Effects Of Alloying And Ultrasonic Treatment On Microstructure And Mechanical Properties Of Nb Containing TiAl Alloys

As a high-temperature structural material with high specific strength,the inherent shortcomings such as low room temperature plasticity,high temperature deformation resistance and low fracture toughness restrict the engineering application of TiAl-based alloy.Alloying is an economic,fast and practical common method.But the coarse microstructures and segregation in the as-cast structures can deteriorate the mechanical properties of the alloys.The research shows that ultrasonic treatment can effectively refine the microstructure and improve the mechanical properties of the alloys.The ultrasonic treatment is an economic,efficient and environmental friendly solidification auxiliary technology in the casting process.This paper systematically studies the influence ofβstable elements such as Fe,Mo on the microstructure and mechanical properties of the Nb-containing TiAl alloy,and obtained the optimized composition.The influence of compounds ZrN and ZrC which containing bothβandαstable elements on phase composition,solidification path,microstructure evolution,precipitates and mechanical properties of the above optimized Nb-containing TiAl alloy are invetigated.The influence mechanisms are revealed,and the optimized compositon of the alloy are obtained.The influence of ultrasonic treatment time on the precipitated phase,microstructure transformation and mechanical properties of the optimized alloy was studied,and the influence mechanisms are revealed.The results show that Fe can refine the as cast colμmnar dendrite structure,form the Fe rich B2 phase and cause the Al segregation in the interdendrites of the Ti-43/46Al-5Nb-0.1B alloys.The refined microstructure is attributed to the undercooling formed by Fe which hinders the growth of dendritics.The Ti-43Al-5Nb-0.1B-0.7Fe and Ti-46Al-5Nb-0.1B-0.3Fe alloys show the higher compressive strength and strain,which result from the solid solution and the refinement of colμmar grains by Fe.But the compressive properties deteriorated when the Fe content is continuously increased.Theγphase formed by the Al segregation in the interdendrites accelerate the crack growth rate which leads to the deteriorated compressive properties.Because the distribution coefficient of Mo betweenβandαis larger than 1,it can be formed B2 phase between the lamellae boundarys when add the Mo to the Ti-43/46Al-5Nb-0.1B alloys.The Ti-43Al-5Nb-0.1B-0.8Mo and Ti-46Al-5Nb-0.1B-1.2Mo alloys exhibit the higher compressive strength and strain,which is attributed to the solid solution and the refinement of colμmar grains by Mo.When further increase of the Mo content,the compressive properties show a downward trend.One of the reasons is atributed to the B2 phase because it can act as the crack source during compression.Another one is atributed to theγphase formed by the Al segregation in the interdendrites,which can accelerate the crack growth rate.The Ti-46Al-5Nb-1.2Mo alloy was selected as the based alloy to invsetigate the influence of ZrN and ZrC on the structure and mechanical properties of it.The results show that the microstructure of Ti-46Al-5Nb-1.2Mo alloy changes from coarse colμmnar to fine dendrite and equiaxed grains with the increasing of ZrN content from0 to 2.0 at.%.When the ZrN content is higher than 2.0 at.%,the granular Ti₂Al N particles precipitate in the alloys,and its content increases with the increasing of ZrN addition.The Ti₂Al N is formed by the reaction of free[Ti]with liquid Al and ZrN.The Ti-46Al-5Nb-1.2Mo-1.0Zralloy exhibits the higher tensile strength and strain.The reasons are attributed to the solution strengthening of Zrand N,fine grain strengthening and second phase strengthening.The tensile properties of the alloy is deteriorated when further increase the ZrN content.It is attributed to the formation of a large nμmber of B2 phase and Ti₂Al N which act as crack source then lead to the premature fracture of the specimen.The microstructure of Ti-46Al-5Nb-1.2Mo alloy transforms from coarse colμmnar crystal to fine dendrite and equiaxed grains,and the relative content of the B2 phase decreased with the increasing of ZrC content from 0 to 2.0 at.%.Ti₂Al C precipitate from the liquid when the content of ZrC is 0.8 at.%,and the content of it increased and agglomerated with further increasing of ZrC content.The Ti₂Al C is formed by the reaction of liquid TiAl₃ with Tiand and ZrC.The Ti-46Al-5Nb-1.2Mo-1.0ZrC alloy exhibits the higher tensile strength and strain at room temperature.The reasons are attributed to the solid solution strengthening of Zrand C,fine grain strengthening and the reduction of B2 phase.When the content of ZrC further increased,large amount of Ti₂Al C is precipiated and it acts as the crack source.Ti₂Al C is easy to dissociate from the lamellar during tension,resulting in cracks and fracture of the alloy and then reducing the strength and plasticity of the alloy.The precipitated of large amount of Fe rich B2 phase and agglomerated Ti₂AlC in the microstructure would deteriorate the mechanical properties of the alloys.The ultrasonic treatment are used to refine the Ti-46Al-5Nb-1.2Fe and Ti-46Al-5Nb-1.2Mo-0.8ZrC alloys.The results show that the microstructure of the Ti-46Al-5Nb-1.2Mo-0.8ZrC alloy refined significantly by ultrasonic treatment.The grain size of Ti₂Al C become smaller and distribute more evenly with the increasing of treatment time.The refinement of the Ti₂Al C is attributed to the undercooling produced by ultrasonic cavitation,which increase the nucleation rate of TiC and reduce the critical radius of TiC particles for the formation of Ti₂Al C.The acoustic streaming effect makes the distribution of TiC in the melt more uniform,then refine the Ti₂Al C and make it distributes more evenly.When increasing of treatment time to 125s the alloy exhibits the highest compressive strength and strain,which are 2648MPa and 27.8%,respectively.The microstructure of the Ti-46Al-5Nb-1.2Fe alloy transformes from colμmnar to equiaxed grains by ultrasonic treatment.It also promote the uniform distribution of Fe among dendrites,reduce the enrichment degree and effectively reduce the size of Fe rich B2 phase.When increasing of treatment time to 125s,the alloy exhibits the highest compressive strength and strain,which are 2648MPa and 27.8%,respectively.It is attributed to the refinement of the microstructures and the Fe rich B2phase,which could reduce the speed of the crack propagation.