Whole martensitic transformation process in Fe–Mn–Si–Cr shape memory alloy by improved characterization of volume resistivity

Stress-induced martensitic transformations and shape memory at nanometer scales

Fe-based shape memory alloy (Fe-SMA) shows the smaller shape memory effect (SME) compared with the widely-used NiTi alloy. However, because its production cost is much lower than the NiTi alloy, Fe-SMA is challenged to be applied in civil engineering fields such as vibration absorbers and joints. A key of the SME is stress-induced martensitic transformation. Thus, it is important to evaluate an amount of martensite, which can control such excellent performance of Fe-SMA, for increasing a reliability of the Fe-SMA. However, until now, it is quite hard to find studies to evaluate the amount of martensite in Fe-SMA experimentally during deformation at various strain rates, especially during high speed deformation. Instead of the evaluation, it is convenient to capture change in volume resistivity, which has a correlation with the amount of martensite, at various strain rates. In the past, the volume fraction of α’ martensite is evaluated by using a resistance measurement based on the four point-probes method. The advantages of the method are quite simple and relatively high precision, however, its disadvantages are a requirement of strictly-precise reference resistor and power supply, and it is easily affected from noise. In this study, at first, a circuit of Kelvin double bridge with a higher precision is assembled. Then, the rate sensitivity of volume resistivity in Fe-28Mn-6Si-5Cr alloy, which is a kind of Fe-SMA, is experimentally estimated by using the assembled circuit of Kelvin double bridge during tensile testing at various strain rates.

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.

The deformation properties of TiNi shape-memory alloy subjected to strain control and stress control were investigated experimentally. The results obtained are summarized as follows. (1) In the case of a full loop, the stress-strain curves under stress-controlled conditions are similar to those under strain-controlled conditions with high strain rate. The overshoot and undershoot do not appear at the start points of the stress-induced martensitic transformation in these curves. (2) In the case of subloop under stress-controlled conditions, temperature decreases and therefore the strain increases owing to the martensitic transformation at the early stage of the unloading process. At the early stage in the reloading process, temperature increases and therefore the strain decreases owing to the reverse transformation. (3) In the case of subloop under stress-controlled conditions, the starting stresses of the martensitic transformation and the reverse transformation in the loading and unloading processes coincide with the transformation stresses under strain-controlled conditions with low strain rate, respectively. (4) The deformation behaviours for a subloop under stress-controlled conditions are prescribed by the condition for progress of the martensitic transformation based on the transformation kinetics. (5) The deformation behaviors subjected to cyclic loading under stress-controlled conditions at constant temperature are also prescribed by the conditions for progress of the martensitic transformation.

TiNi shape memory alloy (SMA) specimens have been subjected to tension carried out at various strain rates. The goal was to investigate a nucleation and development of the stress-induced martensitic transformation by infrared (IR) and acoustic emission (AE) techniques. Therefore, both the infrared radiation and acoustic emission data were recorded using a fast infrared camera and acoustic emission set-up, respectively. It has been shown that the initial, macroscopically homogeneous transformation initiates in the elastic stage of the deformation even before the stress-strain curve knee and formation of the localized transformation bands. It has also been found that the homogeneous transformation occurs at similar stress level for all strain rates applied, while the localized martensitic transformation depends on the strain rate. Nucleation and development of the localized transformation bands, detected by the infrared camera, were confirmed by acoustic emission technique. The differences between the IR and AE activities were recorded during the TiNi SMA loading and unloading process, manifesting different dynamics of the stress-induced martensitic forward and reverse transformation. K e y w o r d s : shape memory alloy, TiNi, superelasticity, martensitic transformation, tension test, acoustic emission

Low-energy tensile-impact behavior of superelastic NiTi shape memory alloy wires

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.

