Displacive precursor phenomena in Ti–22Al–27Nb intermetallic compound prior to diffusional transformation

Abstract A systematic TEM investigation was carried out on the anomalous diffuse diffraction and contrast effects in B2 phase of a Ti–22Al–27Nb compound, the decomposition of which at high temperature into O or/and DO19 phase has so far been believed to be a purely diffusional process. The extinction rule of the diffuse scattering in B2 indicates that all these effects arise from transverse 1 0> displacive waves. Furthermore, evidence for a martensitic transformation in thin film state of the B2 phase was also found. All these facts strongly suggest that the B2 phase of Ti–Al–Nb possesses displacive instability. As a consequence, the decomposition of the B2 phase into O/DO19 is a diffusion-coupled displacive transformation. The diffusional–displacive nature of the transformation in this system not only naturally explains the observed orientation relationships among B2, O and DO19 phases, but also provides new insight into the mechanical behavior of B2+O/DO19 duplex Ti–Al–Nb compounds, such as the large primary creep and superplasticity.