Systematic investigation of abnormal grain growth induced by a cyclic heat treatment in Fe-Mn-Al-Ni-X (X = Cr, Ti) shape memory alloys

对于广泛使用的镍钛诺（Ni-Ti）形状记忆合金（SMA）而言，新的、具有成本效益的替代品的可用性对于SMA的新应用和市场的建立至关重要。成本密集型加工加上镍钛合金相对较高的材料成本是主要的障碍，特别是对于需要大量材料的应用，例如建筑用阻尼元件。铁基形状记忆合金是克服这些问题的有希望的候选者。可以使用钢铁行业的加工路线，材料成本相对较低。特别是新开发的Fe-Ni-Co-Al-X（X = Ta，Nb，Ti）和Fe-Mn-Al-Ni-X（X = Ti，Cr）形状记忆合金具有与Ni-Ti合金相似的相变应力和应变性能，因此具有很高的应用前景。晶粒形态和晶粒取向。特别地，Fe-Mn-Al-Ni-X中的伪弹性行为强烈地依赖于相对晶粒尺寸。晶粒尺寸必须超过样品的横截面，以促进良好的伪弹性，即必须存在寡晶微结构。循环热处理诱导反常晶粒生长（AGG）是一种新的形成低聚晶体组织的方法，与已知的促进二次再结晶的方法相比，循环热处理的优点包括加工过程中的尺寸稳定性和多次热处理循环的可重复性。虽然单一的重要因素，如在治疗过程中形成的子结构的大小已经确定，深入了解的基本机制和它们的相互作用，以及关键的影响因素的评价仍然缺乏公开的文献。此外，在我们对Fe-Mn-Al-Ni-X的初步研究中观察到化学成分对AGG的强烈影响。申请的研究项目旨在深入了解Fe-Mn-Al-Ni-X中循环热处理诱导的AGG。该过程的关键影响参数将是不同的，并不断演变的微观结构的特点。然后，将评估其对AGG的影响，从而确定关键参数。另一个重要方面是研究与四元合金相比，少量铬和钛对AGG的影响。最后，研究结果将分别在循环热处理和AGG过程中描述晶粒生长动力学的模型中实施。

Availability of new, cost-efficient alternatives to widely used Nitinol (Ni-Ti) shape memory alloy (SMA) is crucial for establishing new applications and markets for SMAs. Cost-intensive processing combined with the relatively high material costs for Ni-Ti are major roadblocks, particularly for applications demanding high material quantities, e.g. damping elements for constructions. Iron-based shape memory alloys are promising candidates to overcome these issues. Processing routes from steel industry can be used and material costs are relatively low. In particular, the newly developed Fe-Ni-Co-Al-X (X = Ta, Nb, Ti) and Fe-Mn-Al-Ni-X (X = Ti, Cr) SMAs are high potential candidates as they show SMA performance similar to Ni-Ti in terms of transformation stresses and strains.One of the challenges towards industrial application of both alloys is the establishment of suitable microstructures with respect to phase fractions, grain size, grain morphology and grain orientation. In particular, the pseudoelastic behavior in Fe-Mn-Al-Ni-X strongly depends on the relative grain size. The grain size has to exceed the cross section of the sample to promote good pseudoelasticity, i.e. an oligocrystalline microstructure has to be present. A new method to establish oligocrystalline microstructures is abnormal grain growth (AGG) induced by a cyclic heat treatment.Advantages of the cyclic heat treatment compared to well-known methods promoting secondary recrystallization include dimensional stability during processing and the repeatability over several heat treatment cycles. Although single important factors such as size of substructures formed in the course of treatment have already been identified, a deep understanding of the elementary mechanisms and their interplay as well as critical evaluation of influencing factors are still lacking in open literature. Moreover, a strong impact of the chemical composition on AGG was observed in our preliminary studies on Fe-Mn-Al-Ni-X.The research project applied for aims at a deep understanding of AGG induced by cyclic heat treatment in Fe-Mn-Al-Ni-X. Critical influencing parameters of the process will be varied and evolving microstructures characterized. Afterwards, their impact on AGG will be evaluated and thereby key parameters identified. A further important aspect is investigation of the effect of small amounts of Chromium and Titanium on AGG as compared to the quaternary alloy. Finally, findings will be implemented in a model describing grain growth kinetics in the course of cyclic heat treatment and AGG, respectively.