Microstructural and fractographic observations were systematically done on Fe-30Mn(6-x)Si-xAl (x=0, 1, 2 and 4 mass %) alloys. Optical and transmission electron microscopic observations and X-ray diffractions revealed that the deformation mode continuously shifts from the stress induced fcc/hcp martensitic transformation to the mechanical twinning of the fcc austenite as the Al content increases. It was also clarified by the scanning electron microscopic observations that the microstructural change depending on the Al content is accompanied by the change in the fracture mode from the quasi-cleavage fracture to the ductile fracture. INTRODUCTION Fe-Mn-Si-based shape memory alloys (SMAs) exhibit the shape memory effect (SME) associated with fcc (γ-austenite) / hcp (e-martensite) martensitic transformation [1]. A recoverable strain obtained in a typical Fe-Mn-Si SMA: e. g. Fe-30Mn-6Si (hereinafter compositions are shown in mass%), was reported about 2% in the solution treated condition [2]. This value can be increased to about 4% by so-called the training treatment [3, 4] and fine dispersion of precipitates such as NbC carbides [5-7], etc. One drawback of the alloy was its poor ductility of about 30%. In contrast to this, it was recently reported that the Fe-30Mn-3Si-3Al TWIP (Twinning Induced Plasticity) steel exhibits the ultra-high ductility as much as about 90% [8], but this alloy shows no significant SME. The composition of the Fe-30Mn-3Si-3Al TWIP steel is such that a part of Si in the Fe30Mn-6Si SMA is replaced by Al. In order to systematically investigate the effect of the Al content on the SME and TWIP effect, the present authors prepared four kinds of FeMn-Si-Al alloys by gradually varying the amount of Al substituting Si: i. e. Fe-30Mn(6-x)Si-xAl (x=0, 1, 2 and 4). The following two conclusions were drawn as a result [9]: i) the alloys with x=0 and 1 exhibited similar SME, but no recognizable SME was observed for the alloys with x>2, ii) the ductility linearly increased with increasing the amount of Al. The above-mentioned changing tendencies may originate from the continuous change in the deformation mode from the stress-induced γ → e martensitic transformation to the mechanical γ twinning. However, there has been no systematic study on the effect of the Al content on the deformation and fracture modes between the SMA and the TWIP. In the present paper, microstructural observations using optical microscopy (OPM) and transmission electron microscopy (TEM), phase identification using X-ray diffraction (XRD) and the fractographic observations using scanning electron microscopy (SEM) were carried out to clarify the effect of the Al content on the deformation mode and the corresponding fracture mode of the alloys. EXPERIMENTAL In this paper, hereafter the Fe-Mn-(6-x)Si-xAl (x=0, 1, 2 and 4) alloys are referred to as Al-0,Al-1,Al-2,Al-3 using mass % of Al. The specimens were prepared by vacuum induction melting. After hot forging and rolling at 1270K, the specimens were subjected to solution treatment at 1270K for 3h followed by water quenching. The OPM observations were performed on the samples, which were mechanically and electrolytically polished to obtain smooth surfaces and then extended by about 3%, using a differential interference microscope. The phase constitutions and internal microstructures in the deformed specimens were investigated with a RINT 2500 X-ray diffractometer and with a JEOL 2000FX II transmission electron microscope, respectively. The specimens for TEM observation were carefully prepared to avoid the formation of stress-induced martensite and reverse transformation on heating, using a chemical polishing solution of hydrogen peroxide and hydrofluoric acid mixed in the ratio of 10: 1. The specimens were finally subjected to electropolishing using acetic acid and perchloric acid mixed in the ratio of 20:1 at room temperature, to obtain the TEM foils. Fracture surfaces were examined on the specimens fractured at room temperature with a Hitachi S-3100 scanning electron microscope. RESULTS AND DISCUSSION DEFORMATION BEHAVIOR Figures 1 (a) to (d) show the OPM images observed on the specimens of Al-0 to Al-3, respectively, deformed by tensile strain to 3%. Some grains seen in the figures involves anneal twins. Anneal twin boundaries on {111}γ planes are indicated by arrows in the OPM photos. In each of parent and twin crystals, there are surface striations. It should be noted in Figs. 1(a) to (d) that the width and interval of the striations inside crystals becomes smaller with increase in the Al content. It has been widely accepted that the striations appeared in the Fe-Mn-Si SMAs are formed by the stress-induced γ → e martensitic transformation, while those in the FeMn-Si-Al TWIP steels are due to the mechanical γ twinning. It is inferred from the variation in the microstructures from Figs. 1(a) to (d) that the deformation mode should continuously change from the stress-induced e martensite to the mechanical γ twins, when the Al content is increased from 0 mass % to 3 mass %. Our previous result [9] showing the linear change in the ductility depending on the Al content also supports this speculation. However, it is difficult to distinguish these two deformation products by OPM observations, because both have plate shapes on the {111}γ habit. Figure 1: Deformation microstructures observed by optical microscopy on the specimens of (a) Al-0, (b) Al-1, (c) Al-2, and (d) Al-3. The observations were performed at the tensile strain of about 3%. The phase identification by means of the XRD was, therefore, performed to investigate semi-quantitatively the dependence of the amount of the e phase on the Al content. It was revealed that the intensity of peaks from the e phase relative to that of peaks from the γ phase gradually decreases with increasing the Al content, though not presented here. However, it is impossible to investigate the amount of the mechanical γ twins by the XRD. In order to confirm the existence of the e phase and the γ twins, the electron diffraction pattern analysis using TEM was employed. Figure 2(a) shows the bright field image taken in the Al-0. The plates observed in Fig. 2(a) were identified as the e phase by the corresponding electron diffraction pattern shown in Fig. 2(b). The incident beam is parallel to [011]γ // [21 1 0]e. The diffraction pattern in Fig. 2(b) clearly shows the well known features of the γ → e transformations: i) the S-N orientational relationship between the γ and e crystals, ii) the streaks along γ directions due to small thickness of the e plates. The streaks run in two directions: i. e. and . The (11 1) and (111 ) traces nominal to the corresponding streaks are seen in Fig. 2(a). Figure 2(c) and (d) show an example of the mechanical γ twins (γTM) observation in the Al-3. The zone axes of the diffraction pattern are [011]γ // [011]γTM. A lamella structure consisted of nano-sized twins and the retained austenite is formed in the specimen, being consistent with the previous results in the literature [10]. After a number of careful observations, a very small amount of the e plates were also found even in the Al-3, although it was undetectable in the XRD profile. 200 111 111 MT 200 MT

