Study On Microstructure Optimization And Creep Properties Of TiAl Alloy Containing ? Stable Element

As a new generation of high temperature structrural material,TiAl alloy is expected to replace Ni-based alloy.It has the advantages of low density,good high temperature strength,creep resistance,oxidation resistance,etc.therefore,it has become a vital material in aerospace,automotive and other fields.It is important to improve the mechanical properties of TiAl alloys through microstructure control and optimization.Especially under high temperature service condition,the deformation and damage of materials caused by creep have great influence on the application and life of the alloy,so it is of great significance to study the effect of microstructure on creep properties.Fine fully lamellar microstructure has the best balanced mechanical properties,and fully lamellar microstructure has the best creep properties compared with other typical microstructure of TiAl alloys.But it faces big challenge to get fine fully lamellar microstructure for TiAl alloys,especially for wrought alloys.The microstructure optimization of TiAl alloy and the effect of microstructure on the creep property were studied in this work.?-stable elements were added to TiAl alloy to expand the ? phase region,and then fine fully lamellar microstructure with different lamellar colony size can be obtained by the rapid cooling heat treatment and hot deformation for cast alloys;The creep experiments at high temperature were carried out to analyze the effect of the lamellar structure and the residual ?/B2 phase on creep properties.The main conclusions and innovations are listed as follows:(1)The addition of ? stable element was beneficial to the expansion of ?phase region of TiAl alloy.By rapid cooling from ? phase region to obtain martensite and then tempering,the uniform fine fully lamellar microstructure with the colony size of 25-70?m can be obtained.The martensite was ?2 with hexagonal structure,and formed according to the Burgers orientation relationship with the parent ? phase.With the decrease of quenching cooling rate,the defect density in quenched microstructure became lower.The martensite lath had the largest defect density,and the substructure was consisted mainly of dislocations and stacking faults.(2)The colony size of the microstructure that obtained by rapid cooling and tempering was mainly connected with the quenched microstructures,especially for their substructures,grain size and the residual ?/B2 particles.The higher defect density can be conducive to the occurrence of recrystallization during tempring.In addition,the smaller size of the plate after quenching,and the weaker texture of the martensite variants both were unfavorable to the growth of grain size.Finally,a small amount of residual ?/B2 phase was also beneficial to hinder the growth of lamellar colonies.(3)The addition of ? stable elements imprvoed the lamellar microstructure after tempering of TiAl alloys mainly by controlling the formation and distribution of martensite microstructure after quenching.With the increase of the content of V element,the transformation mode will change from massive to martensite.With the increase of the content of Cr element,the width of martensite lath decreased and the residual ?/B2 phase increased,which was beneficial to the microstructure refinement during the subsequent tempering.However,too much strong ?-stable element addition was not conducive to the formation of the fully lamellar microstructure.(4)The addition of ? stable element can coordinate the deformation by the presence of a small amount of high-temperature ? phase in the microstructure optimization by hot rolling,so as to realize forming of TiAl alloy sheet at different temperatures.Finally,nearly lamellar and fully lamellar microstructure with the lamellar colony size of 65-170?m can be obtained.The refinement of lamellar colony was mainly related to the dynamic recrystallization of a phase.In addition,the recrystallization of some ?/B2 and y grains and the decomposition of ?/B2 phase were both beneficial to the microstructure optimization.(5)The room-temperature tensile property of the alloy with optimized microstructure was improved after hot rolling.The ultimate tensile stress at room temperature can be increased from 483MPa to 858MPa,and the elongation at room temperature can be increased to 0.86%.The fully lamellar microstructure obtained by rolling at a phase region has the best high temperature tensile properties,with the ultimate tensile stress of 744MPa,yield strength of 573MPa and tensile elongation of 6.0%.(6)The fully lamellar microstructure obtained by hot rolling had the best creep property,which was mainly related to the stability of lamellar structure.The results show that the existence of fine B2(?0)grains at grain boundaries and fine lamellar spacing were beneficial to reduce the primary creep strain of the TiAl alloy under higher stress.However,the fine lamellar spacing was easy to cause microstructure degradation and destroy the stability of lamellar structure,thus accelerating the creep rate and reducing the creep life.Therefore,the fully lamellar structure with thick lamellar spacing showed a larger primary creep strain,but a lower creep rate and longest creep life.(7)During the creep process of TiAl alloys,the lamellar microstructure degenerated to different degrees with forming a large number of y grains and precipitating a certain amount of B2(?0).The fine lamellar spacing can enhance the degeneration.The presence of y grains will reduce the creep resistance of the alloy,and the y and B2(?0)grains in the grain boundary will increase the site of the formation of holes before the fracture,and finally produce cracks to cause the fracture.