Microstructure and synergistic strengthening mechanisms of carbon nanotubes and Mg2Si nanoparticles hybrid reinforced Mg matrix composites prepared by powder thixoforming

Journal Pre-proof Microstructure and synergistic strengthening mechanisms of carbon nanotubes and Mg2Si nanoparticles hybrid reinforced Mg matrix composites prepared by powder thixoforming Pubo Li, Bo Cao, Wanting Tan, Mangmang Gao PII:

S0925-8388(19)34171-4

DOI:

https://doi.org/10.1016/j.jallcom.2019.152925

Reference:

JALCOM 152925

To appear in:

Journal of Alloys and Compounds

Received Date: 25 August 2019 Revised Date:

28 October 2019

Accepted Date: 4 November 2019

Please cite this article as: P. Li, B. Cao, W. Tan, M. Gao, Microstructure and synergistic strengthening mechanisms of carbon nanotubes and Mg2Si nanoparticles hybrid reinforced Mg matrix composites prepared by powder thixoforming, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152925. 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. © 2019 Published by Elsevier B.V.

Microstructure and synergistic strengthening mechanisms of carbon nanotubes and Mg2Si nanoparticles hybrid reinforced Mg matrix composites prepared by powder thixoforming Pubo Li*, Bo Cao, Wanting Tan, Mangmang Gao Ningxia Key Laboratory of Photovoltaic Materials, Ningxia University, Yinchuan, 750021, P. R. China * Corresponding author. E-mail: [email protected] (Pubo Li). Abstract AZ91D Mg-based composites containing carbon nanotubes (CNTs), magnesium silicide (Mg2Sip) nanoparticles, or CNTs-Mg2Sip hybrid reinforcements were synthesized via powder thixoforming using blending and pressing procedures in powder metallurgy, followed by partial remelting and thixoforming technology. The thixoformed microstructure of the CNTs-Mg2Sip hybrid reinforced Mg composite consisted of spheroidal primary α-Mg particles, intergranular secondary solidified structures (SSSs), and hybrid reinforcements homogeneously dispersed within the SSSs. A yield strength of 180 MPa was achieved in the Mg matrix composite reinforced by 0.75CNTs-0.75Mg2Sip hybrids, which was 26% and 12% higher than those of composites reinforced with individual 1.5CNTs and 1.5Mg2Sip, respectively (143 and 161 MPa, respectively). This was attributed to the in situ-synthesized Mg2Sip around the CNTs, which not only restricted the aggregation and pulling out of CNTs but also facilitated the synergistic strengthening effect of the CNTs. This work presents a promising strategy for the synthesis of metal matrix composites with impressive mechanical properties by employing hybrids of CNTs and Mg2Sip as reinforcements. Keywords: Mg matrix composites; powder thixoforming; microstructure; mechanical properties; strengthening mechanisms 1. Introduction Mg and its alloys have attracted considerable research interest because of their high

specific stiffness and strength, good dimensional stability, and high damping capacity, making them the most promising materials for aerospace, automotive, and biomedical applications

[1].

However,

Mg

alloys

have

drawbacks,

including

poor

high-temperature strength and creep resistance, which limit their applications as high-performance engineering materials [2]. One way to enhance the overall performance of Mg alloys is to introduce stronger inclusions into the matrix, thereby forming composite microstructures. Traditionally, metal matrix composites reinforced with only a single type of reinforcement exhibit a limited increase in one or two mechanical-performance indicators, while other physical properties, including the electrical and thermal conductivity, are degraded [3]. Using hybrid reinforcements is one of the most promising strategies to resolve this bottleneck by overcoming the insufficient intrinsic properties of a single reinforcement and integrating the advantages of different components [4-6]. Zero-dimensional (0D) magnesium silicide nanoparticles (Mg2Sip) have a low density (1.99 g/cm3), a high melting temperature (1085 °C), and a high elastic modulus (120 GPa), and importantly, Mg2Sip can form in situ Mg alloys, making it an attractive reinforcement for the Mg matrix [7]. One-dimensional (1D) carbon nanotubes (CNTs) have an ultrahigh Young’s modulus (~1 TPa) and strength (~100 GPa) and a good self-lubricant property; thus, they are ideal reinforcements for metal matrix composites [8]. Combining 1D CNTs with 0D nanoparticle reinforcements not only significant increases the well-balanced strength and ductility but also improves the compatibility between its partner reinforcements and the matrix [6, 9]. Therefore, hybrid composites reinforced with a synergistic combination of CNTs and Mg2Sip have great potential to satisfy the ever-increasing demand for lightweight materials with enhanced performance. Extensive investigations have been performed on composites reinforced with CNTs and other types of reinforcements. Rashad et al. [10] fabricated Mg composites reinforced by a combination of CNTs and graphene (Gr). The hybrid composites exhibited higher strength and ductility than those reinforced with Gr or CNTs alone owing to the synergistic strengthening effect. Zhang et al. [9] demonstrated that the

