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.

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.

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.

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.

The two-way shape memory effect with reversible compressive strain up to 5.1

The effect of stress-induced martensite aging on the microstructure, viscoelastic properties, and one-way and two-way shape memory effect in high-temperature Ni50.2Ti37.3Hf12.5 aged polycrystals in tension was investigated. Stress-induced martensite aging for 6 h resulted in significant changes in the viscoelastic properties of the polycrystals. The elastic module of the martensite and austenite increased (by a factor of two), and a large softening of the elastic module and the growth of internal friction by 30% during MT was observed. Aging in martensite leads to the induction of a two-way shape memory effect with a strain of 1.7%, whose degradation is related to the microstructural change. It is established that both the value and the stability of the high-temperature two-way shape memory effect are defined by the time of the stress-induced martensite aging and the maximum value and stability were observed at an aging time of 6 h.

This paper deals with the effect of crystal orientation on the B19’-martensite reorientation stress and yield stress in compression in TiNi single crystals with different Ni contents varying from 50.4 to 51.2 at.%. It was experimentally shown that the martensite yield stress appears to be higher for the [111]B2-oriented single crystals than for the [001]B2-oriented single crystals regardless of Ni content. The difference between martensite yield stress for the two investigated orientations increases with the growth of Ni content. The maximum difference between martensite yield stress σcrM for two investigated orientations is 996 MPa at Ni content of 51.2 at.% (σcrM = 1023 MPa for the [001]B2-orientation and σcrM = 2019 MPa for the [111]B2-orientation). As a result of comparison with the B2-austenite yield stress, it was found that this is not an ordinary case. The [001]B2 orientation is a high-strength in B2-austenite and a low-strength in B19’-martensite. It was experimentally shown that the B19’-martensite reorientation stresses weakly depend on the orientation and chemical composition compared with the martensite yield stress. The reasons for the orientation dependence of the martensite yield stress in compression and the deformation mechanisms of B19’-martensite are discussed.

The study of the influence of the cobalt content on the cyclic stability of superelasticity (SE) was carried out in quenched Ni 44 Fe 19 Ga 27 Co 10 and Ni 39 Fe 19 Ga 27 Co 15 (at.%) single crystals under compression. It is shown that an increase in the cobalt content leads to embrittlement of the material and a decrease in the cyclic stability of SE. In Ni 44 Fe 19 Ga 27 Co 10 single crystals, during the first 20 loading/unloading cycles, the elastic energy relaxation occurs along with the formation of dislocations and residual martensite, which leads to a decrease in critical stress of martensite formation and in stress hysteresis. During the next 80 cycles, SE becomes more stable. Stabilization is accompanied by a slight change in the parameters. On the contrary, Ni 39 Fe 19 Ga 27 Co 15 single crystals are characterized by high-strength characteristics, which lead to high SE stability during the first 20 loading/unloading cycles. However, after 20 cycles, a strong degradation of the SE is observed through the formation of microcracks, which ultimately leads to the destruction of the sample. The results of work are replicable for cycling at different temperatures from all temperature ranges of superelasticity.

The effect of aging (at 773 K for 1 h) on the cyclic stability of superelasticity was studied in preliminarily quenched (after annealing at 1448 K for 1 h) Ni44Fe19Ga27Co10 [001]-oriented single crystals. It was shown that NiFeGaCo single crystals (both quenched and aged) exhibited highly stable superelasticity during more than 105 loading/unloading cycles in compression without destruction. The degradation of the superelasticity curves was divided into stages, and each stage of degradation was carefully considered. The precipitation of ω-phase particles led to a change in the degradation mechanism of superelasticity in the aged crystals. The main degradation mechanism for quenched crystals is a formation of uniform distribution of dislocations; for aged crystals, the microstructural degradation mechanisms consist of a non-uniform distribution of dislocations around the particles and the appearance of residual martensite.

The shape memory effect (SME) during stress-assisted thermal cycles under compressive load in [001]-oriented Ti-50.7at.

