Unified Transformation Pattern of Hexagonal Transition Metal Hemicarbides and Heminitrides from Their Metal Parents

The solid-state phase transformations from body-centered cubic (bcc) transition metals such as V, Nb, Ta, Cr, Mo, and W to hexagonal close-packed (hcp) transition metal hemicarbides and heminitrides (M2C and M2N) were investigated in this work. It is thought that three-dimensional overall lattice distortions induced by complex ordering between interstitial atoms (C, N) and structural vacancies in the transition metal parents are the key-driving force for these structural changes. Moreover, these phase transformation paths were described by ab initio simulations and symmetry analyses. Different transformation paths are categorized, and the reasons for the inability to complete the transformation from bcc to hcp are identified, while all paths that can be transformed into hcp structures follow a unified formation pattern. The number of atoms required for this pattern is defined for a specific set of ordered atoms known as transformation units. These units can be incorporated into the main phase transformation crystallography, describing transformation processes from the atomic-level transformation units to a long-range strain-free interface comprising of the transformation units and interface defects, at microscopic and macroscopic scales. The dynamic origins of the formal crystallographic requirements in interstitials-induced transformations are also elucidated as well. The research can provide some physical significance of coherent terraces within step structures in phase transformation crystallography.