The dependence of the twinned structure of the stabilized L1(0)-martensite variant in Co35Ni35Al28Fe2 single crystals on two-way shape memory effect (TWSME) anisotropy and its work output was investigated. Martensite stabilization was by stress-induced martensite aging at 423 K for 30 min under a compressive stress of 400-500 MPa applied along selected [001](B2)- and < 110 >(B2)-directions. Stabilization of the twinned martensite variant under compression along the [001](B2)-direction resulted in TWSME strain of -3.1% along [001](B2)-direction and + 1.4% along two < 110 >(B2)-directions. In contrast, stabilization of the detwinned martensite variant under compression along the < 110 >(B2)-directions led to excellent TWSME with a high reversible strain of + 6.0% along the [001](B2)-direction and -2.9% along two < 110 >(B2)-directions. Moreover, a thermally-induced B2-L1(0) martensitic transformation after the detwinned martensite stabilization was characterized by narrow temperature intervals for forward and reverse transformations (<18 K) and a maximum TWSME work output of 0.14 J/g, in comparison to 0.014 J/g for twinned martensite stabilization. TWSME anisotropy is a manifestation of the stabilization of martensite variants with different contributions of detwinning, determined by the choice of the stress-induced martensite aging axis.

In the present study, alloying with 2.5 at.% Nb and stress-induced martensite aging (SIM-aging), involving stabilization of stress-oriented martensite was applied to NiTiHf-based shape memory alloy to improve its functional properties. Nb was found to be predominantly segregated along grain boundaries in the as-cast condition, whereas no detectable β-Nb phase was identified by X-ray diffraction in Ni50.3Ti32.2Hf15Nb2.5 alloy. The SIM-aging performed under tensile/compressive stresses of 300/400 MPa at 323K for 6h induced a pronounced two-way shape memory effect with reversible strains of +2.6% and –1.1%, respectively. SIM-aging substantially modified the viscoelastic behavior of the alloy, which is associated with the formation of a different martensitic morphology during subsequent stress-free thermal cycling.

The martensitic transformation in shape memory alloys involves the formation of twinned martensite regions at the austenite-martensite phase boundaries. The twin boundary energy plays a significant role in determining the twinned microstructure, as well as the hysteresis and kinetics of the transformation. We present a systematic microscopy-based approach to analyze the microstructure of twinned martensite plates, enabling the identification of the twin system and extraction of the energies of twin boundaries and martensite plates. The method is applied to the abundant ⟨ 011 ⟩ type II twin boundary in the B-II-1 system in NiTi. The obtained knowledge paves the route for modeling the macroscale evolution of the phase transformation based on material properties measured at the nanoscale.

The effect of cyclic loading/unloading on high-temperature superelasticity and the (B2 + gamma/gamma')-microstructure was studied in quenched Co35Ni35Al28Fe2 single crystals oriented along the [001]B2-direction. They displayed excellent superelastic response in compression over 100 loading/unloading cycles at 423 K. This superelasticity response is attributed to the high-strength crystallographic [001]B2 orientation of the B2-matrix and strengthening of the gamma-phase due to nanosized gamma'-particles. The stress-induced B2-L10 martensitic transformation during loading/unloading cycles was accompanied by plastic deformation via twinning of the secondary gamma/gamma'-phase and dislocation accumulations in B2-matrix around the gamma/gamma'-phase. This results in a reduction in the critical stress for the martensite formation, stress hysteresis and reversible strain during cycling.

The paper reports the temperature dependence of the reversible reorientation of martensite variants during loading/unloading along the [001]B2||[001]L10-direction in compression in stress-induced martensite (SIM) aged at 423K for 0.5h under a compressive stress of 500MPa applied along the [110]B2||[100]L10-direction Co35Ni35Al30 single crystals. A wide temperature range of reversible strain from 203K to 413K (210K) is observed due to the stabilisation of the tetragonal L10-martensite variant V1 ([001]B2||[001]L10) during SIM-aging in these single crystals. The reversible strain in the range of 157K (203KAf=360K occurred due to the stress-induced B2-L10(V2/V3) martensitic transformation (MT). Rubber-like behaviour develops in the range of 52K (203K

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.

