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The two-way shape memory effect with reversible compressive strain up to 5.1% and high cyclic stability is obtained for Ni44Fe19Ga27Co10 (at.%) single crystals by stress-induced martensite aging at 398 K for 1–3 h, under 650 MPa. 100 cycles of thermal treatment at temperatures not exceeding aging temperature, do not affect the temperature of the martensite formation, and reversible strain reduces merely by 0.4%. Thermal cycling with aging overheating significantly decreases reversible strain down to 1% due to the austenite stabilization. [ABSTRACT FROM AUTHOR]

The effect of plastic deformation during the preliminary superelasticity cycle and/or during the stress-induced martensite aging (SIM-aging) on the two-way shape memory effect (TWSME) and rubber-like behavior (RLB) in β- and (β+γ)-single crystals of Ni51Fe18Ga27Co4 alloy with thermoelastic L21(B2)–10M/14M–L10 martensitic transformations is studied. It is experimentally shown that the SIM-aging along the [110]B2-direction at T = 423 K for 1 hour under a compressive stress of 430 MPa (for β-crystals) and 400 MPa (for (β+γ)-crystals) induces large reversible strains in the stress-free cooling/heating cycles (TWSME) and loading/unloading cycles at T < As (RLB) along the [001]B2-direction which is perpendicular to the [110]B2-direction. The chemical stabilization of L10-martensite in the SIM-aged β-crystals leads to the appearance of TWSME with a reversible strain of εTWSME1 = +8.1% due to the oriented growth of martensite in the thermal cycles and RLB with a reversible strain of εRLB1 = –14.2% caused by the reorienting martensitic variants by the moving twin boundaries in compression. A contribution from the plastic deformation in the preliminary superelasticity cycle and/or in the SIM-aging process in (β+γ)-crystals leads to a combination of chemical and mechanical stabilization of L10-martensite. As a result, in the SIM-aged (β+γ)-crystals, firstly, the TWSME strain (εTWSME2 = +4.5%) is 1.8 times smaller than that in β-crystals. Secondly, the loading/unloading cycles of RLB with a reversible strain of εRLB2 = –13.2% are accompanied by the high critical stress values for reorientation of martensitic variants σcr2 = 55 MPa and dissipation energy ΔGirr2 = 924 J/m3, in contrast to β-crystals for which σcr1 = 12 MPa and ΔGirr1 = 456 J/m3. This is due to the contribution of mechanical stabilization of L10-martensite by the pinning of interphase and twin boundaries by dislocations in SIM-aged (β+γ)-crystals. [ABSTRACT FROM AUTHOR]

This work studies the effect of the stress-induced martensite aging at T = 423 K for 1 and 3 h under compressive stress |σ| = 520–590 MPa on the two-way shape memory effect, superelasticity, and elastocaloric effect in [001]L21-oriented Ni49Fe18Ga27Co6 ferromagnetic alloy single crystals containing dispersed γ(γ′)-phase particles. Experiments showed that stress-induced martensite aging causes the two-way shape memory effect, an increase in martensite start temperatures by 64–89 K, and a decrease in strains and stress hysteresis under superelasticity conditions during compression. As the stress-induced martensite aging time increases from 1 to 3 h, the reversible strain increases from 4.1 to 5.1% with the onset of the two-way shape memory effect, and the temperature ranges of forward and reverse martensitic transformation reduce to 1–2 K. It is shown for the first time that as a result of stress-induced martensite aging for 3 h under stress |σ| = 590 MPa the critical stresses of the martensite formation σMs decreases by a factor of 2.5 and stress hysteresis ∆σ decreases by a factor of 1.4 under superelasticity conditions compared to the initial crystals. This contributes to a higher efficiency and cyclic stability of the elastocaloric effect with adiabatic cooling ∆Tad = 5.0 ± 0.5 K in [001]L21-oriented Ni49Fe18Ga27Co6 single crystals after stress-induced martensite aging for 3 h. [ABSTRACT FROM AUTHOR]

In the present study, the effect of γ′-phase dispersed particles on both the L21(B2)-10M/14M-L10 martensitic transformations and the elastocaloric effect in aged Ni54Fe19Ga27 single crystals oriented along the [001]-direction was investigated. It was experimentally shown that aging strongly affects the elastocaloric properties of these crystals. The precipitation of semi-coherent γ′-phase particles up to 500 nm in size in the crystals aged at 773 K for 1 h leads to a 1.4 times increase in the operating temperature range of the elastocaloric effect up to ΔTSE = 270 K as compared with the initial as-grown crystals (ΔTSE = 197 K). The adiabatic cooling values ΔTad are similar for the as-grown crystals ΔTad = 10.9 (±0.5) K and crystals aged at 773 K ΔTad = 11.1 (±0.5) K. The crystals containing incoherent γ′-phase particles sized 5–35 μm (after aging at 1373 K for 0.5 h) possess an operating temperature range of ΔTSE = 255 K with slightly smaller adiabatic cooling ΔTad below 9.7 (±0.5) K. The aged [001]-oriented Ni54Fe19Ga27 single crystals demonstrate high cyclic stability: the number of cycles does not influence the adiabatic cooling values and parameters of loading/unloading curves regardless of the particle size. The ways to improve the elastocaloric cooling parameters and stability of the elastocaloric effect by means of dispersed particles in the NiFeGa ferromagnetic shape memory alloy were discussed. [ABSTRACT FROM AUTHOR]