Study of kinetics and thermodynamics in thin films at the limit between coherent and semi-coherent phasetransformation

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Phase boundaries and stabilities of alloy thin films differ from those of bulk systems. Nowadays, mechanical stress is considered as an important key factor. Recently, Nörthemann et al. calculated that for Niobium-Hydrogen (Nb-H) films of less than d=27 nm thickness, coherent precipitates become energetically favorable. Below this critical thickness d_c, stress release is impossible at the metal-hydride interface. This condition results from the film clamping condition, at 300 K. As a consequence, the properties of thin films should be separated in the two limiting cases: for d< d_c in a coherent high-stress regime and for d > d_c in an incoherent low-stress regime. Due to coherency and stress conditions, thermodynamics and kinetics as well as the hydride morphologies and lateral spreading should abruptly change when crossing this critical thickness.While above d_c, classical Gibbs thermodynamics can describe the film properties, below d_c coherent thermodynamics should be considered. This thermodynamics, as theoretically addressed e.g. by Schwarz et al. for the open system, predicts new features like, inter alia, phase boundaries depending on the direction of the phase transformation (two different miscibility gaps), or a thermodynamic hysteresis.Thus, we expect for d