Thin Film Martensites

9706815 Wuttig这是对约束薄膜中马氏体相演化的定量研究的续期拨款，重点是在恒定应变下的马氏体相变。关于这种转变，很多人已经知道了，因为它发生在恒定应力下，当然，通常情况下，材料在恒定应力和应变条件下的力学行为的差异不是很大。这不是马氏体相变的情况，在这种情况下可能会出现一种特殊的伪弹性。这些研究建立在之前三年NSF拨款期间获得的结果和经验的基础上。具体地说，通过考察单晶形状记忆合金(SMA)薄膜的微结构、应力和弹性系数的同时演化，研究了单晶形状记忆合金(SMA)基片上两相状态α素态和伽马态的演化。模拟了微观组织的演化过程，并与实验观察到的介观两相结构及相关的宏观应力和伪弹性模量值进行了比较。我们期望在介观尺度上实现完全闭合，从而理解约束薄膜中马氏体相变的演化。这些研究最初集中在镍钛薄膜上。在研究的后期阶段，还将研究其他体系，如三元FeNi基合金、无镍SMA、FePT、FEPD和铜基SMA合金。这种现象在基于形状记忆概念的复合薄膜致动器中具有重要的科学意义。***

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. ***