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The low symmetry martensite phase in Nickel-Titanium (NiTi) shape memory alloy (SMA) has a hierarchical microstructure with micro-scale crystallites, which themselves consist of nano-laminates with an internal twin structure. The martensite phase deforms inelastically though reorientation of the twin structure. Furthermore, structural features in the specimens such as cracks or notches interact with the microstructure, influencing the deformation of martensite. In this work, using multi-scale experiments and macro-scale modeling, we demonstrate that the deformation of martensite in NiTi at the macro scale in a notched specimen is largely determined by the structural features, and the hierarchical nature of the microstructure plays a smaller role. At the micro-scale, in-situ measurements indicate that the martensite microstructure continuously evolves during tensile loading. However, the nano-scale twin laminate structure persists despite the large imposed deformation and the presence of stress concentrating features such as a notch. This observation is counter to the intuition of martensite twins reorienting to large domains of the most favored crystallographic variant at larger loads. This persistence of a nano-laminate structure rather than coarsening into large domains of individual variants indicates that the deformation field around cracks/notches in martensite may be modeled for fracture-related phenomena with adequate accuracy using coarse-grained models, without a compelling need for martensite single-variant-scale models.

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 (SE = 325 K) with high cyclic stability compared with the initial quenched crystals SE = 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

This study investigated the effect of stress-induced martensite aging under tensile and compressive stresses on the functional and viscoelastic properties in Ni50.3Ti32.2Hf17.5 polycrystals containing dispersed H-phase particles up to 70 nm in size obtained by preliminary austenite aging at 873 K for 3 h. It was found that stress-induced martensite aging at 428 K for 12 h results in the appearance of a two-way shape memory effect of −0.5% in compression and +1.8% in tension. Moreover, a significant change in viscoelastic properties can be observed: an increase in internal friction (by 25%) and a change in elastic modulus in tensile samples. The increase in internal friction during martensitic transformation after stress-induced martensite aging is associated with the oriented growth of thermal-induced martensite. After stress-induced martensite aging, the elastic modulus of martensite (EM) increased by 8 GPa, and the elastic modulus of austenite (EA) decreased by 8 GPa. It was shown that stress-induced martensite aging strongly affects the functional and viscoelastic properties of material and can be used to control them.

The effects of stress-free and stress-assisted aging at 673 K on microstructures, thermoelastic stress-induced B2-L10 martensitic transformation and superelasticity were investigated in [001]B2-oriented Co35Ni35Al30 (at. %) single crystals in compression. The aging of quenched crystals results in the formation of high-strength nanostructured composites containing spherical nanosized particles and elongated e-Co (HCP structure) particles, which do not undergo martensitic transformation. The stress-free and stress-assisted aged at 673 K for 0.5 h Co35Ni35Al30 crystals, oriented along [001]B2-direction, demonstrate high-yield stress levels for the B2-phase of 1360÷1630 MPa and a wide superelasticity temperature range from 193 K to 563 K with good cyclic stability and narrow stress hysteresis of Δσ = 50–60 MPa. It has been shown experimentally that a difference in the effective Young's modulus of austenite and martensite determinates a decrease in the reversible strain with a test temperature in both quenched and aged crystals. The tensile two-way shape memory effect with a reversible strain of eTWSME = +2.2% along the [001]B2-direction is induced due to the oriented growth of elongated e-Co-particles in stress-assisted aged crystals. These results provide a clear demonstration of the potential of aged CoNiAl single crystals for applications involving high stresses and temperatures.

B2–L21 configurational order resulting from thermal processing of Ni–Co–Mn–In Heusler alloys is one of the main factors defining the characteristics of the martensitic transformation experienced by these alloys. While the effects of the quenched-in disorder on the martensitic transformation have been extensively investigated, the kinetics of the ordering process is not well known and mostly inferred from the indirect results such as the dependence of the martensitic transformation or magnetic transition temperatures to thermal processing. However, recent results suggested that the kinetics of B2– L 2 1 ordering could have a more significant role than expected in the final state of the material and its functional properties. In the current work, an attempt has been made to investigate the kinetics of B2–L21 ordering in order to evaluate the dependence of the quenched-in disorder on the different variables of the thermal procedures. Results showed that ordering rate has a strong dependence on annealing temperature. Below 600 K, slow kinetics is forcing the crystal to always contain a certain amount of quenched-in disorder at room temperature. Between 600 K and 750 K, kinetics is fast enough to reach thermal equilibrium state within tens of minutes during the annealing process. Above 750 K, kinetics is very fast and thermal equilibrium can be reached during heating, so no isothermal holding is required. These predictions have been confirmed using differential scanning calorimetry and in-situ high-energy X-Ray diffraction measurements for non-direct estimations of the degree of order.

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 L10-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 σMs and stress hysteresis Δσ 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 ΔTad/σMs increases by 4.4 times (ΔTad/σMs = 291.9 K/GPa (at Т = 348 K)) and coefficient of performance reaches COP = 24.5 in stress-induced martensite aged crystals as compared with the as-grown crystals (ΔTad/σMs = 62.4 K/GPa (at Т = 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 ΔТad = 10.3–11.0 K in a wide operating temperature range up to 145–197 K. Thus, stress-induced martensite aged Ni54Fe19Ga27 single crystals oriented along the [001]А- direction are expected to be promising materials for elastocaloric application.

