NiTi Shape Memory Alloy Research Articles

Atomistic simulations of structure and motion of twin interfaces reveal the origin of twinning in NiTi shape memory alloys

Shape memory alloys (SMAs) are functional materials featuring unique shape recovery properties that make them suitable for key industrial applications. The essential process controlling this fascinating performance is microstructural twinning. Some twin systems are much more frequently observed than others, yet a fundamental mechanistic understanding of this evidence is missing, which hampers SMA design. Here, we consider the prototypical NiTi SMA to show that the emergence of twinning can be strongly affected by twin interface mobility. In this work, we adopt an integrated approach combining crystallographic theory, state-of-the-art atomistic modelling, topological model, and validation with high-resolution transmission-electron micrographs. Atomistic simulations confirm that the occurrence of twins is dictated by the driving force for twin boundary motion rather than interfacial energy. Moreover, our findings settle long-standing questions by elucidating the propagation mechanisms of twin interfaces, which the established martensite crystallography and kinetic theories could not address conclusively. The newly discovered understanding of the role of interface mobility in twin formation can be used to predict variant selection and guide the design of SMAs with improved functional performance.

Recoverable and plastic strains generated by forward and reverse martensitic transformations under external stress in NiTi SMA wires

Although superelastic NiTi shape memory alloy wire displays very high resistance to plastic deformation in austenite and martensite phases, incremental plastic strains are recorded whenever the cubic to monoclinic martensitic transformation (MT) proceeds under external stress leading to functional fatigue degradation. Therefore, special closed-loop thermomechanical loading tests were performed to shed light on the mechanism by which the incremental plastic strain are generated. These tests revealed that both forward and reverse MTs occurring above certain stress thresholds generate plastic strains specific for the [temperature, stress] conditions under which the MTs occurred. While the forward MT upon cooling does not produce plastic strain or permanent lattice defects up to 500 MPa stress, the reverse MT upon heating starts to generate them from 100 MPa. While plastic strain generated by the forward MT merely elongates the wire, plastic strain generated by the reverse MT also reduces the recoverable strain. Since the reverse MT upon heating generates plastic strains at lower external stresses than the forward MT upon cooling, it is largely responsible for cyclic instability of NiTi actuators. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for specific NiTi wires.

3D Printing Elastocaloric TiNiCu Thermoelectric Shape Memory Alloys.

The development of new thermoelectric conversion and cooling materials is an important means of addressing global climate and heat emissions in the future. While heavy and toxic elements like tellurium and lead are traditionally used to make thermoelectric materials with poor mechanical properties, recent decades have seen a gradual push towards greener and more sustainable alternatives. One such potential alternative material for thermoelectric and thermal management applications would be the Nitinol (TiNi) shape memory alloy, due to their superior mechanical properties. In this study, we have investigated the use of 3D melt printing techniques that can be used to achieve thermoelectric performance and efficiency of elastic memory alloys below 500 °C. The electrical and thermal properties of TiNiCu materials and their relation to morphology were investigated. All the alloys show similar effect sizes, their fatigue behavior is however different. By adjusting the composition of Ti and Ni elements and we have obtained memory alloys with high thermoelectric properties, with a 50% increase in power factor and a 100% increase in ZT values.

Controlling the crystal texture and microstructure of NiTi alloy by adjusting the thermal gradient of laser powder bed melting

The development of a favorable texture can enhance the recoverable strain of polycrystalline NiTi shape memory alloy. Nevertheless, texture tuning of NiTi alloy prepared through additive manufacturing presents more intricate challenges compared to alloys produced using traditional thermomechanical methods, as the solidification process is influenced by various factors. In this study, dense NiTi alloys were fabricated in laser powder bed melting (LPBF) technology. The optimal processing parameters were obtained including a hatching spacing of 24 μm, a forming angle of 90°, and a volume energy density of 139 J/mm3. A robust [001] texture has been developed in NiTi alloys by adjusting the thermal gradient through the precise control of hatching space and energy density. Alloys with this texture manifest a stable and fully recoverable strain of 4.82 %. A novel microstructure evolution mechanism has been proposed to elucidate the texture development during the LPBF manufacturing process under varying thermal gradients. Our findings provide insights into adjusting crystal texture in additive-manufactured NiTi parts through thermal gradient manipulation to achieve superior superelasticity.

