Observation of α phase nucleation within ω precipitates with novel orientation relationship in a low misfit metastable β-Ti alloy through EBSD

During isothermal ageing of metastable β-Ti alloys decomposition of the β matrix occurs through the formation of either the ω or α phases. Recent literature has shown that the presence of ω iso can facilitate α nucleation forming an α variant with a known orientation relationship to the β and ω phases. However, the mechanisms behind such a transformation are unclear, especially in systems with a low ω-β misfit. In this study, the effect of 450 °C ageing on a low misfit alloy, Ti-26Nb (at.%), has been studied using electron backscattered diffraction (EBSD). Isothermal ω precipitates were observed to coalesce along preferred growth directions. EBSD identified the presence of α within the core of the coalesced ω precipitates. The relationship between this α and the parent β was inconsistent with the expected Burgers orientation relationship and did not follow any known crystallographic relationship between α and ω, suggesting a novel transformation pathway.