Interfacial and strain energy considerations on crystallographic characteristics of transition metal semicarbides

Though the crystallographic characteristics, such as orientation relationship, morphology, and defects, of transition-metal semi -carbides (M 2 C, M = V, Nb, Ta, W, Mo) formed by precipitation in alloys have been extensively studied and rationalized as a major crystallographic theory, there is a lack of crystallographic understanding of those prepared by in situ reaction. This study investigates the crystallographic features of M 2 C synthesized in situ from transition-metals and carbon sources. The in situ reaction yields preferred crystal morphologies, high defect densities, heavy matrix deformation, and distinct twinning modes. These results suggest that the crystallographic characteristics of the two semi -carbides formation methods are intrinsically related despite their distinct mechanisms. These characteristics are elucidated via interfacial and strain energy considerations. Specifically, the key role of geometric constraints on the crystallographic characteristics and thermodynamic consequences is examined. The results of these constraints, i.e. , how the transformation strain is accommodated, are characterized and discussed as well. • In-situ formed semi -carbides share crystallographic similarities with those that precipitate. • Atomic interface steps, unusual lattice deformations, and non-classical twins have been detected. • Semi -carbide formation involves displacive characteristics.