Effects of Film Stress and Geometry on Texture Evolution Before and After the Martensitic Transformation in a Nanocrystalline Co Thin Film

Abstract

Using a transmission electron microscopy-based orientation and phase mapping technique we examine effects of film stress and geometry on texture development before and after the martensitic transformation in a nanocrystalline Co thin film. Specimens are annealed for various times at 873 K (600 °C), which is above the equilibrium martensitic transformation temperature [~693 K (~420 °C)], and then cooled to room temperature. The cooled specimens are used for texture analysis. The martensitic transformation is observed to be incomplete, and fcc and hcp structures coexist in all the specimens examined. It is deduced from the crystallographic orientation relationship of the martensitic transformation of Co that, during prolonged annealing at 873 K (600 °C), the major texture in the fcc phase is {101}, followed by {111}. In the cooled structure, the major texture component in the hcp phase is (0001), followed by {2 $$ \overline{1} \overline{1} $$ 0}. The {101} texture is clarified by assuming that the film system is perfect elastic–plastic with the help of the concept of the Taylor factor. We conclude that the (0001) texture is produced by the surface energy minimization, which, however, does not elucidate the development of the {2 $$ \overline{1} \overline{1} $$ 0} texture. The formation of the texture component is understood by considering the strain energy provided by a constraint imposed by neighboring grains. As annealing time increases, the hcp phase expands its area, dominating over the fcc phase after prolonged annealing, which is also clarified by allowing for the geometrical constraint effect. The expansion of the hcp phase supports the possibility of surface nucleation of martensites.