Surface Treatment Strategies and Their Impact on the Material Behavior and Interfacial Adhesion Strength of Shape Memory Alloy NiTi Wire Integrated in Glass Fiber-Reinforced Polymer Laminate Structures.

Over the past few decades, there has been a growing trend in designing multifunctional materials and integrating various functions into a single component structure without defects. This research addresses the contemporary demand for integrating multiple functions seamlessly into thermoplastic laminate structures. Focusing on NiTi-based shape memory alloys (SMAs), renowned for their potential in introducing functionalities like strain measurement and shape change, this study explores diverse surface treatments for SMA wires. Techniques such as thermal oxidation, plasma treatment, chemical activation, silanization, and adhesion promoter coatings are investigated. The integration of NiTi SMA into Glass Fiber-Reinforced Polymer (GFRP) laminates is pursued to enable multifunctional properties. The primary objective is to evaluate the influence of these surface treatments on surface characteristics, including roughness, phase changes, and mechanical properties. Microstructural, analytical, and in situ mechanical characterizations are conducted on both raw and treated SMA wires. The subsequent incorporation of SMA wires after characterization into GFRP laminates, utilizing hot-press technology, allows for the determination of interfacial adhesion strength through pull-out tensile tests.

Transformation pattern of shape-memory NiTi alloy during stress-biased thermal cycling

Despite the superelastic deformation of NiTi has been documented and analyzed elaborately, its shape-memory behavior during stress-biased thermal cycling has not been thoroughly unveiled. This paper examines the evolution of transformation pattern in NiTi upon thermal cycling over a wide range of biasing stress. For the present shape-memory NiTi, both forward and reverse transformations proceed via the growth of localized deformation band (LDB) under low biasing stress (σbias ≤ 150 MPa), while LDB only appears during forward transformation and reverse transformation is uniform under high biasing stress (σbias ≥ 200 MPa). This is the first time that delocalization (conversion of deformation mode from localization to homogeneity) is observed during stress-biased thermal cycling. We clarify that the intrinsic undercooling/overheating of martensitic transformation results in unstable thermomechanical response, and it is the origin of localization in shape-memory NiTi. It is found that transformation-induced plasticity (TRIP), which is dominated by dislocation slip and deformation twining, not only causes irreversibility in the present shape-memory NiTi but also leads to delocalization through stabilization of intrinsic thermomechanical response of reverse transformation.

Stable superelasticity with large recoverable strain in NiTi alloy via additive manufacturing

We report a stable superelasticity with large recoverable strain in Ni51·2Ti48.8 (at.%) shape memory alloy (SMA) via additive manufacturing of laser powder bed fusion (LPBF). Microstructure analysis indicates that the LPBFed SMA samples have a non-homogeneous microstructure of two different grain zones, i.e., the coarse columnar one and the fine cellular one. Specifically, the coarse columnar grain zone accounts for a predominated content as high as ∼79 vol% and has a relatively low dislocation density. In contrast, the fine cellular one has a low content of ∼21 vol% yet a high dislocation density. Especially, all LPBFed non-homogeneous SMA samples exhibit a strong (100) texture with high intensities of 40.2–56.1, accompanied by homogeneous coherent Ti4Ni2Ox nanoprecipitates. Constant-stress cyclic compression shows that the LPBFed sample with 79 vol% coarse columnar grain zone and 52.8 (100) texture presents stable superelasticity with large recoverable strains, 5.71 % from 15 to 30 cycles at high loading of 1200 MPa. Such a large recoverable strain herein is superior to those in NiTi SMAs fabricated by various methods. Basically, the stable superelasticity is attributed to the strong (100) texture against plastic deformation and the pinning effect of the homogeneous coherent Ti4Ni2Ox nanoprecipitates on dislocation motion during cyclic loading. Meanwhile, the large recoverable strain originates from the high ability to accommodate new dislocation in the high content columnar grain zone. This work can provide significant insights into design of high-performance NiTi SMAs and further accelerate their engineering application by additive manufacturing.

