The chemical composition influences the microstructure, phase composition, martensitic transformation temperatures, and superelasticity of NiFeGaCo alloys. Ni47.5Fe16Ga26.5Co10 and Ni47.5Fe15Ga27.5Co10 alloys exhibited high martensitic transformation temperatures (M-s > 330 K). A [001]-oriented Ni47.5Fe16Ga26.5Co10 single crystal contained a large volume fraction of gamma-phase (15 %-20 %), which prevents full reversibility, and superelasticity was observed in a narrow temperature range (423-523 K) with a small reversible strain of 1.5 % and wide stress hysteresis (300 MPa). In contrast, the single-phase Ni47.5Fe15Ga27.5Co10 single crystal demonstrated high-temperature superelasticity over a wide temperature range (from 373 to 573 K and above) with a large reversible strain (up to 5.5 %), low critical stresses (from 37 MPa at 373 K), and a narrow stress hysteresis (from 25 MPa at 373 K).

The effect of a two-phase (B2+γ/γ′)-structure on the corrosion rate is studied in single crystals of the CoNiAl and CoNiAlFe ferromagnetic shape memory alloys. It is shown that the smaller the volume fraction of the secondary γ/γ′-phases, the lower the corrosion rate. It is found out that the Co35Ni35Al30 and Co35Ni35Al28Fe2 single crystals with small volume fractions of γ/γ′-phases have a lower corrosion rate, Ccorr = 1.20–1.86∙10−3 mm/year, compared to that of the Co40Ni33Al27 and Co39Ni32Al27Fe2 single crystals with a large volume fraction of the secondary phases (Ccorr = 1.83–2.98∙10−3 mm/year). This is achieved due to a short length of the interphase B2-γ/γ′-boundary and Fe alloying. The crystals under study demonstrate pitting and intercrystalline corrosion.

The orientation dependence of the two-way shape memory effect (TWSME) in stress-induced martensite aged Co35Ni35Al28Fe2 (B2 + γ') single crystals has been studied. Aging leads to the stabilization of the oriented L10-martensite, which helps to induce the TWSME. By orientation variation of the single crystal, one can stabilize the twinned or detwinned martensite, which determines the regularities of the TWSME manifestation. In the aged [001] crystals, the twinned L10-martensite variant is stabilized. The TWSME in the [001] crystals develops with a large (−2.4

The influence of microalloying with 0.3 at

For the first time Ni44Fe19Ga27Co10 single crystals with high-temperature martensitic transformation (Ms = 336 K) and high-temperature superelasticity (373-548 K) under tension and compression were obtained after annealing at 1448 K for 6 h, followed by fast quenching in ice-cold salt water. The increase in temperature is associated with the precipitation of the large gamma-particles and the substantial decrease in the precipitation of the nanosized w-phase, which was observed in the single crystals after annealing at 1448 K for 1 h with slower water quenching.

The effect of microstructure on stress-induced martensitic transformation and elastocaloric properties was investigated in as-cast and aged at 873 K for 1 h samples of polycrystalline (Ni51Fe18Ga27Co4)(99.7)B-0.3 alloy. This alloy has a columnar grain structure with preferential grain orientation along the [001](A)-direction. Microalloying with 0.3% boron leads to the formation of a multiphase structure consisting of austenite with a B2/L2(1)-structure, as well as borides and gamma-phase particles primarily located along grain boundaries. After aging at 873 K for 1 h, austenite with an ordered L2(1)-structure forms, and gamma '-phase particles (up to 400 nm in size) precipitate inside the grains. The preferential orientation of grains along the [001](A)-direction and the multiphase structure formed by boron microalloying result in high strength with significant plasticity. The as-cast and aged alloys demonstrate the elastocaloric effect with stable adiabatic temperature change, Delta T-ad, over a wide temperature range: Delta T-EC = 125 K and Delta T-EC = 90 K, respectively. The Delta T-ad reaches (9.4 +/- 0.5) K in the as-cast alloy and (8.3 +/- 0.5) K in the aged alloy and corresponds to the experimental values of Delta T-ad = 9-11 K for [001](A)-oriented NiFeGa(Co) single crystals. This enhancement of the elastocaloric properties by creating a multiphase microstructure with defined crystallographic texture of polycrystalline NiFeGa(Co, B) alloys provides important insights into the design of energy-efficient solid-state refrigeration materials.

