NiTi Shape Memory Alloy Research Articles

2D thermo-fluidynamic rotary model of an elastocaloric cooling device: The energy performances

Elastocaloric cooling has been considered a promising solid-state technology for cooling and heat pumping among the alternatives to vapor compression, at room temperature range. The technology is based on elastocaloric effect that is a thermophysical phenomenon occurring in materials like Shape Memory Alloys (SMA). The effect is detectable as a temperature change in the material if adiabatically subjected to a forcing field of mechanical nature. The latter provokes to the materials a stress that can derive from tension, compression, bending, torsion solicitations. As a result, the SMA experiments a structural phase change from austenite to martensite (coupled with heat addition) and temperature rise. Dually when the stress is removed, the SMA releases heat with a temperature decrease. In this paper the SUSSTAIN-EL rotary elastocaloric heat pump has been deeply investigated to test the energy performances while it works on open loop and closed loop, through a 2D numerical model based on the finite element method, for cooling and heating operation modes. The device employs a binary NiTi SMA as elastocaloric refrigerant and air as heat transfer medium. A wide set of working conditions has been considered like variable inlet mass flow rate, rotation frequency and thermal loads. The acquired results demonstrate that, both in open and closed loop, the prototype’s energy performances are promising and highly favourable for the intended macroscale collocation of the device.

Micro-resistance welding of NiTi shape memory alloy wire and MP35N wire

The micro-resistance welding between NiTi shape memory alloy and Co-35Ni-20Cr-10Mo alloy (MP35N) wires was investigated to gage the feasibility of their future applications. Welding current and electrode force significantly affected the joint breaking force. The highest joint breaking force (0.99 kgf) at the optimized welding parameter (0.51 kA, 7.500 kg) was accomplished by maximizing the solid-state bonded area and minimizing the heat-affected zone (HAZ). This highest joint breaking force was higher than that of the micro-resistance welded NiTi wire joint reported in previous studies. Furthermore, the cyclic tensile test revealed that the accumulated residual strain of NiTi wire was reduced after welding. This could be attributed to the reduction in the dislocation density through grain growth in the HAZ of NiTi wire during the welding process.

Temperature dependence of tribological properties in NiTi shape memory alloy: A nanoscratching study

The molecular dynamics method was used to simulate nanoscratching of a monocrystalline NiTi shape memory alloy (SMA) under various temperatures. Parameters, including the normal force, tangential force, friction coefficient, contact area, scratching hardness, atomic displacement, atomic structure, and surface morphology, were comprehensively studied to understand the friction mechanism of the NiTi SMA during operation. It was verified that the temperature dependence of the tribological properties of the NiTi SMA is mainly attributed to the temperature dependence of the tangential force.The beginning temperature of the reverse martensitic transition (As) is the critical temperature for the transition of tribological properties. Below this temperature, the sliding contact region exhibits good normal/tangential projection symmetry, with the atoms on the contact surface arranged in order. A wide range of martensitic strain coordination occurs in the contact region, leading to a low friction coefficient and minimal wear. When the temperature exceeds this threshold, the asymmetry of the contact surface projection gradually increases. The atoms on the contact surface exhibit amorphous atomic characteristics, causing the influence domain of martensitic transformation of scratches to gradually decrease and the friction coefficient to increase. This study offers insights into the atomic-scale perspective for analyzing the temperature-dependent friction behavior of NiTi SMA induced by phase transition.

Phase transformation behavior and mechanical properties of NiTi shape memory alloys fabricated by directed laser deposition

Directed laser deposition shows promise as a method for fabricating complex structure NiTi shape memory alloy parts in engineering, aerospace, automobile, and biomedical fields. The dislocation density has an impact on phase transformation behavior and mechanical properties of NiTi manufactured by directed laser deposition. In this work, NiTi samples were produced by directed laser deposition, and the mechanism of martensitic phase transformation and its interactions with dislocation density were revealed through molecular dynamics. Laser power and scanning speed are the main factors affecting dislocation density, the phase compositions of the samples are B2, B19′, and Ti2Ni phase, and the samples with dislocation density below 2 × 1015 m−2 show single-step B2–B19′ martensitic transformation, stress plateau and improved elongation, the samples with dislocation density above the value exhibit no significant transformation peak and no stress plateau. The movement of (100) twin planes and the growth of advantageous variants are observed in the simulation which can explain stress-induced martensitic reorientation and the stress plateau phenomenon. The bending test suggested that the sample under the optimal parameters has an 85 % recovery ratio within 8 % pre-strain.

