On coherent transformations in steel

The effective grain size of a martensitic steel is the coherence length of structure domains that share the relevant crystallographic features. These, in turn, depend on the property of interest. The crystallographic feature that governs transgranular fracture is the {1 0 0} cleavage plane while that governing dislocation plasticity is normally the {1 1 0} slip plane. However, two adjacent sub-volumes that have different orientation relationships do not necessarily mean two effective grains. In this paper, effective grain size is discussed based on 24 Kurdjumov–Sachs and 12 Nishiyama–Wassermann relationships which can be divided into three groups according to their Bain strains. Stereographic projection showed that only those from different Bain groups will have large-angle misorientations between {1 0 0} cleavage planes while dislocation glide planes {1 1 0} usually do not exhibit large-angle misorientation. Therefore, effective grain sizes for transgranular fracture and plastic deformation are not the same, and do not necessarily respond in the same way to certain grain refinement measures.