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

通过应力诱发马氏体的时效，被称为SIM时效，高温形状记忆合金（HT-SMA）首次实现基于Co-Ni-Ga合金，其初始特征在于低马氏体起始温度（Ms）。马氏体稳定化是由符合马氏体的短程有序（SC-SRO）（一种众所周知的现象，目前仅被视为SMA中的降解机制）施加的，Ms增加了约150 °C。然而，Co-Ni-Ga（-X）合金中马氏体前所未有的稳定性还不清楚。因此，有助于SIM老化影响的基本机制将成为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。由于目前对钴镍镓多晶材料的处理主要是单晶材料，因此，对钴镍镓多晶材料的形变热处理将是第二个研究重点。先前在不同加工条件下获得的结果清楚地揭示了粒度和质地的强烈影响。因此，传统的加工技术，即轧制，型锻和挤压将用于建立强纹理材料。此外，循环热处理将用于获得具有多晶微观结构的Co-Ni-Ga。由于这种过程还没有分析到目前为止，导致异常晶粒长大的基本机制进行了深入的研究。为了能够成功地进一步改进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。透射电子显微镜以及中子衍射将有助于澄清的基本机制，有助于材料的整体响应。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.