Some results concerning the hydrogen effect at electrolytic saturation at a current density of j = 1500 and 3500 A/m2 for 3 h at room temperature on the temperature dependence of the yield stress σ0.1(T) and the shape memory effect (SME) under tension of the [011]-oriented Ti-50.55%Ni (at.%) alloy single crystals are presented. It was shown that hydrogen is in a solid solution and forms particles of titanium hydride TiH2 after hydrogenation at j = 1500 and 3500 A/m2, respectively. Both hydrogen in the solid solution and TiH2 particles led to a decrease in the Ms temperature of the onset of the forward martensitic transformation (MT) upon cooling and the Md temperature (Md is the temperature at which the stresses for the onset of the stress-induced MT are equal to the stresses for the onset of plastic flow of the high-temperature B2 phase), and increased the yield stress σ0.1 of the B2 phase at the Md temperature compared to hydrogen-free crystals. It was found that the SME under stress depends on the tensile stress level and current density. The maximum SME εSME = 10 ± 0.2% at σex = 200 MPa and εSME = 10.5 ± 0.2% at σex = 300 MPa was observed in the hydrogen-free crystals and after hydrogenation at j = 1500 A/m2, respectively, which exceeded the theoretical value of lattice deformation ε0 = 8.95% for the B2-B19′ MT in [011] orientation under tension. At j = 1500 A/m2, the physical reason for the excess of the SME of the theoretical ε0 value was due to the increase in the plasticity of B19′ martensite upon hydrogenation. At j = 3500 A/m2, εSME = 8.0 ± 0.2%, and it was less than ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension. The decrease in SME after hydrogenation at j = 3500 A/m2 was associated with the interaction of two types of B19′-martensite: oriented under stress and non-oriented, formed near TiH2 particles. It was shown that the redistribution of hydrogen in the bulk of the crystals during long-term holding for 168 h at 263 K after hydrogenation at j = 1500 A/m2 increases the SME relative to crystals without long-term holding: 3.5 times at 50 MPa and 1.8 times at 100–150 MPa. After long-term holding, εSME = 9.5 ± 0.2% at 150 MPa, which exceeds the theoretical value ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension.

A study was carried out on the effect of the level of external stresses, σex, and test temperature on the shape memory effect (SME), governed by the FCC ↔ HCP martensitic transformation, in single crystals of the Cr20Mn20Fe20Co34.5Ni5.5 (at.%) high-entropy alloy (HEA) along two different crystallographic orientations, i.e., [1¯23] and [011], under tensile strain. It was shown that the SME depends on the crystal orientation and the level of external stresses, σex, in the “cooling-heating” cycle under constant σex. In the “cooling-heating” cycle under constant σex, a maximum SME of 13.6 ± 0.2% was observed in [011]-oriented crystals at an external tensile stress of 150 MPa while in the [1¯23]-oriented crystals, a SME of 8.4 ± 0.2% was found under an external tensile stress of 170 MPa. In the “stress-strain” cycle, the maximum SME had similar values of 13–14% in studied orientations. General physical factors (the stress level of the FCC phase, short-range order, and change in the value of dislocation splitting in the external stress field) were established and ensured a large SME and its dependence on the crystal orientation in the Cr20Mn20Fe20Co34.5Ni5.5 HEA single crystals. For the studied orientations, a large SME in the FCC ↔ HCP MT was obtained for the first time.

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 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.

Hydrogen’s effect on the shape memory effect (SME) of [1¯17]-oriented Ti49.7-Ni50.3 (at.%) alloy single crystals, with a B2–B19′ martensitic transformation (MT), was studied after being electrolytically hydrogenated at a current density of 1500 A/m2 for 3 h at room temperature under isobaric tensile deformation. It was shown that, under the used hydrogenation regime, hydrogen was in a solid solution and lowered the elastic modulus of B19′ martensite. The hydrogen in a solid solution increased (i) the yield strength σ0.1 of the initial B2 phase by 100 MPa at Md temperature, (ii) the σ0.1 of the stress-induced B2–B19′ MT by 25 MPa at Ms temperature, and (iii) the plasticity of B19′ martensite relative to the hydrogen-free crystals. At the same level of external stresses, the SME in the hydrogenated crystals was greater than that in hydrogen-free crystals. At external tensile stresses σex = 200 MPa, the SME was 4.4 ± 0.2% in the hydrogenated crystals and 1.8 ± 0.2% without hydrogen. Hydrogen initiated a two-way SME of 0.5 ± 0.2% at σex = 0 MPa, which was absent in the hydrogen-free crystals. The physical reasons leading to an increase in the SME upon hydrogenation are discussed.

