Functional stability of polycrystalline Co-Ni-Ga high-temperature shape-meory alloys - On the role of microstructure and martensite stabilization

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Through aging of stress-induced martensite, referred to as SIM-aging, a high-temperature shape-memory alloy (HT-SMA) was for the first time realized based on a Co-Ni-Ga alloy initially characterized by a low martensite start temperature (Ms). Martensite stabilization imposed by symmetry-conforming short-range order (SC-SRO), a well-known phenomenon, by now solely treated as a degradation mechanism in SMAs, Ms was increased by about 150 °C. However, the unprecedented stability of martensite in Co-Ni-Ga(-X) alloy is not understood, yet. Thus, elementary mechanisms contributing to the effects of SIM-aging will be in focus of research in the second funding period. In order to allow for a transfer of results and knowledge, respectively, to robust application, characterization will be conducted on polycrystalline samples. As mainly single crystals have been treated up to know, thermomechanical processing of polycrystalline Co-Ni-Ga will be a second focal point of research. Results obtained on differently processed conditions previously clearly revealed the strong impact of grain size and texture. Consequently, conventional processing techniques, i.e. rolling, swaging and extrusion will be employed for establishing strongly textured material. In addition, cyclic heat treatments will be employed for obtaining Co-Ni-Ga featuring an oligocrystalline microstructure. As such kind of procedure has not been analyzed so far, basic mechanisms leading to abnormal grain growth characterized in depth. In order to be able to succeed in development of further improved functional properties in Co-Ni-Ga HT-SMAs intense collaboration with the other groups will be of utmost importance. Transmission electron microscopy as well as neutron diffraction will assist in clarification of the basic mechanisms contributing to the overall response of the material. Moreover, neutron diffraction will be method of choice for analyses of SC-SRO.