Nucleation model for hcp martensite: Application to Co thin films

Abstract
The martensitic face-centered cubic (fcc) to hexagonal close-packed (hep) transformation was studied in cobalt thin films on silicon substrates in order to understand effects of film constraints, texture and stress state. The martensitic transformation was observed in situ with the substrate curvature method. The transformation manifests itself as a stress drop revealing a positive work. However, with increasing tensile stress, the start of the martensite transformation is shifted to lower temperature. Thus, stress obstructs the transformation in spite of the positive sign of the transformation work. A dislocation-based model for the martensite nucleation is presented and applied to Co thin films. A distinction is made between a microscopic shape change and a macroscopic shape change. The model predicts the stress-induced delayed nucleation due to the microscopic shape change. The positive work results from the macroscopic shape change. Within experimental error, model and experiments agree well.