Evalution Mechanism Of Solidification Microstructure And Metastable Structure Of Ta-containing Hyperperitectic TiAl Alloys

At present,casting Ti-48Al-2Cr-2Nb alloy which has hyperperitectic solidification process has been applied commercially.At present,the working temperature of Ti-48Al-2Nb-2Cr alloy is up to 650℃,which is not enough for the further application of low pressure turbine blade over 800℃.Based on this,it is important to develop a new high-temperature casting y-TiAl based alloy.Recently,βphase stabilization element Ta is found to improve the high-temperature oxidation resistance,strength and lamellar stability.But a great amount of Ta can cause the severe solidification segregation which may increase the cracking tendency at casting process.Thus,in order to control the solidification micro structure reasonably,we consider to add different content of Ta（x=0,0.5,1,2 at.%）into hyperperitectic solidification Ti-48Al-3Nb alloy and discussthe effect of Ta on the solidification behavior.Meanwhile,we explore the effect of Ta on the precipitation of metastable γ_m phase,Widmanst?tten and Feathery-like structures.The refinement of fully lamellar（FL）microstructure of as-cast Ti-Al alloy is successfully achieved only by cyclic air-cooling heat treatment.Room temperature mechanical tests revealed that with the refinement,the mechanical properties are improved.The main method and analysis are showing as follows:We study the solidification process with different solidification rate of Ti-48Al-3Nb-xTa（x=0,0.5,1,2 at.%）alloy,it can be found that the dendrite is gradually refined with the increase of Ta,resulting in the segregation of elements at the dendrite.Meanwhile,the transform temperature of α→γ_L can be increased by the addition of Ta,which would restrain the further growth of α grain,therefore,the lamellar colonies can be refined.Moreover,Ta can induce the lattice distortion of α₂ and γ phase.Through changing the solidification rate,we find with the decrease of cooling rate,the micro structure will change:the angle between the primary dendritic arm and secondary dendritic arm change from 90° to 60°.Combing the calculated phase diagram of Al-equivalent,it can be certified Ti-48Al-3Nb-xTa alloys are in the range of hyperperitectic solidification,during the near equilibrium solidification process,the peritectic process can be take placed completely:L+β→L+α.With the increase of content of Ta and solidification rate,the peritectic process cannot be take placed completely,the solidification process is L+β→L+α+β.And with the decrease of solidification rate,the segregation rate will decrease.We study the microstructural evolution of γ_m phase obtained from different cooling rate.Firstly,after solidification process the microstructure of Ti-48Al-3Nb alloy is fully lamellae,and Ti-48Al-3Nb-1Ta and Ti-48Al-3Nb-2Ta alloy are composed with γ_m phase and α₂/γ lamellae.The diffusion controlled γ lamellae can grow through the diffusionless controlled γ_m phase with 120° rotational relationship.There is a great amount of defects can be found in γ_m phase and two kinds of y laths can be found in ym phase.Secondly,after quenching from single α region of Ti-48Al-3Nb-xTa alloy,the volume fraction of γ_m phase increase with the addition of 0.5 at.%Ta,and decrease with the further increase of Ta.It indicates that as the content of Ta further increase,it can inhibit the movement of Ti and Al atoms,thus the diffusion controlled γ lamellae can be inhibited.And,nano scale subgrain constituted with true twin,120° rotational relationship and APB can be found in Ti-48Al-3Nb-0.5Ta and Ti-48Al-3Nb-1Ta alloys.Combing with the first-principle calculation,it can be certified that the true twin has the lowest interfacial energy than 120° rotational relationship,pseudo twin and APB.Besides,Ta atoms prefer to substitute at A1 atoms position at the true twin,120° rotational relationship and APB interface,due to they have the lowest interfacial energy.Therefore,Ta can promote the formation of subgrains in the γ_m phase.And the hardness can be increased by the nano subgrains,but the Young’s Modulus does not change a lot.Thirdly,the phase transformation behavior between γ_m phase and γ lamellae was studied through interrupted quenching from single α-phase region and α+γ two-phase region in a Ti-48Al-3Nb-0.5Ta alloy.Combining with the thermodynamic analysis,it indicates that the formation of γ_m phase is gradually inhibited with the decrease of quenching temperature,instead fine γ lamellae are preferred.Massive α→γ_m phase transformation temperature T₀ is confirmed to be 1400℃（in α+γ two-phase region）.By using advanced analysis method,we found besides nucleating on α grain boundary,γ_m phase can nucleate on defects in the interior of an α grain with Blackburn orientation relationship or nucleate on fine γ lamellae along close-packed{111}γ with orientation relationship of[121]γlamellae//[（?）11]γ_m1//[10（?）]α₂ at their interface.Furthermore,the ledge growth mechanism of γ_m phase is identified,each step is accompanied by one direction:1/n[121]shear vector.We exploit the three-dimension（3D）atomic details of highly-defected massive γ（γ_m）phase on（001）and（101）plane in TiAl-based alloy,using scanning tunneling microscope（STM）imaging.Remarkable novel structural information was revealed.It can be found the grain boundary of γ_m phase has a large height difference of 18.2 nm.While the terrace-like structure can be observed directly that reveals the growth of γ_m phase is a ledge growth mechanism.Inner one γ_m ledge,the wide of（001）APBs is about 8 nm and the height difference is about 0.76 nm at 3D scale indicating APB is not flat that the interface is connected by atomic slope.The atoms on slopes of APB still belong to（001）plane and the slope is formed by decreasing the misorientation strain to stabilize the ordered L10 structure.Meanwhile,the b of 1/2[101]unit dislocation,[101]superdislocation,and 1/2[110]unit dislocation in γ_m phase can be firstly identified from measuring the crystal indices,height and the dislocation line of 3D-STM atomic images.And,combining the first-principle calculation and the 3D-STM atomic image,it can be found solute atoms Ta,occupying the Al site of L10 phase,can bring defects in the γ_m phase along[121]direction on（111）plane,due to Ta can reduce the stacking fault energy.Combing with the analysis mentioned above,we can find Ta can reduce the stacking faults energy and interfical enrgy.Therefore,metastable Widmanst?tten and Feathery-like structures can be promoted by Ta during air-cooling.It is analyzed that the precipitation of Widmanst?tten and Feathery-like structures originating with several orientations transform to small lamellar colonies on next heat treatment cycle.After five cycles,the coarse FL microstructure of as-cast Ti-48Al-3Nb-1Ta（at.%）alloy can be completely divided to～40 μm small lamellar colonies by these mestable Widmanst?tten and Feathery-like structures.Room-temperature mechanical tests revealed that with the refinement,the ductility and strength are greatly improved compared with those of coarse FL microstructure which are expected to provide a new cast TiAl-based alloy for industrial applications.