Study On Ti₅₅Cu_17.5Ni_17.5Al₁₀/7075 Composites Fabricated By High-energy Milling And Heat Treatment

Particulate-reinforced Al-based metal matrix composites（AMMCs）have high specific strength,high specific stiffness,good corrosion resistance,good dimensional stability and thus have been considered preferred engineering materials for lightweight applications such as in aerospace,defense and automotive industries.But unfortunately,ceramic particles severely degrade the plasticity and machinability of AMMCs（ceramics as reinforcements）.The low wettability and the detrimental interfacial reaction of ceramic particles with the metal matrix also lead to low reproducibility of mechanical properties.Furthermore,the introduction of ceramic particles would make the recycling of AMMCs difficult.These defects give rise to great limitation for their application and commercialization.Metallic glasses have been recently considered as promising candidates for reinforcement due to their high strength,large elastic limit,high corrosion resistance and good wettability with the metal matrix.It is found that the mechanical properties of metallic glasses reinforced with crystalline nanoparticles such as hardness,strength,and ductility are even better than those of the metallic glass matrix.Therefore,the study of metallic glassy particle and metallic glasses reinforced with crystalline nanoparticles as reinforcements are of great significance.In this work,the effects of high energy ball milling on the morphology,microstructure and hardness of the composite powders were studied,in which the titanium-based metallic glassy particles(Ti₅₅Cu_17.5Ni_17.5Al₁₀)were mixed with the 7075 aluminum matrix.The Ti₅₅Cu_17.5Ni_17.5Al₁₀/7075 composite powders milled for different times and followed by hot extrusion.The effect of high-energy ball milling on the microstructure and mechanical properties of the as-extruded and heat treated composites were investigated.The main results are as follows:（1）In the composite powders prepared by high energy ball milling,Ti-based metallic nanoparticles were uniformly dispersed in the Al7075 matrix.High-energy ball milling caused severe plastic deformation at the surface of the coarse composite powders,and formed a deformed surface layer.The metallic galssy microparticles in the deformed layer were fractured into submicro-and/or nano-particles,owing to the large shear deformation.In the process of high-energy ball milling,the composite powder underwent particle growth,particle fracture and balancing state.The deformed layer shows a much higher hardness than the less deformed center part and the hardness of the composite powder increases with increasing milling time.The increase in hardness can be attributed to the collective consequences of dispersion strengthening and strain hardening.（2）The microconstituents of the composites are:nanoparticle reinforcements（⁹8 nm）,MgZn₂ precipitates（⁴0 nm）,residual micro-reinforcements and aluminum matrix.The micro-reinforcements are partially crystallized which contain a large number of nanocrystalline particles.While the nanoparticle reinforcements can be classified into amorphous particles and nanocrystalline particles.An interface layer（50⁸0 nm）is found where nanocrystalline phases formed.The yield strength increased significantly with the addition of reinforcement compared to the pure Al7075 alloy,and the yield strength increased significantly with increasing milling time,from 297 MPa for the Al7075 alloy,to 530 MPa for the composites milled for 10 h and to1014 MPa for 50 h.The fracture mechanism of composites is that cracks primarily originate in the large reinforcement particles,and gradually expand and join with the cracks initiated by other micro-reinforcements,and finally lead to complete failure of the sample.Grain refinement,Orowan strengthening,and dislocation-dislocation interactions give most of the contribution to the increased yield strength.With increasing milling time,these strengthening mechanisms show a stronger strengthening effect.The order of the increment is:?σ_dis>?σ_g>?σ_orowan。（3）The optimized T6 heat treatment of the hot-extruded composites milled for 15 h is:solution treated at 470 ^oC/30 min followed by water-quenched at room temperature,and then artificial aged at 100 ^oC/20 h.After heat treatment,the compressive strength,compressive yield strength and fracture strain of the composite were 942 MPa,635 MPa and 27.2%,respectively.The crystallization of the reinforcement becomes more pronounced as the solution temperature,solution time,aging temperature,and aging time increase.An obvious reaction layer was formed at the interface between the reinforcement and the matrix after the aging treatment.The main compound in the raction layer was the Al₁₈Ti₂Mg₃.The addition of Ti₅₅Cu_17.5Ni_17.5Al₁₀particles promoted the precipitation of second phase in the matrix.