tensile strength of 0.5 vol.% CNTs-0.5vol.% SiCp/Al composites increased by 14% and 56% compared with those of 1.0 vol.% CNTs/Al and 1.0vol.% SiCp/Al composites, respectively. This is because the SiCp promoted the uniform dispersion of CNTs during ball milling, inhibiting the excessive interfacial reaction and delaying CNTs debonding during tensile testing. So et al. [4] fabricated A356Al-based composites reinforced with SiC-coated CNTs. They reported a strength improvement of 15% compared with the matrix due to the enhanced interfacial strength caused by the addition of a SiC coating to the CNTs. Multifunctional bioceramic-based composites reinforced by silica-coated CNTs and Mg2Sip/CNTs thermoelectric and anode composites have also been examined [5, 6, 11]. However, few investigations of the microstructure and mechanical properties of (CNTs+Mg2Sip)/Mg hybrid composites have been performed. Among the various processes for fabricating CNTs hybrid composites, powder metallurgy is the most attractive approach owing to the controlled interfacial reaction and uniformly dispersed reinforcements [9, 12]. However, commercial applications of powder metallurgy have been poorly developed because of the difficulty in fabricating components with complex shapes and compact microstructures. Thixoforming has emerged as a promising technology that is appropriate for near-net-shape forming of such components [13]. In previous studies, we successfully combined powder metallurgy with thixoforming to produce SiC microparticle-reinforced 2024Al composites with excellent performance, which is now known as powder thixoforming. A green compact was first fabricated via blending and pressing procedures commonly applied in powder metallurgy, and then a component was synthesized via partial remelting and thixoforming [14]. We are not aware of any investigations of the microstructure and strengthening mechanisms of CNTs-Mg2Sip/Mg matrix composites fabricated via powder thixoforming. Accordingly, in the present study, Mg matrix composites reinforced with a combination of 0.75 vol.% CNTs and 0.75 vol.% Mg2Sip were synthesized via powder thixoforming. The microstructure, mechanical properties, and synergistic strengthening mechanisms of the thixoformed composites were investigated.

2. Experimental 2.1. Composite fabrication The

starting

spherical

matrix

powders

were

gas-atomized

Mg–9.08Al–0.65Zn–0.23Mn (wt.%, AZ91D), with an average particle size of 35 µm (Fig. 1a, Tangshanweihao Magnesium Powder Co., Ltd., China). The CNTs had an average diameter of 50 nm and a length of 5 µm (Chengdu Institute of Organic Chemistry Co. Ltd., China) (Figs. 1b and c). Spherical Si nanoparticles (Sip) were supplied by Beijing DK Nano Technology Co., Ltd., China and exhibited an average size of 50 nm (Fig. 1d).

Fig. 1. SEM images of (a) AZ91D powders, (b, c) CNTs, and (d) Sip. The method used to fabricate the CNTs-Mg2Sip/Mg hybrid composite is schematically shown in Fig. 2. It mainly included the fabrication of CNTs-Sip/Mg composite powders and the corresponding CNTs-Mg2Sip/Mg hybrid composites. First, 0.75 vol.% CNTs and 0.23 vol.% Sip were dispersed separately in ethanol via ultrasonication for 30 min to fabricate uniformly dispersed suspensions. These suspensions were then blended together and magnetically stirred for 1 h, producing the CNTs/Sip suspension. Additionally, the spherical AZ91D powders were immersed in ethanol using an agitator. CNTs/Sip suspensions were gently added to the Mg alloy powder slurry with constant stirring. After stirring for 1 h, the mixtures were filtered and vacuum-dried for 6 h at 70 °C to obtain the CNTs-Sip/Mg composite powders. These mixed powders were further mixed for 4 h using a QM-QX2 planetary ball milling machine with a ball-to-powder weight ratio of 5:1 and a rotation speed of 200 rpm. The required amount of the synthesized composite powders (70 g) was compacted under a pressure of 200 MPa into a green compact with dimensions of Ø46 mm × 20 mm. Finally, the powder ingot was heated to a semisolid temperature of 585 °C for 60 min in a vacuum furnace, and the semisolid ingot was thixoformed at a punch velocity of 60 mm·s-1, a holding pressure of 224 MPa, and a mold temperature of 350 °C, producing a 0.75 vol.% CNTs-0.75 vol.% Mg2Sip/Mg hybrid composite

product with dimensions of Ø50 mm × 14 mm. For comparison, a 1.5 vol.% CNTs/Mg composite, a 1.5 vol.% Mg2Sip/Mg composite, and AZ91D matrix alloy components were fabricated using the same processes.

