Nickel–titanium (NiTi) shape memory alloys are intermetallic compounds that can exhibit a reversible martensitic phase transformation. While extensively studied for biomedical and actuator applications, their potential as electrocatalysts for the oxygen evolution reaction (OER) remains virtually unexplored. Here, we systematically investigate how phase structure influences OER activity in NiTi alloys by comparing martensite Ni50Ti50 (B19’ monoclinic) and austenite Ni51.2Ti48.8 (B2 cubic). Despite differing by only 1.2 at.% Ni, the investigated specimens exhibit markedly different electrocatalytic behavior. In 1 M KOH containing 15 ppb Fe, the martensitic phase requires 40 mV lower overpotential (450 vs. 490 mV at 10 mA cm−2) and maintains stable operation at 1.56 V vs. RHE for 12 h. This improved activity correlates with phase‐dependent properties: enhanced electrical conductivity, finer surface texture, and markedly increased hydrophilicity (contact angle 21° vs. 71°). The martensitic phase also shows a 10% larger electrochemically active surface area. Under elevated Fe levels (150 ppb), the martensite phase undergoes stronger surface restructuring and achieves a 370 mV lower overpotential, indicating superior Fe incorporation. These findings demonstrate that OER performance in NiTi alloys is systematically tunable via microstructural states, establishing phase engineering as a promising strategy for designing next‐generation water‐splitting electrocatalysts.

High entropy shape memory alloys (HE-SMAs) combine unique high entropy-based properties with the functional advantages of shape memory alloys. Specifically, they feature enhanced strength, adjustable transition temperatures, high recoverable stresses and thermal stability. However, common challenges in multicomponent alloy design such as segregation and secondary phases hinder their functionality. The effectiveness of conventional heat treatments in overcoming these challenges is often limited, resulting in sub-optimal performance of promising alloy systems. This study investigates the benefits of thermo-mechanical processing (TMP) as a strategy to control the homogeneity and microstructure of HE-SMAs and thereby increasing their functionality. It focusses on the NiTi-related alloy Ti16.6Zr16.6Hf16.6Co10Ni20Cu20, evaluating the effects of TMP on its microstructural evolution and mechanical performance with the goal to establish reliable functionality. By employing TMP via hot-extrusion, significant improvements in microstructural homogeneity and both mechanical and functional properties were achieved. The effects of TMP included enhanced structural integrity after forming, a 25

The objective of the present work is to demonstrate that Re-segregation to low-angle grain boundaries (LAGBs) is affected by the misorientation angle associated with these boundaries. For this purpose, Re-segregation at two LAGBs with misorientation angles of 0.5 degrees and 5 degrees in a negative misfit Ni-base single-crystal superalloy is characterized. The LAGBs form during directional Bridgman solidification and consist of dislocation networks, which separate small groups of parallel dendrites. Site-specific specimen preparations for complementary investigations using high-resolution electron backscatter diffraction, scanning electron microscopy, scanning transmission electron microscopy, and atom probe tomography (APT) were performed. APT data suggest that a higher level of misorientation is associated with a higher level of Re-segregation. The results obtained in the present work also allow to conclude that grown in LAGBs act as dislocation sources throughout creep. In the early stages of creep, they provide the dislocations which fill the gamma channels and form the dislocation networks at gamma/gamma ' interfaces. During secondary creep, they are subjected to knitting-in and knitting-out reactions, which allow to establish a stress-dependent average dislocation density. It is proposed that in both stages, Re-segregation at LAGBs slows dislocation processes and thus contributes to creep strength.

The present study explores recycling of NiTi shape memory alloys (SMAs) using vacuum induction melting (VIM). Recycling NiTi is considered challenging due to unavoidable carbon and oxygen pick-up, which affects structural and functional properties. A 1 kg high-purity NiTi ingot was prepared from elemental Ni pellets and Ti blocks using VIM with a graphite crucible. The resulting SMA ingot underwent three additional remelting cycles. Samples for chemical, thermal and microstructural analysis were taken from the original ingot and after each remelting step. The study analyzes how different feedstocks-pure Ni and Ti versus NiTi SMAs-affect melt pool temperatures and VIM durations. It was found that the high heat of mixing during alloy formation serves as an internal heat source, contributing to shorter VIM process durations. In contrast, remelting NiTi alloys, which lacks this heat release, relies entirely on external power, which increases the process duration and thus the time available for impurity pick-up. The VIM process is analyzed using CALPHAD-based thermodynamics, combined with novel atomistic simulations using machine-learning potentials to determine thermodynamic conditions. The study assesses energy balances and contributes to a better understanding of how VIM remelting affects the microstructures and functional properties of NiTi SMAs.

