NiTi Shape Memory Alloy Research Articles

Design and construction of a servomechanism using a memory alloy linear actuator

This work shows the design and construction of a servomechanism of a rotator-type joint based on NiTi Shape Memory Alloys (SMA) with an angular position measurement based on a potentiometer sensor and digital electronic position control. The expected application of this prototype is for the use of small charges that emulate the movement of the human being, being bio-inspired and activated by artificial muscles, their potential applications they will be in medical and humanoid robotics. Computer Aided Design (CAD) allows evaluating and validating the most convenient parameters for construction of servomechanism, experimental results validate allowed us to obtain the values of the range of motion ±20° and a maximum torque of 1.01 kg-cm exerted on the axis of rotation for the prototype.

Mechanism of shape memory effect alteration in Ni50.4Ti49.6 via laser powder bed fusion from the perspective of martensitic twin

ABSTRACT The phase transformation from martensitic twins to austenite phase in NiTi shape memory alloys (SMAs) determines their shape memory effect (SME). However, systematic studies are indeed needed to reveal which martensitic twin type is effective in enhancing the SME for NiTi SMAs due to the diversity of martensitic twins. In this work, based on laser powder bed fusion (LPBF) NiTi SMAs, an excellent strength-ductility combination of 900.5 MPa and 13.9% was obtained by utilising a low laser power (70 W) and scanning speed (200 mm/s), which represents one of the highest values attainable among current LPBF NiTi SMAs. Furthermore, based on transmission kikuchi diffraction, it is found that high-density { 11 1 ¯ } I-type martensitic twins enhanced the SME of LPBF NiTi SMAs significantly, and the maximum SME recovery ratio is 95.8% after 10 cycles at controlled strain (6%) cyclic tensile loading-unloading. Conversely, the formation of { 1 ¯ 11 } I-type martensitic twins in the matrix weakened the SME of LPBF NiTi SMAs remarkably and lost SME after 10 cycles at controlled strain (6%) cyclic tensile loading-unloading. This research offers some valuable insights into the relationship between martensitic twin and SME for LPBF NiTi SMAs and paves the new way for modulation of microstructure to enhance mechanical properties and SME for NiTi SMAs via LPBF.

Abrasive aqua jet machining of NiTiV ternary smart alloy

ABSTRACT Nickel–titanium shape memory alloys (SMAs) are popular due to their usefulness and functional characteristics. The binary NiTi SMA was alloyed with vanadium to improve memory retention properties. In this study, the abrasive aqua jet machining method was adopted to machine the Ni50Ti48V2 ternary SMA to minimize the kerf taper angle (KTA), surface roughness (Ra) of machined Ni50Ti48V2 surface, and enhance the material removal rate (MRR). RSM-BBD optimization using the input variables such as abrasive flow rate (AFR), water jet pressure (PWJ), nozzle traverse speed (VTS), and standoff distance (SFD) predicted that PWJ and SFD highly contributed to MRR, Ra, and KTA responses. Small variation in variation after optimized AWJM conditions, and XRD analysis confirmed no phase changes after AWJM.

Repetitive corrugation and straightening (RCS) of NiTi shape memory alloy strip product

ABSTRACT Repetitive corrugation and straightening (RCS) is a novel method of severe plastic deformation in bulk metallic materials. In this study, the effect of RCS on the properties of NiTi shape memory alloy (SMA) is investigated. Cold-rolled (CR) Ni49.8–Ti50.2 (at. %) strips with varying degrees of cold-work were subjected to RCS and evaluated for transformation, mechanical and functional properties. The study demonstrated that RCS can be used effectively to improve the strength and ductility of the CR SMA strips, while its transformation and functional properties are largely preserved. The ultimate tensile strength (UTS) and elongation to failure of the 30% CR strip post RCS increased from 950 MPa to 1130 MPa and ~19% to ~26%, respectively. The microstructural evaluation confirmed that RCS resulted in notable grain refinement and re-distribution of Ti2Ni second phase in the matrix. Consequent to its high strength, the cyclic response of the RCS strip on stress-free thermal cycling was found to be better than the CR strip.

