Study On Microstructure And Mechanical Properties Of Two-Component TiAl Alloy

TiAl alloy has many excellent physical properties at high temperature,such as low density,high strength,high creep resistance and oxidation resistance,which make it the most promising lightweight high temperature structural materials for aerospace applications.However,the intrinsic brittleness of TiAl alloy leads to poor plasticity at room temperature and difficulty in forming,which seriously limits the extensive application of TiAl alloy in industry.Recent studies have shown that the configuration design of materials can improve the strength and plasticity of materials cooperatively,which is expected to solve the problem of strong plasticity inversion of metal materials.In this paper,based on the concept of configuration design,Ti48Al2Cr2Nb(at.%)and Ti45Al8Nb(at.%)pre-alloyed powders were used as raw materials to prepare TiAl alloy with two-component and grain size classification by powder metallurgy process,aiming at improving the comprehensive mechanical properties of TiAl alloy.The deformation behavior of TiAl alloy at room temperature and high temperature was studied by SEM and EBSD.The densified TiAl alloy was prepared by vacuum hot pressing sintering.The two-component TiAl alloy sintered at 1300?is completely compact,forming a nearly?microstructure with two-component and grain size grading characteristics.The coarse grains are derived from the coarse grains in the original powder,while the fine grains are derived from the multi-point nucleation fine powder and the interlocking between?₂ phase and?phase during hot pressing.The mechanical properties of TiAl alloy were tested by nano indentation,compression at room temperature and tensile at high temperature.The deformation microstructure and crack growth mode were studied.The strengthening and toughening mechanism of TiAl alloy was analyzed.The bicomponent TiAl alloy has excellent compressive strength and plastic combination at room temperature.The compressive strength is 2500MPa and the engineering strain is 48%,which is 1.3 times that of Ti48Al2Cr2Nb alloy under the same process conditions.It is found that a large number of twins are formed and a large number of microcracks occur near?₂.The coarse grains and Ti48Al2Cr2Nb region are subjected to long range compressive stress due to grain size grading and bicomponent microstructure,which improves the work hardening ability of the alloy.The microcracks initiation from the hard and brittle?₂phase to passivation in?phase alleviates the stress concentration,and the microcracks increase in number first and then expand and grow up.This kind of crack growth mode improves the deformation capacity of the alloy.The ductile-brittle transition behavior of TiAl alloy at high temperature is obvious.At 750?,the ductility of the TiAl alloy is low,the deformation mechanism is mainly dislocation slip,and the fracture of the TiAl alloy presents typical rock sugar intergranular brittle fracture.A large number of twins are formed near the tensile fracture at 800?,and the deformation is dominated by twins.Twins also existed near the tensile fracture at 850?,and chain recrystallized grains appeared between the grains.The work hardening and dynamic softening mechanism compete with each other during the whole high temperature deformation process.The ductile-brittle transition temperature of the two-component TiAl alloy is between 750?and 800?.Above this transition temperature,the high strength Ti45Al8Nb in the alloy bears a greater load,and the deformation constraint of Ti48Al2Cr2Nb improves its work hardening ability,while twin and dynamic recrystallization provide plastic strain.