Nickel-titanium that has an austenite to martensite phase transition has been studied extensively in the past as a shape memory alloy, but a lot remains to be learned from such phase transitions [1]. However, single crystals are needed for a detailed characterization of the emerging phase transition. In order to produce NiTi single crystals for research purposes, we have set up a micro-pulling-down (& micro;PD) apparatus. The & micro;PD process is a fast and flexible method for the fabrication of small single crystals [2]. The apparatus is operated in vacuum. By pulling the crystal down through a hole in the crucible bottom, it is possible to reduce oxygen contamination, since oxides float on top of the melt due to their low density. Here we present a detailed characterization of as-grown NiTi crystals by electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), hot gas extraction method and differential scanning calorimetry (DSC). The characteristics of the phase transition in NiTi are very sensitive to dopants and alloying. The & micro;PD method facilitates the introduction of different doping elements into the crystal.

The dependence of transformation patterns in superelastic NiTi tubes on tube outer diameter D and wall-thickness t is investigated through quasi-static uniaxial tension and large-rotation bending experiments. The evolution of outer-surface strain fields is synchronized with global stress-strain and moment-curvature responses using a multi-magnification, high-resolution stereo digital image correlation system at 0.5-2x magnifications. The transformation patterns exhibit systematic size-dependent behaviors. Under tension and for a specific D, as the diameter-to-thickness ratio D/t decreases, a decreasing number of fat/diffuse helical bands emerge, in contrast to sharp/slim bands in thin tubes. Consequently, the austenite-martensite front morphology transitions from finely-fingered to coarsely-fingered with decreasing D/t. Below a characteristic D/t, front morphology no longer exhibits patterning and phase transformation proceeds via propagation of a finger-less front. Moreover, the transformation pattern exhibits an interrelation between D and D/t, where a front possessing diffuse fingers is observed in a thin but small tube. Under bending, both the global moment-curvature response and transformation pattern exhibit D-and D/t-dependence. While wedge-like martensite domains consistently form across all tube sizes, their growth is noticeably limited in smaller and thicker tubes due to geometrical constraints. A gradient-enhanced model of superelasticity is employed to analyze the distinct transformation patterns observed in tubes of various dimensions. The size-dependent behavior is explained based on the competition between bulk and interfacial energies, and based on the energetic cost of accommodating martensite fingers. By leveraging an axisymmetric tube configuration as a reference energy state, the extra energy associated with the formation of fingers is quantified.

Low angle grain boundaries (LAGBs) form during directional solidification of Ni-base single crystal superalloys and extend over distances of one to several average dendrite spacings (mm scale). Their influence on the superalloys' behavior has often been overlooked. In the present work we use the high-resolution rotation vector base line electron back scatter diffraction (RVB-EBSD) technique to locate LAGBs, and focused ion beam (FIB) micromachining to prepare specimens for investigations by scanning transmission electron microscopy (STEM) and atom probe tomography (APT). STEM confirms that LAGBs consists of fine dislocation networks. APT provides experimental evidence for the segregation of Rhenium (Re) to these LAGBs in the as-cast state. RVB-EBSD results show that during a multi-step post cast heat treatment, irregular shaped LAGBs straighten out and move over distances of the order of several 10 mu m. Considering this LAGB movement, and in the light of previous reports (He et al., 2020), we propose that Re segregates to LAGBs during Bridgman processing, redissolves during the high temperature (> 1300 degrees C) solution phase, allowing LAGB migration, and then resegregates during the medium temperature (< 1000 degrees C) precipitation phase of the multi-step heat treatment, which immobilizes the LAGBs during creep loading in the 1000 degrees C temperature range. The finding of the present study shades a new light on the behavior of Re and LAGBs in Ni-base single crystal superalloys.

The present work takes an overarching look on how particles can alter the functional and structural properties of shape memory alloys (SMAs). We consider precipitates which form in binary and ternary NiTi-based alloys, including the high-temperature SMAs NiTiHf and NiTiZr. We also take a look at precipitates which form in Cu–Al–Zn, Cu–Al–Ni, and Co–Ni–Ga–SMAs. We take these alloy systems as examples to review different effects of particles on local alloy chemistry and local stress and strain states at particle/matrix interfaces. Their influence on the nucleation and growth of martensite and on the propagation of a martensitic transformation front is discussed, and the consequences for macroscopic functional and structural properties like phase transformation temperatures and widths of thermal hysteresis are highlighted. Emphasis is also placed on particles which form because elements like C and O are picked up during alloy production and affect functional and structural properties. Finally, it is suggested to take a look at additive manufactured SMAs with added inert nano/micro-particles, which would facilitate the investigation of the mechanical constraints, and which may lead to SMAs with improved structural and functional properties.

The present study demonstrates the potential of laser welding nickel-titanium (Ni-Ti) shape memory alloy wires to copper-tin (Cu-Sn) coated printed circuit boards (PCBs), achieving mechanical strengths of up to 705 MPa +/- 23 MPa. Nanosecond laser pulses enabled the formation of robust joints without visible thermal damage to the thermally sensitive PCB. Scanning and transmission electron microscopy revealed a complex microstructure in the fusion zone, including sharp gradients in grain size, and intermetallic phases such as NiTi (B2), NiTiSn, Cu6Sn5, Sn, and Cu. Despite the presence of these microstructural constituents, mechanical testing confirmed a good structural integrity of the joints. Under the specific actuation fatigue testing conditions considered in this study, fatigue failure primarily occurred in the unprocessed base NiTi material, suggesting that the welded regions did not significantly affect fatigue lives. Thermal imaging revealed that the SMA wires in joint regions did not undergo complete phase transformations as the adjacent Cu-Sn-layers of the PCB act as a heat sink. This contributes to the good fatigue performance. The findings of the present study highlight the potential of this joining method for straightforward and reliable integration of NiTi wires into actuator systems.

Ni-rich NiTi-based shape memory alloys in the pseudoelastic state are promising candidates for various applications such as in cardiovascular stents, sensors, actuators, etc. However, achieving the desired pseudoelasticity for such applications mandates the alloy to be subjected to a critically optimized thermomechanical processing schedule. In this study, cold-rolled Ni-rich NiTi specimens are subjected to aging treatment at three different temperatures that are of industrial concern for shape setting. Systematic microstructural investigation reveals the presence of equiaxed grains with martensite structure in the processed alloys. Distinctly different phase transformation characteristics are noted for the cold-rolled and aged NiTi alloys as well. This is reflected in attaining no pseudoelasticity at all for the cold-rolled alloy, while strain recovery is regained for the aged alloys. Interestingly, subtle variation in aging temperature is noted to significantly affect the mechanical performance of NiTi alloy. Nanoindentation-based investigation reveals that highest aging temperature is beneficial in achieving maximum localized hardness for the NiTi alloy that is particularly related to the occurrence of precipitates. In contrary, best indentation depth recovery is noted for the NiTi alloy aged at an intermediate temperature. This particular aging condition also revealed highest pseudoelastic strain recovery for the studied NiTi alloy, at the global scale under monotonic or cyclic incremental tensile loading. Overall, this systematic investigation plays a pivotal role in identifying the optimized thermomechanical treatment consisting of cold rolling and aging at 525 °C to achieve the best combination of mechanical properties including pseudoelasticity for NiTi alloy that is of industrial significance.

Potentiodynamic polarization measurements combined with acoustic emission (AE) monitoring and in-situ observations are employed to investigate the electrochemical behavior of a CuAlMn shape memory alloy with different microstructures. While the polarization responses are broadly similar across microstructures, the surface film composition, morphology, damage mechanisms, and associated AE activity show strong sensitivity to microstructural variation. The AE activity linked to the surface film life cycle (initiation/propagation, dissolution, and mechanical degradation) intensifies progressively with increasing microstructural complexity, from single-crystalline to polycrystalline austenitic, to two-phase (α + β), and finally to martensitic structures. Clustering analysis of AE events reveals that film initiation/propagation is dominated by low-energy events spanning a broad frequency range, whereas dissolution emits low-frequency (39 –120 kHz), high-energy signals across microstructures. Severe mechanical degradation occurs in the α + β and martensitic microstructures via interfacial decohesion (at the α/β interface) and micro/macro cracking, respectively, both releasing AE signals around ∼ 250 kHz. Notably, micro/macro cracking in the martensitic microstructure generates AE with peak amplitudes up to 7 mV and energies three orders of magnitude higher than other degradation mechanisms. These findings demonstrate that internal interfaces play a critical role in controlling surface film evolution and highlight AE monitoring as a powerful tool for discriminating corrosion processes across different microstructures.

Transport properties can help both characterizing the transformation as well as understanding electronic aspects of the martensitic phase transition. Still, most shape memory research mostly relies on resistivity measurement, even though more and sometimes complementary information can be gained from Hall and Seebeck measurements. Since both are only sparsely used within the shape memory community, this paper aims at introducing Hall and Seebeck effects and their measurement principles. We show how the martensitic transformations in NiTi and NiTi-Cu manifest in different transport measurements and explain how to interpret the results. We hope this manuscript encourages researchers on shape memory materials to utilize transport measurements for improved characterization and understanding of martensitic materials.

Principles behind antiferromagnetic spintronics often claim that an antiferromagnet (AF) cannot be manipulated by external magnetic fields and that domain walls (DWs) are highly mobile and do not suffer strong pinning while maintaining robustness during thermal fluctuations. Using dysprosium (Dy) as an example, we take the opposite view. The functionality of a helical AF in Dy uses two intrinsic properties of chiral DWs: their intrinsic magnetization and their freezing just below the N & eacute;el temperature. Experiments with Dy single crystal show that field cooling through the fluctuation regime converts a helical AF into a domain wall-dominated system with striking properties related with robustness of frozen chiral DWs. On the other hand, chiral DWs are easily controlled by external fields throughout the fluctuation regime.

Ultrashort pulse lasers are common tools for surface structuring and marking of components. However, even finest structures generated by ultrashort laser processes may affect the mechanical properties of components. In the present work, the effects of laser-induced surface modifications on the fatigue behavior of pickled nickel titanium shape memory wires are investigated. Ultrafast Bessel beams have been utilized to create small microstructure-covered grooves on a defined area on the wire’s shell surface. Subsequently, the wires underwent fatigue testing until failure. The impact of laser processing on fatigue behavior and the resulting fracture surfaces were examined using scanning electron microscopy. It was found that laser defects generated with low fluences have a negligible impact on fatigue lives, as microcracks, which may cause failure, typically initiate at other more severe types of surface defects. However, we identified certain laser scanning strategies and other conditions like increased fluences, which can affect wire surface regions to become prone to fatigue crack initiation, resulting in reduced service lives.

Abstract NiTi shape memory alloy (SMA) actuators have gained much attention in recent years because of their ability to combine high actuation forces to a significantly smaller component size. Binary SMAs containing Ni and Ti, however, suffer from relatively poor functional and structural fatigue, which requires training. Binary NiTi SMAs also possess a relatively wide hysteresis gap between the austenite final and martensite final temperatures (ΔTMfAf), which demands more energy to produce each actuation stroke. Zarnetta et al. have shown that small additions of substitutional elements to the NiTi-based SMA can significantly improve those conditions by increasing the coherency between the two transforming crystal structures. Two quaternary NiTiCuPd SMAs were selected based on combinatorial studies from X-ray diffraction data and their results were analyzed concerning their hysteresis width and thermal-mechanical stability.

Microstructural changes induced by a hydrogen permeation into the NiTi superelastic alloy were investigated in-situ using the X-ray synchrotron diffraction. A new design of an electrochemical cell enabled to uncover time and position dependent processes under a flat alloy surface exposed to the cathodic hydrogen. The diffraction data supported by thermo-elastic FEM calculations helped to quantify an evolution of compressive stresses in the B2 austenitic phase hosting hydrogen atoms. The compressive stress state initiates a formation of martensitic phases starting from the exposed surface layer and advancing into the alloy volume with increasing time of hydrogen charging. We have performed the ab-initio DFT study in order to rationalize volumetric changes associated with variations in the B2 austenite and B19’ martensite lattice parameters. The numerical results also contributed to the identification of a new hydride phase with orthorhombic crystal structure and lattice parameters a=0.8505 nm, b=0.7366 nm and c=0.4722 nm.

In the present work we consider the AZ31 Mg alloy for applications as medical implants with bone contact. The alloy has the advantages that it combines a high strength with light weight. Most importantly, it eventually dissolves in the body environment and there is therefore no need to remove implants by additional surgery. However, this dissolution should not happen too fast in order to guarantee mechanical integrity during the healing process. This requires the application of appropriate composite coatings. In a first step, an inorganic protective layer is formed by plasma electrolytic oxidation (PEO) using an electrolyte which contains calcium glycerophosphate (CaGP). Special emphasis is placed on the effect of varying CaGP concentrations in the electrolyte, considering molar concentrations between 0.01 and 0.03 mol/l. In a second step, polymer coatings were applied on composite layers to find out whether the system properties can be further improved. Two types of polymer layers were developed using a spin coating method, based on the biodegradable polymer systems polycaprolactone and polyethylene glycol. The results obtained in the present investigation suggest that introducing CaGP into the electrolyte of plasma electrolyte oxidation induces a composite layer consisting of MgO and CaP compounds. The coatings developed in 0.02 mol/l CaGP electrolyte exhibit superior adhesion strength. A significant tenfold increase in corrosion resistance and the highest cell viability are observed on coatings developed in 0.02 mol/l CaGP electrolyte and then superimposed with PCL. These results suggest that a coating on AZ31, after PEO in an electrolyte with 0.02 mol/l CaGP and then superimposed with PCL, represents a promising candidate for applications as bio-absorbable bone implant material. The coated material by far outperforms the material with no surface treatment.

Stereo-digital image correlation (DIC) measurements can be challenging when working with small specimens. Achieving high-precision data requires careful selection of hardware, stereo-rig design, illumination, speckling, calibration, and minimization of noise levels. This study presents an optimized stereo-DIC setup based on parfocal zoom lenses for full-field measurements at magnifications ranging from 0.5× (14.3 × 16.9 mm2) to 2× (3.5 × 4.2 mm2). The advantages of using parfocal zoom lenses over fixed-focal-length lenses (and extension tubes) for full-field measurements at small fields of view (FOVs) are demonstrated through quantitative comparisons of the temporal evolution of pseudo-strains and null strain analysis from rigid body translation experiments. The optimal speckling parameters for each magnification are determined by analyzing gray-level histograms, mean intensity gradient ( ∇G ), and subset size. The challenges of calibration at high magnifications are discussed, along with strategies for obtaining acceptable results. The accuracy of the presented stereo-DIC setup is evaluated through the study of localized phase transformation on a 1 mm diameter superelastic NiTi wire under tension, column buckling, and compression deformations. The presented setup provides highly consistent full-field data over the 0.5–2× magnification range. The results highlight the benefits of using parfocal zoom lenses for stereo-DIC measurements over a range of small FOVs.

AbstractArc-melting (AM) as a primary method for casting high entropy alloys (HEAs) ensures rapid alloy screening with minimal material input, high cost-effectiveness, and high cooling rates. However, the limitations of AM on a laboratory scale, particularly its constrained sample size and the necessity for remelting steps to ensure homogeneity, hampers thorough mechanical and functional testing of bulk materials. Therefore, this study features a comparative analysis between AM and vacuum induction-melting (VIM) techniques for High Entropy Shape Memory Alloys (HE-SMAs) production, focusing on the senary alloy Ti16.6Zr16.6Hf16.6Co10Ni20Cu20, known for its potential functional applications and high sensitivity to material inhomogeneity. The alloy’s composition, including high-melting point elements like Hf, Ti and Zr, makes it a well-suited candidate for assessing the capabilities of VIM in producing homogeneous bulk materials. The employment of binary pre-alloys in both AM and VIM processes reduced the necessity for remelting steps and ensured better initial quality for subsequent heat treatments. A homogenization treatment at 900 °C for 100 h of an AM-produced senary alloy showed only slight improvements compared to the same alloy produced via VIM, largely due to the slow diffusion of the larger Hf and Zr atoms from the dendrites into the solid solution. This suggests that VIM can achieve comparable levels of homogenization in substantially less time than required for AM-treated samples. The findings finally indicate that by using VIM, when combined with binary pre-alloys, one achieves more homogeneous alloys with reduced heat-treatment time, making it a viable method for HE-SMA production.

Transformation-induced plasticity (TIP) is the source of poor reversibility and functional fatigue in shape memory alloys (SMA)s undergoing martensitic phase transformation. The TIP is believed to originate from the defect generation at the highly-stressed austenite/martensite transition layer. It is suggested that under satisfaction of compatibility criteria (also known as cofactor conditions) the highly-stressed transition layer is removed and the phase transformation reversibility is highly enhanced. In this study, we employ the inclusion problem of micromechanics along with crystallographic theory of martensite to quantify the generated internal stresses associated with the nucleation of martensite in a wide range of SMAs. The micromechanical calculations show that the nucleation of martensite in SMAs with high degree of compatibility and enhanced reversibility generates much reduced internal stresses. For example, the maximum shear stress generated by nucleation of twinned martensite in Ni50.3Ti29.7Zr20 is - 5 GPa, which is reduced to - 400 MPa (about an order of magnitude reduction) in single-variant martensite of Ni39Ti50Pd11 with excellent reversibility and low functional fatigue. It is also shown that the internal stresses are much lower in a supercompatible twinned martensite microstructure (Cu25Au30Zn45) compared with a conventional twinned microstructure such as those formed in NiTi-based alloys. Analysis of the deformation state of the matrix shows that the underlying mechanism behind the reduced internal stresses is the reduction in the maximum shear strain ((lambda 3 - lambda 1)/2) imposed by the nucleation of martensite. The results present a quantified picture of internal (interfacial) stresses in different SMAs to better understand the origin of reversibility and functional fatigue.