Investigation of the Seismic Performance of a Multi-Story, Multi-Bay Special Truss Moment Steel Frame with X-Diagonal Shape Memory Alloy Bars

In this work, the seismic response of a multi-story, multi-bay special truss moment frame (STMF) with Ni-Ti shape memory alloys (SMAs) incorporated in the form of X-diagonal braces in the special segment is investigated. The diameter of the SMAs per diagonal in each floor was initially determined, considering the expected ultimate strength of the special segment, developed when the frame reaches its target drift and the desirable collapse mechanism, i.e., the formation of plastic hinges, according to the performance-based plastic design procedure. To further investigate the response of the structure with the SMAs incorporated, half the calculated SMA diameters were introduced. Continuing, three more cases were investigated: the mean value of the SMA diameter was introduced at each floor (case DC1), half the SMA diameter of case DC1 (case DC2), and twice the SMA diameter of case DC1 (case CD3). Dynamic time history analyses under seven benchmark earthquakes were conducted using commercial nonlinear Finite Element software (SeismoStruct 2024). Results were presented in the form of top-displacement time histories, the SMAs force–displacement curves, and maximum inter-story drifts, calculating also maximum SMA displacements. The analysis outcomes highlight the potential of the SMAs to be considered as a novel material in the seismic retrofit of steel structures. Both design approaches presented exhibit a certain amount of effectiveness, depending on the distribution, with the placement of the SMA bars and the seismic excitation considered. Further research is suggested to fully understand the capabilities of the use of SMAs as dissipation devices in steel structures.

Friction and Wear Resistance of Nanostructured TiNi Shape Memory Alloy

TiNi shape memory alloys with a superelastic effect are widely used in tribological interfaces requiring high wear resistance. One of the common approaches to reducing the wear of various metals is the application of severe plastic deformation (SPD), resulting in structural refinement and corresponding hardening. This paper investigates the tribological behaviour of a nanostructured Ti49.3Ni50.7 shape memory alloy produced using SPD. The friction and wear characteristics of the alloy at room temperature are compared in the coarse-grained, nanostructured, and nanostructured aged states. Through hardness measurement and transmission electron microscopy, it is shown that the transformation of a coarse-grained state into a nanostructured state increases wear resistance and hardness, reduces the coefficient of friction, and changes the friction mechanism. Formed nanoparticles during ageing in a nanostructured state further increase wear resistance.

Cyclic functional degradation of NiTi shape memory alloy wires in wide ranges of strain rate and ambient temperature

Shape memory alloy (SMA) dampers frequently experience cyclic loading over a broad spectrum of strain rates and ambient temperatures, resulting in a significant thermo-mechanical coupling effect during cyclic deformation. Nevertheless, the impact of this coupling effect on the functional degradation of SMAs has not been thoroughly examined, particularly in scenarios involving severe deformation. In this study, a series of strain-controlled cyclic loading–unloading tests were conducted on NiTi SMA wires utilizing various combinations of strain rates ranging from 5 × 10−4 to 6 × 10−2/s and different ambient temperatures (313–393 K). The functional degradation was exacerbated by increasing ambient temperature and loading rate due to the complex interactions among the inelastic deformation mechanisms and thermo-mechanical coupling effects. Furthermore, although the cyclic deformation behavior of NiTi SMA wires varied with the loading rate, this rate dependence diminished with increasing ambient temperature. The synergistic effect of elevated strain rates and high temperatures markedly accelerated the fatigue failure of NiTi SMA wires and substantially altered the underlying microstructural morphology. These findings can provide valuable support for evaluating the service performance of NiTi SMA dampers under various engineering conditions and contribute to developing a cyclic constitutive model for NiTi SMAs.

Optimizing laser parameters and exploring building direction dependence of corrosion behavior in NiTi alloys fabricated by laser powder bed fusion

The microstructure and materials performance of Laser Powder Bed Fusion-fabricated (LPBFed) NiTi shape memory alloys (SMAs) varies depending on the laser parameters and building directions. This study initially optimized the laser processing parameters (laser power and scanning speed) for LPBFed NiTi SMAs to enhance their printing quality and mechanical properties. We determined that with fixed hatch spacing of 80 μm and layer thickness of 30 μm, the laser power of 105 W, and scanning speed of 600 mm/s produced the best results. Subsequently, the corrosion behavior of LPBFed NiTi SMAs built in three different building directions (0°, 45°, 90°) was investigated in 0.9 wt.% NaCl solution at 37 °C. The electrochemical data revealed that the corrosion current density (Icorr) decreased with increasing building direction angle (Icorr-90° <Icorr-45° <Icorr-0°). The corrosion resistance followed the order of 0° sample ˂ 45° sample ˂ 90° sample. These findings were attributed to the difference in the grain size and boundary density. Higher boundary densities enhanced the electron activity capacity and the element diffusion rates, facilitating the rapid formation of a protective film and thus improving the corrosion resistance. This new insight into the anisotropy of corrosion behavior relative to building directions can inform the design of NiTi-SMA products and improve their reliability in tissue engineering applications.

