Effect Of Multiple Heat Treatments On The Microstructure And Toughness Of High-strength Ti55531 Allo

High strength&toughness titanium alloy has become an important structural material in the aerospace industry because of its advantages of ultra-high strength,high hardenability and better matching of plasticity and toughness.With the rapid development of aerospace industry,the strength and toughness matching of existing high strength&toughness titanium alloys has gradually failed to meet the application requirements of high performance bearing structural parts.How to further improve the comprehensive properties of high strength&toughness titanium alloys have become key research directions of modern materials.It is found that multiple heat treatments can further improve the comprehensive mechanical properties of titanium alloys.However,the effects of multiple heat treatments on microstructure evolution and strength and toughness of high strength&toughness titanium alloys are still not systematically understood.In this paper,the effects of multiple heat treatments of"solution+annealing+aging"on the microstructure,strength and toughness of the alloy were systematically studied by taking the high strength&toughness Ti55531alloy as the research object.The main conclusions are as follows:(1)The heterogeneous microstructure of Ti55531 alloy was formed after multiple heat treatments including solution,annealing and aging.The equiaxedα_pphase was transformed into theβphase at the solution stage,and the heterogeneous microstructure including the secondaryα_swith large and small dimensions was formed by annealing treatment.The aging treatment promoted the growth ofα_sand the increase ofαphase content,while the heterogeneous microstructure was preserved.The volume fraction of theαphase in the final microstructure only decreases with increasing aging temperature,the degree of microstructure heterogeneity decreases with increasing solution temperature,and increases with increasing annealing temperature and aging temperature.(2)The temperature parameters of multiple heat treatments control the strong plasticity of the alloy by affecting the degree of microstructure heterogeneity,the size and distribution ofα_sand the volume fraction ofαphase.The main parameter for controlling the strength of the alloy is the size ofα_sin the heterogeneous microstructure,and the plasticity is determined by the size and distribution ofα_sand the volume fraction ofαphase.The intergranular fracture is an important cause of plasticity decline.In addition,the strong plasticity of the alloy is also related to the degree of microstructure heterogeneity.(3)The alloy underwent brittle fracture under impact load,and the difference in impact toughness is dominated by crack initiation work.The average size ofα_sand the volume fraction ofαphase in the microstructure are important parameters affecting the toughness of the alloy.The increase of the average size ofα_sand the volume fraction ofαphase is beneficial to improving the impact toughness,and there is a competition effect between strength and toughness.(4)The microcracks mainly initiated at the interface betweenα_sof large size andβ.The crack tends to propagate to the soft region where the large size ofα_sis distributed.In addition,the orientation of large sizeα_sis also an important factor affecting the direction of crack propagation.In a certain range of orientation,large sizeα_splates can make the crack torsional propagation by changing the angle of the propagation direction through different orientations.