In this work, the regularities and physical causes of the orientation dependence of elastocaloric effect, strength properties of L 10-martensite, and superelasticity parameters are installed in [001]A-, [011]A- and [334]A-oriented single crystals of Ni54Fe19Ga27 alloy. It is shown that high strength properties of L 10-martensite (dislocation yield strength σ0.1M > 1000 MPa) contribute to manifestation of the maximum value of adiabatic cooling ΔTad = (10.3±0.5) K in [001]A-oriented single crystals and ΔTad = (9.3±0.5) K in polycrystals of (Ni54Fe19Ga27)99.7B0.3 alloy with a sharp texture along the [001]A-direction.

The effect of alloying the Ni49.5Ti35.5Hf15 polycrystals with up to 15 at.

This study reveals the impact of the formation mechanism of a two-phase (β + γ) structure during heat treatment on thermoelastic L21(B2)-10M/14M-L10 martensitic transformations and the elastocaloric effect parameters in Ni54Fe19Ga27 alloy polycrystals. It is shown experimentally that annealing of the initial Ni54Fe19Ga27 alloy in the temperature range 1173-1463 K for 0.5 h followed by water quenching leads to the precipitation of the γ phase at the boundaries and in the bulk of grains. As the annealing temperature increases from 1173 to 1463 K, the thickness of the γ-phase layer at the grain boundaries doubles, the particles within the grains enlarge, and their volume distribution becomes inhomogeneous. Simultaneously, the characteristic martensitic transformation temperatures increase by 31-69 K. The inhomogeneous distribution of γ-phase particles and the morphological features of martensite (twin martensite structure refinement) lead to a 5-6-fold increase in the martensitic transformation intervals in crystals after annealing at 1448 K compared to the initial state. After cyclic superelastic tests with 20 to 100 loading/unloading cycles, two-phase (β + γ) polycrystals demonstrate a stable adiabatic cooling value ∆Tad (2.7-3.0 K) and do not crack along grain boundaries, unlike the initial state. Significant fatigue strength together with high efficiency (COP (coefficient of performance) up to 18.3) make Ni54Fe19Ga27 (β + γ) polycrystals promising for practical use in solid-state cooling.

The deformation behavior, yield strength temperature dependence, and plasticity of the high-temperature L21(B2)-phase 1 (B2)-phase during compression in the temperature range 773-1173 K for Ni54Fe19Ga27 54 Fe 19 Ga 27 and (Ni54Fe19- 54 Fe 19- Ga 27 ) 99.7 B 0.3 shape memory alloys was determined for the first time. The studied alloys were characterized by high plasticity (over 70 %) and minimum yield strength level (13-65 MPa) at temperatures from 973 to 1173 K. Possible mechanisms of yield strength decrease, plasticity increase, and microstructure formation during the deformation process depending on test temperature and chemical composition are discussed. The possibility of hot rolling of Ni54Fe19Ga27 54 Fe 19 Ga 27 alloys at elevated temperatures was demonstrated.

Effect of microalloying by boron, added by two different methods, on the microstructure and martensitic transformation in as-cast and aged Ni54Fe19Ga27 alloys was studied. The increase of the aging time of the Ni54Fe19Ga27 alloy from 0.5 to 10 h at 823 K leads to the formation of a large volume fraction of the ordered γ′-phase. Microalloying with boron inhibits the precipitation of the γ(γ')-phase and prevents its ordering. The parameters of martensitic transformation are most stable with increasing aging time of a (Ni54Fe19Ga27)99.7B0.3 alloy when boron is added in the form of a NiB alloy.

The paper studies the alloying effect of the quenched Ni-low (Ni49.5Ti35.5Hf15)100–XNbX and Ni-rich (Ni50.3Ti34.7Hf15)100–XNbX alloys (X = 0, 15 at.

The microstructure and temperature of martensitic transformation of Ni44Fe19Ga27Co10 single crystals after aging at temperatures from 623 K to 1173 K were studied by electron microscopy and differential scanning calorimetry. The temperature ranges of the second phase precipitation, their lattice structure and volume fraction, and also the modification of the nanodomain structure of the L2(1)+B2 high-temperature phase were determined in dependence on aging temperature. The influence of microstructure parameters on the martensitic transformation temperatures, transformation intervals and thermal hysteresis has been discussed.

The deformation behavior, yield strength temperature dependence, and plasticity of the high-temperature L21(B2)-phase during compression in the temperature range 773–1173K for Ni54Fe19Ga27 and (Ni54Fe19Ga27)99.7B0.3 shape memory alloys was determined for the first time. The studied alloys were characterized by high plasticity (over 70%) and minimum yield strength level (13–65MPa) at temperatures from 973 to 1173K. Possible mechanisms of yield strength decrease, plasticity increase, and microstructure formation during the deformation process depending on test temperature and chemical composition are discussed. The possibility of hot rolling of Ni54Fe19Ga27 alloys at elevated temperatures was demonstrated.