The shape memory effect (SME) during stress-assisted thermal cycles under compressive load in [001]-oriented Ti-50.7at. M_s^0 ) for the formation of B19′-martensite, different strain (εrev), different dependences of the interval of forward MT ( Δ_1^ ) and thermal hysteresis ( Δ T_1^σ=A_f^- M_s^ and Δ T_2^σ =A_s^- M_f^ ) on applied stresses, and changes in the morphology of martensite crystals. Practically, these differences do not affect the stresses (σmin and σmax) required to achieve the minimum strain and maximum reversible strain (εrev) and strain growth coefficient (dεrev/dσ). The influence of aging on the dependence of the SME parameters on the chemical composition was analysed in comparison with quenched crystals.

In the present paper, the cyclic stability of the high-temperature two-way shape memory effect was studied in high-strength Ni50.3Ti32.2Hf17.5 polycrystals after various thermomechanical treatments—training (thermocycling under stress) and stress-induced martensite aging. The effect of training and stress-induced martensite aging on the microstructure, the two-way shape memory effect, and its cyclic stability was determined. It was found out that both thermomechanical treatments induce the high-temperature two-way shape memory effect at T > 373 K, with a strain of 1.5% in tension. The influence of cyclic tests (up to 100 stress-free cycles of cooling/heating) on the two-way shape memory effect strain, the transformation temperatures, and the microstructure was established. Different degradation mechanisms of the two-way shape memory effect were established after thermocycling and stress-induced martensite aging.

In this work, the martensitic transformation (MT), shape memory effect (SME) and superelasticity (SE) on [001]-oriented single crystals of Ni44Fe19Ga27Co10, Ni39Fe19Ga27Co15 and Ni34Fe19Ga27Co20 (at %) alloys were investigated. The cobalt content determined any observation of thermal-induced thermoelastic B2-L10 MT and the level of minimum critical stresses necessary to form L10-martensite. Ni44Fe19Ga27Co10 single crystals demonstrated thermal-induced B2-L10 MT, low resistance of the B2-matrix to stress-induced B2-L10 MT, which was observed at 25 MPa, a wide SE temperature range from 263 to 473 K and narrow stress hysteresis of 20–50 MPa. An increase in the cobalt content to 15 at % led to the strain glass transition upon cooling and heating instead of B2-L10 MT and an increase in the critical stresses of L10-martensite formation up to 300 MPa. In Ni39Fe19Ga27Co15 single crystals SE was observed up to 350 K with a narrow stress hysteresis of 20–50 MPa. In Ni34Fe19Ga27Co20 single crystals, no thermal-induced or stress-induced MTs were found and in the temperature interval from 200 to 500 K only plastic deformation of the B2 phase occurred.

Heat treatments of single crystals of Ni44Fe19Ga27Co10 (at.%) shape memory alloys cause various microstructures of the high-temperature phase. The nanodomain structure, consisting of regions of the L21- and B2-phases, and nanosized particles are the main parameters that change during heat treatments and determine the mechanism of nucleation and growth of martensite crystals, the size of thermal-induced martensite lamellae, the temperature Ms, and the temperature intervals of the martensitic transformation. In the as-grown single crystals, the high-temperature phase has only the L21-structure and the MT occurs at low (Ms = 125 K) temperatures due to the motion of the practically single interphase boundary in narrow temperature ranges of 3–7 K. The reduction in the volume fraction of the L21-phase to 40% and the formation of nanodomains (20–50 nm) of the L21-and B2-phases due to annealing at 1448 K for 1 h with quenching causes an increase in the MT temperatures by 80 K. The MT occurs in wide temperature ranges of 40–45 K because of multiple nucleation of individual large (300–500 µm) martensite lamellae and their growth. After aging at 773 K for 1 h, the precipitation of nanosized particles of the ω-phase in such a structure additionally increases the MT temperatures by 45 K. The MT occurs due to the multiple nucleation of packets of small (20–50 μm) martensite lamellae.