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Microstructure and mechanical property of bimodal-size metallic glass particle-reinforced Al alloy matrix composites
Journal of Alloys and Compounds 814 (2020) 152317
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and mechanical property of bimodal-size metallic glass particle-reinforced Al alloy matrix composites M.S. Xie a, b, Z. Wang a, b, *, G.Q. Zhang a, b, C. Yang a, b, W.W. Zhang a, b, K.G. Prashanth c, d, e a
Guangdong Key Laboratory for Processing and Forming of Advanced Metallic Materials, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, China b National Engineering Research Center of Near-net-shape Forming for Metallic Materials, South China University of Technology, Guangzhou, 510640, China c Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajate Tee 5, 19086, Tallinn, Estonia d Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700, Leoben, Austria e CBCMT, School of Engineering, Vellore Institute of Technology, Vellore 632014, Tamil Nadu, India
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a b s t r a c t
Article history: Received 7 October 2018 Accepted 16 September 2019 Available online 16 September 2019
A bimodal size metallic glassy particles reinforced 7075 aluminum composite was fabricated by powder metallurgy via ball milling and hot extrusion. The results show that metallic glassy reinforcements help to improve the densification due to the liquid-like behavior in the supercooled liquid region. The metallic glassy reinforcements were found to have a bimodal size distribution at nanoscale and microscale, and were uniformly distributed in the matrix. An interphase layer with a thickness of 60e80 nm was observed between the reinforcement/matrix resulted from elemental diffusion and chemical reaction. The introduction of bimodal size metallic glassy particles have significantly improved the mechanical properties, where the yield strength and fracture strength increase from 442 and 648 MPa for Al7075 to 869 and 962 MPa for composite with 17 vol% reinforcement, respectively. The strengthening mechanisms of the composites were revealed. © 2019 Elsevier B.V. All rights reserved.
Keywords: Aluminum matrix composites Bimodal sized particles Metallic glasses Mechanical properties Strengthening mechanisms
1. Introduction Al-based metal matrix composites (AMMCs) are expected to be widely used in aerospace, transportation and other fields owing to their superior properties such as high specific strength, high specific modulus, and excellent friction and wear resistance [1e5]. Conventionally, hard and brittle ceramic materials (Al2O3, SiC and B4C etc.) are commonly introduced into the AMMCs [6,7], which give rise to high strength and modulus. However, the long-due unsolved drawbacks still exist in the AMMCs, such as poor wettability, porosity, agglomeration and undesirable interfacial reactions that lead to drastic reduction in ductility and low reproducibility of mechanical properties. This in turn limits the commercial application of AMMCs [8e10]. Recently, a number of novel reinforcements such as metallic glasses [11e15], high entropy alloys [16,17], quasicrystalline [18,19], carbon nanotubes [20e22] along
* Corresponding author. Guangdong Key Laboratory for Processing and Forming of Advanced Metallic Materials, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, China. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.jallcom.2019.152317 0925-8388/© 2019 Elsevier B.V. All rights reserved.
with graphene [23,24] have been considered and developed as promising candidates to avoid the critical drawbacks of ceramic reinforcements. Amorphous alloys/metallic glasses (MGs) exhibit extremely high strength (~2 GPa), show large elastic strain limit (~2%), offer high corrosion resistance and high degree of compatibility with matrix [25e28]. The metallic nature of the amorphous reinforcement and the similar coefficient of thermal expansion (CTE) value with metal matrix are believed to provide improved interfacial bonding between the matrix and the reinforcement, compared to their ceramic counterparts [25]. Furthermore, MGs exhibit a significant decrease of viscosity within supercooled liquid (SCL) region, which contributes to a better consolidation of the composites reaching theoretical density [3,29,30]. Metallic glasses used in MMCs are mainly micron-sized particles, which can cause a direct strengthening effect due the load bearing effect of the micro-MG particles and indirect strengthening effect from the matrix. However, the micron-sized glassy particles are prone to crack initiation and may significantly deteriorate the ductility of the composites [11,31,32]. However recently, it has been observed that composites with bimodal-sized reinforcement (a mixture of coarse and finer particles) can provide the possibility to overcome the
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disadvantage of reduced ductility. Moreover, it can help in achieving high strength and improved ductility compared with the composites reinforced with micron-sized particles [33e37]. In this work, bimodal Ti-based metallic glassy particles reinforced Al7075 alloy matrix are fabricated and the effect of the bimodal distribution on the densification, microstructure and mechanical property was studied.
