Microstructural evolution of hybrid aluminum matrix composites reinforced with SiC nanoparticles and graphene/graphite prepared by powder metallurgy

2020-11-23 09:21JingshnZhngQingLiuShufengYngZhixinChenQingLiuZhengyiJing

Jingshn Zhng,Qing Liu,Shufeng Yng,∗∗,Zhixin Chen,Qing Liu,Zhengyi Jing,∗∗∗

a State Key Laboratory of Advanced Metallurgy,University of Science and Technology Beijing,Haidian District,Beijing,100083,China

b School of Mechanical,Materials,Mechatronic and Biomedical Engineering,University of Wollongong,NSW,2522,Australia

ABSTRACT The dispersion of ceramic nanoparticles is of significance to the microstructure and properties of particulate reinforced metal matrix composites.In this study,two hybrid enhancers,SiC-graphite and SiC-graphene nanosheets(GNSs),were incorporated into aluminum matrix composites using powder metallurgy.The dispersion of the reinforcements and microstructural evolution of the composites were characterized by using Scanning Electron Microscopy,X-Ray Diffraction,Transmission Electron Microscopy,and Raman spectroscopy.The results show that thin GNSs accelerated the deformation of the aluminum particles,and defects were introduced into the carbonaceous phases during the ball milling process.Al4C3 needles generated during hot pressing,and were observed to bridge the aluminum grains.Compared with graphite,GNSs were more uniformly dispersed throughout the composite,which in turn restrained grain growth.As a result,a nanostructured composite(57.7 nm) was successfully produced upon the addition of SiC-GNSs.

Keywords:Microstructure Aluminum matrix composite SiC nanoparticle SiC-Graphene nanosheets Powder metallurgy

1.Introduction

Metal matrix composites (MMCs) have attracted worldwide attention over the past few decades because of their excellent properties which combine the best properties of the metal matrix and reinforcements [1-4].Aluminum is an excellent matrix material for developing light weight,strong,and wear-resistant aluminum matrix composites(AMCs),which have found applications in automotive,aerospace,and electronic components etc.It is well known that the performance of AMCs is highly reliant on the incorporated reinforcements.Some emerging carbonaceous materials,including graphene nanosheets(GNSs)and carbon nanotubes(CNTs),have been reported to show great potential in structural and functional applications due to their extraordinary elastic modulus,high strength,conductivity,transmittance,and self-lubricating performance [1,5,6].GNSs outperform CNTs in terms of their high specific surface area and relatively ease of dispersion in the matrix materials,because plate-shaped GNSs tend to be easier to mix with particles,whereas one-dimensional CNTs are likely to entangle[7-10].Shin et al.[10]comparatively studied the strengthening effect of few-layer GNSs and multi-walled CNTs,and their results showed that few-layer thick GNSs were about 3.5 times more effective.Therefore,GNSs reinforced AMCs have recently received an increasing amount of attention [7].For example,0.3 wt% GNSs have been reported to be able to boost the tensile strength of an AMC by 62%(249 MPa) with 13% elongation [8].Khodabakhshi et al.[11]employed friction stir processing to incorporate GNSs into Al-Mg-based composites and achieved a>300% increase in yield strength without any significant reduction in elongation.Hybrid CNTs and GNSs have been simultaneously introduced into AMCs and their synergistic strengthening effect outperformed individual CNTs or GNSs [12].

One of the major challenges is the uniform dispersion and efficient use of GNSs during the synthesis of graphene reinforced AMCs [9].Clustered or re-stacked thick GNSs are basically graphite-like platelets and consequently,the intrinsic properties of the ultrathin GNSs are lost[3].There are basic two strategies to improve the dispersion of GNSs in composite materials reported in the literature.One is development and optimization of the processing routes used,such as conventional ball milling [13],plasma assisted ball milling [14],electrochemical deposition[15],molecular level mixing[16],solvothermal assisted deposition[17],metal evaporation,and reduction of metallic salts-graphite[3].The other is changing the GNSs into a more dispersion-favorable structure,typically including graphene oxide (GO) and a core-shell structure.Hydroxyl and epoxy groups are distributed along the edges or on the defective sites of GO,which largely enhance the dispersion of GO in solvents and enable the coating of GO onto the charged surfaces of the matrix or other enhancers [3,18-20].A nanoparticle-GNSs core-shell structure has also been developed[14,21-25]because nanoparticles can serve as favorable carriers for GNSs in which the surface energy is minimized [7,21,26].This structure is beneficial toward the uniform dispersion of GNSs in the matrix material.Powder metallurgy is one of the most popular routes used to produce advanced AMCs.However,less is known about the microstructural evolution of hybrid composites reinforced with dispersion-favorable fillers during powder handling and sintering.It is still not clear about the effects of thin GNSs on the microstructure of hybrid composites during processing.