The article reports for the first time the effects of Ti3Ni4 particle variants on the orientation dependence of the yield strength of the B2 phase on Ti-51.5at.

The influence of the crystallographic variants of Ti3Ni4 particles on the cyclic stability of the high-temperature superelasticity of < 0 0 1 > -oriented Ti-51.5at.%Ni single crystals after stress-free and stress-assisted aging at 823 K for 1 h was investigated for the first time. After stress-assisted aging, the single crystals exhibited low cyclic stability of superelasticity (irreversible strain was 5.2 %, and critical stresses decreased by 14 %). After cycling, single dislocations and {1 1 5}(B2) twins were observed, as a consequence of B2-B19 '-B2 martensitic transformations. After stress-free aging, single crystals showed high cyclic stability: critical stresses were practically unchanged, and the irreversible strain did not exceed 0.2 %. No defects were found after cyclic testing; the stress-free aged crystals had the high strength properties of the B2-phase (2100 MPa) and B19 '-martensite (>2700 MPa), in contrast to the stress-assisted aged single crystals, which had the strength properties of the B2-phase, 1630 MPa, and B19 '-martensite, 1650 MPa.

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 Ms>As. The regularities of the stress-induced B2-L10 martensitic transformation change due to an increase in the contribution of the dissipated energy and Msσsσ. 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 the elastocaloric effect and the operating parameters (adiabatic cooling value Delta T-ad, coefficient of performance, operating temperature range) in Ni50.8Ti49.2 and Ni51.5Ti48.5 single crystals oriented along the < 001 >-direction and containing dispersed Ti3Ni4 particles of different sizes were investigated, and the mechanisms of cyclic degradation of the elastocaloric effect were determined. Aged Ni50.8Ti49.2 single crystals containing nanosized Ti3Ni4 particles were shown to possess the optimal combination of operational properties for solid-state cooling. These crystals are characterized by high adiabatic temperature change Delta T-ad of 16.8-21.4 K over a wide temperature range above 160 K, the highest elastocaloric effect cyclic stability, and high coefficient of performance values up to 27.8. Ni50.8Ti49.2 single crystals with semi-coherent particles (400 nm in size) demonstrate maximum values of Delta T-ad = 25.3 K; but, they are not feasible for practical applications because of cyclic degradation caused by the formation of residual martensite and dislocations near large particles as well as low coefficient of performance up to 12.7. The use of Ni-rich Ni51.5Ti48.5 crystals can improve the operating characteristics of crystals with semi-coherent Ti3Ni4 particles and achieve high cyclic stability of the elastocaloric effect by strengthening the crystals through an increase in the volume fraction of particles and a decrease in the distance between them.

The results of the research show that microalloying of polycrystals of the Ni54Fe19Ga27 alloy with boron (0.3 at.%) leads to a 2.5 fold decrease in the average grain size, the precipitation of dispersed particles, and the suppression of grain-boundary cracking during the stress-induced thermoelastic martensitic transformations. Over 100 loading/unloading cycles, the boron-doped (Ni54Fe19Ga27)99.7B0.3 alloy demonstrates high cyclic stability of the elastocaloric effect with a value up to 5.1 K in contrast to the samples of the boron-free Ni54Fe19Ga27 alloy, which crack along grain boundaries after 12 cycles.

The article reports for the first time the effects of Ti3Ni4 particle variants on the orientation dependence of the yield strength of the B2 phase on Ti-51.5at.%Ni single crystals under compression after stress-free and stress-assisted aging at 823 K for 1 h. Understanding the yield strength of the B2 phase is crucial because it determines the superelasticity interval, stress hysteresis, and cyclic stability. A decrease in the number of particle variants from four to one as a result of stress-assisted aging weakened the orientation dependence of the yield strength of the B2 phase. For four particle variants, the sigma(cr)(M-d) differed by 60% in the <001> and <111> orientations (2100 and 1300 MPa). By contrast, for one particle variant, the sigma(cr)(M-d) differed by only 20% (1630 and 1350 MPa). Transmission electron microscopy showed that the number of particle variants did not affect the yield strength of the B2 phase in the case of the {001} <001> and {011} <001> dislocation slip observed in the <111> orientation. However, the number of particle variants strongly influenced the yield strength of the B2 phase in the case of {114}(B2) mechanical twinning, observed in the <001> orientation because of the different arrangement of the twinning planes relative to one or four particle variants and the long-range internal compressive stress formed one particle variant.