Fig. 2. Schematic illustration of the fabrication process for the CNTs-Mg2Sip/Mg hybrid composites. 2.2. Microstructure characterization Microstructural characterizations of the composites were performed using a Talos 200S transmission electron microscope (TEM) and a scanning electron microscope (SEM, JSM-7500FG) equipped with an energy dispersive X-ray spectrometer (EDS, Bruker AXS 5030). Metallographic specimens were machined from the center regions of the thixoformed products parallel to the applied pressure and prepared using standard metallographic techniques. The TEM specimens were prepared for interfacial investigation from the Mg matrix composites via grinding, polishing, and ion milling (Leica EM RES1020508091102, Leica). Moreover, to study the interfacial reaction, the CNTs-Mg2Sip hybrid reinforcements were extracted from the CNTs-Mg2Sip/Mg composite through electrochemical dissolution in an electrolyte comprising 15 vol.% HNO3 in ethanol at a direct-current voltage of 5 V and a current of 2 A. X-ray diffraction (XRD, SmartLab, Rigaku) analysis using Cu Kα radiation at a scan rate of 2 °/min was performed to verify the phase constituents of the Mg matrix composites. The

carbonaceous

structures

were

characterized

via

Raman

spectroscopy

(DXR0304040404, Thermo Fisher, USA) using 514.5-nm-wavelength incident laser light. Dog-bone-shaped tensile specimens were taken parallel to the direction of the applied pressure and possessed a gauge length of 17 mm and a cross section of 5 mm × 1.5 mm. Tensile tests were performed using a WDW-100D material testing machine with a cross-head speed of 0.5 mm·min–1. 3. Results and discussion 3.1 Microstructure of Mg matrix composites

SEM images revealed the morphology of the CNTs/Mg hybrid powders (Fig. 3a). The aggregation of CNTs was severe, and the clustered CNTs that were dozens of micrometers in diameter distributed in the local regions of the surface of the Mg powder (indicated by the arrows in Fig. 3a), which was clearly apparent in Fig. 3b. A similar trend was observed for 1.5Sip/Mg powder mixtures (Fig. 3c). It is known that nano-reinforcements have a strong tendency to agglomerate into clusters, impairing the properties of composites. In particular, CNTs tend to form clusters owing to the strong van der Waals interactions between them and their large specific surface area and aspect ratio. For the CNTs-Sip/Mg powder mixtures, the CNTs and Sip were coated homogeneously and individually onto the Mg powders, and some CNTs were embedded into the Mg powders (indicated by arrows in Fig. 3d). Moreover, Sip were always distributed around CNTs. These results indicate that the dispersion of CNTs was accelerated by the addition of Sip. This phenomenon was also reported by Zhang et al. [9].

Fig. 3. SEM images of the composite powders: (a, b) CNTs/Mg, (c) Sip/Mg, and (d) CNTs-Sip/Mg. The Raman spectra of the CNTs, CNTs-Sip/Mg hybrid powders, and thixoformed CNTs-Mg2Sip/Mg composite were obtained. The CNTs exhibited two peaks at 1334.162 and 1563.65 cm−1, corresponding to the D and G bands, respectively (Fig. 4). Furthermore, the ID/IG ratio increased from 0.801 for the original CNTs to 1.006 for the CNTs-Sip/Mg hybrid powders. The ID/IG ratio can be used to characterize the quality of the C structure, and a high value corresponds to a large number of defects in the CNTs. The ball milling damaged the structure of the CNTs owing to the repeated ball-to-powder impact, which resulted in the unavoidable plastic deformation and fracture of CNTs. The ID/IG value of the CNTs-Mg2Sip/Mg composite increased to 1.243 after partial remelting and thixoforming, indicating that the composite fabrication procedure introduced defects into the CNTs. Compared with the commonly used method of high-energy ball milling and powder metallurgy, followed by hot extrusion procedures, the blending and thixoforming processes used in the

present study induced relatively little structural damage [15]. Additionally, the G band of the CNTs in the CNTs-Mg2Sip/Mg composite was slightly shifted to higher wavenumbers compared with that of the as-received CNTs, which was related to the residual stress and the status of CNTs/Mg interfacial bonding [16]. It was expected that the uniform dispersion and well-maintained CNTs structural integrity enhanced the mechanical properties of the CNTs-Mg2Sip/Mg composites.

