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

铁基形状记忆合金（SMA）由于其低材料成本和与Ni-Ti-FGL相比显著简化的制造条件而一段时间以来一直是关注的焦点。Fe-Ni-Co-Al-X（X = Ti，Ta）和Fe-Mn-Al-Ni合金中的纳米析出相是实现形状记忆效应（SME）所必需的。同时，它们也对所得的机械性能产生重大影响。对于这些合金系统的单晶，沉淀物、相变行为、机械性能和微观结构之间的关系已经很好地理解。通过热机械工艺链的应用，多晶体已经实现了SME的有前途的材料状态。然而，在这两种情况下，用于理解功能和结构疲劳机制的综合数据库仍然缺失。不可逆的过程，如位错运动，不同的马氏体变体的相互作用和与纳米级沉淀物的相互作用，这与相变，在这方面发挥了重要作用。一个很好的方法来表征相变动力学和潜在的微观结构过程是在这些材料的机械测试过程中的声学信号的测量和评估。该项目的目的是通过应用声发射分析来阐明伪弹性和热致相变的不可逆过程的作用。合金系Fe-Ni-Co-Al-Ti-B。声发射是第一次被用来调查的热弹性马氏体相变过程中的机械载荷，以了解正在进行的变形过程的动力学。声发射测量伴随着进一步的补充原位表征技术，如数字图像相关，热成像和X射线衍射，但也通过全面的微观结构调查，使用扫描和透射电子显微镜。研究的出发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的结果作为基准的多晶材料状态的过程的理解。通过热机械过程（轧制、热处理）以这样的方式调节多晶状态，使得<001>存在用于预期良好可逆性的强织构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进行实时筛选，预测功能退化;（ii）预测SMA不同应用场景的合适微结构。

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.