Static investigation of martensitic nucleation criticality in an atomistic model material

ABSTRACT This contribution investigates the nucleation of martensitic transformations in the bulk of austenitic single crystals. The transformation is associated with a decrease in the potential energy that is the driving force for the transformation. The coordinated atom displacements, which generate the martensite, produce large elastic strains in the material, which are associated with an elastic energy that opposes the transformation. A thermodynamic nucleation criterion for this type of transformations is derived. The criterion is evaluated by the method of static lattice relaxation for homogeneous nucleation in an atomistic 2D model material. The potential energy of martensitic clusters of different sizes and shapes, embedded in a stabilised austenitic matrix are calculated. The results exhibit non-monotonous progressions of the potential energy with the cluster size. This indicates that the elastic strain energy scales non-linearly with the size to allow the formation of local energy maxima at critical cluster sizes, interpreted as nucleation barriers. These barriers separate domains of stability from domains of instability in the configurations space, similar to Kelvin's classical theory of spherical fluid phase nucleation. The critical nuclei sizes are in the order of 100–150 unit cells, and their shapes are plate-like with long dimension parallel to the shear direction. The critical size of twinned clusters decreases with the twinning multiplicity.