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

A systematic TEM investigation was carried out on the anomalous diffuse dif fraction and contrast effects in B2 phase of a Ti-22Al-27Nb compound, the d ecomposition of which at high temperature into O or/and DO19 phase has so f ar been believed to be a purely diffusional process. The extinction rule of the diffuse scattering in B2 indicates that all these effects arise from t ransverse < 110 > <1 1(1) over bar0 > displacive waves, Furthermore, eviden ce 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 th e B2 phase into O/DO19 is a diffusion-coupled displacive transformation. Th e diffusional-displacive nature of the transformation in this system not on ly naturally explains the observed orientation relationships among B2, O an d DO19 phases, but also provides new insight into the mechanical behavior o f B2+O/DO19 duplex Ti-Al-Nb compounds, such as the large primary creep and superplasticity. (C) 2001 Acta Materialia Inc. Published hy Elsevier Scienc e Ltd. All rights reserved.