Tension-compression asymmetry of Fe-Ni-Co-Al-X (X= Nb, Ti, Nb-Ti) shape memory alloys – Impact of nanocrystalline precipitates on functional properties

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Since the discovery of iron based (Fe-based) shape memory alloys (SMAs), many efforts have been spent to improve functional performance. In contrary to conventional Ni-Ti, Fe-based SMAs feature lower costs for alloying elements and processing costs, which making them attractive for plenty of industries. Whereas many studies focused the impact of tension-compression asymmetry and orientation dependency in quasistatic tests, only few studies addressed functional fatigue and the impact of gamma´ precipitates on functional properties. Thus, a comprehensive knowledge of the interrelationships between precipitates and functional (cyclic) performance still is missing in literature.This project aims in obtaining a systematical and comprehensive understanding of the factors influencing the functional materials properties. Since it is known that critical precipitate parameters govern the functional properties in Fe-Ni-Co-Al based SMAs this project focuses an approach identifying promising precipitate parameters, improving the performance of Fe-based SMAs. The overall aim is to design a new family of precipitates exhibiting a core-shell character, i.e. a chemical gradient between the core and the shell of a precipitate.Therefore, in a first step novel Fe-Ni-Co-Al-X (X=Ti, Nb) SMAs will be used to systematically vary precipitate parameters. Using tensile and compression tests (quasistatic and cyclically loaded) in different crystallographic orientations the impact of differently tailored precipitates on the nature of martensitic transformation will be studied in depth. In a second step most promising precipitate parameters in the quintenary alloy systems will be deduced and a quasi-quintenary Fe-based SMA system will be introduced, i.e. Fe-Ni-Co-Al(Nb-Ti). The aim is to apply the idea of precipitates exhibiting core-shell character to this SMA system. Following a two-step aging scenario precipitates with a core-shell structure will be promoted in different temperature regimes and aging times, since the diffusion coefficients of Nb and Ti differ.By a systematic variation of stress fields surrounding the precipitates and the local chemistry of both, precipitates and matrix, mechanical and non-mechanical driving forces of the free Gibbs energy for martensitic transformation will be affected using the quasi-quintenary Fe based SMA system. Using this approach the design of precipitates with adjusted properties will become feasible and allows for an improvement of functional properties such as an increase of transformation temperatures, strains and an improvement in cyclic stability. A further decrease in the size of the precipitates in Fe-Ni-Co-Al-(Nb-Ti) SMAs will also lead to novel and unprecedented transformation mechanisms in SMAs including different twinning modes and shape memory effects, resulting from the interaction of nanoscaled precipitates and the related martensite variant selection.