Massive transformations in titanium alloys: Role of relative orientation of adjacent parent grains

Massive transformations occur in both additively and conventionally manufactured titanium (Ti) alloys. Unlike martensitic transformations, massive transformations can result in patch-like massive phases (αm) that traverse the parent prior-β grain boundaries (GBs). However, the conditions favouring the formation of these trans-GB αm-phases in Ti alloys remain largely unexplored. Through characterising the trans-GB αm-phases in α-β Ti alloys fabricated by additive and conventional processes, we find that their formation always occurs when two neighbouring prior-β grains share or nearly share a {110} pole, without exception. These trans-GB αm-phases exhibit concentrated {0001} poles while their {112¯0} poles spread widely. In addition, as metastable phases, they tend to decompose into ultrafine α-β lamellae. The role of relative orientation of adjacent parent grains in massive transformations and the implications for microstructural innovations in α-β Ti alloys are discussed.