THERMAL STABILITY OF THIN Ni-Fe FILMS ON SAPPHIRE
2Department of Materials Science and Engineering, Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA, USA
We deposited thin Ni-Fe bilayers on c-plane (0001) sapphire (α-Al2O3) substrates at room temperature employing the electron beam deposition method. Annealing the films at elevated temperatures led to full intermixing of the layers. The Ni-rich films (Ni-20 at.% Fe) exhibited a strong fiber [111] texture, and their prolonged annealing at a temperature of 1050 °C resulted in full dewetting and formation of [111]-oriented single crystalline faceted particles. The dewetting of the Fe-richer films (Ni-27 at.% Fe and Ni-50 at.% Fe) proceeded differently. At the temperature of 350 °C we observed the onset of abnormal grain growth in the films: sparsely spaced [100]-oriented grains nucleated in the [111]-textured polycrystalline matrix. These abnormal grains grew rapidly and consumed the surrounding matrix. The dewetting of these films proceeded in two stages: the agglomeration began in [111]-textured polycrystalline patches of the film, and then a slower agglomeration process of the [100]-oriented abnormal grains followed. As a result, two distinct populations of small [111]-oriented particles, and larger [100]-oriented particles were formed at the late stages of dewetting. We demonstrated that the nucleation of abnormal grains and their growth are associated with the ordering reaction in Ni-Fe films. Both the existing literature data and our DFT calculations demonstrate that ordering in Ni-Fe alloys is associated with a decrease of the energy of the Ni-Fe (001)-sapphire (0001) interface, as compared to the Ni-Fe (111)-sapphire (0001) interface. We demonstrate that it this energy decrease which allows the nucleation and growth of ordered abnormal grains.
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