On the role of precipitates and lattice defects in the martensitic transformation of high temperature shape memory alloys (HT SMAs)

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The scientific objective of project TP2 is to explain the relation between microstructure and martensitic phase transformations in high temperature shape memory alloys (HT SMAs). The effects of local alloy chemistry and crystallography including point defects, dislocations and precipitates will be studied using transmission electron microscopy (TEM). Research will first focus on the promising polycrystalline Ti-based alloys Ti80Ta20 and Ti75Ta20Al5 (actor alloys) as well as on Co-Ni-Ga (actor alloy and pseudoelastic SMA). Later the investigations will also address optimized ternary and quarternary Ti-Ta(-X-Y) alloys. TP2 uses advanced TEM to study elementary transformation and deformation processes. The microstructural evolution during processing will be documented (ingot metallurgy: melting and forming, thin film technology: sputtering and heat treatment). Special emphasis will be placed on the evolution of the microstructure during the repeated exploitation of the mechanical and thermal shape memory effects (functional fatigue). Project TP2 has for scientific objectives. First, it aims at contributing to a better understanding of the role of alloy chemistry and crystallography including the importance of point defects. Second, the interaction of dislocation plasticity and martensitic transformations will be studied. Third, the influence of precipitation processes on subsequent martensitic transformations will be investigated. As a new topic, high temperature oxidation will be addressed. This includes the formation of oxide layers and the evolution of surface zones of the alloy next to the oxide. The role of the evolving surface layer alloy compositions on the martensitic transformations will also receive attention. These aspects are of particular importance for thin film applications of HT SMAs, where high temperature oxidation can suppress the shape memory effects. TP2 results will directly contribute to the development of robust and damage tolerant HT SMAs.