Structure and Properties of Bioactive Titanium Dioxide Surface Layers Produced on NiTi Shape Memory Alloy in Low-Temperature Plasma.

The NiTi alloy, known for its shape memory and superelasticity, is increasingly used in medicine. However, its high nickel content requires enhanced biocompatibility for long-term implants. Low-temperature plasma treatments under glow-discharge conditions can improve surface properties without compromising mechanical integrity. This study explores the surface modification of a NiTi alloy by oxidizing it in low-temperature plasma. We examine the impact of process temperatures and sample preparation (mechanical grinding and polishing) on the structure of the produced titanium oxide layers. Surface properties, including topography, morphology, chemical composition, and bioactivity, were analyzed using TEM, SEM, EDS, and an optical profilometer. Bioactivity was assessed through the deposition of calcium phosphate in simulated body fluid (SBF). The low-temperature plasma oxidization produced titanium dioxide layers (29-55 nm thick) with a predominantly nanocrystalline rutile structure. Layer thickness increased with extended processing time and higher temperatures (up to 390 °C), though the relationship was not linear. Higher temperatures led to thicker layers with more precipitates and inhomogeneities. The oxidized layers showed increased bioactivity after 14 and 30 days in SBF. Low-temperature plasma oxidation produces bioactive titanium oxide layers on NiTi alloys, with a structure and properties that can be tuned through process parameters. This method could enhance the biocompatibility of NiTi alloys for medical implants.

Designing the mechanical behavior of NiTi self-expandable vascular stents by tuning the heat treatment parameters

The remarkable mechanical properties of nickel-titanium (NiTi) shape memory alloy, particularly its super-elasticity, establish it as the material of choice for fabricating self-expanding vascular stents, including the metallic backbone of peripheral stents and the metallic frame of stent-grafts. The super-elastic nature of NiTi substantially influences the mechanical performance of vascular stents, thereby affecting their clinical effectiveness and safety. This property shows marked sensitivity to the primary parameters of the heat treatment process used in device fabrication, specifically temperature and processing time. In this context, this study integrates experimental and computational analyses to explore the potential of designing the mechanical characteristics of NiTi vascular stents by adjusting heat treatment parameters. To reach this aim, differently heat-treated NiTi wire samples were experimentally characterized using calorimetric and uniaxial tensile testing. Subsequently, the mechanical response of a stent-graft model featuring a metallic frame made of NiTi wire was assessed in terms of radial forces generated at various implantation diameters through finite element analysis. The stent-graft served as an illustrative case of NiTi vascular stent to investigate the impact of the heat treatment parameters on its mechanical response. From the study a strong linear relationship emerged between NiTi super-elastic parameters (i.e., austenite finish temperature, martensite elastic modulus, upper plateau stress, lower plateau stress and transformation strain) and heat treatment parameters (R2 > 0.79, p-value < 0.001) for the adopted ranges of temperature and processing time. Additionally, a strong linear relationship was observed between: (i) the radial force generated by the stent-graft during expansion and the heat treatment parameters (R2 > 0.82, p-value < 0.001); (ii) the radial force generated by the stent-graft during expansion and the lower plateau stress of NiTi (R2 > 0.93, p-value < 0.001). In conclusion, the findings of this study suggest that designing and optimizing the mechanical properties of NiTi vascular stents by finely tuning temperature and processing time of the heat treatment process is feasible.

