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

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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.