Effects of residual stress on the isothermal tensile behavior of nanocrystalline superelastic NiTi shape memory alloy

The residual stress greatly affects the mechanical behavior of a material. In this work, the effect of residual stress on the isothermal tensile behavior of a NiTi shape memory alloy is studied. The focused ion beam and digital image correlation are combined to measure the two-dimensional residual stress in nanocrystalline NiTi plates processed with prestrain laser shock peening. A four-point bending experiment verified the accuracy of this measurement method. The FIB-DIC method is an attractive tool for measuring the two-dimensional residual stress in phase transition nanocrystalline materials. The internal residual stress significantly decreases the phase transition stress, and the mechanism is studied via finite element and theoretical analyses. This work implies that the mechanical behavior of NiTi shape memory alloys can be tailored via residual stress engineering.

Revealing effects of powder reuse for LPBF-fabricated NiTi shape memory alloys

Revealing effects of powder reuse for LPBF-fabricated NiTi shape memory alloys

Effect of Structural Configurations on Mechanical and Shape Recovery Properties of NiTi Triply Periodic Minimal Surface Porous Structures

Based on the advantages of triply periodic minimal surface (TPMS) porous structures, extensive research on NiTi shape memory alloy TPMS scaffolds has been conducted. However, the current reports about TPMS porous structures highly rely on the implicit equation, which limited the design flexibility. In this work, novel shell-based TPMS structures were designed and fabricated by laser powder bed fusion. The comparisons of manufacturability, mechanical properties, and shape recovery responses between traditional solid-based and novel shell-based TPMS structures were evaluated. Results indicated that the shell-based TPMS porous structures possessed larger Young’s moduli and higher compressive strengths. Specifically, Diamond shell structure possessed the highest Young’s moduli of 605.8±24.5 MPa, while Gyroid shell structure possessed the highest compressive strength of 43.90±3.32 MPa. In addition, because of the larger specific surface area, higher critical stress to induce martensite transformation, and lower austenite finish temperature, the Diamond shell porous structure exhibited much higher shape recovery performance (only 0.1% residual strain left at pre-strains of 6%) than other porous structures. These results substantially uncover the effects of structural topology on the mechanical properties and shape recovery responses of NiTi shape memory alloy scaffolds, and confirm the effectiveness of this novel structural design method. This research can provide guidance for the structural design application of NiTi porous scaffolds in bone implants.

Effect of Laser Surface Texturing Process on Dry Sliding Wear Behavior of NiTi Shape Memory Alloy Used in Airplane

In this study, the effect of laser surface texturing (LST) applied to NiTi Shape Memory Alloy (SMA) on the dry sliding wear behavior of the material was investigated. After polishing and cleaning the material surface, a pitted surface texturing process was performed using a femtosecond laser under atmospheric conditions. After the surface texturing process, dry sliding wear tests were performed at room temperature. When the wear behavior of the laser-applied and non-laser-applied test samples was evaluated comparatively, it was determined that the coefficient of friction (COF) of the laser-applied samples under 1N load was approximately 17% lower. It was determined that the decrease in the COF value decreased with increasing load. However, the wear amount of the LSD-applied NiTi SMA was higher than the untreated sample. It was evaluated that this situation was due to thermal softening that occurred depending on the ablation geometry and dimensions.

Numerical and Experimental Study of a Wearable Exo-Glove for Telerehabilitation Application Using Shape Memory Alloy Actuators

Hand paralysis, caused by conditions such as spinal cord injuries, strokes, and arthritis, significantly hinders daily activities. Wearable exo-gloves and telerehabilitation offer effective hand training solutions to aid the recovery process. This study presents the development of lightweight wearable exo-gloves designed for finger telerehabilitation. The prototype uses NiTi shape memory alloy (SMA) actuators to control five fingers. Specialized end effectors target the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints, mimicking human finger tendon actions. A variable structure controller, managed through a web-based Human–Machine Interface (HMI), allows remote adjustments. Thermal behavior, dynamics, and overall performance were modeled in MATLAB Simulink, with experimental validation confirming the model’s efficacy. The phase transformation characteristics of NiTi shape memory wire were studied using the Souza–Auricchio model within COMSOL Multiphysics 6.2 software. Comparing the simulation to trial data showed an average error of 2.76°. The range of motion for the MCP, PIP, and DIP joints was 21°, 65°, and 60.3°, respectively. Additionally, a minimum torque of 0.2 Nm at each finger joint was observed, which is sufficient to overcome resistance and meet the torque requirements. Results demonstrate that integrating SMA actuators with telerehabilitation addresses the need for compact and efficient wearable devices, potentially improving patient outcomes through remote therapy.

