Title: Nucleation and growth of hierarchical martensite in epitaxial shape memory films,
Abstract: Shape memory alloys often show a complex hierarchical morphology in the
martensitic state. To understand the formation of this twin-within-twins
microstructure, we examine epitaxial Ni-Mn-Ga films as a model system. In-situ
scanning electron microscopy experiments show beautiful complex twinning
patterns with a number of different mesoscopic twin boundaries and macroscopic
twin boundaries between already twinned regions. We explain the appearance and
geometry of these patterns by constructing an internally twinned martensitic
nucleus, which can take the shape of a diamond or a parallelogram, within the
basic phenomenological theory of martensite. These nucleus contains already the
seeds of different possible mesoscopic twin boundaries. Nucleation and growth
of these nuclei determines the creation of the hierarchical space-filling
martensitic microstructure. This is in contrast to previous approaches to
explain a hierarchical martensitic microstructure. This new picture of creation
and anisotropic, well-oriented growth of twinned martensitic nuclei explains
the morphology and exact geometrical features of our experimentally observed
twins-within-twins microstructure on the meso- and macroscopic scale.