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（Fe-based shape memory alloy, SMAs）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相反，fe基sma具有更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的拉压不对称性和方向依赖性的影响，但只有少数研究涉及功能性疲劳和γ′沉淀对功能特性的影响。因此，关于沉淀与功能（循环）性能之间的相互关系的全面知识在文献中仍然缺失。本项目旨在系统、全面地了解影响功能材料性能的因素。由于已知关键析出物参数控制着Fe-Ni-Co-Al基sma的功能特性，本项目侧重于确定有前途的析出物参数，提高fe基sma的性能。总体目标是设计一种具有核-壳特征的新沉淀物，即沉淀物的核和壳之间的化学梯度。因此，在第一步中，新型Fe-Ni-Co-Al-X (X=Ti, Nb) sma将用于系统地改变沉淀参数。通过不同晶体取向的拉伸和压缩试验（准静态和循环加载），将深入研究不同定制析出物对马氏体转变性质的影响。在第二步中，将推导出在五元合金体系中最有希望的沉淀参数，并引入准五元铁基SMA体系，即Fe-Ni-Co-Al（Nb-Ti）。目的是将具有核壳特征的沉淀物的思想应用于该SMA体系。在两步时效过程中，由于Nb和Ti的扩散系数不同，在不同的温度和时效时间下，具有核壳结构的析出物会被促进。准五元铁基SMA体系中，析出相周围应力场的系统变化以及析出相和基体的局部化学性质影响了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，设计具有可调整性能的析出物将变得可行，并允许改善功能特性，如提高转变温度、应变和改善循环稳定性。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异选择相互作用造成的。

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.