The two-step ageing of Fe-28Ni-17Co-11.5Al-2.5Nb (at.%) single crystals under and without stress, leads to the precipitation of the γ′- and β-phase particles. Research has shown that γ–α′ thermoelastic martensitic transformation (MT), with shape memory effect (SME) and superelasticity (SE), develops in the [001]-oriented crystals under tension. SE was observed within the range from the temperature of the start of MT upon cooling Ms, to the temperature of the end of the reverse MT upon heating Af, and at temperatures from Af to 323–373 K. It was found that at γ–α′ MT in the [001]-oriented crystals, with γ′- and β-phase particles, a high level of elastic energy, ΔGel, is generated, which significantly exceeds the energy dissipation, ΔGdis. As a result, the temperature of the start of the reverse MT, while heating As, became lower than the temperature Ms. The development of γ–α′ MT under stress occurs with high values of the transformation hardening coefficient, Θ = dσ/dε from 2 to 8 GPa and low values of mechanical Δσ and thermal ΔTh hysteresis. The reasons for an increase in ΔGel during the development of γ–α′ MT under stress are discussed.

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 °C to 500 °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 °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 °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.

A stable tensile two-way shape memory effect with a strain of (+7.0 ± 0.3) % along the [001]B2-direction upon stress-induced martensite ageing was obtained for single crystals of an Co35Ni35Al30 ferromagnetic alloy. Stress-induced martensite ageing at 398 K for 1.0 h under a compressive stress of 500 MPa along the [110]B2-direction led to the stabilization of L10-martensite variant, which contributes to sample elongation along the [001]B2-direction during stress-free cooling and reversible recovery upon subsequent heating. The tensile two-way shape memory effect was observed to be stable during stress-free thermal cycling through the phase transformation range. The characteristics of the two-way shape memory effect (reversible strain, martensitic transformation temperatures, and thermal hysteresis) did not change during 100 thermal cycles. The characteristics of the tensile two-way shape memory effect remain unchanged even after superelasticity cycling under a compressive stress of 1025 MPa applied along the [001]B2-direction at a temperature of 423 K, which is close to the stress-induced martensite ageing temperature.

The near equiatomic NiTi alloy is the most successful shape memory alloy by a large margin. It is widely and increasingly used in biomedical devices. Yet, despite having a repeatable superelastic effect and excellent shape-memory, NiTi is very far from satisfying the conditions that characterize the most reversible phase-transforming materials. Thus, the scientific reasons underlying its vast success present an enigma. In this paper, we perform rigorous mathematical derivations and accurate density-functional theory calculations of transformation mechanisms to seek previously unrecognized twinlike defects that we term involution domains, and we observe them in real space in NiTi by aberration-corrected scanning transmission electron microscopy. Involution domains lead to an additional 216 compatible interfaces between phases in NiTi, and we theorize that this feature contributes importantly to its reliability. They are expected to arise in other transformations and to alter the conventional interpretation of the mechanism of the martensitic transformation.

The main disadvantage of fcc (face-centred cubic lattice) high-entropy alloys is the low stress level at the yield point (σ0.1) at a test temperature above room temperature. This restricts their practical application at high test temperatures from 773 K to 973 K. In this study, we found that a high stress level was reached at the yield point σ0.1 ≈ G/100–G/160 (G is the shear modulus) of the [001]- and [1¯44]-oriented crystals of the Co23.36Cr23.29Fe23.80Ni21.88Al7.67 (Al0.3CoCrFeNi) high-entropy alloy (HEA) within a wide temperature range of 77–973 K under tension, due to the occurrence, of nanotwins, multipoles, dislocations under plastic deformation at 77 K and the subsequent precipitation of ordered L12 and B2 particles. It was shown that grain boundaries are not formed and the samples remain in a single-crystal state after low-temperature deformation and subsequent ageing at 893 K for 50 h. Achieving a high-strength state in the Al0.3CoCrFeNi HEA single crystals induces the orientation dependence of the critical resolved shear stresses (τcr) at T ≥ 200 K (τcr[1¯44] > τcr[001]), which is absent in the initial single-phase crystals, weakens the temperature dependence of σ0.1 above 573 K, and reduces plasticity to 5–13% in the [1¯44] orientation and 15–20% in the [001] orientation.

The oxidation behaviours of polycrystalline and monocrystalline Ni-base alloys (Ni-16Cr-8Fe (wt%)) were investigated in situ by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The oxide layer formed is duplex: an inner chromium-rich layer and an outer layer which is rich in nickel and iron. Time evolution of the composition of inner layer from pure Cr2O3 towards spinel (NiCr2O4) was observed. The oxidation kinetics was accelerated by alloy grain boundaries, which can favour the diffusion of chromium. Both the parabolic and volatilization constants increased with increasing temperature, with kp varying from 1.9×10−3 to 1.6×10−2 nm2.s−1, and kv from 1.0×10−3 to 2.9×10−3 nm.s−1.

The effect of two-stage ageing in a stress-free state and under tensile stress on the precipitation processes of dispersed particles, temperature range of γ–α′ martensitic transformations (MT), superelasticity and shape memory effect was investigated on [001]-oriented single crystals of the Fe–28% Ni–17% Co–11.5% Al–2.5% TiNb (at %) alloy. It was shown that two-stage ageing leads to simultaneous precipitation of γ′- and β-phase particles. β-phase particles were not found to inhibit the γ–α′ MT and led to the appearance of a tensile two-way shape memory effect (TWSME), which was 3.8% in stress-assisted aged crystals. The reasons for the appearance of a TWSME were discussed.

