The contrasting fracture behaviour of twin boundaries and general boundaries – A first principles study based on experimental observation

Abstract

Experimental observations of grain boundary fracture behavior have been used as exemplars, and boundaries with the same crystallography as the experiments were developed. These boundaries were non-symmetrical, and this necessitated the first ever study of non-symmetrical boundaries in magnesium using density functional theory. The broad agreement of the calculated boundary cohesion values with the experimental observations of fracture behavior showed the simulations to be good approximations of real behavior, and from this point the simulations were further interrogated to understand the differences between the boundary types and solute species. Solutes with both larger and smaller radii than magnesium had a preference for segregation to the grain boundary. The boundary cohesion was examined by the parameter known as embrittlement potency, and it was found that solutes smaller than magnesium had a toughening effect, while those solutes larger than magnesium had more tendency to embrittle. The two boundaries studied in most detail, the {101‾2} twin boundary, and a general grain boundary observed experimentally, showed different cohesive behaviors. Although the symmetrical twin boundary behaved in a similar manner to previous reports, the non-symmetrical boundary showed more complex cohesive behavior, highlighting the importance of studying the non-symmetrical boundaries that predominate real materials.