Single crystalline materials have the potential to exhibit superior performance because they exclude grain boundaries, which increase susceptibility to creep, oxidation, and corrosion, and make thermal and electronic transport inefficient. However, single crystal properties vary significantly with crystallographic orientation, making the ability to control the orientation critical for their use in applications. The complex nature of crystal nucleation and growth processes makes such control challenging. Here we report a new crystal reorientation mechanism that results in abrupt and massive orientation changes in bulk single crystalline and oligocrystalline alloys via solid state thermal processing. We demonstrated this method in two alloy systems, FeMnAlNi and CuAlMn, and achieved repeated, massive orientation changes in the solid state. These findings offer a new strategy for manipulating the orientation of large single crystals on demand in order to take advantage of their superior and highly anisotropic properties.

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.

The cyclic stability of superelasticity in compression in [001]B2-oriented Ti49.3Ni50.7 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: TR temperature increases by 4 K; Ms and Mf 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.

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.

Effect of the surface oxide layer on the shape memory effect (SME) and superelasticity (SE) after marforming (deformation in the martensitic state, followed by annealing at 713 K for 0.5 h in an inert helium gas and in dry air) was investigated on Ti-50.1Ni (at.%) single crystals, oriented along [011]-direction, under compression. Quenched [011]-oriented crystals of the Ti-50.1Ni alloy experience a one-stage B2-B19′ martensitic transformation (MT) without SE under compression. Marforming leads to a two-stage B2-R-B19′ MT and creates conditions for SE. A thin TiO2 oxide layer of 170 nm thick was formed on the sample surface upon annealing at 713 K for 0.5 h in dry air. In [011]-oriented crystals without and with an oxide layer, maximum of the SE value reached 4%, and the SME was 2.4 and 2.6%, respectively. Appearance of an oxide layer upon annealing in dry air: (i) reduces the stresses of B2-phase by 50 MPa from Md to 473 K; (ii) decreases Θ = dσ/dε from 6.5 GPa in crystals without an oxide layer to 2.0 GPa with an oxide layer and (iii) does not affect the SME and SE values.

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 degrees C to 500 degrees 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 degrees 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 degrees 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.

The cyclic stability of superelasticity in a wide temperature range was investigated on aged at 573 K for 0.5 h Co35Ni35Al30 single crystals oriented along the [001]-direction in compression. Aged single crystals exhibited superelasticity in a wide temperature window from 223 to 548 K (ΔTSE = 325 K) with high cyclic stability compared with the initial quenched crystals ΔTSE1 = 175 K (from 373 to 548 K) due to the strengthening of the high-temperature B2-phase by nanoscale particles. It is shown that the aged single crystals demonstrate excellent cyclic stability of superelasticity at room temperature during 10 000 loading/unloading cycles without any significant degradation of reversible strain, critical stresses for the stress-induced martensitic transformations and stress hysteresis. The main mechanisms of the superelasticity degradation during cyclic tests have been determined.

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 L1(0)-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 sigma(Ms) and stress hysteresis delta sigma 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 delta T-ad/sigma(Ms) increases by 4.4 times (delta T-ad/sigma(Ms) = 291.9 K/GPa (at T = 348 K)) and coefficient of performance reaches COP = 24.5 in stress-induced martensite aged crystals as compared with the as-grown crystals (delta T-ad/sigma(Ms )= 62.4 K/GPa (at T = 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 delta T-ad = 10.3-11.0 K in a wide operating temperature range up to 145-197 K. Thus, stress-induced martensite aged Ni(54)Fe(19)Ga(27 )single crystals oriented along the [001](A)- direction are expected to be promising materials for elastocaloric application.