Reverse Martensitic Transformation Research Articles

Reverse ε→γ transformations of temperature-induced and stress-induced martensites in Fe–Mn–Si alloys with shape memory effect

In this work we argue that the reverse martensitic transformation (MT) of the stress-induced martensite (SIM) of Fe–Mn–Si-based alloys for moderate prestrains occurs in essentially thermoelastic-like way, in contrast to non-thermoelastic MT of temperature-induced martensite (TIM). We compare, using a number of complementary experimental techniques, the temperatures and the kinetics of the reverse ε→γ martensitic transformation of SIM and TIM for commercial and model Fe–Mn–Si alloys after various thermomechanical treatments. We claim that two studied phenomena are generic in Fe–Mn–Si-based alloys of different compositions and microstructure: first, opposite to the conventional effect of stabilization of martensite by prestrain, the reverse MT during heating of alloys prestrained in bending and tension starts well below (up to 100 K) the start temperature of the reverse MT for the TIM of the same alloy. The difference between engineering temperatures of the start of the reverse transformation, As, for SIM and TIM reaches ca. 50 K, pointing to a need to consider different As for TIM and SIM, AsTIM and AsSIM. Second, the reverse transformations of TIM and SIM show very different kinetics. The reverse MT of SIM is diffuse over the temperature range exceeding 100 K. The reverse MT of TIM, on the contrary, is well localized within the temperature range of ca. 20 K. The experimental observations can be interpreted assuming that, due to the differences in the formation mechanisms of SIM and TIM, the latter shows non-thermoelastic behaviour, whereas the SIM shows similarities with the thermoelastic martensites stabilized by prestrain.

Transformation pathway of NiTi shape memory alloy and its modulation based on grain size gradient: A molecular dynamics study

The reduction of grain size to nanoscale is a common approach to enhance material properties, such as fatigue resistance and overall strength of Shape Memory Alloys (SMAs), but it normally decreases the volume fraction of the material undergoing reversible martensitic transformation (MT), leading to a reduction in the reversible strain amplitude. This paper explores the atomic relationship between specific transformation pathways and grain size/gradient of NiTi polycrystal based on molecular dynamics simulation. It is found that during a tensile loading-unloading cycle, four different transformation pathways can be observed, with their volume fractions being dependent on grain size. Importantly, these transformation pathways serve as the medium of the grain size effects on the global mechanical properties of NiTi. The gradient nanograined (GNG) NiTi with large gradients can maintain a high stress level while reducing the residual strain by adjusting the fraction of different transformation pathways, thus achieving both advantages of small residual strain and high strength during cyclic loading with large strain amplitude. The results provide atomic insights into the grain size effects on the transformation pathways of SMAs and the development of high-performance SMAs based on the grain size gradient.

Phase-field damage simulation of subloop loading in TiNi SMA

In practical applications, TiNi shape memory alloys (SMAs) exhibit behavior that can pose a challenge with current constitutive models and their implementations in finite element method (FEM) software. TiNi SMA devices typically operate in the forward or reverse martensitic transformation regime, which is known as subloop loading. During such cyclic loading–unloading, the hysteresis stress–strain loop changes because of material damage, which can be considered the fatigue of TiNi SMAs. During both the loading and unloading processes, the stress plateau decreases. At the same time, the accumulated (residual) martensitic transformation strain increases. In this study, the experimental investigation results and observations of the aforementioned phenomena are presented. Next, the phase-field damage model is employed, along with a modified Lagoudas constitutive model, to simulate the change in stress–strain hysteresis. Furthermore, a fatigue function is used to simulate the accumulation of martensitic transformation strain. The experimental stress–strain response is compared with the simulation results, and good quantitative and qualitative agreement is obtained. The damage and martensitic volume fraction with respect to strain are discussed for full-loop and subloop loading. The observations and conclusions, as well as open questions, are presented. Possible directions for future research are provided.

