Formation mechanism of an α2 phase-rich layer on the surface of Ti-22Al-25Nb alloy

In order to investigate the formation mechanism of the alpha(2)-rich layer in the surface of the Ti-22A1-25Nb alloy, systematic analyses are carried out on the surface of the alloy by using SEM, XRD and EBSD. The results reveal that this alpha(2)-rich layer is formed by oxidation of the surface layer. In this surface oxidation layer, there are abundant Ti2O3 particles similar with the alpha(2) phase in structure. These Ti2O3 particles serve as nucleation points and lower the nucleation energy of the alpha(2) phase, promoting the precipitation and growth of alpha(2) phase on the surface layer of the alloy. This surface layer has a hardness of 684 HV, significantly exceeding the value of 312 HV for the interior of the alloy and thus considerably improves the surface properties of the alloy, serving as a high-performance, self-generated surface strengthening layer.