Preparation And Hot Deformation Behavior Of TiAl-based Alloys With Bimodal Grain Size Distribution

TiAl-based alloys have attracted much attention in national defense industry and civil industry such as aerospace industry and marine industry due to their low density,excellent high-temperature strength,creep resistance and oxidation resistance,and have attracted widespread attention form scientific researchers which become one of the most promising light-weight high-temperature materials.However,this kind of material has the intrinsic brittleness and high resistance to high-temperature deformation,so it is difficult to process and form,which seriously restricts its further development and application.In this paper,TiAl alloy with bimodal structure was prepared by high-energy ball milling and hot pressing sintering.On this basis,the hot deformation experiment was carried out on the TiAl alloy with bimodal structure to study its hot deformation behavior and explore the suitable thermal processing range.In this experiment,Ti-48Al-2Cr-2Nb alloy was used as the research object.By adjusting the ball milling process parameters,the bimodal TiAl-based alloys composed of different coarse powders and fine powders ratios was prepared.The powders were sintered at1200?.The microscopic morphology and phase of the powders and the samples after sintering were studied.It was found that the bimodal structure consisted of two phases of?-TiAl and?₂-Ti₃Al,and the content of coarse and fine grains were different.At the strain rate of 5×10^-5s^-1,the tensile mechanical properties were tested at 800?.It was found that the bimodal structure TiAl alloy 30FG with the ratio of coarse grains to fine grains was 7:3,which exhibited excellent comprehensive mechanical properties.The yield strength was408.61 MPa,the tensile strength was 455.03 MPa,and the yield strength was 55%higher and the tensile strength was 49%higher than that of the Pre-alloyed powder sintered sample 00FG.The elongation rate was 6.91%,which was 3.8 times of the elongation of the sintered sample with the ratio of coarse grain and the fine grain of 5:5(50FG).The fracture mode of 30FG sample was intergranular fracture and transgranular fracture.The back stress was strengthened due to the accumulation of dislocations at the interface of coarse grains and fine grains,which provided additional strength and plasticity to the alloy.30FG sample was compressed at different deformation temperatures(900?1100?)and different deformation rates(0.1?0.001 s^-1)to study their thermal deformation behavior.It was found that there were three different stages in the true stress-strain curve.The stress value decreased as the deformation temperature increased and the strain rate decreased.Through calculation,the hot deformation activation energy Q=390.543 KJ·mol^-1of Ti-48Al-2Cr-2Nb alloy,and the constitutive relation equation at high temperature was established.According to the stress value of different true strain during high temperature deformation,the thermal processing map was drawn,and the microstructure of Ti-48Al-2Cr-2Nb alloy after hot deformation was analyzed.It was found that the deformation temperature and stain rate had a significant effect on the dynamic recrystalliazation process.The suitable thermal processing zone of the alloy with bimodal grain size distribution were 980?1020?/0.001?0.003 s^-1(=58%)and1050?1100?/0.001?0.003 s^-1(=66%)by power dissipation diagram and instability diagram.