Thin Film Martensites

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9706815 Wuttig This is a renewal grant for quantitative studies of the evolution of the martensitic phase in constrained thin films with an emphasis on the martensitic transformation at constant strain. Much is known already about this transformation as it occurs at constant stress and, of course, normally the differences in mechanical behavior of materials under conditions of constant stress and strain are not very large. This is not the case for the martensitic transformation, where a special kind of pseudo-elasticity can occur. The studies build on results and experiences gained during the previous three-year NSF grant. Specifically, the evolution of the two phase state, alpha prime and gamma, is investigated by examining the simultaneous evolution of the microstructure, stress and modulus of single crystalline shape memory alloy (SMA) films on a substrate which serves to constrain this state. Microstructural evolution is modeled and the results are compared to the experimentally observed mesoscopic two-phase structure and related macroscopic stresses and pseudo- elastic moduli. It is expected that complete closure will be achieved leading to an understanding of the evolution of the martensitic phase transformation in constrained films on the mesoscale. The studies focus initially on NiTi films. In later stages of the research other systems, such as ternary FeNi-based alloys, Ni-free SMAs, FePt, FePd and Cu- based SMA alloys will be investigated. %%% This phenomenon is of scientific interest and important in composite thin film actuators based on shape memory concepts. ***