Propagation of phase transformation fronts during the stress induced martensitic (SIM) transformation at impact strain rates, on the order of 10 s -1 , was observed in situ by measuring changes in infrared radiation on the surface of superelastic NiTi wires. The exothermic/endothermic character of the forward/reverse SIM transformation changes the local temperature making visible the nucleation and propagation of phase transformation fronts. The nucleation during forward SIM transformation usually occurs at both ends of the sample near the grips, where stress concentrations are unavoidable. During unloading, nucleation of the reverse SIM transformation takes place at point where the forward SIM transformation was finished. At impact no more nucleations were observed so that only the phase transformation fronts arising from the mentioned nucleations appear. In the non-transformed zone, the temperature remains similar to that observed for the test in which only the elastic deformation of the austenitic phase occurs. This feature shows that the SIM transformation at impact strain rates is inhomogeneous and, similarly to that observed at very low strain rates, lower than 10 -4 s -1 , when the deformation may be considered as an isothermal process and the temperature in the sample remains almost constant, no multiple transformation fronts appear as is observed when strain rate is on the order of 10 -4 -10 -2 s -1 . For specimens cycled at impact, the nucleation at the beginning of the SIM transformation occurs at several locations and during the deformation more nucleation points arise since the stress necessary to initiate another nucleations is lower than necessary to continue the propagation of the active fronts. These locations are similar for different cycles showing that they do not arise by chance, but rather because there are locations more favourable for the nucleation. the present work, the phase transformation fronts at impact strain rates, on the order 10 s -1 , were observed on the surface of NiTi wires via thermographic observations. The stress-strain response was simultaneously registered with the thermographic observations in order to link the evolution of the transformation fronts with the mechanical behaviour. 2. Experimental procedure Infrared thermographic pictures were taken at a frame rate of 1250 Hz with a high speed thermographic camera Flir Titanium 550M during tensile deformation of NiTi wires at impact strain rates. The impact tests were carried out with an instrumented tensile impact device. The whole experimental set-up is shown in figure 1. It consists on an impactor which deforms the sample by hitting a mobile grip at which the sample is attached. The impact force is measured by a piezoelectric sensor ICP a quartz force ring attached to the other grip which is fixed. The measurement of the deformation during the impact was carried out with a laser± based noncontacting measurement equipment Polytec OFV± 505. More detailed information of the instrumented tensile impact test applied to SMA wires may be found in (7). During deformation, simultaneous measurements of infrared

