A molecular dynamics study of dislocation-interface boundary interactions in lath martensite

This study aims at extending the current understanding on the nano-scale deformation mechanisms in lath martensite. The microstructure of lath martensite is set by the prior austenite grain size and each grain is divided into packets of martensite laths. Each packet comprises blocks and even sub blocks, and these in turn are constructed from parallel laths of martensite. Lath boundaries within a block are low angle, taking up the small misorientation. Block boundaries are high angle twist boundaries. Understanding and modelling the details of the dislocation-grain boundary (GB) interactions is a key for understanding softening behavior and strengthening mechanisms of lath martensite and is of basic scientific and practical importance. In the present paper we investigate the atomic structure of several low and high-angle twist GBs and study the interaction between dislocation and GBs via Molecular Dynamics (MD) simulations. Our simulations reveal a difference in dislocation transmission across high-angle and low-angle GBs. At low-angle lath boundaries dislocation motion is hindered by dense network of intersecting screw dislocation segments. As a critical stress is reached the junctions between the dislocations forming the network unzip and a high density of dislocations is emitted into the adjacent grain. At a high-angle block boundary, it is not possible to distinguish individual interfacial dislocations. The incoming dislocation is simply absorbed into the GB plane where the line defect is not possible to be distinguished as an individual interfacial dislocation. With increasing the orientation angle the blocking strength of the GBs increases. As a critical stress is reached low density of 1/2 [111]screw segments is emitted into the adjacent grain.