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The research paper presents a study of the effect of stress-induced martensite aging along the [001]A-deformation axis of the sample on both elastocaloric cooling capacity and superelasticity in Ni54Fe19Ga27 single crystals in compression. It has been experimentally shown that L10-martensite stabilization after the stress-induced martensite aging enhances the superelasticity parameters for the elastocaloric performance in the studied single crystals. In the stress-induced martensite aged Ni54Fe19Ga27 single crystals, the stress level of martensite formation σMs and stress hysteresis Δσ decrease by 130 MPa and by 16–17 MPa, respectively, compared with the as-grown crystals. Therefore, the stress-induced martensite aging increases the material efficiency for solid-state cooling systems: specific adiabatic temperature change per unit stress ΔTad/σMs increases by 4.4 times (ΔTad/σMs = 291.9 K/GPa (at Т = 348 K)) and coefficient of performance reaches COP = 24.5 in stress-induced martensite aged crystals as compared with the as-grown crystals (ΔTad/σMs = 62.4 K/GPa (at Т = 348 K), COP = 21.7). Smaller stress hysteresis corresponds to less energy dissipation in an operating cycle, which is also certainly useful for optimizing elastocaloric properties of a material. Moreover, both as-grown and stress-induced martensite aged crystals demonstrate high cyclic stability during loading/unloading cycles and weak temperature dependence of the elastocaloric cooling capacity ΔТad = 10.3–11.0 K in a wide operating temperature range up to 145–197 K. Thus, stress-induced martensite aged Ni54Fe19Ga27 single crystals oriented along the [001]А- direction are expected to be promising materials for elastocaloric application.

In the present study, a two-step thermo-mechanical processing consisting of cold work and heat treatment steps was performed to increase the operating temperature of 316 austenitic stainless steels. A hierarchical microstructure of thermally-stable, nano twin bands was achieved forming into bundles in elongated grains. The mechanical response of the samples with this microstructure was evaluated through uniaxial tension tests at temperatures ranging from 20 °C to 500 °C and compared with those from the fully annealed samples. The results demonstrate that such hierarchical microstructure leads to a significant increase in the elevated temperature yield strengths due to the presence of nano-twin boundaries and resulting decrease in dislocation mean free path and increase in dislocation storage capacity. In fact, the yield strength ratio of the twinned and annealed samples increases with increasing temperature up to 500 °C, indicating the effectiveness of pre-existing thermally-stable twin boundaries as the strengthening source at temperatures as high as 0.46 homologous temperature. The hierarchical microstructure also led to irregular serrations through dynamic strain aging in the stress-strain response at 500 °C, which is attributed to the bi-modal microstructural length-scales present in the structure affecting the diffusion distances during dynamic strain aging. This structure also increases the tensile strength, and without a total loss in ductility, even though the flow stress of the twinned samples surpasses the tensile strength of the annealed samples, especially at elevated temperatures. Total hardening rate is consistently higher in the twinned samples as compared to the annealed samples, indicating the positive role of nano-twin boundaries in the dislocation storage capacity at elevated temperatures. Overall, the present study clearly demonstrate the positive role of thermally stable nano-twins on the elevated temperature mechanical response of austenitic stainless steels.