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

通过应力诱导马氏体时效（sim时效），首次实现了以低马氏体起始温度（Ms）为特征的Co-Ni-Ga合金为基础的高温形状记忆合金（HT-SMA）。对称性符合短程有序（SC-SRO）是一种众所周知的现象，目前被认为是sma中的一种降解机制，Ms升高了约150°C。然而，Co-Ni-Ga（-X）合金中前所未有的马氏体稳定性尚不清楚。因此，促进sim老化效应的基本机制将是第二个资助期的研究重点。为了将结果和知识分别转移到稳健的应用中，将对多晶样品进行表征。由于迄今为止主要是对单晶进行热处理，多晶Co-Ni-Ga的热机械加工将成为研究的第二个重点。先前在不同加工条件下获得的结果清楚地揭示了晶粒尺寸和织构的强烈影响。因此，传统的加工技术，即轧制，挤压和挤压将被用于建立强纹理材料。此外，循环热处理将得到具有低晶结构的Co-Ni-Ga。由于这一过程至今尚未得到深入的分析，导致晶粒异常生长的基本机制有待探讨。为了能够成功地开发进一步改善Co-Ni-Ga ht - sma的功能特性，与其他小组的密切合作将是至关重要的。透射电子显微镜和中子衍射将有助于澄清导致材料整体响应的基本机制。此外，中子衍射将是SC-SRO分析的首选方法。

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.