Preferred orientation relationship of intra- and inter-granular precipitates in titanium alloys

In a previous study by one of the authors, it had been reported that intra-granular α (HCP) precipitates have a Burgers orientation relationship (O.R.) with the β (BCC) matrix in Ti–22V–4Al and Ti–15V–3Cr–3Sn–3Al alloys. On the other hand, grain boundary α precipitates were shown to have a tendency for parallelism of the {110}β//{11̄01}α planes, i.e. they display a Potter O.R. with one of the conjugate β grains. A three-dimensional near-coincidence site lattice (3-D-NCS) model, coupled with transmission electron microscope (TEM) observations, has been developed to explain these observations. The shape of an embryo predicted by the 3-D-NCS analysis is compared for the Burgers and Potter O.R., assuming the morphology is only determined by the anisotropy of the structural part of the interfacial energy. An intra-granular precipitate with Burgers O.R. has a lower free energy of formation for the critical nucleus than one with the Potter O.R., in agreement with the observations. However, α precipitates with the Potter O.R. are preferred at grain boundaries, again in agreement with the experimental observations.