Investigating the reversible nature of the magnetocaloric effect under cyclic conditions of the Ni50Mn34In15Ga1 magnetic shape memory alloy

This study explores the reversibility of the martensitic transformation (MT) and magnetocaloric (MC) response of the Ga-doped Ni50Mn35In15 magnetic shape memory alloys (MSMAs) with a Heusler structure. The direct and reverse MT occurs between temperatures TM ≈ 257 K and TA ≈ 266 K, respectively. The large MC effects resulting from the magnetic-field-induced first-order MT render these materials promising for room-temperature magnetic refrigeration. On account of both a low thermal hysteresis of MT (ΔThyst = 4 K) at a magnetic field of 2 T and a highly reproducible peak value of the adiabatic temperature change at MT (|∆Tad| ≈ 1.3 K for µ0H = 1.9 T), Ni50Mn34In15Ga1 MSMA emerges as a benchmark material for studying a cyclic stability of MC effects. During the first magnetic switching cycle, a reduction of ∆Tad at MT by approximately 1.3 is observed, significantly lower than the reported one for other MC materials undergoing similar first-order phase transitions. Subsequent cycles revealed a consistent stability of the magnetic-field-induced ∆Tad even after more than 200 magnetic field switching cycles. These findings suggest a notable degree of reversibility of the MT in the studied MSMA, which was also confirmed in the present work by a Temperature-First Order Reverse Curve distribution analysis.

A Giant Magneto-Superelasticity of 5% Enabled by Introducing Ordered Dislocations in Ni34Co8Cu8Mn36Ga14 Single Crystal.

Elasticity, featured by a recoverable strain, refers to the ability that materials can return to their original shapes after deformation. Typically, the elastic strains of most metals are well-known 0.2%. In shape memory alloys and high entropy alloys, the elastic strains can be several percent, as called superelasticity, which are all triggered by external stresses. A superelasticity induced by magnetic field, termed as magneto-superelasticity, is extremely important for contactless work of materials and for developing brand-new large stroke actuators and high efficiency energy transducers. In magnetic shape memory alloys, the twin boundary motion driven by magnetic field can output a strain of several percent. However, this strain is unrecoverable when removing the magnetic field and hence it is not magneto-superelasticity. Here, a giant magneto-superelasticity of 5% in a Ni34Co8Cu8Mn36Ga14 single crystal is reported by introducing arrays of ordered dislocations to form preferentially oriented martensitic variants during the magnetically induced reverse martensitic transformation. This work provides an opportunity to achieve high performance in functional materials by defect engineering.

Hyper‐Elastic Deformation via Martensitic Phase Transformation in Cadmium Telluride

Cadmium telluride (CdTe) is a highly promising material for photovoltaics (PV) and photodetectors due to its light‐absorbing properties. However, efficient design and use of flexible devices require a deep understanding of its atomic‐level deformation mechanism. Herein, uniaxial compression deformation of CdTe monocrystalline with varying crystal orientations is investigated using molecular dynamics (MD) with a newly developed machine‐learning force field (ML‐FF), alongside in‐situ micropillar compression experiments. The findings reveal that CdTe bulk deformation is dominated by reversible martensitic phase transformation, whereas CdTe pillar deformation is primarily driven by dislocation nucleation and movement. CdTe monocrystals possess exceptional super‐recoverable deformation along the <100> orientation due to hyper‐elastic processes induced by martensitic transformation. This discovery not only sheds light on the peculiarities observed in micropillar experimental measurements, but also provides pivotal insights into the fundamental deformation behaviors of CdTe and similar II–VI compounds under various stress conditions. These insights are crucial for the innovative design and enhanced functionality of future flexible electronic devices.

Endowing low fatigue for elastocaloric effect by refined hierarchical microcomposite in additive manufactured NiTiCuCo alloy

NiTiCu-based shape memory alloys have been considered as ideal materials for solid-state refrigeration due to their superb cycling stability for elastocaloric effect. However, the embrittlement and deterioration caused by secondary phase and coarse grains restrict their applications, and it is still challenging since the geometric components are required. Here, bulk NiTiCuCo parts with excellent forming quality were fabricated by laser powder bed fusion (LPBF) technique. The as-fabricated alloy exhibits refined three-phases hierarchical microcomposite formed based on the rapid cooling mode of LPBF, composed of intricate dendritic Ti2Ni–NiTi composite and nano Ti2Cu embedded inside the NiTi-matrix. This configuration endows far superior elastocaloric stability compared to the as-cast counterpart. The low fatigue stems from the strong elastic coupling between the interphases with reversible martensite transformation, revealed by in-situ synchrotron high-energy x-ray diffraction. The fabrication of NiTiCuCo alloy via LPBF fills the bill of complex geometric structures for elastocaloric NiTiCu alloys. The understanding of interphase micro-coupling could provide the guide for designing LPBF fabricated shape memory-based composites, enabling their applications for special demands on other functionalities.