When thermochromic material meets shape memory alloy: A new smart window integrating thermal storage, temperature regulation, and ventilation

Traditional windows have poor thermal insulation performance, resulting in significant indoor heat loss in winter and outdoor heat entry in summer. Thermochromic smart windows can effectively block solar radiant heat by automatically adjusting light transmittance, thereby reducing air conditioning loads and leading to significant energy savings. In this study, the poly N-isopropyl acrylamide (PNIPAm)-based thermochromic hydrogel, modified MXene nanoparticles, and NiTi shape memory alloy (SMA) are integrated to endow the smart window with heat storage, temperature control, and ventilation. The smart window achieves 88.6% visible light transmission and 70% solar modulation. The inclusion of MXene nanoparticles further enhances photothermal response efficiency, while the ventilation system ensures efficient and fresh indoor air circulation. Compared to the common glass, the smart window reduces the indoor temperature by 8 °C, demonstrating its excellent temperature regulation ability. Simulation results indicate that in Shanghai, Cairo, Singapore, and Kuwait, the employment of thermochromic smart windows can reduce heating, ventilation, and air conditioning energy consumption (HVAC) by 32.6%, 49.9%, 42.7%, and 34.1%, respectively. This versatile thermochromic smart window is expected to significantly improve building efficiency and occupant comfort, offering a sustainable solution for future building designs.

Thermodynamic ripening induced multi-modal precipitation strengthened NiTi shape memory alloys by directed energy deposition

The functional properties of shape memory alloys made by additive manufacturing can be used in numerous applications. Adjusting the precipitation can be one way to tailor the functional properties. In this work, the NiTi shape memory alloys are fabricated through Directed Energy Deposition (DED) technology and facilitate Ni4Ti3 and NiTi2 precipitates by heat treatment. Experimental results show that increasing the heat treatment temperature can switch the precipitates distributed from the grain boundaries to the interior of the grains, accompanied by the increasing size and content of precipitates. The results pressent that the tailored microstructure affects the mechanical performance, manifested by the tensile recovery strain of ∼2 %, the tensile strength of ∼749 MPa, and the strain of ∼11 %. Our findings can provide information for designing enhanced shape memory properties and further microstructural studies of NiTi shape memory alloys.

Molecular dynamics simulation for nanometric cutting of NiTi shape memory alloys at elevated temperatures

In nanometric cutting, the deformation mechanism is closely related to the cutting temperature. This study investigates the cutting mechanism at elevated temperatures via molecular dynamics simulation of nanometric cutting on equiatomic NiTi SMAs. Mutual transformation between martensite and austenite during the indentation and cutting process, and effects of the cutting temperature are analyzed. The results indicate that the NiTi workpiece atoms experience martensite phase transformation and reverse phase transformation during the indentation and cutting process, with different orientation of martensite facilitating the transformation. Besides, the average resistance coefficient decreases with increasing cutting temperature due to the elevated temperature lubrication effect, and the cutting forces and phase transformation in the processing area also experience a gradual reduction. Some of the dislocations inside the NiTi workpiece show plugging and tangling phenomena at temperatures of 400 and 600 K. Cutting simulations reveal that fewer dislocation slips and less residual stress occur at elevated temperatures and lead to significantly reduce of residual martensite phase after cutting. From an atomic perspective, this investigation explains the cutting mechanism of difficult-to-machine materials NiTi SMAs at elevated temperatures and gives guidance for improving its machinability.

NiTi shape memory alloys under nanoindentation with different atomic compositions

ABSTRACT Molecular dynamics simulations were conducted to explore nanoindentation-induced martensitic transformation in NiTi shape memory alloys. Three atomic compositions and two indenter sizes were considered. During loading, the B2 structure underwent martensitic transformation to the B19' structure, accompanied by atomic rearrangement in the form of bands. After unloading, a portion of the B19' phase underwent reversible transformation, while the remainder remained in the form of bands, resulting in residual strain. This effect was accentuated by the more localised deformation promoted by the smaller indenter. The influence of atomic composition was observed not to play a key role in the mechanical and structural properties, contrary to previous reports on tension and compression tests. Our findings shed light on the intricate interplay of factors influencing the mechanical performance of NiTi shape memory alloys at the atomic scale, providing crucial insights for the design and understanding of such materials.

