Preparation And Microstructure And Properties Of Mixed Crystal TiAl-based Alloys

Among many intermetallic compounds,TiAl-based alloy has good high temperature resistance and oxidation resistance.Its modulus of elasticity and creep resistance are similar to those of Ni-based superalloy,but its density is less than half of that of Ni-based superalloy.TiAl-based alloys are considered to be a very potential high temperature structural material.In recent years,TiAl-based alloys have gradually become a research and development hotspot,and have broad application prospects in aviation,aerospace,national defense,military and other fields.However,TiAl-based alloys have great room temperature brittleness,which has become the main bottleneck restricting their application.Based on the idea of multi-peak grain strengthening and toughening,TiAl-based alloys with mixed crystal structure were prepared by high energy ball milling+hot pressing sintering technology in order to improve plasticity while maintaining high strength.In this paper,Ti-48A1-2Cr-2Nb alloy was used as the research object.Fine and ultrafine Ti-48A1-2Cr-2Nb pre-alloy powder was prepared by high-energy ball milling for25 h and 35 h.Ultrafine powder of 0 wt.%,20 wt.%,40 wt.%,60 wt.%,80 wt.%and 100wt.%were added to the pre-alloyed powder without ball milling,and ultrafine powder of 0wt.%,20 wt.%,40 wt.%,60 wt.%,80 wt.%and 100 wt.%were added to the fine powder.TiAl-based alloys with mixed crystal structure were prepared by hot pressing sintering after homogeneous mixing.The effects of different mixed crystal structure characteristics on the room temperature and high temperature properties of TiAl-based alloys were studied.The effect of the proportion of fine powder added to the pre-alloyed powder without ball milling on the microstructure and mechanical properties of sintered samples was studied.The results show that ultrafine/coarse-grained bimodal grain TiAl-based alloys with duplex microstructure can be obtained when sintered at 1200?.With the increase of the mass ratio of fine powder,the phase of?₂-Ti₃Al in sintered samples increases,the yield strength and compressive strength at room temperature increase,but the compressive fracture strain decreases.At room temperature,the samples prepared by mixing 80 wt.%fine powder show higher yield strength,compressive strength and good plastic deformation ability,the yield strength is 1485 MPa,compressive strength is 2590 MPa and compressive fracture strain is 14.09%;At room temperature,the compressive fracture is typical brittle fracture,and the main fracture forms are translaminar fracture and intergranular/interlaminar fracture;Coarse grains in ultrafine grains effectively disperse the stress and hinder the propagation of microcracks thus improve the plasticity of materials.Under high temperature,at the temperature of 750?,the yield strength and compressive strength of sintered samples with full fine powder are the highest,but the plasticity is the lowest;The yield strength and compressive strength of 80 wt.%fine powder sintered samples are higher than that of full coarse powder sintered samples;At 850¹050?,the yield strength and compressive strength of sintered samples with full coarse powder are the highest,and the yield strength of sintered samples with 80 wt.%fine powder is slightly higher than that of sintered samples with full fine powder;the high temperature deformation mechanism is mainly grain boundary slip,rotation and twist of lamellar clusters,grain growth and dynamic recrystallization.The effect of the proportion of ultrafine powder added to the fine powder on the microstructure and mechanical properties of sintered samples was studied.The results show that ultrafine mixed crystal structure TiAl-based alloys with dual phase structure can be obtained when sintered at 1200?;There are much?₂-Ti₃Al phase in the sintered samples of full ultrafine powder,and the microstructures are composed of a large number of lamellar structures.At room temperature,the yield strength and compressive strength of sintered samples increase with the increase of the mass ratio of ultrafine powder,and the compressive fracture strain decreases;When mixed with 80 wt.%ultrafine powder,it exhibits high yield strength,compressive strength and good plastic deformation ability.The compressive fracture strain is 4.85%,the yield strength is 2047 MPa and the compressive strength is 2693 MPa;The main fracture mechanisms at room temperature are translaminar fracture and intergranular/interlaminar fracture.Under high temperature,at the temperature of 750?,the compressive fracture strain of sintered samples of full-fine powder,80 wt.%ultrafine powder and all ultrafine powder is less than 40%;The compressive strength of sintered samples of full ultrafine powder is the highest,and there is no obvious plastic deformation process;The yield strength and compressive strength of sintered samples of80 wt.%ultrafine powder are higher than that of sintered samples of full fine powder;At850¹050?,the yield strength of sintered samples with full ultrafine powder are the highest,while 80 wt.%of ultrafine powder sintered samples are slightly higher than that of full fine powder sintered samples;The main deformation mechanisms at high temperature are grain boundary slip,rotation and twist of lamellar clusters,grain growth and dynamic recrystallization.