Microstructural evolution and phase transformation of a Ti-5Nb-5Al alloy during annealing treatment

Abstract

The microstructural evolution and phase transformation of a Ti-5Nb-5Al alloy during isothermal annealing treatment were studied in this paper. The microstructural evolution was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with X-ray energy dispersive spectroscopy (EDS) and electron backscattering diffraction (EBSD). The frst-principle calculation by density functional theory method was performed to analyze the effect of niobium diffusion on the a to ß transformation. The results showed that the a phase initially grew by merging neighboring grains and then by boundary splitting with increasing the solution time below the ß-transus temperature. However, the a phase disappeared absolutely above the ß-transus temperature. According to the thermodynamic analysis by DFT calculations, the diffusion of niobium from ß to a phase can promote the a to ß transformation. After quenching in water, the acicular a' phase precipitated from ß matrix with an orientation relationship of 110bcc||0001hcp and <111glt;bcc||<1120>hcp, and thus increased the hardness of the alloy.