In the present study the superelasticity (SE) under tension/compression was investigated in the oligocrystals of Fe42.5Mn34Al15Ni7.5Ti1 (1% Ti), Fe42Mn34Al15Ni7.5Ti1.5 (1.5% Ti) and Fe41.5Mn34Al15Ni7.5Ti2 (2% Ti) (at %) alloys undergoing α (bcc)–γ′ (fcc) martensitic transformation. It was optically established that, first, with a growth in the Ti concentration from 1 to 2% the average and maximum grain sizes increase. Second, the optimal Ti concentration necessary to suppress the precipitation of the γ-phase particles during quenching is no more than 1.5%. SE strain studied at room temperature depends on the Ti concentration, heat treatment and stress state—tension/compression. In the quenched state the SE with value of 6% was observed only in 1% Ti oligocrystal under compression. Quenched oligocrystals was brittle and broke at the yield stress under tension. Aging at 473 K for 3 h leads to an increase in the SE value to 6.8% in 1% Ti oligo-crystal under compression, observation of the SE of 3.1% in 1.5% Ti oligocrystal and suppression of brittle fracture at the yield stress under tension. The SE strain in aged 1% Ti and 1.5% Ti oligocrystals under tension was eSE(1% Ti)=1.2% and eSE(1.5% Ti)=2%, respectively. SE was not found in a 2% Ti oligocrystal under tension/compression.

In the present study, the necessary conditions for the two-way shape memory effect (TWSME) and its stress-free thermal cycling stability in the Co 35 Ni 35 Al 30 single crystals, oriented along the [001] B2 direction, were investigated. The TWSME is attributed to the internal stress fields created through the generation of stable dislocation structures and retained martensite in two ways. First, the training involved 100 loading/unloading cycles at room temperature T=295 К with the maximum given strain of 6.0% in compression. Second, the ageing at T=423 К for 1.0 h under a stress of 500 MPa was applied to the martensitic state along the [001] B2 ||[110] L10 direction. It was experimentally shown that the aged crystals demonstrated the best properties of the TWSME response when compared with the trained crystals: i.e., a higher operating temperature range (250-297 K), the high cyclic stability of the reversible strain e TWSME =(3.1±0.3)% during thermal cycles and the low value of the temperature hysteresis (ΔT=24 К). In contrast, in the trained crystals the operating temperature range of the TWSME is shifted towards a low temperature region (220-275 K), after 100 thermal cycles the reversible strain e TWSME is significant decreased from 3.0 to 2.0 % and the temperature hysteresis ΔT is increased from 33 to 50 K.

The effects of stress-free and stress-assisted ageing at 873 K for 3 h on the crystal microstructure, the B2-B19' martensitic transformation (MT), the critical stress level for the stress-induced martensite formation, and the superelasticity (SE) in Ni-rich Ni 51.0 Ti 37.3 Hf 12.5 (at.%) single crystals, oriented along the [001] B2 direction, were investigated. For the first time, it is experimentally demonstrated that ageing, under a compressive stress of 150 MPa along the [001] B2 direction, leads to the oriented growth of H-phase particles. The temperature range of SE and the critical stress level for martensite formation are determined by variation in the size (40-150 nm) and the number of crystallographic variants of the H-phase particles. Reversible stress-induced B2-B19' MTs at high levels of applied compressive stress up to 1750-1970 MPa, with the reversible strain of 1.1%, are observed in all investigated Ni 51.0 Ti 37.3 Hf 12.5 single crystals.

[ 1 ¯ 12 ]-oriented single crystals of Ti-51.3Ni (at. %) alloy, aged at 823 K for 1.5 h under a tensile stress of 150 MPa, have been investigated regarding the two-way shape memory effect (TWSME) in a hydrogen-free state and after hydrogenation. It has been established that a TWSME, with a value of -1.1 %, is observed in hydrogen-free [ 1 ¯ 12 ]-oriented single crystals with one variant of dispersed Ti3Ni4 particles. The physical reason for the TWSME occurrence is associated with the advent of orienting internal compressive stresses from one variant of Ti3Ni4 particles, which results from ageing under a tensile stress of 150 MPa. It has been shown that hydrogen suppresses the TWSME. Therefore, it has been confirmed that suppression of the TWSME occurs in [ 1 ¯ 12 ]-oriented single crystals of Ti-51.3Ni (at. %) alloy with one variant of dispersed Ti3Ni4 particles, which are associated with shielding the stress fields from one variant of Ti3Ni4 particles, and with the multiple generation of the R- and B19'-martensite variants in crystals with the presence of hydrogen.

It was experimentally shown that the reversible strain at the shape memory effect, the critical stress level of the martensite formation and the work output were determined by the stress state in [ 1 ¯ 17]-oriented Ni 45.3 Ti 29.7 Hf 20 Pd 5 (at. %) single crystals, which were quenched at 1173 K for 3 h. These single crystals demonstrate large values of the reversible strain (up to 5 %) and the work output (up to 27 J/sm 3 ) under tensile stress level of 500 MPa. However, the reversible strain does not exceed 0.8 % under compressive stress levels of 600 MPa. The thermal hysteresis regarding the stress-induced martensitic transformation almost certainly does not depend on the stress state.

