Effect Of Heat Treatment On Superplastic Deformation Behavior Of Ti₂AlNb Alloy Sheet

Ti₂AlNb alloy is a new type of Ti-Al intermetallic compound with both light weight and excellent high temperature properties.It has become an advanced lightweight structural material with great application prospects in the field of aerospace.However,the poor formability of the alloy limits its wide application.The superplastic forming process is an ideal process for forming intermetallic compounds that are difficult to deform.In this paper,the microstructure of rolled Ti₂AlNb alloy sheet was characterized,and the phase content in the alloy was successfully controlled by different heat treatment processes.The superplastic tensile properties of the alloy sheets before and after heat treatment were tested.The microstructure and fracture morphology at different positions were observed.The evolution of the microstructure of the alloy during the deformation process and its influence on the superplastic properties were analyzed.The superplastic deformation mechanism of Ti₂AlNb alloy was described.The rolled Ti₂AlNb alloy is composed of three phases B2,O andα₂.The two forms of theα₂ in the plate come from the forging process.The elements in the sheet are evenly distributed,the average grain size of B2 is 0.49μm.After the solution treatment at 970℃,only the B2 andα₂ exist in the sample.The average hardness of the alloy after solution treatment decreased from 340 HV to 310HV.But the fluctuation of the hardness value greatly.In superplastic deformation,it is easy to produce uneven deformation and cause premature fracture.After stress relief annealing treatment at 650℃,the O is re-precipitated.The average hardness of the sample is 367 HV,and the fluctuation of the hardness value decreased.Under the deformation conditions of 950℃and 1×10^-3s^-1,the elongation of the As-received rolled sheet is 379%,and the elongation of the heat-treated rolled sheet is 208%.They all show superplastic forming ability.During the deformation process,the B2 has mainly responsible for the deformation,α₂ has limited deformation ability and it is stretched in the stretching direction.During the deformation process,α₂ will gradually deform.After deformation to a certain extent,some ofα₂ particles will recrystallize to maintain deformation.However,in areas whereα₂ is concentrated in large quantities,voids will occur at the phase boundary due to local stress concentration.In the end,the voids grow up and aggregate,causing fracture.