2. Experimental procedure The starting MG powders (<38 mm) with a composition of Ti55.5Cu18.5Ni17.5Al8.5 (at.%) were prepared by mechanical alloying (MA) using a high-energy planetary ball mill (QM-2SP20) equipped with hardened steel balls and vials, which was performed under Ar atmosphere for 35 h with a speed of 245 rpm and a ball-powderratio of 10:1 at room temperature. The obtained MG powders were mechanically mixed with the gas-atomized Al-7075 matrix (~48 mm) through ball milling for 10 h using a speed of 245 rpm. Different volume fractions of MG powders, 0%, 2%, 6%, 11% and 17%, were mixed with the Al-7075 matrix, respectively. The composite powders were subsequently consolidated in the form of cylindrical billets of 13.5 mm diameter and ~23 mm height by cold pressing. The green compacts were subsequently hot extruded at 673 K and 590 MPa, using an extrusion ratio of 11.4 to produce rods with a diameter of 4 mm and a length of ~135 mm. The density of the extruded samples was evaluated by Archimedes principle. X-ray diffraction (XRD) was performed (D8-Advance, Bruker, Karlsruhe, Germany, Cu-Ka radiation, l ¼ 0.154056 Å) on the samples with a step size of 0.02 and a counting time of 19.2 s per step. The thermal stability of the samples was determined using a differential scanning calorimeter (DSC; STA449C, NETSCH, Bavaria, Germany) with a heating rate of 10 K/min. The microstructural studies were conducted using a scanning electron microscope (SEM, Nova 2000, FEI, Germany) equipped with an energy-dispersive X-ray spectroscopy (EDS) and an analytical transmission electron microscopy (TEM, Tecnai G2 F20, FEI, Germany). Uniaxial compression tests were performed at CMT5504 testing facility at room temperature and at a strain rate of 0.5 mm/min. Cylindrical samples with 3 mm in diameter and 6 mm in length were used for compression tests. Three samples for each composite and unreinforced Al7075 were measured for data reproducibility.
3. Results and discussion 3.1. Microstructure observation Fig. 1a shows the XRD patterns obtained on the consolidated aluminum matrix composites reinforced with different volume fractions of Ti55.5Cu18.5Ni17.5Al8.5 particles. Some sharp Bragg peaks seen in the patterns were identified as fcc-Al. However, the typical abroad maximum peak corresponding to amorphous phase is difficult to differentiate from the XRD patterns due to low volume fraction of MG. Some extra crystalline Bragg peaks were found in the composites which are possibility corresponding to the intermetallic phases such as MgNi2, Al3Ti, MgZn2, TiZn16, in which the intensity increases gradually with increasing the content of reinforcement. The thermal stability of the composites and unreinforced Al7075 are shown in Fig. 1b. The Al-based composites display an exothermic event with the onset at ~710 K, in which the peak area increases with increasing volume of metallic glassy reinforcement, indicating that the presence of amorphous phase in the reinforcement. The results of XRD and DSC tests indicate that the metallic glassy reinforcement experienced partial crystallization during consolidation. Fig. 2 shows the SEM micrographs of the as-extruded composites. Fig. 3 shows the SEM micrograph of the composite reinforced with 6 vol% Ti55.5Cu18.5Ni17.5Al8.5 particles and corresponding EDS mapping images. The bright particles correspond to the metallic glassy reinforcement (which are rich in Ti, Cu, Ni element and poor in Al), while the dark phase corresponds to the aluminum alloy matrix. The bright particles are observed in two different particle size regimes (micro-sized and nano-sized), where the nano-sized reinforcing particles are resultant from the breaking of the large particles during ball milling and hot extrusion [38]. The nano-sized reinforcing particles can be confirmed in Fig. 4eeh, which show the presence of irregular shaped particles rich in elements like Ti, Cu and Ni and at the same time depriving Al. Both micro-sized and nano-sized particles are homogeneously dispersed in the Al7075 matrix. Nanoscale MgZn2 precipitates (circle shape and marked as yellow arrows) were also found in the Al7075 matrix, which was confirmed by XRD and TEM observations as discussed in Ref. [4]. At lower reinforcement content, i.e. 2 vol% and 6 vol% (Fig. 2a and b), reinforcing particles are homogeneously dispersed in the aluminum matrix and no particle clustering and pores are detected,
Fig. 1. (a) XRD patterns and (b) DSC curves of the consolidated composites with varying volume fractions of Ti55.5Cu18.5Ni17.5Al8.5 particles.
corroborating a high densification (99.2% for the composite with
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Fig. 2. (aed) SEM micrographs of the as-extruded composites with 2, 6, 11 and 17 vol% of Ti55.5Cu18.5Ni17.5Al8.5 particles, respectively; (e) Density and relative density of the composites.
Fig. 3. SEM micrograph of the composite reinforced with 6 vol% Ti55.5Cu18.5Ni17.5Al8.5 particles and corresponding EDS mapping images.