Selective laser melting fabrication of body-temperature shape memory NiTi alloy for 4D-printed orthopedic implants

ABSTRACT SLM fabrication of NiTi alloy is receiving increasing attention. However, the relatively high austenite transformation finish (Af) temperature is the main problem for biomedical applications. In this study, it was found that phase transformation temperature (PTT) increased with increasing laser power, decreasing scanning speed, and decreasing hatch spacing. The combination of PPs with an energy density of less than 75 J/mm³ is expected to yield the Af temperature below 37 °C. P = 80 W, v = 600 mm/s, and h = 70 μm were identified as the optimal combination because of a good tensile strength of 612 MPa and a peak shape recovery rate of 96.94%. Biocompatibility assessment showed that SLM-NiTi porous scaffolds supported pre-osteoblast survival and proliferation. A patient-specific mesh-like supporting prosthesis was designed to show the prospect of 4D-printed orthopedics implant. In clinical application, SLM-NiTi prosthesis could reduce bony window and facilitate easier insertion through compressive deformation.

Phase transition in the shape memory alloy NiTi described by the SCAN meta-GGA functional.

Climbing the ladder of density functional approximations has long been proposed to systematically improve the accuracy of first-principles calculations employing the density functional theory (DFT); however, up until now, the Perdew-Burke-Ernzerhof (PBE) functional at the second rung of the ladder, has dominated. Here, we present a study of the martensitic phase transition in NiTi based on abinitio molecular dynamics simulations and thermodynamic integration using the third-rung approximation of the strongly constrained and appropriately normalized (SCAN) meta-generalized gradient approximation (GGA). Although the predicted equilibrium lattice constants and formation enthalpy agree well with experimental data, the martensitic transition temperature (MTT) is overestimated by 94% (or 324K too high), compared with only 22% (77K) overestimation by PBE. The latent heat (q) is severely overestimated by SCAN as well. This deteriorated performance originates from the enlarged energy difference (ΔE) between the austenite and martensite phases, compared with the PBE result. Furthermore, a large variation (over 50meV/atom) in ΔE using different meta-GGAs indicates large variations in computed MTTs (∼400-500 K) and q, i.e., the predicted thermodynamic properties depend sensitively on the choice of meta-GGA. This would pose a serious problem when upgrading DFT calculations to the third rung. One possible solution is to add NiTi as a normsystem so that the revised SCAN meta-GGA could reproduce the PBE results of the relevant energy difference.

Fabrication of porous Ni-Ti shape memory alloy via binder jet additive manufacturing and solid-state sintering

This paper explores the additive manufacturing of nickel-titanium (Ni-Ti) shape memory alloys (SMAs) using binder jetting and subsequent solid-state sintering under an Ar atmosphere. The consolidated 3D-printed Ni-Ti parts exhibit a correlation between pore morphology, pore fraction, and phase formation at different solid-state sintering temperatures. At a sintering temperature of 1175 °C, NiTi grains with TiC precipitates along grain boundaries are observed, accompanied by irregular, interconnected pores constituting ∼15 % of the volume. Increasing the sintering temperature to 1185 °C leads to the formation of a minor phase of Ni4Ti3 within NiTi grains, along with grain boundary TiC precipitates, and isolated pores with a volume fraction of ∼5 %. The higher sintering temperature corresponds to a higher average nanohardness value (8.1 GPa or 763 Hv compared to 6.9 GPa or 654 Hv), indicating the presence of a greater abundance of secondary phases and higher densification at the elevated sintering temperature. While the transformation temperatures (TTs) were undetectable in the bulk-printed samples, a different structure featuring designed channels and thin struts, sintered under similar conditions, exhibited detectable TTs, with a martensite start temperature of 34 °C. Biocompatibility tests demonstrated cell spreading and attachment on both sintered Ni-Ti samples. These findings offer valuable insights into the potential use of binder jetted Ni-Ti for medical implants and tissue engineering applications.

