The complexity of {112¯1} twin interface structure and energetics in HCP materials

The quest for a deeper understanding of the mechanical properties of HCP alloys has been an ongoing effort. Due to the lack of a sufficient number of slip systems, twin nucleation and migration originate as a substantial plastic deformation mechanism in HCP metals. The paper uncovers the energetics and atomistic mechanism of the {112¯1}〈112¯6〉 which is one of the most prominent twin modes in HCP materials that has not been fully understood. In view of corrugated {112¯1}interfaces, the positioning of lattice/motif atoms needs to be clarified. First, we establish the initial twin boundary (TB) structure unambiguously via the lattice offsets resulting in volume invariancy and correct relative positioning of the twin variants. Secondly, the TB migration mechanism is determined from a crystallographic (zero net shuffle, Σs→=0) and energetic standpoints (minimum energy path). The Generalized Planar Fault Energy (GPFE) curves are calculated for Ti, Hf, Mg, and Zr. Such GPFE curves for this twin mode are calculated for the first time, and point to unusually high unstable energies (300-850 mJ/m²), and low migration barriers (<60 mJ/m²) for twinning accompanied with large shuffle magnitudes, |s→|, comparable to the Burger's vector (|s→|≅b_e>0.44Å).