Deformation-induced martensitic transformation from face-centred-cubic γ-austenite to hexagonal close-packed e-martensite and body-centred-tetragonal α'-martensite has attracted much attention owing to the transformation-induced plasticity effect in austenitic stainless steels, high- and medium-Mn austenitic steels, ferrous shape memory alloys, and high entropy alloys. For better understanding of the complicated triple γ/e/α' phase deformation microstructure, various structures at the intersection of different variants of deformation-induced e-martensite are examined. A type-316 austenitic steel single crystal is compressively deformed along the [0 0 1]γ axis at a cryogenic temperature (173 K), and a deformation microstructure on the (1 1 0)γ surface is observed by means of scanning electron microscopy equipped with an electron backscattering diffraction analysis system. Depending on the shear angle with respect to the intersection axis, either 90o (Type I) or 30o (Type II), three types of atomic rearrangements of the intersection volume are observed: (1) reverse transformation into the γ-phase, which is rotated by 90o from the parent γ-austenite (Type I); (2) 10-12 twinning of either crossing e-martensite variants (Type I); (3) secondary martensitic transformation into α'-martensite (Type II). Transformation matrices for the intersection products are built, which are then used to successfully calculate their crystallographic orientations starting from the orientation of the parent γ-phase. In addition, the atomic rearrangements and orientational changes are visualized by the distortion and/or kinking of Thompson's regular tetrahedron representing the initial atomic arrangement and the orientation of the parent γ-phase.

In this work, we measure the microstructure response of the austenite phase during martensitic phase transformation tens of micrometers beneath the surface of a bulk single crystal nickel-titanium shape memory alloy. Using an emerging dark-field X-ray microscopy (DFXM) technique, the austenite phase fraction, relative misorientation, and elastic lattice plane strain are measured in the interior of the microstructure with a spatial resolution of 108 nm. The results show that some defects consistently induce forward transformation and delay reverse transformation, while other defects consistently impede the propagation of both forward and reverse transformation fronts. We also show that the austenite undergoes an orientation splitting wherein the austenite near the transformation front is constrained from rotating and the austenite far from the transformation front is free to rotate. Finally, we measure interfacial strain fields at the transformation front that extend tens of micrometers into the material. We use an analytical model to show how these strain fields can be explained by a lack of kinematic compatibility between the austenite and martensite phases at the austenite-martensite interface.

Two-way shape memory effect (TWSME) and its stability under cooling/heating cycling through the martensitic transformation (MT) temperatures in stress-free condition were investigated in mono-phase [001]-oriented single crystals of the ferromagnetic Co49Ni21Ga30 (at %) alloy. TWSME is induced by thermomechanical training– cooling under different external compressive stress and subsequent stress-free heating. It is shown that the TWSME strain eTWSME depends on the applied stress in the training cycle. The maximum of the eTWSME equal to 4.2(±0.2)% is achieved after training under σext=125 MPa and then decreases with increase of σext. The value of eTWSME = 4.2(±0.2)% is equal to the theoretical transformation strain for [001]-orientation at B2-L10 MT in compression. In crystals after training under σext=125 MPa with maximum eTWSME, the stability of the TWSME strain and martensitic start temperature Ms is observed up to 150 cooling/heating cycling. From 150 to 300 thermocycle, TWSME strain decreases from 4.1(±0.2)% to 3.5(±0.2)%, and the Ms temperature decreases from 251 to 242 K.

For [001]-oriented single crystals of Fe-28Ni-17Co-11.5Al-2.5Ti (at %) alloy in tension, it was shown that particles of the γ’-phase with a size of 4–6 nm lead to the appearance of superelasticity, with a strain of eSE=4.5% and a shape memory effect of eSME=5.9%. Increasing the particle size to 10–12 nm reduces eSE to 3.6% and eSME to 3.5%. The paper discusses the causes of the influence of particle size on the reversible strain.

Near-field and far-field high-energy diffraction microscopy and microcomputed tomography X-ray techniques were used to study a bulk single crystal nickel-titanium shape memory alloy sample subjected to thermal cycling under a constant applied load. Three-dimensional in situ reconstructions of the austenite microstructure are presented, including the structure and distribution of emergent grain boundaries. After one cycle, the subgrain structure is significantly refined, and heterogeneous {\Sigma}3 and {\Sigma}9 grain boundaries emerge. The low volume and uneven dispersion of the emergent {\Sigma} boundaries across the volume show why previous transmission electron microscopy investigations of {\Sigma} grain boundary formation were inconsistent., Comment: accepted in Scripta Materialia

WOS: 000374615700008, The shape memory behavior of [111]-oriented Ni51Ti49 (at.%) single crystals was investigated after stress assisted aging at 500 degrees C for 1.5 h under a compressive stress of -150 MPa. It was found that a single family of Ni4Ti3 precipitates with two crystallographically equivalent variants was formed after aging under compressive stress. Stress-assisted aging resulted in tensile two-way shape memory effect strain of 1.56% under -5 MPa. Thermal cycling under -600 MPa resulted in a transformation strain of -2.15%, while the subsequent thermal cycling under -5 MPa resulted in a tensile two-way shape memory effect strain of 2.2%., RFBR Project [10-03-0154-a]; NASA [NNX11AQ31A], This work was supported in part by the NASA Fundamental Aeronautics Program, Supersonics Project and the NASA EPSCOR program (Grant No. NNX11AQ31A), and RFBR Project (Grant No. 10-03-0154-a).