High thermal stability effect of vanadium on the binary CuAl base alloy for a novel CuAlV high-temperature shape memory alloy

Abstract In this study, two high-temperature shape memory alloys (HTSMAs) of CuAlV with unprecedented chemical compositions were fabricated using the arc melting technique, followed by traditional ice-brine water quenching after the melting process. To characterize the shape memory properties and structure of the alloys, a series of tests including differential calorimetry (DSC and DTA), EDS, optical microscopy, and XRD were conducted. The DSC tests, performed at different heating and cooling rates, demonstrated highly stable reversible martensitic phase transformation peaks at high temperatures, which were also confirmed by the results of DTA tests. Microstructural XRD and optical microscopy tests were conducted at room temperature, revealing the martensitic structure of the alloys in both cases. Based on all the results, the effects of different minor amounts of vanadium additives directly on the CuAlV alloy were discussed.

A low-density polymer/CrMnFeCoNi composite with high strength and high damping capacity

A novel damping composite was successfully prepared by taking a porous CrMnFeCoNi high-entropy shape memory alloy as the skeleton and filling its pores with the composite composed of carbon nanotubes and polyurethane/epoxy interpenetrating polymer networks. When the porosity, pore size and CNT loading are 80 %, 1.2 mm and 2 wt%, respectively, the compressive strength, elastic modulus and energy absorption capacity of the composite are 35.7 MPa, 1.31 GPa and 23.1 MJ/m3 (ε = 65 %), respectively. Furthermore, it has a mere density of 2.525 g/cm3. Its loss factor is greater than 0.093 and can reach a maximum of 0.145 within the temperature and frequency range of 20 ∼ 150 ℃ and 0.1 ∼ 200 Hz. A triple-phase micromechanical model was utilized to explore the damping mechanism of composites. Results indicate the coupling of multiple damping mechanisms is the reason for the high ground-state damping of composites, and interface damping is the primary damping mechanism. The superposition of the ε → γ reverse martensite transformation peak of the CrMnFeCoNi HESMA skeleton and the glass transition peak of the CNTs/polymer composite matrix realizes the wide damping temperature range of the composite.

Improving ductility of high strength nanotwinned steel with reverse transformation of epsilon martensite inside nanotwin lamellae

In this work, the reverse transformation of ε-martensite located inside nanotwin lamellae has been introduced to improve the ductility of a high-strength Fe–Mn–C steel with a high density of nanotwins. The steel was designed to have a low stacking-fault energy (SFE, 17.8 mJ/m2) to introduce high density of nanotwins through pre-deformation. Meanwhile, the fine ε-martensite was also obtained in nanotwin lamellae. The steel was strengthened significantly, with an increase in yield strength from 357 to 1275 MPa by the introduction of nanotwins. A subsequent intermediate annealing was applied to control the reverse phase transformation from ε-martensite to austenite to enhance the deformation ability of the nanotwinned steel (rising tensile elongation from 5 to 16%). The nanotwins with a width of 32 nm on average were stable during the heat treatment. The high strength was retained because the size of the austenite transformed from ε-martensite was restrained by the stable nanotwins. This strategy holds promise and can be widely applied to alloys with low SFE.

Nondissipative Martensitic Phase Transformation after Multimillion Superelastic Cycles.

Superelastic alloys used for stents, biomedical implants, and solid-state cooling devices rely on their reversible stress-induced martensitic transformations. These applications require the alloy to sustain high deformability over millions of cycles without failure. Here, we report an alloy capable of enduring 10×10^{7} tensile stress-induced phase transformations while still exhibiting over 2% recoverable elastic strains. After millions of cycles, the alloy is highly reversible with zero stress hysteresis. We show that the major martensite variant is reversible even after multimillions of cycles under tensile loadings with a highly coherent (11[over ¯]0)_{A} interface. This discovery provides new insights into martensitic transformation, and may guide the development of superelastic alloys for multimillion cycling applications.

