Texture Evolution During Crystallization Of Thin Amorphous Films
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TEXTURE EVOLUTION DURING CRYSTALLIZATION OF THIN AMORPHOUS FILMS Q. K.K. Liu and G. Schumacher Hahn-Meitner-Institut Berlin, Glienicker Straße 100, D-14109 Berlin ABSTRACT Stress and energy distributions for crystallization of a thin amorphous film are calculated by means of 3D finite element method. The changes in energy are caused by elastic strain induced by different thermal expansion of the film and the substrate and by different mass densities of the crystal and the surrounding amorphous matrix. The calculations were performed for cubic crystal structure and for disc shaped crystals. Three crystal orientations (001), (011) and (111) were considered. Based on strain energy considerations the (001) orientation of crystals with respect to the film plane is energetically more favorable than (011) and (111) orientations. Interfacial and surface energies are certain to play a part in these effects as well. INTRODUCTION Crystallization of amorphous alloys has been studied extensively in the past. However, our understanding about crystallization in thin supported films is still limited. A few studies have suggested that crystallization in thin films differs appreciably from crystallization in thick specimens. Harris and coworkers [1] studied, e.g., crystallization in films of amorphous Fe75B25 with a thickness of 15 nm and measured a crystallization temperature which is about 200 K lower compared to the crystallization temperature in bulk specimens [2]. Ikari and coworkers [3] studied the crystallization behavior in films of amorphous Ni81P19 and found a metastable hexagonal phase which generally is not detected in thick specimens with the same composition. The orientation of the c-axis of this phase was found to depend on the film thickness. While the driving force for crystallization in thick specimens is dominated by the volume energy and by the strain energy induced by the different mass densities of the crystalline phase and of the amorphous phase, the total energy driving the phase transformation in thin films is increasingly affected by the free surface of the film and by the film-substrate interface. Additionally, thin supported films are elastically strained during heating due to the different thermal expansions of film and substrate. The energy driving the phase transformation in thin films is, therefore, affected by the volume energy, by the surface energy, by the interfacial energies and by thermal strain energy. The change in mass density during crystallization also has to be taken into account. This work presents 3-dimensional finite-element method (3-D FEM) calculations of the influence of grain orientation on the total change in energy during the phase transformation under thermal strain and under strain induced by the different mass density of the crystalline and of the amorphous phase. The crystal structure was assumed to be cubic and the crystals were assumed to have the shape of a disc. It was assumed that the {001}-, the {011}- or the {111}lattice planes of the crystals were parallel to t
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