This study focuses on the microstructural evolution of aluminum matrix composites reinforced with two different fillers,including SiCgraphite and SiC-GNSs nanoparticles,using both ball milling and hot pressing.The effects of the carbonaceous additions on the microstructural development were analyzed during the powder-metallurgy process,including powder morphology,carbide formation,and crystalline structure.Meanwhile,the carbonaceous defects and distribution were investigated in the composites.

2.Experimental procedures

2.1.Material processing

A355 alloy (Al-Si alloy) powder was used as the matrix and supplied by Haoxi Nanotechnology.A355 is a type of hypoeutectic 4000 series aluminum alloy which has been widely used as a structural material in automotive and aircraft industries.The composition of the alloy was Al-4.6Si-0.8Cu-0.5Mg-0.2Fe (wt.%).The as-received A355 powder had a spherical morphology with>99% purity,as shown in Fig.1(a).Two types of hybrid reinforcers were used.One was a mixture of as-received β SiC nanoparticles(~45 nm,Alpha Aesar)and graphite flakes(<20 μm,Aldrich Sigma)in a volume ratio of 5:1.The other was obtained by ball milling the above-mentioned mixture for 50 h to produce SiC-GNSs nanoparticles.The lateral size of the as-produced GNSs ranged from several nanometers to several hundred nanometers.The thickness of the GNSs was less than 10 nm and parts of the very thin GNSs were wrapped on the SiC nanoparticles forming a core-shell structure.The detailed information on the production of SiC-GNSs nanofillers can be found in the literature [23].

A355 powder was mixed with 5.0 vol% SiC+1.0 vol%GNSs(named as Al-Gn) and 5.0 vol% SiC+1.0 vol% graphite (named as Al-Gt) respectively to compare the effect of carbon addition on the microstructural evolution of the hybrid composites.Typically,15 g of the mixed powder was ball milled with 2 wt % stearic acid under an argon atmosphere using a planetary ball mill.Stearic acid works as a process control agent.Stainless steel balls were used as milling balls at a ball-to-powder mass ratio of 15:1.The powder was first ball milled at 180 rpm for 30 min to roughly mix the powder and then at 250 rpm for 20 h.Samples were taken at 2,5,10,15,and 20 h during the ball milling process and used for characterization.The ball milling process was conducted for 5 min followed by a 15 min pause in each cycle to avoid overheating and structural damage in the GNSs from occurring.The mechanically ball milled powder(around 7 g)was compacted using a Ф 20 mm steel die under 350 MPa at room temperature.The resulting disk was transported into a graphite die for hot pressing.The graphite papers were placed on the top and bottom surfaces of the sample.The disk was heated up to 400°C and maintained at this temperature for 2 h to degasify the stearic acid using a hot-pressing machine (BTF-1200CVP) under vacuum (0.1 Pa),followed by hot pressing under 50 MPa at 500 °C for 1 h.

2.2.Characterization

The morphologies of the ball milled powders were observed on a JEOL JSM-7500FA microscope.X-ray diffraction of the ball milled powder was conducted on a GBC diffractometer using Cu-Kα radiation.The X-ray source was operated at 35 kV and 26.8 mA with a scanning speed of 1.5°/min and step size of 0.02°.The particle size of powder was measured using a Malvern Panalytical Matersizer 2000.The produced disks were ground and polished on a Struers grinding/polishing machine to reach a mirror-like finish.Raman spectroscopy was recorded on a WITec® alpha 300R confocal Raman microscope (532 nm laser).TEM samples were prepared using a FEI Helios nanoLab G3 CX dual beam microscope and then observed on a JEOL JEM-2011.At least 150 grains from a dozen random TEM images were measured manually to calculate the grain size distribution.