The authors studied the development of thermoelastic martensite transformation from FCC-γ of high-temperature phase to OCT-α¢ martensite after the aging at 973 K during 20 hours on the single-crystals of Fe – 28 % Ni – 17 % Co – 11.5 % Al – 2.5 % Nb (Nb) and Fe – 28 % Ni – 17 % Co – 11.5 % Al – 2.5 % Nb – 0.05 % B (NbB) (at. %) alloys oriented for the extension along [001]-direction. It is shown that boron causes: the retardation of the aging processes: in NbB-crystals, the particles of γ¢-phase have the size of 12–14 nm and in Nb-crystals – 18–25 nm; the decrease of M s onset temperature of the martensite transformation: M s =108 K in NbB-crystals and M s =116 K in Nb-crystals; the increase of the stress level at the temperature M s : at M s temperature, the stresses are equal to 70 MPa in NbB-crystals and 31 MPa in Nb-crystals. The level of stresses of high-temperature phase is determined by the size of γ¢-phase particles: in Nb-crystals with bigger size of γ¢-phase particles, the high-temperature stresses are higher than in NbB-crystals where γ¢-phase particles have the smaller size. When developing thermoelastic γ–α¢ martensite transformation under the load, in Nb- and NbB-crystals after the aging during twenty hours at 973 K, the form memory effect of 2.6 % and 2.2 % respectively was observed, and the superelasticity was not observed. It is shown that the physical reason for the superelasticity absence in Nb-crystals is caused by the brittle β-phase that results into the crystals destruction at the beginning of γ–α¢-MT under the load and in NbB-crystals – by the increase of mechanical hysteresis.

The crystallographic anisotropy of elastocaloric effect (ECE) and relative cooling power (RCP) in Ni54Fe19Ga27 shape memory alloy single crystals are studied via compression tests. Single crystals are studied along the [001], [123], and [011] austenite directions and yield different ECE behaviors and maximum RCPs for various strain levels. A thermodynamic framework using the Helmholtz free energy is employed to analyze the total entropy change as a function of strain. Thermodynamic losses are computed from the mechanical hysteresis of superelasticity experiments to quantify the strain dependent RCP. It is found that the [001] orientation generates the highest maximal RCP of 738 J kg−1 when unloaded from 200 MPa. This is attributed mainly to the large superelastic temperature window of 45 K. However, loading the crystals to stresses higher than 200 MPa causes a multistep transformation in the [011] direction, thus reducing the alloy's overall RCP by 135 J kg−1. This is a consequence of the negative entropy change and large transformation hysteresis generated by the second‐stage transformation in the [011] direction. Interestingly, if only the first‐stage transformation in [011] is employed for the ECE, the [011] direction yields the highest RCP compared to [001] and [123] for any strain up to 3.5%.

Modern X-ray diffraction techniques are now allowing researchers to collect long-desired experimental verification data sets that are in situ, three-dimensional, on the same length scales as critical microstructures, and using bulk samples. These techniques need to be adapted for advanced material systems that undergo combinations of phase transformation, twinning and plasticity. One particular challenge addressed in this article is direct analysis of martensite phases in far-field high-energy diffraction microscopy experiments. Specifically, an algorithmic forward model approach is presented to analyze phase transformation and twinning data sets of shape memory alloys. In the present implementation of the algorithm, the crystallographic theory of martensite (CTM) is used to predict possible martensite microstructures (i.e. martensite orientations, twin mode, habit plane, twin plane and twin phase fractions) that could form from the parent austenite structure. This approach is successfully demonstrated on three single- and near-single-crystal NiTi samples where the fundamental assumptions of the CTM are not upheld. That is, the samples have elastically strained lattices, inclusions, precipitates, subgrains, R-phase transformation and/or are not an infinite plate. The results indicate that the CTM still provides structural solutions that match the experiments. However, the widely accepted maximum work criterion for predicting which solution of the CTM should be preferred by the material does not work in these cases. Hence, a more accurate model that can simulate these additional structural complexities can be used within the algorithm in the future to improve its performance for non-ideal materials.

The proposed dislocation model describes the orientation dependence of the critical resolved shear stress (CRSS) and deformation mechanisms on the yield point in single crystals of austenitic stainless steel with nitrogen impurities. The model takes into account the following: the change of the interstitial atom position in the lattice from octahedral interstice to tetrahedral site owing to passage of a leading Shockley’s partial dislocation; the change in the separation width between two partial dislocation in external stress field; the relationship between the width of the extended dislocation and the elastic interaction of the extended dislocation with the impurity atoms.

Using Co35Ni35Al30(at.%) single crystals the effects of crystals axis orientation and ageing at 673 K for 0.5 h in free state on superelasticity responses in compression are investigated. Ageing of these single crystals is found to result in strengthening of the high-temperature В2 phase and L10martensite, twofold increasing of temperature interval of SE response and weakening of the orientation dependence and improving the cycle stability of superelasticity response. In [011]- and [123]-oriented single crystals the values of the stress hysteresis decrease a nearly threefold as compared to the quenched state. The combination of high-strength [001] crystal axis orientation and precipitation of coherent nanoscale particles leads to the best stability of SE response in aged Co35Ni35Al30single crystals.