Fig. 4. Raman spectra of the initial CNTs, CNTs-Sip/Mg powders, and thixoformed CNTs-Mg2Sip/Mg composite. Figure 5 shows the thixoformed microstructure of the CNTs-Mg2Sip/Mg hybrid composites. The microstructure consisted of spheroidal primary α-Mg particles (indicated by arrow A in Fig. 5a) and intergranular secondary solidified structures (SSSs; indicated by arrow B in Fig. 5a). The SSSs included the secondary α-Mg phase (indicated by arrow C in Fig. 5b) and the eutectic α-Mg phase and β-Mg17Al12 phase (indicated by arrow D in Fig. 5b). The average particle size of the primary α-Mg was approximately 40 µm, and that of the SSSs was approximately 5 µm, indicating a bimodal microstructure. The formation mechanisms of the thixoformed microstructure were extensively discussed in our previous works: (i) the formation of a continuous liquid layer on the primary particle surface due to partial remelting of the alloy powders; (ii) liquid solidification through attachment of the secondary α-Mg phase to the primary α-Mg phase surfaces during thixoforming; and (iii) the formation of fine equiaxed secondary α-Mg phases nucleated in the regions far from the primary particles when the requirements for heterogeneous nucleation were satisfied [14, 17]. Figures 5c–f show the EDS surface scanning analysis results corresponding to Fig. 5b. These results indicate that the SSSs were rich in elemental Al, confirming the existence of Al-rich eutectic phases between the primary α-Mg particles (Fig. 5d). Additionally, elemental C and Si were homogeneously distributed in the SSSs (Figs. 5e and f), indicating that the CNTs and Mg2Sip (the in situ-synthesized Mg2Sip from the reaction between Si and the Mg matrix is shown in Fig. 6 and 7 in the following section) remained homogeneous after being partially remelted and thixoformed.

Fig. 5. (a, b) SEM images of the CNTs-Mg2Sip/Mg composites and (c–f) EDS surface scanning analysis for (b). In XRD analyses, the hybrid CNTs-Sip/Mg powders exhibited diffraction peaks corresponding to Si, CNTs, and α-Mg and Mg17Al12 phases, and a new Mg2Si phase was detected for the thixoformed CNTs-Mg2Sip/Mg matrix composite (Fig. 6). Thus, Sip reacted with the Mg matrix during partial remelting, and the resulting reinforcements of the composite were CNTs-Mg2Sip hybrids. TEM was employed to further study the microstructure of the CNTs-Mg2Sip/Mg composites (Fig. 7). The CNTs remained intact and exhibited no obvious agglomeration, despite undergoing the partial remelting and thixoforming (Fig. 7a), which agrees well with the SEM observations (Fig. 5). There was good CNTs/Mg matrix interfacial adhesion, with no debonding or microcracks, and no obvious reaction product was observed. However, MgO was observed in the local regions of the CNTs/Mg interface. This phenomenon was also indicated by the XRD analyses, which revealed a novel diffraction peak corresponding to MgO for the thixoformed composite. A small amount of the residual O atoms derived from the functional groups of CNTs or the oxidation of the matrix could have chemically bonded with the metal ions, forming metal-O-C bonds that efficiently accommodated the load transfer between the matrix and CNTs [18]. The in situ Mg2Sip were uniformly dispersed around CNTs (Fig. 7c). The products extracted from the composite shown in Fig. 7d indicated that the fiber reinforcement attached to the grey phase and the spherical nanoparticles were interconnected, forming an interlocked network. The EDS results in Fig. 7d indicated that this structure was rich in C, Si, Mg, and O (Fig. 7e–h). Thus, the interconnected 1D CNTs, with some undissolved eutectics and uniformly dispersed 0D Mg2Sip, remained after the electrochemical dissolution, confirming the successful synthesis of the Mg2Sip-CNTs hybrid network structure in the CNTs-Mg2Sip/Mg composite. Therefore, CNTs and Sip were distributed uniformly and individually onto the Mg powder surface through the blending procedures employed in this study. Sip reacted with the Mg matrix to form Mg2Sip during partial remelting. The in situ-synthesized Mg2Sip and CNTs maintained

their structural integrity and were homogeneously distributed between the primary α-Mg particles in the thixoformed CNTs-Mg2Sip/Mg composite. The CNTs/Mg interface was free of pores and reaction products, with the exception of some MgO in local interfacial regions.

Fig. 6. XRD spectra of the CNTs-Mg2Sip/Mg composites. Fig. 7. (a–c) High-resolution TEM images of the interfaces between the CNTs, Mg2Sip, and Mg matrix of the CNTs-Mg2Sip/Mg matrix composite. (d) TEM image of the CNTs-Mg2Sip hybrid reinforcements extracted from the composite. (e–h) Elemental-mapping images for (d). 3.2 Mechanical properties of Mg matrix composites The tensile curves of the Mg matrix composites with different contents of CNTs-Mg2Sip hybrids are shown in Fig. 8. The mechanical properties of both reinforced

and

unreinforced

Mg

alloy

are

presented

in

Table

1.

