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

自从铁基形状记忆合金（SMA）被发现以来，人们一直在努力提高其功能性能。与传统的Ni-Ti合金相比，铁基SMA具有较低的合金元素成本和加工成本，这使得它们对许多行业具有吸引力。虽然许多研究集中在准静态测试中拉伸-压缩不对称性和方向依赖性的影响，但只有少数研究涉及功能疲劳和γ沉淀物对功能特性的影响。因此，沉淀物和功能（循环）性能之间的相互关系的全面知识仍然是literature.This项目的目的是在获得一个系统和全面的了解的因素影响功能材料的性能。由于它是已知的，关键的沉淀参数支配的功能特性在Fe-Ni-Co-Al基SMA本项目的重点是一种方法，确定有前途的沉淀参数，提高性能的铁基SMA。总体目标是设计一种新的具有核-壳特征的沉淀物族，即沉淀物的核和壳之间的化学梯度。因此，在第一步中，将使用新型Fe-Ni-Co-Al-X（X=Ti，Nb）SMA来系统地改变沉淀物参数。使用拉伸和压缩试验（准静态和循环加载）在不同的晶体取向的不同定制的沉淀物的马氏体相变的性质的影响将进行深入研究。在第二步中，将推导出五元合金体系中最有希望的析出参数，并将引入准五元铁基形状记忆合金体系，即Fe-Ni-Co-Al（Nb-Ti）。其目的是将沉淀物表现出核-壳特性的想法应用于该SMA系统。在两步时效方案之后，由于Nb和Ti的扩散系数不同，具有核-壳结构的沉淀物将在不同的温度制度和时效时间中得到促进。通过沉淀物周围的应力场以及沉淀物和基体两者的局部化学的系统变化，使用准五元Fe基SMA系统将影响用于马氏体相变的自由吉布斯能的机械和非机械驱动力。使用这种方法，具有调整的性质的沉淀物的设计将变得可行，并允许功能性质的改善，如转变温度的增加，应变和循环稳定性的改善。Fe-Ni-Co-Al-（Nb-Ti）SMA中析出相尺寸的进一步减小也将导致SMA中新颖的和前所未有的转变机制，包括不同的孪生模式和形状记忆效应，这是由纳米级析出相和相关的马氏体变体选择的相互作用引起的。

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.