6 vol% reinforcement) of the consolidated specimens. However, with the higher amount of reinforcement (11 and 17 vol%), clustering of reinforcing particles are visible and a few pores can be observed at the particles/matrix interface (marked by the red arrow in Fig. 2c and d). The main reason for the agglomeration of large particles is that large particles can be easily moved and get
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contacted and connected with its neighbor due to the shear stress occurring during the hot extrusion process. Moreover, large particle fracture and interfacial debonding occur owing to the high local stress concentration near the large particles, which leads to the formation of pores and cracks. The local stress can be attributed to the accumulation of the shear deformation near the large reinforcement particles and reinforcement agglomeration. The volume fraction of the micro-sized and the nano-sized metallic glassy reinforcement was calculated by measuring the dark area shown in the EDS mapping of Al element (e.g. the dark area shown in Fig. 3e), which is ~4.3 vol % and 1.7 vol% for the metallic glassy particle with a size larger than 1 mm and smaller than 1 mm, respectively. The higher magnification image (see inset in Fig. 2b) displays a good metallurgical bond and is free of any pores at the matrix/ reinforcement interface, suggesting the good compatibility between Ti55.5Cu18.5Ni17.5Al8.5 particles and the aluminum matrix. Fig. 2e shows the density and relative density of the composites as a function of volume fraction of the reinforcing particles. The relative density of the composites increases firstly with increasing reinforcement content (up to 6 vol%) and then decrease with further increase in reinforcement content, in which the composites containing 6 vol% reinforcement shows the highest density of ~99.2%. The improved relative density indicates that metallic glassy particles facilitate to obtain a higher densification rate in the composites, which is unusual for the metal matrix composite where the ceramic reinforcements are detrimental for its densification. The improved densification is mainly because that the metallic glassy particles are in the liquid-like super cooled liquid state during hot extrusion at the SCL region, where the viscosity drops drastically. Therefore, the pores and cracks in the composites are easier to be filled by the liquid-like metallic glassy reinforcement by the shear deformation during the hot extrusion. However, when the volume fraction of the metallic glassy reinforcement is too high, agglomeration takes place leading to pores between the particles, which decreases the relative density of the composites. The microstructure of the composite reinforced with 6 vol% Ti55.5Cu18.5Ni17.5Al8.5 particles was further investigated by TEM observations. Fig. 4a shows the Al-7075 matrix and reinforcement phase. The large dark particle represents the micro-sized reinforcement phase. Although it shows a clean interface between the metallic glassy reinforcement and matrix in the SEM images, an interphase layer (60e80 nm) is visible in the TEM images as a result of elemental diffusion and chemical reaction, as shown in the dotted line in Fig. 4a. The selected area electron diffraction (SAED) taken from the white circle in Fig. 4a shows the presence of a broad halo, proving that the micro-sized reinforcement was partially crystallized, as shown in Fig. 4b. The nano-sized reinforcement (highlighted by the blue arrows in Fig. 4e and f), which is irregular shape and was broken from the micro-sized particles, can be fully amorphous or partially amorphous or nanocrystalline. The reinforcement was furthermore confirmed by the EDS analysis, shown in Fig. 4g and h taken from area 1, area 2 of Fig. 4e, respectively. Fig. 4d is the inverse fast Fourier transform (IFFT) image of the Al alloy matrix of the area contained within the red dash rectangular in Fig. 4c. It can be clearly seen that many dislocations are found in the Al alloy matrix, which may be from the severe plastic deformation during ball milling [39] and hot extrusion. The short hot extrusion time also limits the grade of recovery. Large reinforcing particles in the composites are continuously fragmented into smaller nano-sized reinforcing particles due to the high-energy ball milling, as pointed by the red arrow in Fig. 4e and f. The composite with 6 vol% bimodal sized metallic glassy particles shows an average grain size of 250 nm (calculated from the TEM micrograph), which is smaller than that of the unreinforced Al7075 (3 mm), indicating that metallic glassy particles resulted into grain
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Fig. 4. (aed) TEM micrographs of the composite reinforced with 6 vol% Ti55.5Cu18.5Ni17.5Al8.5 particles; (e) Dark-field TEM micrograph; (f) Bright-field TEM micrographs; (g) and (h) are the EDS of the areas marked as number 1 and number 2 in (e), respectively.