On single-phase [ 1 ¯ 23 ] -oriented single crystals of ferromagnetic Co 49 Ni 21 Ga 30 (at.%) alloy shape memory effect (SME) and superelasticity (SE) in compression are studied. The value of SME and SE are both equal to −3.2%. SE is observed in the temperature range of T=318–473 K. After realization of SE in [ 1 ¯ 23 ] -oriented crystals the two-way SME was found. Two-way SME is equal in value to one-way SME, SE and the lattice stain, |e 0 |, for the crystals of a given orientation at B2–L1 0 martensitic transformation (MT) under compression deformation. The physical reason for the existence of the two-way SME in [ 1 ¯ 23 ] -oriented crystals after realization of SE is associated with the occurrence of internal stress fields due to the appearance of the crystal structure defects at T>423 K.

The temperature dependence of the critical stresses and transformation strain upon cooling/heating under a constant tensile stress in the temperature range T = 77–573 K are studied on single crystals of Fe–28% Ni–17% Co–11.5% Al–2.5% (Ti+Nb) (at %) alloy oriented along the [001]-direction under tensile deformation. It is shown that at the start martensitic transformation temperature Ms the critical stress for the development of stress-induced γ–α′-martensitic transformations under isobaric and isothermal conditions differs by 7 times. The critical stress for the development of stress-induced γ–α′-martensitic transformations under isobaric and isothermal conditions become equal to each other at the external stress σext above 400 MPa. The maximum reversible strain was 4.5% at a given value of the transformation strain 9.8%.

For the first time, a study of the two-way shape memory effect (the TWSME) for the quenched and aged at 673 K for 0.5 h [001]-oriented Co35Ni35Al30 single crystals in compression was carried out. In the quenched CoNiAl crystals, the TWSME with a reversible strain of (-3.1 ± 0.3) % was induced by training procedure in the superelasticity temperature range. The physical reason of the TWSME is the creation of internal stress fields due to the formation of dislocations next to γ-phase particles, and an interface between the B2-matrix and the γ-phase in the quenched crystals. It was experimentally shown that aging at 673 K for 0.5 h under a compressive stress of 100 MPa along the [110] direction creates the conditions necessary for observing the TWSME, without additional training with a reversible strain of (+2.2 ± 0.3) %. The conditions for observing the TWSME are determined by the action of the internal stress fields of 25 MPa created by the oriented arrangement of the dispersed particles in the stress-assisted aged crystals. The TWSME is not observed in the stress-free aged crystals with non-oriented precipitation of particles, both before and after training.

The conventional shape memory effect (SME) and pseudoelasticity (PE) in as-grown [100] single crystals of Co49Ni21Ga30 alloy under compression are reported. The parent single crystals exhibit about 5% transformation strain at compressive stress levels as low as 4 MPa, and a pseudoelastic strain of 4.5%. Complete PE was observed in the temperature range from 35 to 285°C, along with increasing stress hysteresis with temperature. The latter is attributed to increasing number of variants and the corresponding variant–variant interactions. We demonstrate that the current material can be utilized in applications that demand high strength at elevated temperatures. Moreover, the current results also indicate the potential of this material to exhibit magnetic shape memory effect, which could broaden the scope of utility of this material upon further research.

The low stacking fault energy and high carbon content in Hadfield steel make twinning the basic deformation mechanism from the onset of plastic deformation in [1¯11] and [011] oriented single crystals in tension at T = 77 – 300 K. Alloying with aluminium (2.7 Al in wt.%) results in an increase of stacking fault energy from 0.03 J · m2 to 0.05 J · m−2 and moves twinning to higher degrees of deformation (∊pl > 15 %). In aluminium-free [1¯23] crystals twinning starts after 20 % strain. For [1¯23], [001] orientations, aluminium additions change the dislocation arrangement from a uniform distribution to a planar dislocation arrangment and also suppress twinning. Intersections of dislocation pile-ups were found to be the governing factor for hardening in the aluminium-alloyed [001] crystals.

We present experimental results on the instrumented Vickers micro-indentation and compression of solutionized Ni-rich NiTi single crystals. The tests are conducted at room temperature where the solutionized Ti-50.9 at percent Ni material is 18 degrees above Af and the solutionized Ti-51.5 at percent Ni material is more than 100 degrees above Af. Aside from elastic deformation, it is discovered that dislocation motion and a reversible stress-induced martensitic transformation influence the micro-indentation response of Ti-50.9 at percent Ni, while the micro-indentation of Ti-51.5 at percent Ni only induces irreversible dislocation motion. The effect of the surface normal orientation on material hardness was negligible in the Ti-51.5 at percent Ni and followed trends anticipated by the activation of favorable slip systems in the Ti-50.9 at percent Ni. Compression tests on the identical Ti-50.9 at percent Ni samples revealed deformation by coupled stress-induced martensite and plastic flow, depending on the crystallographic orientation. The trends in hardness with surface normal orientation were not commensurate with the orientation dependence of the uniaxial compressive transformation or “yield” strength. The ramifications of the results in terms of comparing micro-indentation and macro-compression and the interactions between plasticity and the stress-induced martensitic transformation are discussed.

A near single-variant martensitic state of Co[Formula: see text]Ni[Formula: see text]Ga[Formula: see text] alloy was induced by a static axial stress of 66[Formula: see text]MPa. Upon application of a magnetic field pulse, the single-variant state of stressed specimen was transformed into a hierarchic martensitic structure at micron, submicron and nanometer scales. The martensitic structures, which were induced during the magnetic field pulse, did not disappear after the field was switched off. This stability of the field-induced martensitic structure is attributed to the appearance of additional twinning systems with hardly mobile twin boundaries.

