Research on the Deformation Mechanism of Ti-22Al-25Nb Alloy Sheet

Author:Shao Bin

Supervisor:zong ying ying


Degree Year:2019





Ti2AlNb-based alloys have attracted extensive attention due to their low density,high specific strength,high specific stiffness,oxidation resistance,corrosion resistance and high temperature mechanical properties.It is expected to replace the existing high-density nickel-based alloys as high-temperature structural materials for aerospace engines.The Ti-22Al-25Nb alloy has optimal composition ratio among the latest generation of Ti2AlNb based alloys.It has best comprehensive properties and wide application future.However,the application of this alloy is strongly restricted by its poor forming ability and microstructure property control difficulty during processing.The Ti-22Al-25Nb alloy is composed of orthorhombic O phase,hcpα2 and bcc B2phase.The alloy phase content,morphology and size change diversely,making the phase transformation process complex.There is no systematic research on its phase transformation,leading to an insufficient thorough understanding of alloy plastic deformation behaviors.The three phase microstructure alloy has optimal service properties,but the phase transformation process in the three phase region is hard to characterize and the research is deficient.A pressing demand is needed on the systematic and in-depth research of alloy phase transformation process,especially the phase transformation process in the three phase region.The researches on the deformation mechanism of O,α2 and B2 phases as well as their interaction mechanism reveal the alloy plastic deformation behaviors from an intrinsic mechanism level,providing theoretical guidance for the determination of hot processing parameters and microstructure property control.Based on this purpose,systematic researches are performed on the phase transformation process and plastic deformation behavior of the Ti-22Al-25Nb alloy,revealing its plastic deformation mechanism.In order to systematically elaborate alloy phase transformation process,research methods,such as OM,SEM,TEM and EBSD etc.,are used to investigate the warming and cooling phase transformation process in the temperature interval of 7001200℃.The researches reveal element diffusion controlled O phase spheroidization,O→α2,B2→O phase transformation process and establish a phase transformation mechanism map,settling foundations for subsequent researches on the mechanical properties and the relationship between plastic deformation behavior and microstructure.The research results show that at phase transformation in the three phase region,Nb in the O phase at B2 grain boundary preferentially diffuses into B2 phase,resulting in O phase spheroidization.Theα2 phase nucleates and grows in the spheroidized O phase,gradually replacing O phase,completing the O→α2 phase transformation process.The Ti2O3 particles generated by surface layer oxidation during heat treatment lead to the concentration ofα2 phase at alloy surface,forming a high hardnessα2-phase rich layer.This is a special phase transformation process.However,theα2-phase rich layer is torn at large plastic deformation.By times of tension-unloading quasi in situ process,combining the EBSD analysis of slip lines of the specimens after unloading,the room temperature deformation behaviors of the Ti-22Al-25Nb alloy with B2+α2,B2 texture+α2,B2+O+α2microstructure morphology are investigated.The respective deformation mechanism of B2,α2 and O phases,and the interaction mechanism ofα2 with B2 and O with B2 are clarified.A slip line analysis model and interaction model of O with B2 are built.It is found that B2 phase has 4 kinds of slip ways including single system slip,double system slip,triple system slip,and cross slip.In the 24{110}<111>and{112}<111>slip systems of B2 phase,two of them can induce O phase slip and 12 of them induce O phase twin and 10 of them can’t induce O phase deformation.Temperature has great significance to the Ti-22Al-25Nb alloy.The service temperature of this alloy is 650750℃and hot processing temperature is above 900℃.In order to investigate the effect of temperature on the deformation mechanism of the alloy with different microstructure morphology,the effect of three temperatures(room temperature,750℃and 930℃)and four microstructure morphologies(B2 single phase microstructure,double phase microstructure consisting of B2 fine grained+granularα2phase at grain boundaries,three phase microstructure consisting of B2+strip/spheroidized O+granularα2 and B2+strip/acicular O+stripα2)on the deformation mechanism of this alloy is quantitatively investigated.It is found that B2grain boundary has elevated temperature embrittlement phenomenon andα2 phase can strengthen B2 grain boundary.Temperature has little effect on this process.The strengthening effect of O phase decreases with increasing temperature and the strengthening effect of O phase disappears at 930℃.B2 phase dynamic recrystallization is a gradually changing process.At 930℃,regional dynamic recrystallization preferentially takes place at B2 grain boundary and the extent of recrystallization continuously increases with increasing temperature.At 990℃,complete dynamic recrystallization process takes place.The spheroidization of O phase and fragmentation of stripα2 phase both can promote the dynamic recrystallization process of B2 phase.