Insensitivity of nucleation rate to order-disorder interfaces in reversible thermoelastic martensitic transformations

Insensitivity of nucleation rate to order-disorder interfaces in reversible thermoelastic martensitic transformations

Phase field study on phase stability at high temperatures in nanograined Fe–Ni alloy prepared by plastic deformation

The phase stability at high temperatures in nanograined Fe–Ni alloy prepared by surface mechanical attrition treatment (SMAT) and ball milling (BM) modes are drastically different. For example, the onset temperature (As) of reverse martensitic transformation (RMT) of nanocrystal prepared by SMAT is higher than that of coarse crystal, while BM exhibits the opposite result. The elastoplastic phase field model for RMT was first proposed to investigate the correlation between RMT and its forward martensitic transformation (FMT), in turn, the above experimental phenomena were revealed in this paper. The prepared nanocrystals with the same diameter (15 nm) by SMAT and BM mode were taken as representative examples. The simulation results indicated that the strain energy is relaxed by the applied strain during FMT in nanocrystalline prepared by smaller plastic deformation mode (e.g. SMAT), but it is enhanced by larger plastic deformation mode (e.g. BM). The stored strain energy in BM mode is much larger than that in SMAT mode, and hence the driving force provided for subsequent RMT is also larger in BM mode. Therefore, the phase stability of the martensitic nanocrystals prepared by BM mode at high temperatures is much lower than the counterpart by SMAT mode, which agrees well with the experimental measurements.

A study of the reverse martensitic transformations in austenitic stainless steels by calorimetry and isothermal heat treatments

A study of the reverse martensitic transformations in austenitic stainless steels by calorimetry and isothermal heat treatments

Pressure tuning reverse martensitic transformation in the Mn0.9Co0.1NiGe half-Heusler alloy

Here we investigate the structural properties of the Mn0.9Co0.1NiGe half-Heusler alloys under pressure up to 12 GPa by Synchrotron angle-dispersive x-ray diffraction (XRD). At room temperature and pressure, the compound exhibits only the hexagonal NiIn2-type structure. Lowering the temperature to 100 K at ambient pressure induces an almost complete martensitic phase transformation to the orthorhombic TiNiSi-type structure. With increasing pressure, the stable orthorhombic phase gradually undergoes a reverse martensitic transformation. The hexagonal phase reaches 85% of the sample when applying 12 GPa of pressure at T = 100 K. We further evaluated the bulk modulus of both hexagonal and orthorhombic phases and found similar values (123.1 ± 5.9 GPa for hexagonal and 102.8 ± 4.2 GPa for orthorhombic). Also, we show that the lattice contraction induced is anisotropic. Moreover, the high-pressure hexagonal phase shows a volumetric thermal contraction coefficient α v ∼ −8.9(1) × 10−5K−1 when temperature increases from 100 to 160 K, evidencing a significant negative thermal expansion (NTE) effect. Overall, our results demonstrate that the reverse martensitic transition presented on Mn0.9Co0.1NiGe induced either by pressure or temperature is related to the anisotropic contraction of the crystalline arrangement, which should also play a crucial role in driving the magnetic phase transitions in this system.

Heat Treatment Effect on Thermal, Micro-Crystal Structure and Magnetic Behavior of Ni45Mn40Sn10Cu5 Heusler Shape Memory Alloy

Ni-Mn-Sn-based Heusler alloys are important materials that have potential applications as environmentally friendly smart materials with beneficial properties as well as being magnetic. Both the magnetic field-induced reverse martensitic transformation and the high operating temperature are crucial for the applications of Ni-Mn-Sn-based magnetic shape memory alloys. In this study, martensite transformation temperatures were determined by applying different heat treatments (700 °C, 900 °C, 1100 °C) to Ni45Mn40Sn10Cu5 sample produced by Arc Melter melting method. While reverse conversion was observed in T2 (no heat treatment) and 700°C heat treatment, no reverse conversion was observed at 900°C and 1100°C. When the X-ray diffractogram was examined, martensite phase and γ phase were determined. When the magnetic hysteresis of the samples was examined, it was observed that the magnetic saturation went towards zero with the increase in heat treatment and accordingly, there was a decrease in the magnetization effect.

