Effects Of Alloying And Ultrasonic Treatment On Microstructure And Mechanical Properties Of Ti46Al4Nb1Mo Alloys

TiAl-based alloys have the advantage of lower density and high elasticity modulus,and it has good performance at high temperature,such as good creep resistance and high strength.Therefore,they have important applications in the turbine blades of aero-engines.However,TiAl alloys have poor plasticity at room temperature,which hinders the application of TiAl alloys on more severe service conditions.Alloying is a convenient and effective method to solve the above mentioned problems of TiAl alloys.As Co expands theβphase region,N expands theαphase region,B and Ho have no effects on the phase region,therefore,this study selects Ti46Al4Nb1Mo alloy as the based alloy to investigate the effects by adding above elements.The results show that when the content of Co is 0.7%,a small amount of Co-rich B2phase appears on theα₂/γlamellae and the grain boundary of the columnar dendrites.When the content of Co reaches to 1.4%,the secondary dendrite arms of the columnar grains are more obvious,and a large number of massive,worm-like and long strip Co-rich phases appear.With 0.15%Ho addition,the average spacing ofα₂/γlamellae is reduced from 493±65nm to 366±37nm,and micro-nano scales Ho₂O₃ particles form at the grain boundaries.When the addition of Ho increases to 0.3%,the secondary dendrite arms are more obvious,and irregular Al₂Ho phases form at the grain boundaries.When the content of N is more than 0.8%,Ti₂Al N reinforcements are formed,and the network B2 phase disappears.With more addition of N content,the number of Ti₂Al N and massiveγphases increases,and the size ofα₂/γlamellae reduces.When the content of B is 0.4%and 0.8%,the secondary dendritic arms of columnar grains are more obvious,and there are long straight Ti B phases located around grain boundaries.With more than 1.2%B content,the columnar dendrites transform to the equiaxed grains,the long straight Ti B phases are reduced,and the tiny Ti B phases are increased and coarser.It is found that theβsolidification mode is changed toβ+αsolidification mode by 0.4%N and 0.4%B addition,and the network B2 phase disappears.With increasing addition of N and B,the number of Ti₂Al N and Ti B phases increases,and the size ofα₂/γlamellae is reduced.The columnar grains are refined and the obvious secondary dendrite arms are mainly attributed to the undercooling of the Co,Ho and B elements,and the transformation of the columnar grains to equiaxed grains is mainly attributed to the heterogeneous nucleation of Ti₂Al N and Ti B phase.The compressive strength and strain at room temperature increase by alloying with0.7%Co,which results from the solid solution of Co and the refinement of columnar grains;The addition of 0.15%Ho improves the room temperature compressive strength,and the refinement ofα₂/γlamellae,the strengthening effect of Ho₂O₃ particles and the reduction ofα₂/γthickness contribute to the improvement of the compressive strength at room temperature,and the main strengthening effect at high temperature is the second phase strengthening of Ho₂O₃ particles.When the content of N is 1.6%,the maximum compressive strength at room temperature reaches to 2312MPa,which is due to the strengthening effect ofα₂/γlamellae refinement、Ti₂Al N particles and disappearance of B2 phase.The refinedα₂/γlamellae weaken the effect of grain refinement strengthening due to grain boundary softening at high temperature,and strengthening by Ti₂Al N particles has the dominant effect on improving the peak stress,and the compressive strength at 900°C and 1000°C reaches to 956MPa and 746MPa,respectively.The maximum compressive strength and strain are 2339MPa and 33.7%with 1.6%B addition,respectively,and the compressive strength and strain are mainly enhanced via refinement of lamellar colony and formation of Ti B.The maximum compressive strength is obtained with 1.2%N and 1.2%B content,which reaches to 2493MPa.The synergistic effect of Ti₂Al N and Ti B phases and grain boundary strengthening contribute to the improvement of room temperature compressive strength.In view of the large size of Ti₂Al N and Ti B reinforcements and the serious segregation of the B2 phase by Co addition,the ultrasonic treatment is used to further optimize the microstructure and mechanical properties.With the increase of ultrasonic treatment time,it is found that the region of shrinkage porosity,the aspect ratio of Ti₂Al N、Al segregation in Ti46Al4Nb1Mo1.6N alloy are reduced,the length of Ti B is reduced and the columnar dendrites transform to the equiaxed dendrites in Ti46Al4Nb1Mo0.8B alloy,the size of elongated B2 phase and columnar dendrites are reduced in Ti46Al4Nb1Mo1.4Co alloy.When the ultrasonic treatment time is 90s,the maximum compressive strength of Ti46Al4Nb1Mo1.6N alloy reaches to 2462MPa,and the maximum compressive strain reaches to 28.5%with ultrasonic treatment time for 120s.When the ultrasonic treatment time is 90s and 120s,the maximum compressive strength and strain of Ti46Al4Nb1Mo0.8B alloy reach to 2238MPa and 31.9%,respectively.When the ultrasonic treatment time is 120s,the maximum compressive strength and strain of Ti46Al4Nb1Mo-1.4Co alloy reach to 1773MPa and 27.5%,respectively.The change of microstructure is mainly due to the ultrasonic cavitation effect,which increases the undercooling of nucleation,reduces the nucleation radius of the reinforcements and promotes the transformation of the columnar dendrites to equiaxed dendrites.The dendrite trunk orientation distributes more randomly,which is due to the uniform temperature field by the acoustic streaming effect.