Role of dislocations in the bcc-hcp transition under high pressure: A first-principles approach in beryllium

We study the impact of dislocations on the bcc-to-hcp-to-bcc phase transition cycle using density-functional theory. The transformation is studied under two external constraints: first under pressure, and second under uniaxial shear. In both cases, we find that the elastic strain created by +/- 1/2[111] screw dislocations induces a shear deformation which initiates the bcc-to-hcp transformation through the Burgers mechanism, as suggested by the location of the phases and their orientations. For the pressure-induced transformation, a hysteresis appears in the P-V curve and the analysis of structures reveals that only the three hcp variants topologically compatible with the screw dislocations emerge. Our calculations thus capture characteristics of microstructures containing grains (variant hcp) and defects (triple junction and grain boundaries). Interestingly, a small bcc inclusion is present in the parent bcc after reversion. A careful analysis of the underlying deformation reveals that its origin is explained by three reversion transformations which are self-accommodating and thus stable. Under shear, the strain field induced by the dislocation decreases the energetic barrier considerably.