Investigations on the interaction of laser parameters for efficient microdrilling of NiTiV smart alloy system

Ni–Ti shape memory alloys (SMAs) are popular in current research due to their usefulness and mechanical properties. At different temperatures, Ni–Ti alloys transition from austenite to martensite. To restore high-temperature memory in nickel-titanium SMAs, vanadium (V) is added as an alloying element. For Ni–Ti-based SMAs, the fiber laser is one of the best machining procedures for bio-implants, actuators, and aircraft engine parts. Using a Box–Behnken design to experiment with laser power, nozzle distance, cutting speed, and frequency, this study examines fiber laser micro-drilled Ni50Ti48V2 SM alloy material removal and hole taper angle. By increasing power (P), frequency (F), and cutting speed (C S ), Ni50Ti48V2 alloy material removal rate (MRR) increased by 75.79%. The hole taper angle (HTA) dropped 75.33% when cutting speed, laser power and frequency decreased. Lowering cutting speed and laser power increases micro-hole circularity and reduces HTA. Upon surface topographical inspection, debris and molten materials were found on the drilled surface. The flow of nitrogen gas caused materials to diffuse on the Ni50Ti48V2 alloy’s entry and exit surfaces, changing surface roughness. High parameters influence surface roughness, HTA, and circularity due to nitrogen gas flow. The material’s DSC and XRD tests confirmed its suitability for biomedical microhole production.

Low-fatigue large elastocaloric effect in NiTi shape memory alloy enabled by two-step transition

NiTi shape memory alloys are promising elastocaloric materials owing to their substantial adiabatic temperature change (ΔTad). However, the simultaneous attainment of large ΔTad and low fatigue poses challenges due to the significant hysteresis and severe functional fatigue associated with the autocatalytic avalanche-like martensitic transformation. This study demonstrates a continuous two-step transition in Ni50.8Ti49.2 (at.%), showcasing cyclic-stable superelasticity with large recoverable strain (5.9 %), substantial ΔTad (19.1 K) and high coefficient of performance. In-situ loading analysis indicates a stress-induced continuous transition from the B2 to R and subsequently to B19′. Nanoscale lattice analysis exposes heterogeneous strain network, harboring metastable R phase preceding the B19′ phase. By exploiting differences in critical stress and transformation potential between R and B19′ phases, this study demonstrates the possibilities to synergistically integrate functional performances of different martensitic phases in NiTi into a sequential and continuous two-step transition to provide controlled strain release with unprecedented properties.

Challenges in Chemical Vapour Deposition of Graphene on Metallurgical Alloys Exemplified for NiTi Shape Memory Alloys

Atomically-thin two-dimensional (2D) materials like graphene have been suggested as ultimately thin corrosion barriers and functional coatings for modern metallurgical alloys. The challenges of chemical vapour deposition (CVD) of such 2D materials, particularly graphene, on modern metallurgical alloys are discussed and reviewed here, focusing on the key problems with the metallurgical alloys’ often limited catalytic activity towards 2D materials growth and the key need to preserve the metallurgical alloys’ bulk properties during the high temperature 2D materials CVD processes. Using graphene CVD on NiTi (Nitinol) shape memory alloys as a case study, we illustrate the constraints arising from low catalytic activity and tendency to form oxides due the Ti in the NiTi alloy in terms of graphene growth results. We show that, by using a scalable low-temperature CVD process at 650 to 750 °C, we can deposit fully covering carbon films on the NiTi, albeit at limited structural quality. Notably, we also demonstrate that our CVD process does not degrade the bulk microstructure of the NiTi during carbon deposition and, importantly, leaves the crystallographic shape memory effect evolution intact. This underscores the potential of CVD for depositing graphene films on NiTi alloys while emphasizing the necessity for further exploration of CVD conditions to achieve high-quality graphene deposits akin to those on prior widely investigated dedicated (often sacrificial) high-purity metal substrates such as Ni.

