Abstract
The phase transformation and microstructure in Ti-22Al-25Nb alloy are extremely complex.In this work,the morphology evolution of the O phase during the heating and cooling process was investigated by electron backscatter diffraction (EBSD) and first-principles calculations.The results show that the O→α2 phase transformation process during the heating process is as follows:spheroidization of the O phase occurs first,then the α2 phase nucleates in the spheroidized O phase,grows and replaces the O phase,completing the O→α2 phase transformation.In the meanwhile,the diffusion of Nb from Nb-poor O to Nb-rich B2 phases is a back-diffusion process.According to first-principles calculations,the driving force of the O→α2 phase transformation is the difference in the free energies of formation for the two phases(0.09eV/atom).When the Nb content is greater than 15.625 ％,the lattice distortion of the α2 phase sharply increases,and the distortion energy drives the back-diffusion of Nb.During the cooling process,the α2→O phase transformation is difficult and slow due to the difficult diffusion of Nb from the B2 to α2 phases.When holding for 60 min at 960 ℃,the coarse α2 phase gradually transforms to the O phase from the margin to the inside,forming a dispersed mixed structure of the O and α2 phases.During the B2→O transformation,the nucleation of the O phase induces a high stress region,in the range of approximately 200 nm.
基金项目
National Natural Science Foundation of China(52005129)
NETL
NSTL
维普
万方数据