Effect of Al doping on the kinetics of reverse martensitic transformation in Ni-Mn-In Heusler alloys

In this work, the effects of Al on the microstructure and kinetics of reverse martensitic transformation in Ni50Mn34In16-xAlx (x = 0, 0.5, 1, and 1.5 (% at)) Heusler alloy was studied. The structural and phase transformation behavior of the samples was evaluated by XRD, FE-SEM, and DSC techniques. In order to evaluate the transformation kinetics parameters in the non-isothermal state upon the heating rates of 5, 10, and 15 K/min, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and isoconversional methods were utilized. The results indicated that the reverse martensitic transformation temperatures and activation energy shifted to higher values due to doping the Al content up to 1 at.%. Conversely, the substitution of In by 1.5 at.% Al leads to a decrease in the transformation temperatures and activation energy. Furthermore, due to the lack of dependence on the activation energy and Avrami exponent on the degree of transformation, the reverse martensitic transformation is single-stage for all samples.

Realizing reversible phase transformation of shape memory ceramics constrained in aluminum

Small-scale shape memory ceramics exhibit superior shape memory or superelasticity properties, while their integration into a matrix material and the subsequent attainment of their reversible tetragonal-monoclinic phase transformations remains a challenge. Here, cerium-doped zirconia (CZ) reinforced aluminum (Al) matrix composite is fabricated, and both macroscopic and microscopic mechanical tests reveal more than doubled compressive strength and energy absorbance of the composites as compared with pure Al. Full austenitization in the CZ single-crystal clusters is achieved when they are constrained by the Al matrix, and reversible martensitic transformation triggered by thermal or stress stimuli is observed in the composite micro-pillars without causing fracture in the composite. These results are interpreted by the strong geometric confinement offered by the Al matrix, the robust CZ/Al interface and the local three-dimensional particle network/force-chain configuration that effectively transfer mechanical loads, and the decent flowability of the matrix that accommodates the volume change during phase transformation.

Achieving enhanced cryogenic toughness in a 1 GPa grade HSLA steel through reverse transformation of martensite

The effect of microstructure on crack resistance and cryogenic toughening in a 3.5 wt% Ni high-strength low-alloy (HSLA) steel was investigated. Multistage heat treatments involving quenching (Q), lamellarization (L), and tempering (T) were applied to prepare the HSLA steels with various microstructures, focusing on the reverse transformation and reconfiguration of martensite, as well as its influence on impact crack formation and propagation behavior by multi-scale characterizations. The results indicate that lamellarization treatment has little influence on tensile properties, but significantly improves impact toughness in the QL and QLT specimens, which exhibit over 30 % increment in Charpy V-notch (CVN) absorbed energy Et tested in range from RT to −196 °C, over 25 °C decrement in ductile-brittle transition temperature (DBTT), and much higher crack propagation energy Ep and higher ratio of Ep/Et, as compared with the as-quenched (AQ) and QT specimens. The lamellarization treatment also contributes to a significant refinement effect on martensitic block size, caused by fresh martensite transformation from the reversed austenite, resulting in an increment in high angle grain boundaries (HAGBs), with introduction of a small amount of retained austenite (RA) as well. Therefore, the impact crack resistance and cryogenic toughness is improved in specimens processed with lamellarization treatment, due to the enhancement in crack deflection and hindering effect of the HAGBs, as well as toughening effect by in situ austenite-to-martensite transformation of the RA. Based on the present study, a 1 GPa grade HSLA steel with high ductility and excellent cryogenic toughness can be produced.

Influence of NiTi Spring Dimensions and Temperature on the Actuator Properties

Nitinol (NiTi) is used in many medical applications, including hard tissue replacements, because of its suitable characteristics, including a close elastic modulus to that of bones. Due to the great importance of the mechanical properties of this material in tissue replacements, this work aims to study the hysteresis response in an attempt to explore the ability of the material to remember its previous mechanical state in addition to its ability to withstand stress and to obtain the optimal dimensions and specifications for the manufacturer of NiTi actuators. Stress-strain examination is done in a computational way using a mutable Lagoudas MATLAB code for various coil radii, environment temperatures, and coil lengths. The computational methodology was done by varying the dimensions and the ambient temperature of the simulated NiTi spring actuator. The hysteresis loop is studied by increasing the external stress for a reversible martensitic transformation. The coil radius, spring height, and wire radius affect the spring force and deformations. In the same way, these parameters affect the strain and stress point values. These changes are shown through the martensite and austenite start and finish values. The NiTi hysteresis loop narrows with increasing ambient temperature or initial spring height. At a higher temperature, the force supplied to the actuator must be less for the same deformation; therefore, a higher ambient temperature provides more efficiency for the shape memory devices and a longer lifetime for the actuator.