In this study, the property improvement of shape memory effect (SME), pseudoelasticity (PE) and stress-strain (σ-e) cycling of Ti49.3Ni50.7 and Ti50Ni50 shape memory alloys (SMAs) is investigated. Ti49.3Ni50.7 SMA aged at 300℃×100h and 400℃×8h can reach the maximal precipitation-hardening with the hardness of the former being higher than that of the latter. Tensile test indicates that the specimen aged at 300℃×100h has better SME/PE and σ-e cycling properties than that aged at 400℃×8h. Cold-rolling effect on the property improvement is studied on Ti50Ni50 SMA. Experimental results show that the degree of cold-rolling lower than 20% is insufficient to strengthen the SMAs to improve their properties, such as the σ-e cycling stability and the recoverable storage energy in σ-e curve. If the annealing of cold-rolled specimen is over, the SMAs’ properties can also be deteriorated. At the same time, the σ-e cycling test indicates that, after 20th cycles, both R-phase and B19’ martensitic transformations are depressed due to the dislocations pile-up during the cycling, and the B2→R transformation is more depressed than R→B19’ one. In this study, the maximal PE strain induced by stress-induced martensite (SIM) is found to be lower than ~7% and the plasticity deformation occurs if the strain is higher than 7% which will deteriorate the SMAs’ PE property. For the strain rate (e ) effect on the property improvement of Ti50Ni50 SMA, in the e range of 2.5×10-4s-1~1.0×10-2s-1, the σ-e cycling with higher e will be more beneficial to the forward SIM transformation, instead of the reverse SIM transformation during the cycling.

The purpose of this paper is to explain the thermodynamical and shape memory behaviour in comparison with structural parameters for Zr-based intermetallics - a new class of potential shape memory alloys (SMA) for high-temperature applications [1]. Electrical resistivity, structural, shape memory and calorimetric measurements were carried out for the Zr2CuNi - Zr2CuCo quasibinary cross-section. It was shown that the martensitic transformation (MT) of the high -temperature B2 phase resulted in the simultaneous formation of the two martensitic phases belonging to the P21/m (B19' type) and Cm space groups in the case of Zr2CuNi similar to ZrCu [2]. Co for Ni substitution causes the changes in the martensite volume fractions up to formation of only B19' type martensite in Zr2CuCo compound without significant changes in lattice parameters for both martensites. Such substitution also decreases generally the transformation heats and energy dissipated during the full cycle of MT. The non-thermoelastic behaviour that was observed in [l] for Zr2CuNi changes to a thermoelastic one in the case of MT in Zr2CuCo. Shape memory effect (SME) is nearly complete for alloys with high Ni content (not less than 85% of shape recovery ratio (Ksme)). It becomes complete at Co additions. The effect of the interaction between two martensites on the non-thermoelastic MT behaviour and SME in Zr-based intermetallics is discussed in the present paper.

Shape memory alloys take place in a class of advanced smart materials by exhibiting a peculiar property called shape memory effect. This phenomenon is initiated with thermomechanical treatments on cooling and deformation and performed thermally on heating and cooling, with which shape of materials cycles between original and deformed shapes in reversible way. Therefore, this behavior can be called Thermoelasticity. This is plastic deformation with which strain energy is stored in the material and releases on heating by recovering original shape. This phenomenon is based on thermomechanical transformations, thermal and stress induced martensitic transformations. Thermal induced transformations are exothermic reactions and occur on cooling with the cooperative movement of atoms in {110} -type directions on {110}-type planes of austenite matrix, along with lattice twinning and ordered parent phase structures turn into twinned martensitic structure. Twinned structures turn into detwinned martensite by means of stress induced martensitic transformation with deformation. Also, reverse austenitic transformation is an endothermic austenitic transformation, and occurs on heating, and detwinned martensite structures turn into the ordered parent phase structure. These transformations are driven by lattice invariant shears. Martensitic and austenitic transformations are solid state transformation, and these transformations do not start at the equilibrium temperature at Gibbs Free Energy Temperature Diagram and a driving force is necessary for the transformations. These alloys exhibit another property called superelasticity, which is performed with stressing and releasing the material in elasticity limit at a constant temperature in parent phase region, and shape recovery occurs upon releasing, by exhibiting elastic material behavior. Stress-strain curve exhibit non-linear behavior, stressing and releasing paths are different, and hysteresis loop refers to the energy dissipation. Superelasticity is the result of stress-induced martensitic transformation, and parent phase structures turn into the completely detwinned martensite structures with stressing. Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures. Lattice invariant shears and lattice twinning are not uniform in these alloys, and the ordered parent phase structures undergo long-period layered structures with martensitic transformation. These structures can be described by different unit cells as 3R, 9R or 18R depending on the stacking sequences on the close-packed planes of ordered lattice of parent phase. In the present contribution, x-ray diffraction and transmission electron microscopy studies were carried out on copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections inherited from parent phase due to the displacive character of the transformation. X-ray diffractograms taken in a long-time interval show that diffraction angles and intensities of diffraction peaks change with the aging time at room temperature. This result refers to a new transformation in diffusive manner. Keywords: Shape memory effect, martensitic transformations, thermoelasticity, superelasticity, lattice twinning and detwinning.