The

0.75CNTs-0.75Mg2Sip/Mg matrix composite exhibited the best strength improvement. Its ultimate tensile strength (UTS), yield strength (YS), and fracture elongation (εf) were 232 MPa, 180 MPa, and 6.1%, respectively, representing increases of 21% and 30% and a decrease of 20%, respectively, compared with the Mg matrix alloy prepared via the same processes. Thus, the incorporation of the reinforcements strengthened the matrix alloy, while the elongation was reduced, exhibiting a traditional tradeoff between strength and ductility of the composite. The maximum UTS, YS, and εf of the 1.5CNTs/Mg composite were 197 MPa, 143 MPa, and 4.4%, respectively, and those of the 1.5Mg2Sip/Mg composite were 212 MPa, 161 MPa, and 5.6%, respectively. Although the strength of the CNTs and Mg2Sip-reinforced Mg matrix composites was improved compared with that of the Mg matrix, the strengthening efficiency was lower than that of the CNTs-Mg2Sip hybrids. In addition, the elongation of the CNTs-Mg2Sip/Mg composite was 39% and 9% higher than those of the CNTs/Mg and Mg2Sip/Mg composites, respectively. These results indicate that

the strengthening and toughening capability of these hybrids was exploited effectively during tensile testing, compared with the individual CNTs and Mg2Sip-reinforced composites. Furthermore, the strength and elongation of the Mg2Sip/Mg composite were higher than those of the CNTs/Mg composite, implying that the Mg2Sip strengthening phase was also efficient in strengthening the Mg matrix. This phenomenon agrees well with the results of Zhang et al., who reported that the strengthening effect of a 1.5 vol.% CNTs/Al composite was inferior to that of a 1.5 vol.% SiCp/Al composite [9]. Generally, CNTs have been regarded as the most promising reinforcement in metal matrix composites owing to their ultrahigh strength and outstanding physical properties. However, the degree of strengthening was clearly degraded and not commensurate with the excellent properties of CNTs at a high content (~2 vol.%), because of reinforcement aggregation [19, 20]. Therefore, to achieve excellent tensile properties, a novel Mg matrix composite was fabricated by hybridizing Mg2Sip and a low content of CNTs .

Fig. 8. Tensile stress–strain curves of the Mg matrix composites. Table 1 Mechanical properties of the Mg matrix composites.

3.3 Fracture behavior Figure 9 shows the fracture surfaces of the Mg matrix composites. The CNTs-Mg2Sip/Mg composite exhibited a ductile fracture morphology with small dimples (Fig. 9a), coinciding with the moderate elongation of the composite in the tensile tests. Convex structures and pits with spherical morphologies and a similar size to the primary α-Mg particles were observed (indicated by arrows in Fig. 9a). A side view of the fracture surface indicated that cracks predominantly propagated across the primary particles during tensile testing (indicated by arrow A in Fig. 10a). Only a small number of cracks occasionally propagated along the SSSs between the primary particles (indicated by arrow B in Fig. 10a), and concave surfaces appeared when the

primary particles were debonded and remained on the fracture surface of the other half of the test sample. Thus, the concave depression corresponded to the pits on the fracture surface. High-magnification micrographs indicated that the debonded Mg2Sip was always uniformly dispersed around the CNTs (indicated by arrows in Fig. 9b), suggesting that the good CNTs/Mg and Mg2Sip/Mg interfacial bonding was favorable for load transfer from the Mg matrix to the hybrid reinforcements. During tensile testing, the Mg matrix was preferentially deformed. The stress was concentrated around the reinforcement/matrix interface as the tensile testing continued, owing to the efficient load transfer. The interface debonding absorbed energy and relaxed the stress, reducing the rate of crack propagation and thus strengthening the composite. The generated cracks then propagated across the deformed primary α-Mg particles, resulting in the fracture of the composite.

Fig. 9. Fractographs of the (a, b) CNTs-Mg2Sip/Mg composite, (c, d) CNTs/Mg composite, and (e, f) Mg2Sip/Mg composite. Fig. 10. Side views of fracture surfaces of the (a) CNTs-Mg2Sip/Mg composite and (b, c) Mg2Sip/Mg composite. The fracture surfaces of the CNTs/Mg and Mg2Sip/Mg composites were characterized by a large number of convex structures and pits, and microcracks always existed between them (Figs. 9c and e). CNTs and Mg2Sip aggregated into clusters and were debonded from the Mg matrix (indicated by arrows in Fig. 9d and f), which was contrary to the good dispersion shown in Fig. 9b. A side view of the fracture surface of the Mg2Sip/Mg composite indicated that cracks propagated between the primary particles (indicated by arrow B in Fig. 10b), and many microcracks were simultaneously generated near the fracture surface (indicated by arrows in Fig. 10b), indicating that the fracture changed to the intergranular mode, which differed from the transgranular regime of the CNTs-Mg2Sip/Mg hybrid composite (Fig. 10a). High-magnification micrographs indicated that microcracks were initiated from the aggregated Mg2Sip regions (Fig. 10c). As discussed previously,

