Improvement of superelasticity by controlling NiAl precipitates in polycrystalline Fe-Ni-Co-Al-based alloy

Abstract

The superelastic effect (SE) in polycrystalline Fe-Ni-Co-Al-based alloys primarily arises from the growth and recovery of thermoelastic martensite. The martensitic transformation process is influenced by intragranular Ni₃Al nanoprecipitates and NiAl precipitates along grain boundaries. Highly coherent Ni₃Al plays a significant role in elastic accommodation during transformation process, whereas NiAl impedes the recovery of martensite. Low-energy grain boundaries (LEGBs) inhibit the nucleation of NiAl, and the greater the texture strength, the less likely for NiAl to accumulate at grain boundaries. This paper utilizes two methods to control the proportion and texture of LEGBs: cold rolling after hot rolling followed by rotary cold rolling (CT + RCR), and direct rotary cold rolling after hot rolling (RCR). The results show that the CT + RCR sample produces a RD∥〈100〉 texture, while the RCR sample obtains a {203}〈100〉 texture with a higher strength. The proportions of LEGBs for the CT + RCR and RCR are 24.8 % and 43.6 %, respectively. In the CT + RCR, the coarse NiAl precipitates at high-angle grain boundaries (HAGBs) restrict the recovery of most martensites, resulting in a SE of only 0.6 %. In the RCR, NiAl precipitates at HAGBs are smaller than those in the CT + RCR. Due to the combination of favorable texture orientation and a higher proportion of LEGBs, a large amount of stress-induced martensite can grow and recover during cyclic loading and unloading, achieving a 6 % SE.