanosheets/AlCu composites with enhanced mechanical properties, Mater. Sci. Eng. A 742 (2019), 201-210. [24] A. El-Ghazaly, G. Anis, H.G. Salem. Effect of graphene addition on the mechanical and tribological behavior of nanostructured AA2124 self-lubricating metal matrix composite, Compos. Part A 95 (2017), 325-336. [25] M.K. Habibi, A.S. Hamouda, M. Gupta. Hybridizing boron carbide (B4C) particles with aluminum (Al) to enhance the mechanical response of magnesium based nano-composites, J. Alloys Compd. 550 (2013), 83-93. [26] A.F. Boostani, R.T. Mousavian, S. Tahamtan, S. Yazdani, R.A. Khosroshahi, D. Wei, J.Z. Xu, D. Gong, X.M. Zhang, Z.Y. Jiang. Graphene sheets encapsulating SiC nanoparticles: A roadmap
towards enhancing tensile ductility of metal matrix composites, Mater. Sci. Eng. A 648 (2015), 92-103.
Figure Captions: Fig. 1. Schematic of the FPT technology. Fig. 2. Dimensions of the tensile test specimen (unit: mm). Fig. 3. Micrographs of (a) spherical 2024Al powders, (b) 2024Al flakes, (c) GO, (d, e, f) 2.4wt.%RGO/Al hybrid powders prepared by FPT and (g, h) 0.4wt.%RGO/Al hybrid powders prepared by HEM. Fig. 4. Raman spectra of (a) RGO/Al hybrid powders and (b) RGO/Al composites. Fig. 5. SEM images of FPT-0.4wt.%RGO/Al green compact heated at 625 °C for (a, b) 0 min and (c, d) 30 min. Fig. 6. (a, b) SEM images of the FPT-0.4wt.%RGO/Al composite. (c, d) EDS spectra of Area 1 and Area 2 in Fig. 6b, respectively. (e, f) SEM images of the electrolytic polished cross section of FPT-0.4wt.%RGO/Al composite. Fig. 7. TEM images of the FPT-0.4wt.%RGO/Al composite. Fig. 8. Stress-strain curves of the 2024Al alloy and RGO/Al composites. Fig. 9. SEM images of the fracture surface of (a, b) 2024Al alloy, (c, d) FPT-0.4wt.%RGO/Al composite and (e, f) HEM-0.4wt.%RGO/Al composite. Fig. 10. Side views of fracture surfaces of (a,b) 2024Al alloy, (c, d) FPT-0.4wt.%RGO/Al composite and (e, f) FPT-0.4wt.%RGO/Al composite.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Flake powder thixoforming technology was developed. A FPT-0.4wt.%RGO/Al composite with hierarchical microstructure was fabricated. The strengthening and toughening are achieved by tailoring the dispersion of RGO.
Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. Pubo Li wrote the paper and contributed to all activities. Luyao Chen performed the experiments under Pubo Li’s guidance. Bo Cao, Luyao Chen, Keren Shi contributed to analysis tools and the interpretation and discussion of experimental results.