refinement. 3.2. Mechanical properties Fig. 5a displays the room temperature compression stress-strain curves of the composites containing different volume fraction of Ti55.5Cu18.5Ni17.5Al8.5 particles along with the curves for the Al7075 alloy with and without milling under quasistatic loading. Fig. 5b summarizes the corresponding mechanical data from Fig. 5a. Yield strength (sy) and ultimate compressive strength (smax) were improved almost linearly with increasing reinforcement volume, which increases from 478 to 763 MPa for the composite with 2 vol% reinforcement to 869 and 962 MPa for the composite containing 17 vol% reinforcement, respectively. The main strengthening mechanisms are found to be dislocation strengthening [40], which is closely related to the severe plastic deformation during the preparation process [38]. In addition, the introduction of more micro-sized Ti55.5Cu18.5Ni17.5Al8.5 particles give rises to higher load transfer effect [41]. The fracture strain (εf) of the composites with 2 vol% and 6 vol% reinforcements were ~40% and ~37%, which is even higher than that of the unreinforced Al7075 (~30%). This can be attributed to the improved densification as shown in Fig. 2e. It is interesting to note that the yield strength and the ultimate strength of the Al7075-BM exhibit a significant increase with respect to the Al7075, which increase from 297 to 535 MPa for the Al7075 to 442 and 648 MPa for the Al7075-BM, respectively. The results reveal that ball milling has a positive effect on the strength due to the
work hardening, which was studied in detail elsewhere [4]. The significant improvement of strength can be attributed to load transfer effect, grain refinement, Orowan strengthening and dislocation strengthening. Firstly, the presence of the micron-sized particles is mainly contributed to the load transfer effect, in which the load transferred from the soft Al-7075 matrix, across the matrix/reinforcement interface, to the harder micro-reinforcement particles [42]. As a result, the harder micro-reinforcements are subjected to bear a large part of the external stresses and contribute to the strengthening of the Al-7075 matrix. Secondly, the addition of nano-reinforcement gives rise to Orowan strengthening by serving as obstacles to the passing of dislocations and thus promotes Orowan loops around the particles [40]. Thirdly, the grain refinement of the matrix results from the preparation method used (high-energy ball milling) and the nano-sized reinforcing particles acting as pinning points, hampering or retarding grain growth [35,43]. In addition, micro-reinforcement can stimulate dynamic recrystallization (DRX) nucleation during hot deformation process [36], thus contributes to the grain refinement. It can be inferred that the grain size of the matrix decreased gradually with increasing volume fraction of Ti55.5Cu18.5Ni17.5Al8.5 particles. Fourthly, the dislocations generated primarily during the preparation process (high-energy ball milling and hot extraction) have a significant effect on the increase of strength. It is noteworthy that in the composite examined in this work, dislocation strengthening as a result of the thermal expansion mismatch is negligible since the difference of the CTE between the reinforcement and the matrix is
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Fig. 5. (a) Room-temperature compressive stress-strain curves of the composites reinforced with different volume fractions of Ti55.5Cu18.5Ni17.5Al8.5 particles; (b) corresponding mechanical data; (c) and (d) are the SEM micrographs of the lateral surface for the composite with 2 vol% and 17 vol% reinforcements after compressive test, respectively; (e) and (f) are the schematic diagrams showing the fracture behavior of composites reinforced with lower and higher reinforcements, respectively.
much smaller than that of the ceramic reinforced metal matrix composites [32,44]. Fig. 5c and d shows the fracture morphology (after polishing) of the composites with 2 vol% and 17 vol% of Ti55.5Cu18.5Ni17.5Al8.5 particles after compression testing, respectively. It can be seen that fracture occurs more easily in the micro-sized reinforcing particles than the nano-sized ones during deformation (Fig. 5c), which is because that larger reinforcing particles have a higher local stress concentration and a greater statistical probability of containing intrinsic flaws [45]. Noteworthy, the cracks generated in the microsized reinforcing particles do not propagate along the interface, further proving that strong interfacial bonding strength obtained in the composites. Fig. 5e and f shows the schematic diagrams of fracture behavior for the composites. In the composites with lower reinforcement content (Fig. 5e), cracks originated in the microsized reinforcing particles are rarely connected with each other due to crack blunting effect by the soft matrix. Therefore one major crack occurred along the maximum shear stress direction (red
arrow in Fig. 5e). However, with increasing the content of reinforcements (Fig. 5f), cracks originated in the micro-sized reinforcing particles can be connected and form several major cracks which are deviated from the maximum shear stress direction. The main reason for this is that the closely spaced reinforcing particles give rise to higher stress concentration, thus microcracks propagated easily and connected rapidly, leading to the premature failure of the composites during the compressive deformation. It can be seen that the bimodal-sized metallic glassy particles reinforced composites can provide a reasonable combination of strength and ductility. It is known that monolithic micron-sized metallic glassy particle reinforced composites show relatively low ductility [4,28,29]. Although nano-sized particles reinforced composites showed enhanced strength and exhibit accepted ductility [41,42], the bimodal-sized metallic glassy particle reinforced composites have their unique advantages such as high strength obtained by direct loading strengthening effect and Orowan strengthening effect, and relatively low cost compared to the
