*4.1. Enhanced* α *Phase Precipitation*

A significant difference in the evolution of α phase in the non-deformed and the HPT-deformed conditions was observed. α phase is known to precipitate preferentially along the grain boundaries as so-called grain boundary α (GB α) [31]. Enhanced α phase precipitation was also found in pre-deformed materials due to the high dislocation density [32,33]. High concentration of defects in the HPT-deformed condition reduce the energy barrier for the nucleation of α phase. The growth of an α nuclei and its coarsening is controlled by the diffusion of Mo (β stabilizing element) in the β matrix [34]. It is well-known known that the pipe diffusion along dislocation cores as well as the diffusion rate along grain boundaries are several orders of magnitude higher than the bulk diffusion [35]. As a consequence, the growth of the α precipitates along grain boundaries is also accelerated. The enhanced precipitation of the α phase in severely deformed metastable β Ti alloys was reported in several studies [36,37]. In the coarse-grained material, α phase particles precipitate in the form of lamellae, because certain mutual orientations of neighboring α and β lattices are associated with the significantly lower interfacial energy and therefore, lamellar shape is optimal for the reduction of the total interfacial energy of a precipitate [38]. On the other hand, α particles in the HPT-deformed materials are equiaxed, but not round—detailed inspection of Figure 5 reveals that particles are rather polygonal and sharp edged. It is assumed that α phase particles nucleate at triple junctions and all observed α particles are in fact GB α.

Ageing of HPT-deformed material (particularly at 400 ◦C/16 h) resulted in an inhomogeneous precipitation of α particles. Such inhomogeneity was reported to be caused by shear bands formed in the HPT deformed material [36,37,39,40]. However, we did not find any shear bands in the HPT material. On the other hand, we observed chemical inhomogeneities both in the non-deformed and in the HPT-deformed sample. In the latter case, the inhomogeneities are extended in the direction of HPT deformation (cf. Figure 3). As a consequence, the nucleation of the α phase particles may be therefore promoted in the areas depleted in Mo, even if the shape and the scale of precipitation inhomogeneities in Figure 5 cannot be directly compared to Mo-depleted regions in Figure 3 due to very different magnification (zone of observation).

In the specimen aged at 500 ◦C, ω phase was retained in the non-deformed material while it was completely absent in the HPT-deformed specimen. Enhanced precipitation of the α phase results in the rejection of the β stabilizing Mo to the surrounding β matrix causing a thermodynamic stabilization of the β matrix and suppression of the formation of the ω phase HPT deformed material [26]. Similar behavior, i.e., the preferred α phase precipitation over the formation of the ω phase, was observed in Ti-25Nb-2Mo–4Sn alloy deformed by cold-rolling [41].