Accelerating lPBF process optimisation for NiTi shape memory alloys with enhanced and controllable properties through machine learning and multi-objective methods

ABSTRACT NiTi shape memory alloys (SMAs) prepared by the laser powder bed fusion (LPBF) technology have demonstrated promise in aerospace and medical applications. Nevertheless, ensuring repeatability and customised design in printed parts remains challenging. This paper addressed this challenge by introducing a machine learning model that effectively predicted the performance of NiTi SMAs across diverse LPBF processing and equipment conditions. Trained on a dataset of 195 entries from 23 publications, the model accurately predicted critical metrics, including density, ultimate tensile strength, elongation, and thermal hysteresis. Validation using data from eight experimental groups confirmed its reliability and generalisation capability. Multi-objective optimisation identified processes yielding synergistic improvements, achieving a tensile strength of 783 ± 8 MPa, an elongation of 13.7 ± 0.8% and a low hysteresis of 15.1 K. This study also discussed strategic applications of the model for LPBF process optimisation and proposed a method for constructing tailored LPBF process maps for specific NiTi alloy performance attributes.

Topological optimisation and laser additive manufacturing of force-direction-sensitive NiTi porous structures with large deformation recovery behaviour

ABSTRACT Optimising the cell type and configuration is a key approach for overcoming stress concentration and irreversible deformation in lightweight lattice structures with high strength. In this work, the topology optimisation method was utilised to design force-direction-sensitive structures, including face-centre loaded (FCL), central-edge loaded (CEL), and hybrid-loaded (HL) structures, which were manufactured by laser powder bed fusion (LPBF) using Nitinol (NiTi) shape memory alloy. Results indicated that the HL lattice exhibited the highest initial peak force of 7.6 kN with higher specific compressive strength (41.74 MPa·cm3/g) and specific energy absorption (6.82 J/g), which was benefit from the layer-by-layer deformation mechanism. Furthermore, the HL lattice also exhibited the best damping properties and energy absorption capacity, while achieving a high shape recovery ratio of 85%. This work offers insights into a design strategy for functional structures with high strength and large deformation recovery capacity.

Transformation Behaviors and Microstructure Modifications in Hydrogen-Charged Ti–Ni Shape Memory Alloy

AbstractThe effect of hydrogen charging duration on the transformation behavior, microstructural evolution, and dynamic microstructural changes associated with thermoelastic martensitic transformation in Ti–Ni shape memory alloy was investigated. Compared with the uncharged specimen, the martensitic transformation start (Ms) and reverse transformation finish (Af) temperatures increased with charging time, whereas the martensitic transformation finish and reverse transformation start temperatures remained almost unchanged. In situ SEM results were consistent with these behaviors. Upon cooling, the transformation progressed from the center to the surface in charged specimens, indicating a higher transformation temperature in the center than the surface. The latent heat of transformation decreased with increasing charging time, quantitatively attributed to an untransformed region consisting of hydrogen-induced martensite and a hydrogen-affected layer. The hydrostatic effect from those layers on the interior B2 phase was proposed as the origin of the increased Ms and Af temperatures.

Complementary In Situ Characterization of Superelasticity in Fe–Mn–Al–Ni–Ti Shape Memory Alloys by Acoustic Emission, Digital Image Correlation and Infrared Thermography

AbstractCoupled in situ investigations were conducted on a Fe–Mn–Al–Ni–Ti single crystal deformed in compression and two Fe–Mn–Al–Ni–Ti oligo-crystals deformed in tension. Acoustic emission measurements were employed to monitor the degradation of superelasticity and the stabilization of martensite due to dislocation processes. These observations were corroborated by the application of digital image correlation and infrared thermography measurements. A poor strain reversibility and a premature plastification of the parent phase were observed in case of the single crystal due to an unfavourable crystal orientation. A contradictory transformation behaviour of the two oligo-crystals was observed, with one specimen showing a promising strain reversibility and characterisitic signs of degradation, and the other specimen exhibiting a limited strain reversibility due to an unusual confinement of the martensitic phase transformation to an unfavourably oriented grain. In the former case, an increase in the dislocation density within five cycles was detected through a shift of the acoustic signals’ median frequencies. In the latter case, a strong coupling between martensite nucleation and dislocation generation led to a pronounced martensite stabilization after one loading cycle. For all specimens, temporal sequence effects related to the coupling of martensite nucleation and dislocation generation were detected by means of acoustic emission.