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monolithic nano-sized particle reinforced composites which needed much longer milling time or special technique to obtain nano-sized reinforcement. 4. Conclusions Ti-based metallic glassy particle reinforced 7075 aluminum composites were successfully produced using ball milling and hot extrusion. The metallic glassy reinforcement shows a bimodal size distribution (in micro- and nano-scale), yet homogeneously dispersed in the Al-7075 matrix. An interphase layer with 60e80 nm thickness is formed between the reinforcement and the Al alloy matrix as a result of elemental diffusion and chemical reaction. Metallic glassy reinforcements help to improve the densification due to the liquid-like behavior in the supercooled liquid region. However, when the volume fraction of the metallic glassy reinforcement is too high, the relative density of the composites decreases owning to agglomeration and subsequent formation of pores. The introduction of bimodal size metallic glassy particles significantly improves the mechanical properties, where the yield strength and fracture strength increase with increasing reinforcements, from 442 and 648 MPa for 7075Al alloy to 869 and 962 MPa for composite with 17 vol% reinforcement, respectively. Four strengthening mechanisms are proposed for the improved strength, where the micro-reinforcement is beneficial to the load transfer effect, the nano-reinforcement is mainly favorable to the grain refinement and Orowan strengthening, and the generation of the dislocations during ball milling and hot extrusion is the main reason for the enhance dislocation density strengthening. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51701075), the Science and Technology Program of Guangzhou (Grant No. 201804010365), the Fundamental Research Funds for the Central Universities. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and mechanical property of bimodal-size metallic glass particle-reinforced Al alloy matrix composites M.S. Xie a, b, Z. Wang a, b, *, G.Q. Zhang a, b, C. Yang a, b, W.W. Zhang a, b, K.G. Prashanth c, d, e a
Guangdong Key Laboratory for Processing and Forming of Advanced Metallic Materials, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, China b National Engineering Research Center of Near-net-shape Forming for Metallic Materials, South China University of Technology, Guangzhou, 510640, China c Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajate Tee 5, 19086, Tallinn, Estonia d Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700, Leoben, Austria e CBCMT, School of Engineering, Vellore Institute of Technology, Vellore 632014, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2018 Accepted 16 September 2019 Available online 16 September 2019
A bimodal size metallic glassy particles reinforced 7075 aluminum composite was fabricated by powder metallurgy via ball milling and hot extrusion. The results show that metallic glassy reinforcements help to improve the densification due to the liquid-like behavior in the supercooled liquid region. The metallic glassy reinforcements were found to have a bimodal size distribution at nanoscale and microscale, and were uniformly distributed in the matrix. An interphase layer with a thickness of 60e80 nm was observed between the reinforcement/matrix resulted from elemental diffusion and chemical reaction. The introduction of bimodal size metallic glassy particles have significantly improved the mechanical properties, where the yield strength and fracture strength increase from 442 and 648 MPa for Al7075 to 869 and 962 MPa for composite with 17 vol% reinforcement, respectively. The strengthening mechanisms of the composites were revealed. © 2019 Elsevier B.V. All rights reserved.
Keywords: Aluminum matrix composites Bimodal sized particles Metallic glasses Mechanical properties Strengthening mechanisms
1. Introduction Al-based metal matrix composites (AMMCs) are expected to be widely used in aerospace, transportation and other fields owing to their superior properties such as high specific strength, high specific modulus, and excellent friction and wear resistance [1e5]. Conventionally, hard and brittle ceramic materials (Al2O3, SiC and B4C etc.) are commonly introduced into the AMMCs [6,7], which give rise to high strength and modulus. However, the long-due unsolved drawbacks still exist in the AMMCs, such as poor wettability, porosity, agglomeration and undesirable interfacial reactions that lead to drastic reduction in ductility and low reproducibility of mechanical properties. This in turn limits the commercial application of AMMCs [8e10]. Recently, a number of novel reinforcements such as metallic glasses [11e15], high entropy alloys [16,17], quasicrystalline [18,19], carbon nanotubes [20e22] along
* Corresponding author. Guangdong Key Laboratory for Processing and Forming of Advanced Metallic Materials, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, China. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.jallcom.2019.152317 0925-8388/© 2019 Elsevier B.V. All rights reserved.
with graphene [23,24] have been considered and developed as promising candidates to avoid the critical drawbacks of ceramic reinforcements. Amorphous alloys/metallic glasses (MGs) exhibit extremely high strength (~2 GPa), show large elastic strain limit (~2%), offer high corrosion resistance and high degree of compatibility with matrix [25e28]. The metallic nature of the amorphous reinforcement and the similar coefficient of thermal expansion (CTE) value with metal matrix are believed to provide improved interfacial bonding between the matrix and the reinforcement, compared to their ceramic counterparts [25]. Furthermore, MGs exhibit a significant decrease of viscosity within supercooled liquid (SCL) region, which contributes to a better consolidation of the composites reaching theoretical density [3,29,30]. Metallic glasses used in MMCs are mainly micron-sized particles, which can cause a direct strengthening effect due the load bearing effect of the micro-MG particles and indirect strengthening effect from the matrix. However, the micron-sized glassy particles are prone to crack initiation and may significantly deteriorate the ductility of the composites [11,31,32]. However recently, it has been observed that composites with bimodal-sized reinforcement (a mixture of coarse and finer particles) can provide the possibility to overcome the
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disadvantage of reduced ductility. Moreover, it can help in achieving high strength and improved ductility compared with the composites reinforced with micron-sized particles [33e37]. In this work, bimodal Ti-based metallic glassy particles reinforced Al7075 alloy matrix are fabricated and the effect of the bimodal distribution on the densification, microstructure and mechanical property was studied.
