Microstructural Evolution and Phase Transformation of a Ti-5Nb-5Al Alloy During Annealing Treatment

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 first-principle calculation by density functional theory method was performed to analyze the effect of niobium diffusion on the alpha to beta transformation. The results showed that the alpha phase initially grew by merging neighboring grains and then by boundary splitting with increasing the solution time below the beta-transus temperature. However, the alpha phase disappeared absolutely above the beta-transus temperature. According to the thermodynamic analysis by DFT calculations, the diffusion of niobium from beta to alpha phase can promote the alpha to beta transformation. After quenching in water, the acicular alpha' phase precipitated from beta matrix with an orientation relationship of {110}(bcc)parallel to{0001}(hcp) and <111>(bcc)parallel to<11<(2)over bar>0>(hcp), and thus increased the hardness of the alloy.