Acoustic emission screening of thermo-mechanically processed shape memory alloys in tension and compression – The role of irreversible processes in Fe-Ni-Co-Al-Ti-B SMA

与Ni-Ti-FGL相比，铁基形状记忆合金（SMA）由于材料成本低，制造条件大大简化，一段时间以来一直是人们关注的焦点。Fe-Ni-Co-Al-X （X = Ti, Ta）和Fe-Mn-Al-Ni合金中的纳米级析出物是实现形状记忆效应（SME）的必要条件。同时，它们也对最终的力学性能产生重大影响。对于这些合金体系的单晶，析出相、转变行为、力学性能和微观组织之间的关系已经很好地了解了。通过热-机械工艺链的应用，已经在多晶材料中实现了有前景的SME材料状态。然而，在这两种情况下，了解功能和结构疲劳机制的综合数据库仍然缺失。与相变相关的不可逆过程，如位错运动、不同马氏体变体的相互作用以及与纳米级析出相的相互作用，在这一背景下发挥了重要作用。表征相变动力学和潜在微观结构过程的一个极好的方法是在这些材料的力学测试过程中测量和评估声信号。该项目的目的是通过声发射分析来阐明不可逆过程在伪弹性和热诱导相变中的作用。采用Fe-Ni-Co-Al-Ti-B合金体系进行研究。首次利用声发射技术研究了机械载荷作用下的热弹性马氏体相变过程，以了解变形过程的动力学。声发射测量伴随着进一步补充的原位表征技术，如数字图像相关，热成像和x射线衍射，以及使用扫描和透射电子显微镜进行全面的微观结构研究。研究的起点是具有高度可逆性的不同取向的单晶。在这些状态下的声发射分析结果可以作为理解多晶材料状态过程的基准。通过热机械工艺（轧制、热处理）调整多晶态，使其具有<001>的强织构（具有预期的良好可逆性）或具有高度不可逆性的状态。该项目的长期目标是(i)在sma应用过程中通过声发射实时筛选来预测功能退化，（ii）预测适合sma不同应用场景的微结构。

Iron-based shape memory alloys (SMA) have been in the focus of interest for some time due to their low material costs and significantly simplified manufacturing conditions compared to Ni-Ti-FGL. Nano-scaled precipitates in the alloys Fe-Ni-Co-Al-X (X = Ti, Ta) and Fe-Mn-Al-Ni are indispensable for the realization of shape memory effects (SME). At the same time, they also have a significant impact on the resulting mechanical properties. For single crystals of these alloy systems, the relationship between precipitates, transformation behavior, mechanical properties and microstructure is already well understood. Through the application of thermo-mechanical process chains, promising material states for SME have already been achieved for polycrystals. However, in both cases a comprehensive database for understanding functional and structural fatigue mechanisms is still missing. Irreversible processes such as dislocation motion, interaction of different martensite variants and interactions with nano-scaled precipitates, which are associated with phase transformation, play a significant role in this context. An excellent method to characterize the kinetics of phase transformation and the underlying microstructural processes is the measurement and evaluation of acoustic signals during mechanical testing in these materials. The aim of the proposed project is to elucidate the role of irreversible processes for both pseudoelasticity and thermally-induced phase transformation by applying acoustic emission analysis. The alloy system Fe-Ni-Co-Al-Ti-B is used for the investigations. For the first time, acoustic emission is used to investigate the processes of thermoelastic martensitic phase transformation under mechanical loading in order to understand the kinetics of the ongoing deformation processes. Acoustic emission measurements are accompanied by further complementary in situ characterization techniques such as digital image correlation, thermography and X-ray diffraction but also by comprehensive microstructural investigations using scanning and transmission electron microscopy. Starting point of the investigations are differently oriented single crystals with a high degree of reversibility. The results of the acoustic emission analysis at these states serve as a benchmark for the understanding of the processes in polycrystalline material states. The polycrystalline states are adjusted by thermomechanical processes (rolling, heat treatment) in such a way that either a strong <001> texture for expected good reversibility or states with a high degree of irreversibility are present. The long-term goal of the project is (i) the prediction of functional degradation using real-time screening by means of acoustic emission during the application of SMAs and (ii) the prediction of suitable microstructures for different application scenarios of SMAs.