Effect of Ti content on abnormal grain growth of Fe–Mn–Al–Ni–Ti shape memory alloy

In this paper, the effects of Ti content on the solvus temperature of γ-phase and abnormal grain growth (AGG) in Fe43.5−x Mn34Al15Ni7.5Ti x (x = 0, 0.5, 1 and 1.5) shape memory alloys (SMAs) were investigated. It is found that, the increase of Ti content leads to a significant reduction of the solvus temperature of γ-phase, a significant refinement of γ-phase, and a decrease of subgrain size. After 3 times cyclic heat treatments, the average grain size of Fe42Mn34Al15Ni7.5Ti1.5 SMA reaches about 9.0 mm, which is about twice of that for Fe42.5Mn34Al15Ni7.5Ti1 SMA. This is attributed to the small subgrains can provide a higher subgrain boundary energy (ΔG s) and grain boundary (GB) migration rate. The subgrain size of Fe42Mn34Al15Ni7.5Ti1.5 SMA (9.7 μm) is significantly smaller than that of Fe42.5Mn34Al15Ni7.5Ti1 SMA (21.3 μm). Thereby, the ΔG s (15.3 × 10−2 J mol−1) and GB migration rate (11.3 × 10−6 m s−1) of Fe42Mn34Al15Ni7.5Ti1.5 SMA are significantly higher than those of Fe42.5Mn34Al15Ni7.5Ti1 SMA (7.1 × 10−2 J mol−1, 6.3 × 10−6 m s−1). In addition, when the applied strain was up to 10%, the maximum superelastic strain of Fe42Mn34Al15Ni7.5Ti1.5 and Fe42.5Mn34Al15Ni7.5Ti1 were 5.5% and 5.1%, respectively. In summary, the addition of 1.5 at.% Ti in Fe–Mn–Al–Ni–Ti SMA can promote the AGG with relatively small loss in superelasticity.

Origins of functional fatigue and reversible transformation of precipitates in NiTi shape memory alloy

The work clarifies several key questions in shape memory research that have eluded previous studies. The findings show that dislocation slip emanates at austenite-martensite interfaces during unloading and aligns with the internal twin boundary interface of martensite. It was observed that the type II internal twins of the martensite become parallel dislocations in the austenite. During reloading, these dislocations act as nucleation sites for the martensitic twins, reducing the nucleation barrier and the transformation stress. The precipitates facilitate martensite nucleation but also act as an obstacle to martensite front motion, restrict detwinning, and pin the interfacial dislocations during unloading, thereby contributing to residual strains and martensite stabilization. Martensite nucleation is not suppressed by the size of the thin film, which is of the order of 85 to 105 nanometers thick, and repeated transformation occurred cycle after cycle. Single crystals deformed in the <101>LD exhibited the best recoverability of up to 5.5 % and tensile stresses of up to 1.4 GPa. It was demonstrated for the first time that, when favorably oriented, Ni4Ti3 precipitates undergo a reversible phase transformation to R-phase and can accommodate up to 4 % reversible strains.

Effect of Ni4Ti3 precipitates on the functional properties of NiTi shape memory alloys: A phase field study

Ni4Ti3 precipitates, which generally exist in aged Ni-rich NiTi shape memory alloys (SMAs), can have a profound effect on material properties. However, the fundamental insights into the effects of Ni4Ti3 precipitates on the functional properties, including the superelasticity (SE), elastocaloric effect (eCE), and shape memory effects (SMEs), are not well understood yet, especially those originating from the B2-B19′ martensite transformation (MT). In this work, a phase field model coupling the precipitation of Ni4Ti3 and the B2-B19′ MT is proposed, where the thermo-mechanical coupling effect and grain size effect are considered. The precipitate-dependent SE, eCE, one-way SME (OWSME), and stress-assisted two-way SME (SATWSME) of single-crystal, polycrystalline, and gradient-nanograined NiTi SMAs are simulated. The effects of the precipitate density, grain orientation range (texture), and gradient-distributed precipitate are examined, and the underlying microscopic mechanisms are revealed. The simulation results and new findings not only contribute to a more comprehensive insight into the effect of Ni4Ti3 precipitates on the MT, martensite reorientation, and functional properties of NiTi SMAs but also provide a reference for the development of excellent SMA-based solid refrigerants or SMA smart materials with designable functional properties.