3.Results and discussion

3.1.Microstructure of the composite powders during ball milling

1) SEM analysis

SEM images of the ball milled Al-Gt and Al-Gn powders taken at different milling times are shown in Figs.1 and 2 respectively.It is shown that all the powder mixtures went through six typical steps[27]of starting spherical powders,flattened lamellae,welding,equiaxed particle formation,random welding and steady state during the ball milling process.Ductile aluminum powders were flattened to form lamellae,while the brittle particles(Si/SiC/GNSs)were fragmented.With increasing milling time,the aluminum particles became work hardened and the lamellae became convoluted and refined [28].The presence of brittle particles generated local stress and promoted work hardening,convolution,and refining.However,the addition of different reinforcements influenced the ball milling process.During the first flattening stage,as shown in Fig.1,the powder mixtures of Al-Gt were flattened in the first 10 h and then the Al-Gn particles turned into lamella shape at 5 h,and the equiaxed particle formed at 10 h,as shown in Fig.2.This means the presence of thin GNSs intensified the flattening and work hardening of aluminum particles when compared with graphite particles.GNSs had significantly fewer layers and were inherently harder,which allows GNSs to generate stronger local stresses.Graphite was quite soft and could be delaminated during the initial stage of the ball milling at the cost of consuming a part of the ball milling energy.As a result,graphite could delay the ball milling process during the flattening stage.Subsequently,aluminum flakes were convoluted and welded into large-size lumps,as presented in Fig.1(e) at 15 h and in Fig.2(d) at 10 h,respectively.The size of the lumps reached~50 μm,which was even larger than the starting size (~36 μm).With further milling,the lumps were fragmented into smaller-sized and equiaxed particles at 20 h.

The particle size distributions of the composite powders are given in Fig.3.The average particle size(D0.5)of the Al-Gn powder was 7.9 μm,which is 26.2% smaller than that observed for Al-Gt (10.7 μm).This could be attributed to the fact that the GNSs accelerate the whole milling progress by generating local stresses and intensify the flattening,welding,fracturing,and re-welding steps.It was also noted that the SiC-GNSs fillers were dispersed and embedded into the aluminum matrix,as shown in the inset of Fig.2(f).

2) XRD observations

XRD is a powerful tool used to examine the morphological and microstructural properties of ball milled powders.The XRD patterns obtained for the composite powders at different milling times are shown in Fig.4.The peak intensity of aluminum decreased and the peaks were broadened upon increasing the ball milling time from 2 to 20 h.These indicate a reduction in crystalline size and accumulated lattice strain [29].It was also notable that no peak assigned to Al4C3was detected.To analyze the microstructural evolution using XRD analysis,the intensity ratio of (111) to (200) [I(111)/I(200)],has been used to illustrate the changes in the crystallographic orientation of the particles in FCC metals [30].Fig.4(c) shows that the ratio was 2.43 prior to the ball milling process.The intensity ratio of I(111)/I(200)observed for Al-Gn decreased rapidly to 1.90 at 5 h,recovered to 2.52 at 10 h,and finally fluctuated at~2.5.The variation in I(111)/I(200) is related to the anisotropy of the crystalline materials.When ductile particles were flattened,they tended to arrange themselves in a way that the flat side of the particles was almost parallel to the same surface[30].As a result,the flattened particles could give rise to a textured XRD pattern,which is the case observed at 2 h and 5 h for Al-Gn.Further milling fractured the flattened flakes and rewelded them into equiaxed particles,which implies their relatively good isotropy and is in accordance with the SEM observations shown in Fig.2.A similar tendency also applies to Al-Gt,as shown in Fig.4(c).The difference is that the decreasing I(111)/I(200) ratio (particle flattening) terminated at 10 h and recovered to~2.4 at 15 h and 20 h(formation of equiaxed particles),which is also in agreement with the morphological evolution observed using SEM,as shown in Figs.1 and 2.

3) Raman spectroscopy observations on the carbonaceous defects

Raman spectroscopy was employed to investigate the effect of the ball milling process on the carbonaceous structure.Fig.5 shows the variation in the Raman spectra observed for Al-Gn and Al-Gt as well as the change in ID/IG(the intensity ratio of D band over G band) at different milling times.The intensity ratio,ID/IG,is a well-accepted index to probe change in the defects of GNSs and graphite.The increase in ID/IGcould be attributed to the exfoliation and fragmentation of graphite into thinner and smaller platelets,because more edge defects are created [23].As seen in Fig.5(c),a progressive milling time led to an increase in ID/IGfor both Al-Gt and Al-Gn,which indicates the accumulative defects within the carbonaceous structure and an increase in amorphous fraction [31].The ID/IGratio of the GNSs averaged at 0.94 prior to the ball milling process,and slightly increased to 1.12 at 2 h.Due to the ductile nature of aluminum powder,GNSs could be embedded into the aluminum particles,which helps protect the GNSs from further damage and maintain their intrinsic structure [31,32].As a result,only a very slight change was observed from 2 to 20 h,as seen in Fig.5(c).A similar tendency was also observed for the variation in ID/IG,when graphite was incorporated into the aluminum matrix.Ball milling severely damaged the crystalline structure of the graphite during the first 2 h and resulted in a sharp increment in ID/IGfrom 0.03 to 0.6,as shown in Fig.5(b).And then the ID/IGratio gradually increased to 0.94 at 20 h.

