Microstructural adjustments of superelastic iron based shape memory alloys for structural applications

Iron based shape memory alloys attract a lot of attention since they are promising alloys for structural applications like damping elements for bridges and skyscrapers. Especially, Fe-Mn-Al-Ni is in focus of current research since it shows superelastic properties which are comparable to Ni-Ti and it possess a low temperature dependence of the critical stress for martensitic transformation between -196°C and about 150°C. Other advantages are the availability of relatively simple processing routes from steel industry and the low material costs. However, the alloy suffers from low recoverability if the grains do not completely cover the sample cross-section. To overcome this issue, we show that small amounts of titanium are suitable to enhance abnormal grain growth in this system, whereas small amounts of chromium lead to a strong inhibition of abnormal grain growth. Due to the findings regarding the elementary mechanisms of this kind of abnormal grain growth, it is possible to produce single crystalline Fe-Mn-Al-Ni-Ti bars with a length of more than 200 mm. A comprehensive characterization of the superelastic properties reveals the high potential of this alloy system for future applications in structural engineering.