CNTs entanglement and Sip aggregation in the CNTs/Mg and Sip/Mg mixed powders were apparent (Figs. 3a–c). It is expected that the reinforcement aggregation was also severe in the zones of the SSSs after partial remelting and thixoforming; thus, the intergranular SSSs of the CNTs/Mg and Mg2Sip/Mg composites became weak points. Cracks were preferentially initiated in the vicinity of the agglomerated CNTs and Mg2Sip and then propagated along the SSSs during the tensile testing, causing the premature fracture of the matrix and thus the deterioration of the mechanical properties. Therefore, the contributions from individual CNTs or Mg2Sip to the mechanical properties were insufficient because of the weak strengthening effect. Accordingly, the variation in the mechanical properties of the Mg matrix composites could be well interpreted. 3.3 Strengthening mechanisms of Mg matrix composite It is commonly acknowledged that the strengthening mechanisms of composites include direct and indirect strengthening [12, 21-23]. Direct strengthening results from load transfer from the matrix to the reinforcement through shear stress (∆σLt). Indirect strengthening is due to the influence of the reinforcing phase on the microstructure of the matrix, which includes grain refinement (∆σGr), Orowan strengthening (∆σOr), and thermal-mismatch strengthening (∆σCt). For hybrid composites, the YS is generally estimated by accumulating the strengthening contribution of each reinforcement using the rule of mixtures. Thus, the strength of the CNTs-Mg2Sip/Mg hybrid composites was calculated using the following equation [24]: σc = σm + ∆σGr + ∆σ dMg2Si + ∆σ LtMg2Si + ∆σ dCNTs + ∆σ LtCNTs

(1)

where σc and σm represent the YS values of the composite and Mg matrix, respectively. ∆σd represents the strength increment due to dislocation strengthening and can be expressed as: ∆σd = ∆σOr 2 + ∆σCt 2

(2)

where ∆σOr and ∆σCt represent the strengthening contributions from Orowan

strengthening and thermal mismatch, respectively. The addition of reinforcements reduced the grain size of the matrix, increasing the YS, in accordance with the Hall–Petch relationship [21]. The strength increment due to grain reinforcement (∆σGr) was calculated using the following equation:

(

∆σGr = K dc -1 2 - dm -1 2

)

(3)

where K is a coefficient (0.28 MPa·m1/2 [25]), and dc and dm represent the average grain sizes of the composite and the matrix (including the primary particles and the SSSs), respectively. According to Eq. (3), the strength contribution for the primary particles was 3.0 MPa, and that for the SSSs was 26.2 MPa. According to the rule of mixtures, the total strength increment due to grain refinement ranged between these two values, depending on the fractions of the two microstructures. The calculated ∆σGr value for the hybrid composite was 14.6 MPa when the liquid fraction was 50 vol.%, as shown in Table 2. A high dislocation density was generated at the reinforcements/matrix interface because of the discrepancy in the coefficients of thermal expansion between the matrix and the reinforcement phase during the cooling from the fabrication temperature to room temperature, increasing the tensile strength of the matrix. The strength increment ∆σCt was calculated as follows [12, 23]: ∆σCt = abG m 12Vc∆α∆T [bd c(1 - Vc )]

(4)

where a is a strengthening coefficient (0.5) [12], b represents the Burgers vector of the Mg matrix (0.32 nm), Gm represents the shear modulus (17.7 GPa) [26], Vc represents the volume fraction of the reinforcement, ∆α represents the difference between the coefficients of thermal expansion of the matrix and reinforcements (αMg = 25 × 10-6 K-1, αCNTs = 2.7 × 10-6 K-1, and αMg2Si = 7.5 × 10-6 K-1) [7, 12, 27], and ∆T represents the difference between the thixoforming temperature and room temperature (560 K). For the CNTs-Mg2Sip/Mg hybrid composite, the thermal-mismatch strengthening ∆σCt for 0.75 vol.% Mg2Sip (21 MPa) was greater than that for 0.75 vol.% CNTs (10.3 MPa) (Table 2). According to the shear lag model proposed by Kelly and Tyson for short

fiber-reinforced composites, the applied stress is transferred from the matrix to the reinforcement via interfacial shear stress during tensile testing, enhancing the strength of the composites [28]. If the length of CNTs is smaller than the critical length (lc), the CNTs are pulled out during tensile testing. Otherwise, the CNTs fail in a fracture mode. lc can be calculated as: lc =

dcσr 2τm

(5)

where τm represents the matrix shear strength (0.5σm), and σr represents the strength of the CNTs (30 GPa) [22]. The calculated lc (10.9 µm) was larger than the measured length of the CNTs (5 µm); thus, the fracture mode of the CNTs was the pull-out mode, which agrees well with the experimental results shown in Fig. 7. In the pull-out mode, the strength increment due to load-transfer strengthening ∆σLt can be calculated as follows [22]: ∆σLt = σrVc (l / 2lc ) − σmVc