Deformation of nickel rich (51.5%Ni) Ni–Ti single crystals are investigated over a wide range of temperatures (77–440 K) and strain levels in compression as high as 9%. These alloys combine high strength with an unusually wide pseudoelasticity temperature interval (near 200 K) and can be exploited to suit specific applications. The slip deformation in [001] orientation can not occur due to the prevailing slip systems, as confirmed by transmission electron microscopy. Consequently, the [001] orientation exhibited pseudoleastic deformation at temperatures ranging from 77 to 283 K for the solutionized case and 273–440 K for the aged condition respectively. The critical transformation stress levels were in the range 800–1800 MPa for the solutionized case, and 200–1000 MPa for the aged case depending on the temperature and specimen orientation. These stress levels are considerably higher compared to the near equiatomic Ni compositions of these class of alloys. On the other hand, the maximum transformation strains, measured from incremental straining experiments in compression, were lower compared to both the phenomenological theory with Type II twinning and the previous experimental work on 50.8%Ni NiTi crystals. A new theory for compound twinning is introduced with lattice invariant shear as a solution, and relies on the successive austenite phase (B2) to intermediate phase (R) to martensite phase (B 19′) transformation. The compound twinning model predicts lower transformation strains compared to the Type II twinning case lending an explanation of the experimental transformation strain levels.

Room temperature monotonic and cyclic stress–strain curves for single crystal and polycrystalline NiTi shape memory alloys containing Ti3Ni4 precipitates are presented. The tensile and compressive single crystal results illustrate the importance of crystallographic texture, the unidirectional nature of the martensitic transformation, and martensite detwinning on tension-compression stress–strain asymmetry in polycrystalline NiTi. Moreover, results on the fatigue of NiTi single crystals demonstrate the fundamental characteristics of cyclic deformation in NiTi alloys and the importance of texture on the fatigue of polycrystalline NiTi. © 2001 Elsevier Science B.V. All rights reserved.

We study the instrumented Vickers micro-indentation of single-crystal Ti–50.9 at% Ni shape-memory alloys with systematically varied surface normal orientations ([100], [210], [111] and [221]) and Ti3Ni4 precipitate sizes (0 nm, 10 nm, 50 nm, 100 nm, 300 nm and 500 nm). Based on transmission electron microscopy observations, indentation of solutionized NiTi induces inelastic deformation via dislocation activity and a stress-induced martensitic transformation. The room-temperature hardness, Hv, and recoverable energy, Er, of NiTi are shown to be a maximum for very small precipitate sizes, decrease for intermediate precipitate sizes, and increase for large precipitate sizes. The maximization of Hv and Er at small precipitate sizes (10 nm) is attributed to the relatively high resistance to both dislocation motion and a recoverable stress-induced martensitic transformation. The decreases in Hv and Er at intermediate precipitate sizes (50–300 nm) are attributed to a decrease in the resistance to dislocation motion and a measured increase in the transformation temperatures with respect to the indentation temperature. The increases in Hv and Er at large precipitate sizes (500 nm) are attributed solely to measured decreases in the transformation temperatures with respect to the indentation temperature, since the resistance to dislocation motion remains constant as the precipitates grow from 300 nm to 500 nm. For nearly all heat treatments, the [100] and [221] surfaces demonstrate the highest and lowest values of Hv and Er, respectively, an effect attributed primarily to orientation of favorable slip systems.

Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loading conditions at room temperature which is above the M s (martensite start) temperature of −30°C. The single crystals exhibited remarkable cyclic hardening under zero to compression strain control experiments. The stress range under strain control increased by as much as a factor of 3 in compression. The increase in stress range is primarily due to the increasing strain hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was undertaken with a strain range of 3% which is far below the theoretical transformation strains levels exceeding 6%. The maximum stress levels reached in the experiments are below those that cause martensite slip. Therefore, the stress–strain response is governed by transformation from the austenite to the martensitic phases and the dislocation structure evolution in the austenite domains. Two single crystal orientations [148] and [112] were examined during the experiments with single and double CVP (correspondent variant pair) formations respectively. The strain hardening in compression cases is rather substantial with the stress range in the double CVP case surpassing the single CVP case. Two heat treatments were selected to produce coherent and incoherent precipitates in the microstructure respectively. The influence of the coherent precipitates on the stress–strain response is significant as they lower the transformation stress from austenite to martensite, and at the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue.

The fracture mechanisms in single crystal and polycrystalline Ti-50.8at%Ni shape memory alloys containing Ti3Ni4 precipitates are studied using the scanning electron microscope (SEM). Aged materials with three different precipitate sizes (50 nm, 150 nm, and 400 nm), which have interfaces ranging from semi-coherent to incoherent, are considered. The mechanisms of material fracture identified in the single crystal NiTi are: 1. Nucleation, growth, and coalescence of voids from the Ti3Ni4 precipitates, 2. Cleavage fracture on {100} and {110} crystallographic planes, 3. Nucleation, growth, and coalescence of voids from fractured Ti-C inclusions. Cleavage and ductile tearing mechanisms also operate in polycrystalline NiTi, however, since the Ti-C inclusions are an artifact of single crystal growth processes, mechanism 3 was not discovered in the polycrystalline materials. Cleavage fracture and ductile tearing are found to act in conjunction, with the relative dominance of one over the other depending on the local precipitate size and concentration. As the Ti3Ni4 precipitate size increases to about 400 nm, the overall fracture is dominated by failure mechanism 1, and the cleavage markings become diffuse. Finally, we assert that the high tensile ductility of drawn NiTi polycrystals is due partially to the fact that drawn bar and wire stock usually have a strong {111} fiber texture. Such a texture promotes the initiation of the transformation at low stresses and concurrently prevents primary cleavage on the {100} or {110} planes.

