Reduced functional degradation of a magnetic shape memory alloy by aging under stress

与托木斯克国立大学西伯利亚物理技术研究所合作提出的研究项目的目标是全面分析CoNiGa磁性形状记忆合金的功能退化。以往对传统NiTi形状记忆合金的研究表明，NiTi单晶在应力作用下时效可以获得只具有某些析出变种的组织。这反过来又导致功能疲劳行为的显著改善。初步工作已经证明，这种微结构的定制在CoNiGa单晶中也是可行的。然而，这种材料也可以表现出磁场诱导的应变，因此，可以实现比传统NiTi更高的功率密度。对于设想的应用，微结构的循环稳定性是关键问题，然而，这一问题尚未被研究。拟议项目的假设是，与传统的NiTi系统类似，只有某些类型的沉淀物变体的微观结构应该表现出显著改善的抗功能降解能力。在拟议的研究中，将研究析出变种对传统形状记忆效应和磁场诱导形状记忆效应的影响。首先，俄罗斯的项目合作伙伴将利用透射电子显微镜分析在应力辅助时效过程中形成的沉淀变体的演变。随后，将分析各种微结构对传统形状记忆效应稳定性的影响。德国队将使用大电流和短脉冲来产生高磁场强度(高达40Koe)。这将允许系统地研究具有不同类型的沉淀物变体的微结构对磁场诱导的形状记忆效应的影响。这里的动机是了解在许多周期中提供最大可逆场致应变的微观结构条件。因此，将研究具有不同程度功能退化的样品的微观结构演变，以揭示相关的损伤机制。这也将提供数据，可用于以后开发一个有效的模型，允许在与实际服务相关的条件下进行寿命预测。

The objective of the research project proposed in cooperation with the Siberian Physical Technical Institute at Tomsk State University is a comprehensive analysis of the functional degradation in a CoNiGa magnetic shape memory alloy. Previous studies on a conventional NiTi shape memory alloy showed that aging of NiTi single crystals under stress can be used to obtain a microstructure that features only certain precipitate variants. This in turn resulted in a substantial improvement in functional fatigue behaviour. Preliminary work already demonstrated that such a tailoring of the microstructure is also feasible in CoNiGa single crystals. This material, however, can also show magnetic field-induced strains, and thus, a drastically higher power density could be realized than possible in conventional NiTi. With respect to the envisaged applications, cyclic stability of the microstructure is the key issue, which has, however, not been studied yet. The hypothesis of the proposed project is that - similar to the conventional NiTi system - a microstructure that features only certain types of precipitate variants, should demonstrate significantly improved functional degradation resistance. In the proposed research, the effect of precipitate variants on both the conventional as well as the magnetic field-induced shape memory effect will be studied. Firstly, the Russian project-partner will analyse the evolution of the precipitate variants that form during stress-assisted aging by transmission electron microscopy. Subsequently, the influence the various microstructures have on the stability of the conventional shape memory effect will be analysed. The German team will use high currents with short pulses to generate high magnetic field strengths (up to 40 kOe). This will allow for a systematic study of the effects that the microstructures with different types of precipitate variants have on the magnetic field-induced shape memory effect. The motivation here is to understand the microstructural conditions that provide for maximum reversible field-induced strains over many cycles. Therefore, the microstructural evolution will be studied in samples that feature different degrees of functional degradation in order to uncover the relevant damage mechanisms. This will also provide for data that can be used later on to develop a validated model that allows for life prediction under conditions relevant for actual service.