(6)

where σm represents the YS of the matrix, and l represents the length of the reinforcement. When the essential parameters were substituted into Eq. (6), the calculated ∆σLt for 0.75 vol.% CNTs was 50.7 MPa. Additionally, the Mg2Sip contributed to the strength enhancement from load bearing considerations, and this strengthening effect was calculated using a modified shear lag model [29]:

∆σLt = 0.5σmVcSe

(7)

where Se represents the effective aspect ratio of the reinforcement. For a spherical reinforcement, Se = 1. The calculated ∆σLt for 0.75 vol.% Mg2Sip (0.5 MPa) was less than 1 MPa, which was significantly smaller than the strengthening contribution from the CNTs. Orowan strengthening is another strengthening mechanism for composites. The formation of dislocation loops surrounding the nano-reinforcement generates stress concentration that prevents the generation of dislocations during tensile testing. The strength increase due to Orowan strengthening ∆σOr is given by [30]: ∆σOr =

0.13Gmb

λ

In

de b

(8)

where de represents the equivalent diameter of the reinforcement, and λ represents the

effective interparticle spacing, which is related to the size and volume fraction of the reinforcement and can be calculated using a spherical model. The interparticle spacing of Mg2Sip (λMg2Si) was calculated as follows [12]:

(

)

λMg Si = dMg Si 3 1 2VMg Si - 1 2

2

2

(9)

For rod-shaped CNTs, the equivalent diameter (de-CNTs) should be substituted for the average diameter of the CNTs (dCNTs) and is given as follows [31]: de - CNTs = 3 3LCNTsd 2 CNTs 2

(10)

where LCNTs represents the length of the CNTs. The effective interparticle spacing of the CNTs (λCNTs) differed from that of Mg2Sip and was given by [31]:

(

λCNTs = de - CNTs 3 π 6VCNTs

)

(11)

According to Eqs. (8)–(11), the predicted value of ∆σOr for 0.75 vol.% CNTs was only 4.5 MPa, which was significantly smaller than the corresponding strengthening contribution (24.4 MPa) from 0.75 vol.% Mg2Sip. Fig. 11a shows the contributions for each strengthening effect for each of the composites, indicating that load transfer was the most important strengthening mechanism for the CNTs reinforcement and the Orowan strengthening was the most significant strengthening mechanism for the Mg2Sip. Although the equivalent diameter de of the CNTs (265.7 nm) was small, the CNTs having a large aspect ratio (Se of 100) contributed little to the Orowan strengthening, which is attributed to dislocation sink rather than dislocation loops formed around the long CNTs [31].

Table 2 Strengthening contributions of different mechanisms (MPa) and the corresponding strength increase compared with the Mg matrix (%).

Fig. 11. Theoretical calculation of different strengthening mechanisms. (a) Strengthening contribution and (b) comparison between calculation and experiment values. The predicted strength of the CNTs-Mg2Sip/Mg composite (243.8 MPa) was higher than the experimental result of 180 MPa (Fig. 11b). The different strengthening

mechanisms interacted with each other, and their synergistic strength contributions may not be superimposed linearly through Eq. (1). Additionally, Raman analyses indicated that the structure of the CNTs was slightly damaged (Fig. 4), which reduced the strength of the composite, but this detrimental effect was not considered in the analysis. Thus, it is reasonable that the predicted values were higher than the experimental results. For the 1.5CNTs/Mg composite, a large difference between the calculated values and the experimental results for the YS was observed; the discrepancy was as large as 131 MPa. The main reason for this is that the reinforcements were assumed to be uniformly dispersed throughout the matrix in the strengthening model, while the aggregation of the CNTs became increasingly significant as their content increased. In previous studies, the maximum volume fraction of CNTs introduced into the matrix was less than 2%, even though different types of dispersal procedures were used [9, 12, 19, 20]. In the present study, cracks were preferentially initiated in and propagated along the reinforcement clustering regions, thereby neutralizing the contributions of different strengthening mechanisms (Fig. 10). Thus, the calculated value for the 1.5CNTs/Mg composite significantly deviated from the experimental value due to the formation of CNTs clusters and the superior strengthening ability of the CNTs were adequately realized at low reinforcement contents. Additionally, the calculated value for the composite reinforced with Mg2Sip alone was approximately 24% lower than that for the hybrid composite. This is because the nanoparticles around the CNTs not only restricted the debonding of the CNTs during plastic deformation but also facilitated the synergistic strengthening effect of the CNTs. Similarly, Zhang et al. reported that the strengthening efficiency of the CNTs-SiCp hybrid reinforced Al composites was higher than those of composites reinforced with CNTs or SiCp alone [9]. Therefore, this study provides an efficient strategy for fabricating Mg-based composites with high performance by promoting the linkages between individual CNTs and in situ Mg2Sip reinforcements.