Due to attractive mechanical properties, metastable β titanium alloys have become very popular in many industries including aerospace, marine, biomedical, and many more. It is often the complex interplay among the different deformation mechanisms that produces many of the sought-after properties, such as enhanced ductility, super-elasticity, and shape memory effects. Stress induced martensitic transformation is an important deformation mechanism for these alloys. Understanding of it and the influence it has on the microstructural evolution of materials is of great importance. To this end we have developed a crystal plasticity based constitutive model which accounts for both martensitic phase transformation and slip based plasticity simultaneously in metastable β titanium alloys. We present a new formulation for the evolution of martensite transformation, based on physical principles and crystal plasticity theory. To understand and demonstrate this feature of the model, a parametric assessment of the newly developed constitutive model is conducted. This is followed by first of its kind analyses of stress induced martensitic transformation in metastable β titanium alloys. We firstly present validations against uniaxial loading experiments for different metastable β titanium alloys exhibiting stress induced martensite transformation. As part of this, single crystal simulations in metastable β titanium alloys are used for the first time to investigate the interaction of individual transformation systems during unconstrained transformation. This study shows good agreement between the experimental and simulated responses during all stages of deformation in which elastic, transformation and finally the slip stage are exhibited. Relatively ‘strong’ and ‘weak’ orientations for transformation are observed, consistent with experimental studies. The work done here demonstrates the ability of this crystal plasticity finite element method to capture physical mechanisms while bringing new insight about the interaction of different deformation mechanisms in metastable β titanium alloys.

Aichi Institute of Technology, 1247 Yachigusa, Yakusa-cho, Toyota, 470-0392, Japan; tobushi@aitech.ac.jp Mechanical characteristics and temperature changes related to stress-induced martensite transformation developing in TiNi shape memory alloy have been presented. Exothermic martensite forward and endothermic reverse transformations have been recorded with use of three kinds of infrared cameras, including very fast and sensitive Therma-Cam Fenix DTS. It was found that the temperature distribution on the surface of the specimens was uniform during straining below the austenite start temperature, while investigating shape memory effect, whereas bands of higher temperature corresponding to localized martensitic transformation were recorded during the process carried out above the alloy austenite finish temperature. The shape memory effect (SME) and superelasticity (SE) are the main phenomena which appear in a shape memory alloy (SMA), depending on the test temperature T. They are controlled by two material parameters: the austenite finish (Af) and the austenite start (As) temperatures. If T is higher than Af, the SE appears, and if T is lower than As, the SME appears. The behavior is caused by the stress-induced reversible martensite transformation (MT) which takes place during the SMA loading and unloading. In the case of SE, almost complete reverse transformation occurs during the SMA unloading Figs 4-9. In the case of SME, quite significant residual strain is observed after the SMA unloading (Figs 2,3). The strain related to the martensitic phase disappears, if the specimen is heated after the unloading above the Af temperature. The energy storage and the energy dissipation due to the SE in SMA are very large and the recoverable stress and strain are quite large compared to the traditional metals. The described properties enable the SMA many applications, e.g., as damping elements, driving force of actuators or main parts of heat engines. The MT in general can be induced by variation in temperature or stress, so the SMA behavior depends on the thermomechanical loading conditions (1-4). The main point studied in the paper is the homogeneity of the martensite transformation process, carried out in various conditions, since the homogeneity usually assures higher reliability of the SMA applied systems.

The effects of dynamic interactions between hydrogen and a stress-induced martensite transformation on the recovery of deteriorated tensile properties by ageing in air at room temperature have been investigated for a Ni–Ti superelastic alloy. A specimen is subjected to single stress-induced martensite and reverse transformations immediately after hydrogen charging. Upon tensile testing, brittle fracture occurs in the latter half of the elastic deformation region of the martensite phase after the stress-induced martensite transformation. Upon ageing before the tensile test, fracture occurs during the stress-induced martensite transformation. In addition, the nano- and micro-morphologies of the brittle outer part of the fracture surface of the specimen are changed by ageing. Thus, the tensile properties markedly deteriorate, rather than recover, by ageing. The present results clearly indicate that dynamic interactions between hydrogen and the stress-induced martensite transformation have serious after-effects on hydrogen embrittlement of Ni–Ti superelastic alloy.