2. Experimental procedure The starting MG powders (<38 mm) with a composition of Ti55.5Cu18.5Ni17.5Al8.5 (at.%) were prepared by mechanical alloying (MA) using a high-energy planetary ball mill (QM-2SP20) equipped with hardened steel balls and vials, which was performed under Ar atmosphere for 35 h with a speed of 245 rpm and a ball-powderratio of 10:1 at room temperature. The obtained MG powders were mechanically mixed with the gas-atomized Al-7075 matrix (~48 mm) through ball milling for 10 h using a speed of 245 rpm. Different volume fractions of MG powders, 0%, 2%, 6%, 11% and 17%, were mixed with the Al-7075 matrix, respectively. The composite powders were subsequently consolidated in the form of cylindrical billets of 13.5 mm diameter and ~23 mm height by cold pressing. The green compacts were subsequently hot extruded at 673 K and 590 MPa, using an extrusion ratio of 11.4 to produce rods with a diameter of 4 mm and a length of ~135 mm. The density of the extruded samples was evaluated by Archimedes principle. X-ray diffraction (XRD) was performed (D8-Advance, Bruker, Karlsruhe, Germany, Cu-Ka radiation, l ¼ 0.154056 Å) on the samples with a step size of 0.02 and a counting time of 19.2 s per step. The thermal stability of the samples was determined using a differential scanning calorimeter (DSC; STA449C, NETSCH, Bavaria, Germany) with a heating rate of 10 K/min. The microstructural studies were conducted using a scanning electron microscope (SEM, Nova 2000, FEI, Germany) equipped with an energy-dispersive X-ray spectroscopy (EDS) and an analytical transmission electron microscopy (TEM, Tecnai G2 F20, FEI, Germany). Uniaxial compression tests were performed at CMT5504 testing facility at room temperature and at a strain rate of 0.5 mm/min. Cylindrical samples with 3 mm in diameter and 6 mm in length were used for compression tests. Three samples for each composite and unreinforced Al7075 were measured for data reproducibility.
3. Results and discussion 3.1. Microstructure observation Fig. 1a shows the XRD patterns obtained on the consolidated aluminum matrix composites reinforced with different volume fractions of Ti55.5Cu18.5Ni17.5Al8.5 particles. Some sharp Bragg peaks seen in the patterns were identified as fcc-Al. However, the typical abroad maximum peak corresponding to amorphous phase is difficult to differentiate from the XRD patterns due to low volume fraction of MG. Some extra crystalline Bragg peaks were found in the composites which are possibility corresponding to the intermetallic phases such as MgNi2, Al3Ti, MgZn2, TiZn16, in which the intensity increases gradually with increasing the content of reinforcement. The thermal stability of the composites and unreinforced Al7075 are shown in Fig. 1b. The Al-based composites display an exothermic event with the onset at ~710 K, in which the peak area increases with increasing volume of metallic glassy reinforcement, indicating that the presence of amorphous phase in the reinforcement. The results of XRD and DSC tests indicate that the metallic glassy reinforcement experienced partial crystallization during consolidation. Fig. 2 shows the SEM micrographs of the as-extruded composites. Fig. 3 shows the SEM micrograph of the composite reinforced with 6 vol% Ti55.5Cu18.5Ni17.5Al8.5 particles and corresponding EDS mapping images. The bright particles correspond to the metallic glassy reinforcement (which are rich in Ti, Cu, Ni element and poor in Al), while the dark phase corresponds to the aluminum alloy matrix. The bright particles are observed in two different particle size regimes (micro-sized and nano-sized), where the nano-sized reinforcing particles are resultant from the breaking of the large particles during ball milling and hot extrusion [38]. The nano-sized reinforcing particles can be confirmed in Fig. 4eeh, which show the presence of irregular shaped particles rich in elements like Ti, Cu and Ni and at the same time depriving Al. Both micro-sized and nano-sized particles are homogeneously dispersed in the Al7075 matrix. Nanoscale MgZn2 precipitates (circle shape and marked as yellow arrows) were also found in the Al7075 matrix, which was confirmed by XRD and TEM observations as discussed in Ref. [4]. At lower reinforcement content, i.e. 2 vol% and 6 vol% (Fig. 2a and b), reinforcing particles are homogeneously dispersed in the aluminum matrix and no particle clustering and pores are detected,
Fig. 1. (a) XRD patterns and (b) DSC curves of the consolidated composites with varying volume fractions of Ti55.5Cu18.5Ni17.5Al8.5 particles.
corroborating a high densification (99.2% for the composite with
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Fig. 2. (aed) SEM micrographs of the as-extruded composites with 2, 6, 11 and 17 vol% of Ti55.5Cu18.5Ni17.5Al8.5 particles, respectively; (e) Density and relative density of the composites.
Fig. 3. SEM micrograph of the composite reinforced with 6 vol% Ti55.5Cu18.5Ni17.5Al8.5 particles and corresponding EDS mapping images.