4. Conclusions

In this work, the microstructure and mechanical properties of the CNTs-Mg2Sip hybrid reinforced Mg-based composites were studied. The conclusions are as follow: (1) The in situ-synthesized Mg2Sip and CNTs maintained their structural integrity and were homogeneously distributed between the primary α-Mg particles in the thixoformed CNTs-Mg2Sip/Mg composite. (2) The UTS and YS of the CNTs-Mg2Sip/Mg hybrid composite were 232 MPa and 180 MPa, respectively, which were 21% and 30% higher than those of the Mg matrix prepared by the same process. For the Mg matrix composites with the same concentration of reinforcements, the strengthening capability of CNTs-Mg2Sip hybrids was 12 and 26% higher than those of composites reinforced with individual Mg2Sip and CNTs, respectively. The enhanced mechanical properties of the CNTs-Mg2Sip/Mg composite were mainly related with the improved dispersion of reinforcements and the high synergistic strengthening efficiency. (3) The fracture of the Mg2Sip/Mg and CNTs/Mg composites was intergranular mode due to serious reinforcement aggregation existed in the zones of SSSs, which was different from the transgranular regime of the CNTs-Mg2Sip/Mg hybrid composite. (4) The CNTs-Mg2Sip hybrids exhibited a promising way for the development of the metal matrix composites with impressive performances.

Acknowledgments The authors acknowledge the financial support provided by the Natural Science Foundation of Ningxia University (Grant No. ZR1702).

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Table captions: Table 1 Mechanical properties of the Mg matrix composites. Table 2 Strengthening contributions of different mechanisms (MPa) and the corresponding strength increase compared with the Mg matrix (%).

Table 1 Materials

YS (MPa) UTS (MPa) εf (%)

AZ91D

138

192

7.6

CNTs/Mg

143

197

4.4

Mg2Sip/Mg

161

212

5.6

(CNTs+Mg2Sip)/Mg

180

232

6.1

Table 2 Sample

∆σGr

∆σCt-CNTs

∆σCt-Mg2Sip

∆σLt-CNTs

∆σLt-Mg2Sip

∆σOr-CNTs

∆σOr-Mg2Sip

σc

CNTs-Mg2Sip/Mg

14.6 (10.6%)

10.3 (7.5%)

21.0 (15.2%)

50.7 (36.7%)

0.5 (0.36%)

4.5 (3.3%)

24.4 (17.7%)

248.3

CNTs/Mg

16.7 (12.1%)

14.6 (10.6%)

–

102.5 (74.3%)

–

4.1 (%)

–

273.8

Mg2Sip/Mg

15.2 (11.0%)

–

29.7 (21.5%)

–

1.0 (0.72%)

–

33.5 (24.3%)

200.1

Figure Captions: Fig. 1. SEM images of (a) AZ91D powders, (b, c) CNTs, and (d) Sip. Fig. 2. Schematic illustration of the fabrication process for the CNTs-Mg2Sip/Mg hybrid composites. Fig. 3. SEM images of the composite powders: (a, b) CNTs/Mg, (c) Sip/Mg, and (d) CNTs-Sip/Mg. Fig. 4. Raman spectra of the initial CNTs, CNTs-Sip/Mg powders, and thixoformed CNTs-Mg2Sip/Mg composite. Fig. 5. (a, b) SEM images of the CNTs-Mg2Sip/Mg composites and (c–f) EDS surface scanning analysis for (b). Fig. 6. XRD spectra of the CNTs-Mg2Sip/Mg composites. Fig. 7. (a–c) High-resolution TEM images of the interfaces between the CNTs, Mg2Sip, and Mg matrix of the CNTs-Mg2Sip/Mg matrix composite. (d) TEM image of the CNTs-Mg2Sip hybrid reinforcements extracted from the composite. (e–h) Elemental-mapping images for (d). Fig. 8. Tensile stress–strain curves of the Mg matrix composites. Fig. 9. Fractographs of the (a, b) CNTs-Mg2Sip/Mg composite, (c, d) CNTs/Mg composite, and (e, f) Mg2Sip/Mg composite. Fig. 10. Side views of fracture surfaces of the (a) CNTs-Mg2Sip/Mg composite and (b, c) Mg2Sip/Mg composite. Fig. 11. Theoretical calculation of different strengthening mechanisms. (a) Strengthening contribution and (b) comparison between calculation and experiment values.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Fig. 11

A novel powder thixoforming technology is developed. The Mg2Sip distributed uniformly around CNTs with maintained structural integrity. The strengthening mechanism is analyzed by a strengthening model. Hybridizing CNTs with Mg2Sip effectively improves the mechanical properties.

Declaration of interests The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: