Data and metadata from additively manufactured shape memory alloys (AM SMAs) were collected and utilized to illustrate trends and anomalies of current state-of-the-art additive manufacturing (AM) processes. Input parameters, output parameters, and properties were documented from six alloy families spanning nearly two decades of research and development: NiTi, NiMn, NiCo, FeMn, CuAl, and others. Data analyses revealed several key challenges and opportunities for advancing the field, including standardization of terminology, increased reporting of significant process parameters, and inclusion of more feedstock material information. These analyses demonstrate that improvements in these areas in future work can help ensure that reported results are easier to reproduce. Still, to date, microstructures and thermomechanical properties of SMAs have shown promising results, in some cases comparable with conventionally processed materials. This review documents some of these trends and opportunities while providing suggestions toward more data-centric reporting practices in AM SMA research. Furthermore, the dataset that was curated for this review can be analyzed in future investigations using data science approaches to further SMA alloy design and AM processes.
Shape memory alloys (SMAs) have gained much attention in the fields of aerospace engineering and elastocaloric refrigeration due to their improved performance and high work output density. Unfortunately, NiTi SMAs still suffer from poor phase compatibility issues, affecting their thermomechanical stability and functional fatigue. Previous studies have shown that small additions of substitutional elements to NiTi-based SMAs can improve those areas by further improving the degree of phase compatibility between the austenite and martensite phase transformation. The primary goal of this study was to investigate the additions of Cu and Pd to a NiTi-based SMA, and to identify processing routes to produce thermomechanically stable low-hysteresis shape memory alloys (LHSMAs) for actuator applications. The results were compared regarding hysteresis width and thermomechanical stability by way of differential scanning calorimetry, scanning electron microscopy with energy dispersive spectroscopy, and compressive thermomechanical testing. Two quaternary NiTi-based SMAs containing small additions of Cu and Pd exhibited promising results regarding low-hysteresis width and excellent thermomechanical stability.
The influence of the Hf/Zr ratio on the martensitic transformation and functional behaviour was studied in Ti30(Hf,Zr)20Ni30Cu10Co10 and Ti16(Hf,Zr)34Ni16Cu17Co17 high-entropy shape memory alloys. Hf/Zr concentration ratios were equal to 1/3, 1/1, and 3/1. The study findings revealed that substituting Hf atoms with Zr atoms increased the transformation temperatures in high-entropy Ti–Hf–Zr–Ni–Cu–Co shape memory alloys. On cooling under stress of 400 MPa, the Ms temperature increased from −123 to −45 °C when Hf/Zr ratio changed from 1/1 to 1/3 in Ti16(Hf,Zr)34Ni16Cu17Co17 alloys. Overall, this improved the functional behaviour of the alloys due to a larger volume fraction of martensite forming upon loading or cooling under stress. So, a change in Hf/Zr ratio from 3/1 to 1/3 increased the maximum recoverable strain from 0 to 6
The present paper focuses on the evolution of micro- and nanostructures during a cyclic heat treatment in a Fe41Mn34.2Al14.1Ni7.7Cr3 (at.-
This study investigates the thermomechanical behavior of Ni50.3Ti29.7Hf20 high-temperature shape memory alloys (HTSMAs), examining the effect of different aging treatments. Samples were produced via vacuum induction melting and extrusion, and aged at 500, 525, and 550 °C for 3 h. Material characterization involved differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and Vickers microhardness. Small punch testing (SPT) was employed to assess mechanical performance at 22 and 150 °C. Results indicated that aging promoted the formation of nanoscale H-phase precipitates, with their maximum presence and intensity at 525 °C. This correlated with the highest transformation temperatures and peak hardness values. SPT revealed temperature-dependent deformation behavior: Martensite-rich samples (22 °C) demonstrated higher fracture resistance, more cycles before failure, and greater ductility compared to austenite-rich samples (150 °C), which showed earlier degradation and brittle fracture. Cyclic SPT confirmed better loop stability and recoverability for martensitic samples. Aging at 525 °C optimizes the balance of transformation temperature, hardness, and mechanical resilience for HTSMA applications, contributing to the validation of SPT as an efficient characterization method for these alloys.
Directed energy deposition using a laser beam and wire feedstock (DED-LB/w) is a promising route for fabricating NiTi components, yet its usable parameter space is narrow because melt pool dynamics, evaporation, and thermal gradients can modify composition and functional behavior. A three-dimensional multiphysics CFD model is developed to simulate melt pool evolution during DED-LB/w of NiTi. The novelty of this work lies in developing and experimentally validating, to the authors’ knowledge, among the first DED-LB/w-specific multiphysics CFD frameworks for NiTi that simultaneously resolves free-surface evolution, Marangoni-driven flow, buoyancy, evaporation-induced heat loss, and recoil pressure while quantitatively linking these transient melt pool physics to experimentally measured single-track geometries. The model solves the coupled mass, momentum, and energy equations using a VOF free-surface formulation with enthalpy–porosity solidification, temperature-dependent properties, Marangoni convection, buoyancy, evaporation, and recoil pressure. Simulations were performed over 600–1200 W and 300–700 mm/min, and validation was conducted using experimentally measured melt pool depth, width, and buildup height from transverse cross-sections. The 600 and 800 W validation cases are presented in the main text, while the 1000 and 1200 W cases are included in the Supplementary Material. Increasing power shifts the melt pool from compact, conduction-dominated behavior to larger, convection-dominated pools with stronger unsteadiness and evaporation, whereas increasing scan speed reduces thermal exposure and raises the risk of insufficient fusion. Beyond validating melt pool dimensions, the study introduces a process physics-based interpretation of DED-LB/w NiTi through temperature-history analysis, heat flux–temperature correlations, and correlation heatmaps of thermophysical and flow-related variables, providing a more mechanistic basis for process window selection than geometry comparison alone. The results suggest a practical operating window at 600–800 W and 500–600 mm/min that balances adequate penetration with reduced overheating and provide a foundation for coupling DED-LB/w thermal histories with microstructure and phase transformation models for functional-property prediction.
Nitinol technology, besides utilizing the functional thermomechanical properties derived from the B2 cubic to B19′ monoclinic martensitic transformation, also exploits the excellent plastic deformability of NiTi in the martensite state. It originates from the unique mechanism of plastic deformation of the B19′ martensite by kwinking involving dislocation slip-based kinking assisted by deformation twinning. Although the mechanism of plastic deformation of martensite by kwinking was revealed only very recently, various unusual phenomena that can only be rationalized by kwinking have been reported in literature in the last 50 years. These phenomena include the following: (1) cold working with a high degree of reduction without introducing cracks, (2) excellent plastic deformability in the martensite state (plastic deformation up to 80
This paper describes the development and application of an outreach workshop on shape memory material for 10- to 14-year old children. The aim is not only to demonstrate the extraordinary capabilities of shape memory and superelastic materials but also more specifically to have the children design their own shape memory part and have fun during doing so. In other words, to take the step from the simple demonstration of a scientific phenomenon to the full participation in the development of a shape design. Moreover, by allowing them to take home their own design, they can still enjoy and even demonstrate the magic of the shape memory to their friends and family in the weeks and months to come.
High-load actuators based on shape memory alloys (SMAs) hold significant potential for industrial applications. When controlled precisely, such actuators are suitable for machine tool applications, including automated fine alignment of guiding rails or fine positioning of large workpieces. Unlike conventional SMA wire actuators, these devices have large cross-sections, operate under compressive loads, and generate high actuation forces. However, their accuracy is limited by the nonlinear material behaviour during the martensite-to-austenite phase transformation. Additionally, the large SMA cross-section results in considerable thermal inertia, leading to notable temperature gradients and spatial variations in phase fractions. To support the development of advanced control concepts, a comprehensive model of these actuators has been derived. The model introduces martensite fraction as a state variable to describe hysteresis in the phase transformation, utilising a Preisach model to capture the behaviour of the Ni45Ti50Cu5 alloy. Thermomechanical properties such as heat capacity, thermal conductivity, thermal expansion, and temperature-dependent stress–strain behaviour were identified experimentally. A thermal lumped-parameter network model, coupled with a mechanical model, is developed to account for thermal gradients and axial stress distribution, providing accurate predictions validated by thermographic measurements. The material and hysteresis models can be applied to various high-load actuator configurations with similar alloy compositions.
Elastocaloric cooling (eC) using shape memory alloy (SMA) is a greenhouse gas-free alternative to conventional vapor-compression refrigeration. Stress-induced eC effect requires that the SMA refrigerants structure have both high heat transfer efficiency and robust mechanical stability. In this work, we fabricate porous refrigerants from NiTi SMA thin sheet using laser cutting and stamping. A maximum adiabatic temperature drop of 23.4 K under 900 MPa compression is achieved in NiTi refrigerant. Compared to existing wire-cutting techniques, our method increases processing speed by at least one order of magnitude and achieves 90
The superelasticity in shape memory alloys is a useful property for many applications. It is due to a martensitic transformation induced by applied stress and can reach up to about 10
Risk assessment of NiTi requires careful consideration of Ni release for safe application. Although testing standards have been updated, fundamental aspects affecting Ni release from NiTi remain debated. This study aims to contribute to the fundamental understanding of Ni release by investigating the influence of thermomechanical pre-treatment on short- and long-term Ni release from electropolished NiTi wires. The wires were heat treated and strained + released before immersion in Ringer’s solution. Ni release was determined using inductively coupled plasma mass spectrometry. Microscopic analysis showed a smooth and homogeneous surface before straining, followed by formation of cracks and local removal of oxide after deformation and immersion testing, respectively, correlated with higher Ni release. Higher short-term Ni release was observed for all conditions, decreasing over time regardless of heat treatment duration. Compared to previous release tests on mechanically polished wires, electropolished wires showed lower short-term release of Ni. The results are discussed regarding factors influencing Ni release, including pre-deformation, oxide layer characteristics and the formation of metastable Ni-rich phases beneath the surface oxide. The highest Ni release was observed for conditions correlating with the formation of metastable Ni4Ti3 and NiTi supersaturated in Ni beneath the oxide layer.
This study is a comparative investigation on the structures of temperature-induced and stress-induced R phase in a Ti50Ni48Fe2 alloy by means of in situ X-ray. It is found that the thermally induced transformation proceeds in two distinct stages: an initial first-order transition, characterized by a sudden increase in both lattice distortion and atomic shuffle, followed by a second stage wherein these parameters increase continuously with lowering the temperature. In comparison, the structure of the stress-induced R phase remains unchanged with increasing stress. A direct comparison reveals that the structural parameters, i.e., lattice distortion and atomic shuffle, of the stress-induced R phase correspond closely to those of the thermally formed R phase at the testing temperature.
NiTi shape memory alloys (SMAs) are widely used in applications ranging from medical devices to aerospace and automotive structural actuators, yet their performance is often limited by large hysteresis and highly nonlinear pseudoelasticity, which can reduce efficiency and hinder precise control. This study presents a computational investigation of two microstructural engineering strategies aimed at achieving a more linearized and predictable stress–strain response. The first strategy, termed soft confinement, introduces concentration modulations (CMs) produced by dissolving Ni4Ti3 nanoprecipitates, generating smooth spatial gradients in the martensitic start temperature (Ms) through controlled variations in the Ni concentration field. The second strategy, hard confinement, employs amorphous–crystalline composite microstructures in which a non-transforming amorphous phase serves as a robust physical barrier to martensitic progression. Phase-field simulations are used to systematically evaluate the effectiveness of each approach. The results demonstrate that both soft and hard nanoconfinement strategies successfully regulate the otherwise avalanche-like martensitic transformation (MT), enabling controlled strain release and yielding a more linear, stable, and tunable superelastic response.
Current additive manufacturing methods for joining and fabricating shape memory alloy (SMA) components cause heat affected zones that alter nitinol’s original properties and limit large-scale application of nitinol (NiTi) parts. This work examines the feasibility of using solid-state cold spray additive manufacturing to join SMA NiTi to similar and dissimilar materials. Ni50.3Ti at.
All-d-metal Heusler alloys are emerging functional materials in which magnetic ordering, lattice distortion, and mechanical behavior are strongly coupled through d–d electronic interactions. This study systematically investigates the structural, thermal, magnetic, and mechanical properties of Mn₂FeCu synthesized within a Heusler-type compositional framework. SEM/EDS revealed a dual-phase FCC-based microstructure consisting of Mn–Fe–rich and Mn–Cu–rich domains, while XRD confirmed FCC symmetry with compositional partitioning rather than full L2₁ ordering. Differential scanning calorimetry identified partial melting of the Cu-rich phase near 900 °C. Dilatometry showed a thermoelastic FCC → FCT transformation at 770–780 °C with a recoverable strain of 0.067
This paper presents the phase transformation behaviors and mechanical properties of nanocrystalline Ni51Ti49−xTax (x = 1, 2, 3) shape memory alloys as alternatives to binary NiTi for enhanced X-ray imaging visibility for medical applications. The alloys were fabricated via severe cold drawing (70
Preconditioning of a Nitinol implant is a critical procedure prior to delivery of the device to its intended site. This preconditioning, due to assembling, crimping, or other processes, results in a history of deformation in the Nitinol. This strain history can influence the fatigue and performance of the device due to residual stresses (strains) after the preconditioning steps. To accurately predict fatigue behavior, finite element analysis (FEA) must account for this change in the constitutive property while estimating the fatigue strains (or stresses). Additionally, FEA constitutive material models typically use an averaging law for the Austenitic and Martensitic ratio to match the global behavior; however, local strains measured by digital image correlation (DIC) are found to differ. The aim of this paper is to provide a precise characterization of the preconditioning strain for Nitinol implants and evaluate its impact on fatigue strains and implant geometry following preconditioning loads through FEA. Furthermore, we compare both local and global strains measured using DIC with FEA and demonstrate the effect of spatial and temporal resolution on the measured strains to facilitate the model credibility assessment of FEA fatigue life analysis procedures.
Nickel–titanium (NiTi) alloy lattice structures demonstrate significant potential for reusable impact protection applications due to their shape memory effect. However, their energy absorption behavior and shape recovery properties under dynamic impact loading have not been investigated. This study systematically examines the energy absorption behavior and shape recovery characteristics of two additively manufactured NiTi alloy lattice structures, the single helical lattice (SHL) and the double helical lattice (DHL), under dynamic impact loads. Experimental results show that as impact velocity and drop hammer mass increase, the macroscopic strain of both structures gradually increases, while their stress levels and energy absorption behavior remain nearly unchanged. The shape recovery rates of both structures decrease with increasing impact velocity and drop hammer mass. Finite element analysis results indicate that the stress distribution in both structures is relatively uniform. The DHL structure exhibits superior stress distribution and load-bearing efficiency compared to the SHL structure. During cyclic impact, the velocity–time response of the DHL structure gradually deviates from linearity, showing slower acceleration changes at the early stage and faster changes at the later stage. After three impact-heating cycles, the DHL structure maintains a shape recovery rate above 80