6 vol% reinforcement) of the consolidated specimens. However, with the higher amount of reinforcement (11 and 17 vol%), clustering of reinforcing particles are visible and a few pores can be observed at the particles/matrix interface (marked by the red arrow in Fig. 2c and d). The main reason for the agglomeration of large particles is that large particles can be easily moved and get
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contacted and connected with its neighbor due to the shear stress occurring during the hot extrusion process. Moreover, large particle fracture and interfacial debonding occur owing to the high local stress concentration near the large particles, which leads to the formation of pores and cracks. The local stress can be attributed to the accumulation of the shear deformation near the large reinforcement particles and reinforcement agglomeration. The volume fraction of the micro-sized and the nano-sized metallic glassy reinforcement was calculated by measuring the dark area shown in the EDS mapping of Al element (e.g. the dark area shown in Fig. 3e), which is ~4.3 vol % and 1.7 vol% for the metallic glassy particle with a size larger than 1 mm and smaller than 1 mm, respectively. The higher magnification image (see inset in Fig. 2b) displays a good metallurgical bond and is free of any pores at the matrix/ reinforcement interface, suggesting the good compatibility between Ti55.5Cu18.5Ni17.5Al8.5 particles and the aluminum matrix. Fig. 2e shows the density and relative density of the composites as a function of volume fraction of the reinforcing particles. The relative density of the composites increases firstly with increasing reinforcement content (up to 6 vol%) and then decrease with further increase in reinforcement content, in which the composites containing 6 vol% reinforcement shows the highest density of ~99.2%. The improved relative density indicates that metallic glassy particles facilitate to obtain a higher densification rate in the composites, which is unusual for the metal matrix composite where the ceramic reinforcements are detrimental for its densification. The improved densification is mainly because that the metallic glassy particles are in the liquid-like super cooled liquid state during hot extrusion at the SCL region, where the viscosity drops drastically. Therefore, the pores and cracks in the composites are easier to be filled by the liquid-like metallic glassy reinforcement by the shear deformation during the hot extrusion. However, when the volume fraction of the metallic glassy reinforcement is too high, agglomeration takes place leading to pores between the particles, which decreases the relative density of the composites. The microstructure of the composite reinforced with 6 vol% Ti55.5Cu18.5Ni17.5Al8.5 particles was further investigated by TEM observations. Fig. 4a shows the Al-7075 matrix and reinforcement phase. The large dark particle represents the micro-sized reinforcement phase. Although it shows a clean interface between the metallic glassy reinforcement and matrix in the SEM images, an interphase layer (60e80 nm) is visible in the TEM images as a result of elemental diffusion and chemical reaction, as shown in the dotted line in Fig. 4a. The selected area electron diffraction (SAED) taken from the white circle in Fig. 4a shows the presence of a broad halo, proving that the micro-sized reinforcement was partially crystallized, as shown in Fig. 4b. The nano-sized reinforcement (highlighted by the blue arrows in Fig. 4e and f), which is irregular shape and was broken from the micro-sized particles, can be fully amorphous or partially amorphous or nanocrystalline. The reinforcement was furthermore confirmed by the EDS analysis, shown in Fig. 4g and h taken from area 1, area 2 of Fig. 4e, respectively. Fig. 4d is the inverse fast Fourier transform (IFFT) image of the Al alloy matrix of the area contained within the red dash rectangular in Fig. 4c. It can be clearly seen that many dislocations are found in the Al alloy matrix, which may be from the severe plastic deformation during ball milling [39] and hot extrusion. The short hot extrusion time also limits the grade of recovery. Large reinforcing particles in the composites are continuously fragmented into smaller nano-sized reinforcing particles due to the high-energy ball milling, as pointed by the red arrow in Fig. 4e and f. The composite with 6 vol% bimodal sized metallic glassy particles shows an average grain size of 250 nm (calculated from the TEM micrograph), which is smaller than that of the unreinforced Al7075 (3 mm), indicating that metallic glassy particles resulted into grain
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Fig. 4. (aed) TEM micrographs of the composite reinforced with 6 vol% Ti55.5Cu18.5Ni17.5Al8.5 particles; (e) Dark-field TEM micrograph; (f) Bright-field TEM micrographs; (g) and (h) are the EDS of the areas marked as number 1 and number 2 in (e), respectively.
refinement. 3.2. Mechanical properties Fig. 5a displays the room temperature compression stress-strain curves of the composites containing different volume fraction of Ti55.5Cu18.5Ni17.5Al8.5 particles along with the curves for the Al7075 alloy with and without milling under quasistatic loading. Fig. 5b summarizes the corresponding mechanical data from Fig. 5a. Yield strength (sy) and ultimate compressive strength (smax) were improved almost linearly with increasing reinforcement volume, which increases from 478 to 763 MPa for the composite with 2 vol% reinforcement to 869 and 962 MPa for the composite containing 17 vol% reinforcement, respectively. The main strengthening mechanisms are found to be dislocation strengthening [40], which is closely related to the severe plastic deformation during the preparation process [38]. In addition, the introduction of more micro-sized Ti55.5Cu18.5Ni17.5Al8.5 particles give rises to higher load transfer effect [41]. The fracture strain (εf) of the composites with 2 vol% and 6 vol% reinforcements were ~40% and ~37%, which is even higher than that of the unreinforced Al7075 (~30%). This can be attributed to the improved densification as shown in Fig. 2e. It is interesting to note that the yield strength and the ultimate strength of the Al7075-BM exhibit a significant increase with respect to the Al7075, which increase from 297 to 535 MPa for the Al7075 to 442 and 648 MPa for the Al7075-BM, respectively. The results reveal that ball milling has a positive effect on the strength due to the
