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 ΔT_SE = 270 K as compared with the initial as-grown crystals (ΔT_SE = 197 K). The adiabatic cooling values ΔT_ad are similar for the as-grown crystals ΔT_ad = 10.9 (±0.5) K and crystals aged at 773 K ΔT_ad = 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 ΔT_SE = 255 K with slightly smaller adiabatic cooling ΔT_ad 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.

This paper deals with the martensitic transformation and functional properties in the quenched single crystals of the Co₃₅Ni₃₅Al₂₈Fe₂ medium-entropy alloy, oriented along the [001]_B2-direction. The microstructure and chemical composition of the single crystals have been studied in detail using transmission and scanning electron microscopy. The {111}_L10 martensite twins up to 10-20 nm width and γ/γ′-phase precipitations larger than 100 μm are detected. The thermoelastic B2-L1₀ martensitic transformation upon stress-free cooling/heating in single crystals of Co₃₅Ni₃₅Al₂₈Fe₂ alloy is characterized by the accumulation of elastic energy, which is the driving force of the reverse martensitic transformation, and the low dissipation energy. The reverse transformation starts at lower temperatures than the forward transformation M_s>A_s. The regularities of the stress-induced B2-L1₀ martensitic transformation change due to an increase in the contribution of the dissipated energy and M_s^σ<A_s^σ. There is shape memory effect with the reversible strain (3.2±0.3)% and high temperature superelasticity with the reversible strain (3.3±0.3)% in the temperature range from 323 K to ≥548 K in the [001]_B2-oriented single crystals. These crystals withstand stress up to 1200 MPa in compression without destruction.

The cyclic stability of superelasticity in compression in [001]_B2-oriented Ti_49.3Ni_50.7 single crystals is considered in this paper. The crystals were aged at 823 K for 1.0 h in air and helium. It has been experimentally shown that a two-layered surface thin film, consisting of a Ni-free oxide layer and a Ni-rich sublayer, appears after the oxidation at 823 K in air. The surface layers have a weak effect on the forward B2-R-B19’ martensitic transformation temperatures: T_R temperature increases by 4 K; M_s and M_f temperatures decrease by 6 K. The oxide layer does not affect either the superelasticity response during fatigue tests or the temperatures of reverse B19’-B2 martensitic transformation. The cracking of the surface oxide layer during fatigue tests was not found in [001]_B2-oriented single crystals aged in air. This is contributed by the relaxation of internal stresses. Such internal stresses are caused by both the formation of an oxide layer during aging and the matrix deformation at the stress-induced martensitic transformation. The main relaxation mechanisms of the internal stresses are the oriented growth of Ti₃Ni₄ precipitation near a thin surface film at aging in air, the formation of dislocations near the precipitation-matrix interface and a fine twinned B19’-martensite at fatigue tests.