Low temperature effect on cyclic behavior of shape memory alloy U-shaped dampers

Although Nickel-Titanium (NiTi) shape memory alloys (SMAs) have been widely studied as seismic response mitigation devices in civil structures, their temperature sensitivity due to the coupled thermo-mechanical behavior prevents their practical implementation in cold temperature environments. This paper investigated the effects of varying ambient temperatures (particularly low ambient temperature) on the hysteretic behavior and self-centering (SC) ability of shape memory alloy U-shaped dampers (SMAUDs) that were made of NiTi showing superelasticity (SE) at room temperature. Thermo-mechanical properties of SMAUDs were identified through differential scanning calorimetry (DSC) tests. Cyclic loading experiments on SMAUDs were conducted at a wide ambient temperature range from −40 °C to 20 °C. The variations in the hysteretic characteristics of the SMAUDs, including partial superelasticity (SE), strength, stiffness, SC, and energy dissipation capabilities, at different ambient temperatures were investigated. In general, the decreasing ambient temperature leads to the degradation of the SC ability. At ambient temperatures below the phase transformation temperature, the SMAUDs lose the SE but still maintain certain levels of strength and energy dissipation, which is different from common axial-type SMA elements. Meanwhile, the SMAUDs can restore their SC ability and strength after the recovery from low temperatures to room temperature, making them suitable for use in a cold environment.

Preparation of tantalum-containing coatings on NiTi shape memory alloys with enhanced in vitro cytocompatibility and antibacterial effectiveness

In this work, the magnetron sputtering method is used to deposit tantalum-containing coatings on NiTi shape memory alloys under various sputtering atmospheres including (i) 100% argon, (ii) 16.6% oxygen and 83.4% argon, and (iii) 16.6% nitrogen and 83.4% argon. This research examines how the sputtering atmosphere can impact surface morphology, roughness, wettability, biocompatibility, in vitro cytocompatibility, and antibacterial performance of tantalum-containing coatings. The calculation of nickel ions release by coupled inductively coupled plasma-optical emission spectrometry displayed that none of the coated samples showed any traces of nickel, unlike the bare NiTi substrate. This has led to an improvement in biocompatibility. Topographical measurements demonstrated the formation of a smooth surface under the argon-oxygen atmosphere, whereas the deposited layer in the argon-nitrogen atmosphere exhibited irregular ups and downs. The coatings deposited under the argon-nitrogen atmosphere result in a hydrophobic surface with many hills and valleys which can considerably enhance protein and cellular absorption. Besides, the samples coated under an argon-oxygen atmosphere with a smooth surface exhibited excellent antibacterial efficiency only as early as 6 h of incubation. This is probably due to the formation of amorphous tantalum pentoxide.

Characterization of the temperature-dependent superelastic and elastocaloric effects of a NiTi tube under compression at 293–330 K

Comprehensive characterizations of the superelastic and elastocaloric effects of NiTi and NiTi-based shape memory alloys (SMA) in the operation temperature region are highly desirable for using them in elastocaloric coolers with a large temperature lift. In this article, we report the superelastic and elastocaloric effects of a commercially available superelastic polycrystalline NiTi SMA tube with an outer diameter of 5 mm and a wall thickness of 1 mm between 293 and 330 K. The NiTi tube sample was subjected to a training of 250 cycles to stabilize its superelastic and elastocaloric effects. We observed that temperature dependencies existed for both superelastic and elastocaloric effects of the NiTi tube, and stress–strain curves differed much between isothermal and adiabatic loading conditions. The largest temperature rise and temperature drop measured at 293 K under an applied strain of 3.66% and a strain rate of 0.1 s−1 during loading and unloading were 21 and 11 K, respectively. The loading conditions (loading function and holding time) also impacted the superelastic effect of the NiTi tube. We identified two major reasons for the irreversibility of the adiabatic temperature change: the hysteresis heat dissipation and the temporary residual strain after unloading, and they affected the cooling performance of the elastocaloric cooler in different ways. We investigated the dependencies of the superelastic and elastocaloric effects on the maximum applied strain and the temperature distribution on the NiTi tube during loading and unloading. The results are beneficial to the modeling of elastocaloric coolers with large temperature lifts.