The influences of grain boundaries and relative grain misorientations on stress-induced martensitic transformations in NiTi are studied using unique experiments and finite element modeling. Tensile and compressive mechanical tests reveal that polycrystalline NiTi with a dominant fiber texture and single crystal NiTi oriented along the [111] direction exhibit nearly identical stress–strain curves during a stress-induced martensitic transformation. Micro-mechanical finite element simulations of fiber textured polycrystals and single crystals undergoing a multi-variant martensitic transformation confirm the relative indifference of the macroscopic transformation attributes to the presence of grain boundaries. On the microscale, the finite element simulations further reveal that the insensitivity of the transformation to intergranular constraint is linked to the local stress disturbance created by transforming grains. The transformation of grains that are favorably oriented with respect to the loading axis creates local stresses that invariably assist the transformation in neighboring grains, effectively lowering the influence of grain misorientations and boundaries on the macroscopic transformation behavior.

Transmission electron microscopy is used to determine the microstructures of a Ti-50.8 at% Ni alloy given different aging treatments. Two different peak-aging treatments are shown to result in disk shaped semi-coherent Ti{sub 3}Ni{sub 4} precipitates with a diameter ranging from 50 nm to 200 nm depending on the aging temperature. In the peak-aged materials, strong strain fields are clearly visible on TEM micrographs. An Eshelby based model is used to predict the local stress fields due to the differences in the lattice parameters of the precipitates and surrounding matrix. The position dependent local stress fields are then resolved onto the 24 different martensite correspondence variant pairs (CVP`s). It is further demonstrated that due to the unique orientation relationship that exists between the precipitate variants and the martensite CVP`s, the local resolved shear stresses are extremely large on some CVP`s and negligible on others. When the Ni rich NiTi is over-aged, it is found that the precipitates coarsen to approximately 1000 nm, they become in-coherent, and the local stress fields disappear. It is also determined that after over-aging in the average composition of the matrix drops from 50.8 at% Ni to approximately 50.4 at% Ni. In a subsequent paper (part 2)more » the results here are used to explain the dependence of the critical transformation stress levels and martensite start temperatures on the aging treatment.« less

An experimental study was performed to determine the effect of aging on martensitic transformations in NiTi, Polycrystalline and single crystal NiTi ([100], [110], and [111] orientations) were both considered, Stress-induced transformations in polycrystalline NiTi were found to closely resemble transformations in single crystals of the [110] and [111] orientations. Solutionized and over-aged single crystals exhibited a strong orientation dependence of the critical stress required to trigger the transformation, σcr The Schmid law was able to accurately predict the orientation dependence of σcr in the solutionized and over-aged single crystals. Peak-aged single crystals demonstrated a much weaker orientation dependence of σcr and in general, the Schmid law was not obeyed. By considering the local stress fields outside of the semi-coherent precipitates, the decrease in the orientation dependence of σcr was accounted for. The martensite start temperatures, Ms, in aged single crystal and polycrystalline NiTi were much higher than in solutionized samples In peak-aged NiTi the increase was primarily attribited to the local stress fields outside the coherent precipitates which create preferential nucleation sites for the martensite. In the over-aged NiTi the increase in Ms was primarily attributed to the decrease in the average Ni concentration of the matrix surrounding the coarsened precipitates.

A combined experimental and theoretical investigation of the transformation temperature and transformation strain behaviors of a promising new Ni_(50.3)Ti_(29.7)Hf_(20) high-temperature shape memory alloy was conducted. Actuation behavior of single crystals with loading orientations near [001]_(B2), [110]_(B2), and [111]_(B2), as well as polycrystalline material in aged and unaged conditions was studied, together with the superelastic, polycrystalline torsion response. These results were compared to analytic calculations of the ideal transformation strains for tension, compression, and torsion loading of single crystals as a function of single crystal orientation, and polycrystalline material of common processing textures. H-phase precipitates on the order of 10–30 nm were shown to increase transformation temperatures and also to narrow thermal hysteresis, compared to unaged material. The mechanical effects of increased residual stresses and numbers of transformation nucleation sites caused by the precipitates provide a plausible explanation for the observed transformation temperature trends. Grain boundaries were shown to have similar effects on transformation temperatures. The work output and recoverable strain exhibited by the alloy were shown to approach maximums at stresses of 500–800 MPa, suggesting these to be optimal working loads with respect to single cycle performance. The potential for transformation strain in single crystals of this material was calculated to be superior to binary NiTi in tension, compression, and torsion loading modes. However, the large volume fraction of precipitate phase, in part, prevents the material from realizing its full single crystal transformation strain potential in return for outstanding functional stability by inhibiting plastic strain accumulation during transformation. Finally, calculations showed that of the studied polycrystalline textures, [001]_(B2) fiber texture results in superior torsion performance, while [011]_(B2) fiber texture results in superior tensile behavior, and both [011]_(B2) and random textures will result in the best possible compression performance.