Research On Microstructure Control And Strength-plasticity Mechanisms Of Ti₃AlC₂/Al Composites

Ti₃AlC₂ combines high modulus and hardness of ceramics and high damage tolerance of metals,and can be deformed by basal sliding,bending,and kinking.As a novel reinforcement for aluminum matrix composites,it is expected to increase the strength while endowing the material with considerable plasticity and toughness.However,as an unstable system,Ti₃AlC₂/Al is prone to generate a large number of brittle phases.In addition,the deformation and fracture of Ti₃AlC₂ have strong orientation sensitivity,and the weak basal bonding can easily lead to basal plane cleavage at specific orientation.Therefore,the effective control of both the Ti₃AlC₂/Al interfacial reaction and the orientation of Ti₃AlC₂ particles is the key to improving the properties of Ti₃AlC₂/Al composites.In order to solve the above problems,we took 20vol.%Ti₃AlC₂ particle reinforced commercial pure Al composites as the research object,and optimized the microstructure of the composites by adjusting the preparation process.The hot working parameters was determined using hot compressive simulations.The orientation of Ti₃AlC₂ particles and microstructure of the matrix were improved by hot extrusion.Finally,the mechanical behavior and high plasticity mechanisms of the composites at different orientations were analyzed,and the deformation and fracture behavior of Ti₃AlC₂ particles with different orientations were clarified.The low-temperature rapid densification as well as effective suppression of interfacial reaction of Ti₃AlC₂/Al composites was achieved by means of the local melting of Al particles using spark plasma sintering.The interfacial reaction of pure Al reinforced with micron-sized Ti₃AlC₂ particles was effectively regulated by changing the sintering temperatures.When the sintering temperature was 550–600℃,there was no interface reaction product,the density was low,and the Ti₃AlC₂ particles were not preferentially arranged.When the sintering temperature was 620–640℃,Ti Al₃ whiskers were formed at the interface,and the size increased with temperature,and the basal plane of Ti₃AlC₂ particles was perpendicular to the sintering direction.When sintered at620℃,the as-sintered TA-620 had the best strength and plasticity performance.The hot deformation behavior of TA-620 at various temperatures and strain rates was investigated.The hot deformation constitutive equations and processing maps for different deformation stages were established using friction corrected hot compressive curves,and the microstructure evolution of hot deformation was analyzed.The hot flow behavior of the composites was accurately described using constitutive equations of hyperbolic sine form.The strain distribution within the hot deformed samples was not uniform,and the strain in the central region was the largest,where the Ti₃AlC₂ flakes had the highest preferred orientation degree.The optimal region for hot working was410–500℃,0.18–1s^-1,where the dynamic recrystallization ratio of Al matrix was the highest.The highly unstable region was 410–500℃,0.003–0.02s^-1,huge voids and interface cracks were generated within the samples.Based on the optimized hot working parameters,the hot extrusion of TA-620 with different extrusion ratios was carried out at 460℃/1s^-1.Increasing the extrusion ratio can increase the alignment degree of Ti₃AlC₂ flakes and reduce Ti₃AlC₂ polycrystalline clusters,but too high deformation led to the increase of Ti₃AlC₂ prismatic plane cracks.Compared with TA-620,the strength and plasticity of the extruded composites were improved simultaneously.Fine-grain strengthening of the matrix and load transfer strengthening of the Ti₃AlC₂ particles were the main strengthening mechanisms.Refinement of the matrix and Ti₃AlC₂ particles were the main mechanisms to improve plasticity and toughness.Both Ti₃AlC₂ clusters and cracks on the prismatic plane led to a decrease in plasticity and toughness.When the extrusion ratio was 20:1,the as-extruded ER-20 had both high strength and high plasticity.The microstructure anisotropy of the as-sintered TA-620 was mainly the preferred orientation of Ti₃AlC₂ flakes.The microstructure anisotropy of the as-extruded ER-20concluded the<111>texture of Al and the preferred orientation of Ti₃AlC₂ particles.Both the sintered and extruded composites have anisotropic mechanical behaviors.The strength and plasticity of the TSS sample in TA-620 were higher.For ER-20,as the angle between the tensile direction and the extrusion direction increased,the strength decreased and the elongation increased significantly.When the tensile direction was perpensicular to the extrusion direction,the elongation was 30.9%,and the yield strength and tensile strength was respectively 149.2MPa and 195.8MPa.Compared with the pure Al matrix,the yield strength and ultimate tensile strength were respectively increased by 206.6%and 109.0%,and the elongation was only slightly decreased.Using X-ray micro-CT technology,the three-dimensional shape and spatial orientation of Ti₃AlC₂ particles in TA-620 and ER-20 samples were quantitatively characterized.The three inertial axes of the Ti₃AlC₂ particles were determined to be1.00×0.60×0.42,the orientation angles of TA-620 and ER-20 were about 25°and 19°,respectively.Combined with the shear-lag model including the orientation angle,the load transfer efficiencies of TA-620 and ER-20 at different orientations were quantitatively calculated.The efficiencies of TSS and TTS samples of TA-620 were113.1%and 107.8%,respectively.The efficiencies of ED-00,ED-45,and ED-90samples of ER-20 were 119.3%,113.4%,and 107.4%,respectively.The calculated and experimental values were in good agreement.The high plasticity of the composites stemmed from the coordinated deformation behaviors of Ti₃AlC₂ particles including basal slipping,bending,and kinking.The coordinated deformation and fracture mechanisms of Ti₃AlC₂ particles at different orientations were elucidated by in-situ tensile tests.In ED-00,the particle orientation angle was small,the particles mainly deformed by in-plane bending,and the fracture mainly accoured on the prismatic planes.In ED-45,the long axis of the particle was close to 45°to the tensile direction,and the larger force couple made the basal plane cleavage more likely to occur,leading to the side bending and cleavage of the particle,and the fracture mainly occurred on the basal planes.In ED-90,the force couple effect was small,the proportion of basal plane slipping and shearing was higher,and the fracture also mainly occurred on the basal planes.