3.2.Microstructure of the bulk composites

The XRD patterns of the bulk composites presented in Fig.6 show that several weak peaks could be detected in addition to the sharp peaks of aluminum.These weak peaks were mainly assigned to Al4C3,which implies that a small amount of Al4C3formed during the vacuum hot pressing via Eq.[1],which is energetically favorable at 500 °C(ΔG=−61756 kJ/mol),as reported [33-35].However,the intrinsic defect-free graphitic plane is energetically stable and does not react with aluminum even at very high temperatures[34].It is believed that the formation of Al4C3is normally promoted by the reaction between defective sites(e.g.edges and voids)of graphitic planes and aluminum.Therefore the TEM morphology of the Al4C3is observed as needle-like(~80 × 8 nm) as presented in Fig.5(b).

The distribution of reinforcing phases plays a critical role in the microstructure and properties of composite materials [7,36].Raman scanning offers a reliable method to show the distribution of carbonaceous materials.The Raman mapping results recorded in randomly selected areas are shown in Fig.7,where the bright areas indicate the presence of carbon-rich materials.The added graphite and GNSs were retained after the ball milling process and hot pressing,however,their distributions seem to be quite different.Fig.7(a) and(b)show that the distribution of graphite was not uniform and a cluster (size~10 μm)was observed in the Al-Gt sample,which is owing to insufficient exfoliation of the thick graphite flakes during the ball milling process.SiC nanoparticles can serve as carriers for GNSs and relief the agglomeration of GNSs [21].The distribution of GNSs shown in Fig.7(c) and (d)suggests that the GNSs were well dispersed in the bulk Al-Gn thanks to both the larger specific area of the GNSs than the thick graphite flakes and the ease of dispersion of the core-shell structure during the ball milling.

As it is known,hot pressing involves the consolidation and densification of powders into bulk materials.Fine grains of the powders tend to grow into larger ones during the long period of thermal exposure in consolidation.TEM was employed to obtain a good understanding of the microstructure.The typical TEM images and Al grain size distribution histograms presented in Fig.8 show that the grains of Al-Gt were relatively coarse with an average size of 168 nm and extend to 1 μm.This means that the mixing was still insufficient after ball milling with the SiC-graphite particles.It is shown in Fig.8(c)and(d)that Al-Gn displayed nanostructured/ultrafine grains with SiC nanoparticles embedded along the grain boundaries.The average grain size of Al-Gn was 57.7 nm.The formation of nano-sized grains may be firstly attributed to the above-mentioned grain refinement by accelerated strain from the SiC-GNSs during the ball milling process.More importantly,the uniform distribution of SiC-GNSs decreased the interplanar spacing of the GNSs,which exerted a strong pinning effect and thus restrained grain growth during the sintering process.This indicates the advantage of SiC-GNSs in producing nanostructured aluminum composites.

4.Conclusions

1) The SEM and XRD results are in good agreement in terms of microstructural evolution observed during the ball milling process.The addition of SiC-GNSs particle intensifies the deformation of the aluminum particles compared with SiC-graphite.

2) Ball milling brings about carbonaceous defects in the graphite flakes and GNSs,especially at the initial flattening stage.

3) Al4C3needles form and have been observed to be bridging between the aluminum grains due to the favorable thermodynamic conditions that occur during the hot-pressing process.

4) Raman scanning results indicate that the SiC-GNSs are uniformly dispersed in the bulk Al-Gn sample.The added graphite flakes are partially exfoliated into thinner ones,and the clusters of graphite with the size as large as 10 μm can still be observed in the Al-Gt sample.

5) The well dispersed SiC-GNSs nanoparticles restrain grain growth during hot pressing,and a nanostructured composite (57.7 nm) can be obtained using the powder metallurgy process.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (No.FRF-GF-18-007B).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.pnsc.2020.01.024.