Abstract: Fe-Mn-Al-Ni based shape memory alloys (SMAs) are able to exhibit superelasticity in a wide range of temperatures, and their superelastic stresses are extremely low temperature dependent, which holds potential applications in inclement fields such as deep space. However, the difficulty in obtaining large single crystals and the shortcoming of natural aging in this alloy system are hindering their practical application. Based on this, an ultra-large Fe-Mn-Al-Ni-Mo single-crystal SMA with nearly zero natural aging effect was successfully fabricated in this work via bifunctional Mo-segregation engineering. Firstly, at elevated temperatures, the grain boundary segregation of Mo atoms in this alloy effectively facilitated the abnormal grain growth during cyclic heat treatment. Based on this, a large-scale Fe-Mn-Al-Ni-Mo single-crystal bar with a diameter of approximately 15.5 mm and a length of approximately 95 mm was obtained. Meanwhile, the Mo atoms with a low diffusion coefficient effectively hindered the coarsening of coherent B2 nanoprecipitates during natural aging. This led to the Fe-Mn-Al-Ni-Mo single crystals close to [001] orientation to exhibit a huge superelastic strain of 8.5% even after 1.5 years of natural aging. In contrast, B2 nanoprecipitates in the Mo-free Fe-Mn-Al-Ni SMA grew from ∼7.7 to ∼10.1 nm after natural aging for 1.5 years. This study provides a unique insight into the development of high-performance functional alloys using elemental segregation engineering.

Key words: Shape memory alloy, Single crystal, Natural aging effect, B2 nanoprecipitates, Superelasticity