Phase transformation mechanism of α2→O in Ti2AlNb alloy during rapid thermal cycling

: Upon rapid thermal cycling into the B2 phase region, the α 2 phase in Ti 2 AlNb alloy does not dissolve directly but rather partially transforms into the thermodynamically less stable O phase, forming a mixed α 2 /O structure. Transmission electron microscopy (TEM) analysis reveals that the metastable O phase forms through two steps: first, elemental diffusion and random compositional fluctuations cause local thermodynamic instability within the α 2 phase, leading to spinodal decomposition and the formation of Nb-lean and Nb-rich regions; second, the enrichment of Nb results in severe lattice distortion within the α 2 phase, which can no longer maintain the symmetry of the hcp structure. Driven by lattice distortion stress, the Nb-rich α 2 phase transforms into the O phase through atomic shifts. Specifically, the α 2 phase elongates along the [11 0] direction and contracts along the [1 00] direction, transforming into the [100] O and [010] O crystallographic orientations, respectively.