High-Number-Density Coherent Nanoprecipitates Induce Superelasticity in a Fe-Ni-Co-Al-Based Alloy

In this paper, the evolution of microstructure and mechanical properties in the Fe-28Ni-20Co-11.5Al-2.5Ta-0.05B (at.%) alloy were systematically studied. The findings elucidate that the absence of thermoelastic martensitic transformation following cyclic loading signifies the lack of superelasticity in solid solution state. Subsequent to the 72 h aging process at 600°C, the emergence of small (4 ± 2 nm) Ni₃Al precipitates with an L1₂ structure was observed within the grains. The degree of misfit between the precipitate and matrix was found to be only 0.28%. The precipitates contribute to the strength through ordering strengthening, ultimately increasing the strength of austenite. In the method this study introduces, there is neither a need for significant cold rolling deformation and prolonged annealing to form specific recrystallization texture nor preparation of large grain sized specimens equivalent to single crystals. The coherent relationship between the Ni₃Al phase and matrix induces the occurrence of thermoelastic martensitic transformation, which results in a 1.3% superelasticity.