Constitutive modeling of functional fatigue with tension–compression asymmetry for superelastic NiTi shape memory alloy

Under cyclic loads, superelastic shape memory alloys (SMAs) exhibit stress–strain responses featured by functional fatigue, i.e., degradation of superelasticity and accumulation of irrecoverable deformation as cycling number increases, together with an asymmetry between tensile and compressive responses. Comprehensive understanding and modeling of these material complexities are crucial for the design and analysis of various superelastic SMA structures in practical applications. This work has developed a novel constitutive model based on irreversible thermodynamics to account for functional fatigue with tension–compression asymmetry. A potential function, defined as a weighted sum of two potentials that are calibrated against the tensile and compressive responses respectively, is employed to generate the asymmetric responses, and functional fatigue is represented by degradation of superelastic properties and growth of plastic strain as martensitic transformation accumulates. The model is adopted in numerical simulations for superelastic SMA tubes under cyclic lateral compression, which is experimentally investigated as a model problem. The agreement between simulations and experiments shows the validity and effectiveness of this constitutive modeling. Through additional finite element simulations incorporating this model, the effects of tension–compression asymmetry under cycling and diameter-to-thickness ratio of the tubular geometry upon mechanical responses of laterally compressed SMA tubes are also unveiled.

Martensitic transformation induced by cooling NiTi wire under various tensile stresses: Martensite variant microstructures, textures, recoverable strains and plastic strains

Whenever the forward and/or reverse B2 cubic to B19’ monoclinic martensitic transformation in NiTi shape memory alloy wire proceeds under external stress above certain threshold, it generates incremental plastic strains which accumulate during thermomechanical cyclic loading and lead to functional fatigue preventing many promising engineering applications from realization. In this work, unique thermomechanical loading experiments were performed on NiTi shape memory wire with the aim to reveal the mechanism by which the forward martensitic transformation upon cooling under external stress generates plastic strain. Recoverable transformation strains and plastic strains generated by the forward transformation on cooling under various tensile stresses were evaluated, martensite variant microstructures in grains were reconstructed by nanoscale orientation mapping in TEM, martensite textures after cooling at room temperature were evaluated by in-situ synchrotron x-ray diffraction and permanent dislocation defects in martensite were analyzed by TEM. Based on the obtained experimental evidence, it is proposed that the forward martensitic transformation on cooling under stress proceeds via habit plane interfaces between austenite and second order laminate of (001) compound twins in martensite. Depending on the magnitude of the applied stress, the induced martensite reorients, partially detwins and deforms plastically via [100](001) dislocation slip in martensite in extent permitted by the requirement for compatible deformation of grains in nanocrystalline NiTi wire via a single deformation system. During the forward MT upon cooling under highest stresses 600 MPa, the martensite deforms via kwinking deformation enabling generation of large plastic strains 8 %.

Determination of the Optimum Conditions for Machining NiTi Shape Memory Alloys by Electrical Discharge Machining

Determination of the Optimum Conditions for Machining NiTi Shape Memory Alloys by Electrical Discharge Machining

Microstructure, Mechanical Properties and Corrosion Performance of Laser-Welded NiTi Shape Memory Alloy in Simulated Body Fluid.

Laser-welding is a promising technique for welding NiTi shape memory alloys with acceptable tensile strength and comparable corrosion performance for biomedical applications. The microstructural characteristics and localized corrosion behavior of NiTi alloys in a simulated body fluid (SBF) environment are evaluated. A microstructural examination indicated the presence of fine and equiaxed grains with a B2 austenite phase in the base metal (BM), while the weld metal (WM) had a coarse dendritic microstructure with intermetallic precipitates including Ti2Ni and Ni4Ti3. The hardness decreased from the BM to the WM, and the average hardness for the BM was 352 ± 5 HV, while it ranged between 275 and 307 HV and 265 and 287 HV for the HAZ and WM, respectively. Uni-axial tensile tests revealed a substantial decrease in the tensile strength of NiTi WM (481 ± 19 MPa), with a reduced joint efficiency of 34%. The localized corrosion performance of NiTi BM was superior to the WM, with electrochemical test responses indicating a pitting potential and low corrosion rate in SBF environments. The corrosion rate of the NiTi BM and WM was 0.048 ± 0.0018 mils per year (mpy) and 0.41 ± 0.019 mpy, respectively. During welding, NiTi's strength and biocompatibility properties changed due to the alteration in microstructure and formation of intermetallic phases as a result of Ti enrichment. The performance and safety of welded medical devices may be impacted during welding, and it is essential to preserve the biocompatibility of NiTi components for biomedical applications.