Basal dislocations in MAX phases: Core structure and mobility

Abstract

The MAX phases, layered ternary carbides and nitrides, plastically deform via exclusively basal slip systems. This constraint leads to their well-documented plastic anisotropy and deformation mechanisms such as kinking. Basal dislocations govern the incipient plastic deformation, and therefore understanding them is a key step to utilizing the unique properties of MAX phases. As such, we performed the first fully atomistic calculations of the core structures and mobilities of basal dislocations using a bond-order interatomic potential. Calculations show that the weakly bonded A-layer can have a large impact on core structure, with several configurations exhibiting non-planar cores and being an order of magnitude less mobile than their planar counterparts. Additionally, due to the unique stacking sequence and stacking faults in MAX phases, basal dislocations preferentially nucleate and glide as same-signed pairs on adjacent slip planes. These pairs dissociate as zonal dislocations over much larger distances, enabling greater core spreading and therefore increased mobility. Edge pairs have Peierls stresses more closely resembling those in metals than in ceramics, suggesting they control the incipient plasticity of MAX phases at lower temperatures. Overall, these results provide a foundation for better understanding MAX phase incipient plasticity through basal dislocations and thus enabling them to be better engineered for specific applications.