work hardening, which was studied in detail elsewhere [4]. The significant improvement of strength can be attributed to load transfer effect, grain refinement, Orowan strengthening and dislocation strengthening. Firstly, the presence of the micron-sized particles is mainly contributed to the load transfer effect, in which the load transferred from the soft Al-7075 matrix, across the matrix/reinforcement interface, to the harder micro-reinforcement particles [42]. As a result, the harder micro-reinforcements are subjected to bear a large part of the external stresses and contribute to the strengthening of the Al-7075 matrix. Secondly, the addition of nano-reinforcement gives rise to Orowan strengthening by serving as obstacles to the passing of dislocations and thus promotes Orowan loops around the particles [40]. Thirdly, the grain refinement of the matrix results from the preparation method used (high-energy ball milling) and the nano-sized reinforcing particles acting as pinning points, hampering or retarding grain growth [35,43]. In addition, micro-reinforcement can stimulate dynamic recrystallization (DRX) nucleation during hot deformation process [36], thus contributes to the grain refinement. It can be inferred that the grain size of the matrix decreased gradually with increasing volume fraction of Ti55.5Cu18.5Ni17.5Al8.5 particles. Fourthly, the dislocations generated primarily during the preparation process (high-energy ball milling and hot extraction) have a significant effect on the increase of strength. It is noteworthy that in the composite examined in this work, dislocation strengthening as a result of the thermal expansion mismatch is negligible since the difference of the CTE between the reinforcement and the matrix is
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Fig. 5. (a) Room-temperature compressive stress-strain curves of the composites reinforced with different volume fractions of Ti55.5Cu18.5Ni17.5Al8.5 particles; (b) corresponding mechanical data; (c) and (d) are the SEM micrographs of the lateral surface for the composite with 2 vol% and 17 vol% reinforcements after compressive test, respectively; (e) and (f) are the schematic diagrams showing the fracture behavior of composites reinforced with lower and higher reinforcements, respectively.
much smaller than that of the ceramic reinforced metal matrix composites [32,44]. Fig. 5c and d shows the fracture morphology (after polishing) of the composites with 2 vol% and 17 vol% of Ti55.5Cu18.5Ni17.5Al8.5 particles after compression testing, respectively. It can be seen that fracture occurs more easily in the micro-sized reinforcing particles than the nano-sized ones during deformation (Fig. 5c), which is because that larger reinforcing particles have a higher local stress concentration and a greater statistical probability of containing intrinsic flaws [45]. Noteworthy, the cracks generated in the microsized reinforcing particles do not propagate along the interface, further proving that strong interfacial bonding strength obtained in the composites. Fig. 5e and f shows the schematic diagrams of fracture behavior for the composites. In the composites with lower reinforcement content (Fig. 5e), cracks originated in the microsized reinforcing particles are rarely connected with each other due to crack blunting effect by the soft matrix. Therefore one major crack occurred along the maximum shear stress direction (red
arrow in Fig. 5e). However, with increasing the content of reinforcements (Fig. 5f), cracks originated in the micro-sized reinforcing particles can be connected and form several major cracks which are deviated from the maximum shear stress direction. The main reason for this is that the closely spaced reinforcing particles give rise to higher stress concentration, thus microcracks propagated easily and connected rapidly, leading to the premature failure of the composites during the compressive deformation. It can be seen that the bimodal-sized metallic glassy particles reinforced composites can provide a reasonable combination of strength and ductility. It is known that monolithic micron-sized metallic glassy particle reinforced composites show relatively low ductility [4,28,29]. Although nano-sized particles reinforced composites showed enhanced strength and exhibit accepted ductility [41,42], the bimodal-sized metallic glassy particle reinforced composites have their unique advantages such as high strength obtained by direct loading strengthening effect and Orowan strengthening effect, and relatively low cost compared to the
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monolithic nano-sized particle reinforced composites which needed much longer milling time or special technique to obtain nano-sized reinforcement. 4. Conclusions Ti-based metallic glassy particle reinforced 7075 aluminum composites were successfully produced using ball milling and hot extrusion. The metallic glassy reinforcement shows a bimodal size distribution (in micro- and nano-scale), yet homogeneously dispersed in the Al-7075 matrix. An interphase layer with 60e80 nm thickness is formed between the reinforcement and the Al alloy matrix as a result of elemental diffusion and chemical reaction. Metallic glassy reinforcements help to improve the densification due to the liquid-like behavior in the supercooled liquid region. However, when the volume fraction of the metallic glassy reinforcement is too high, the relative density of the composites decreases owning to agglomeration and subsequent formation of pores. The introduction of bimodal size metallic glassy particles significantly improves the mechanical properties, where the yield strength and fracture strength increase with increasing reinforcements, from 442 and 648 MPa for 7075Al alloy to 869 and 962 MPa for composite with 17 vol% reinforcement, respectively. Four strengthening mechanisms are proposed for the improved strength, where the micro-reinforcement is beneficial to the load transfer effect, the nano-reinforcement is mainly favorable to the grain refinement and Orowan strengthening, and the generation of the dislocations during ball milling and hot extrusion is the main reason for the enhance dislocation density strengthening. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51701075), the Science and Technology Program of Guangzhou (Grant No. 201804010365), the Fundamental Research Funds for the Central Universities. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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