Phase-field damage simulation of subloop loading in TiNi SMA

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

Design and Experimental Evaluation of Passive Ankle-Foot Orthoses Hinges Using Superelastic NiTi Shape Memory Alloy

Design and Experimental Evaluation of Passive Ankle-Foot Orthoses Hinges Using Superelastic NiTi Shape Memory Alloy

Features of functional behavior of TiNi shape memory alloy after uniaxial high speed magnetic pulse tension

This study investigates the influence of high strain rate on the functional properties of TiNi shape memory alloys. Specimens were deformed in tensile mode using a magnetic pulse loading setup equipped with an optical system to measure the loading time. The technique allows to obtain different strain rates without changing the working part of the specimens, i.e. the residual strains remained the same. The results show that the shape memory effect decreases by 10–25% with increasing strain rate. However, a high strain rate has a positive effect on the two-way shape memory, which increases by 10–25%.

Laser and Electron-Beam Surface Processing on NiTi Shape Memory Alloys: A Review

Laser and Electron-Beam Surface Processing on NiTi Shape Memory Alloys: A Review

Relationship between ceramic additives and improved biodegradability, osseointegration, and surface roughness of Ni–Ti shape memory alloy for use in load-bearing bone site applications

To make metallic biomaterial implants acceptable for use in bone replacement applications, this research aimed to identify a potential means of improving the poor wear resistance of implants as well as their limited osseointegration capability and rapid biodegradation. In this regard, mechanical alloying was used to make a nickel-titanium (Ni–Ti) shape memory alloy with increasing amounts of hardystonite (HT) and niobium carbide (NbC). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to examine the phase composition and microstructure of the sintered samples. To test osseointegration and biodegradation, samples were immersed in simulated body fluid (SBF). They were then analyzed using SEM, weight loss measurements, and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The SEM pictures indicated that increasing HT and NbC helped build apatite layers on their surfaces, anticipating successful osseointegration if utilized as bone implants. ICP findings showed that HT and NbC significantly reduced Ni2+ ion release, preventing tissue damage. In contrast to electrical conductivity, these ceramics improved the compressive strength and microhardness of the examined samples. The improvement in their strength was 10.81, 27.02, 21.62, and 31.89 %. SEM results confirmed the role of ceramic materials to enhance the wear resistance of the tested samples. Based on these findings, the prepared nanocomposites achieved the main goal of this study.

Additive manufacturing of NiTi architected metamaterials

Additive manufacturing has revolutionized the creation of complex and intrinsic structures, offering tailored designs for enhanced product performance across various applications. Architected cellular or lattice structures exemplify this innovation, customizable for specific mechanical or functional requirements, boasting advantages such as reduced mass, heightened load-bearing capabilities, and superior energy absorption. Nonetheless, their single-use limitation arises from plastic deformation resulting from localized yield damage or plastic buckling. Incorporating NiTi shape memory alloys (SMAs) presents a solution, enabling structures to recover their original shape post-unloading. In this study, an NiTi architected metastructure, featuring auxetic behavior and a negative Poisson's ratio, was designed and fabricated via laser powder bed fusion (LPBF). The samples exhibit promising superelastic performance with recoverable deformation strains at room temperature. Comprehensive characterization processes evaluated the functional performance of the fabricated metastructures. The metastructure geometry promoted microstructure formation primarily along the wall thickness. Cycling compression tests, conducted at three applied force levels, demonstrated stable cyclic behavior with up to 3.8 % reversible deformation strain, devoid of plastic buckling or yielding damage. Furthermore, the NiTi metastructures displayed robust energy absorption capacity and damping behavior, underscoring their potential for reusable energy dissipators in various industries including aerospace, automotive, construction, and etc.

Reactive Ti/Ni multilayer composites produced via accumulative roll-bonding (ARB): post-annealing treatment

ABSTRACT Ti/Ni multilayers, produced through the accumulative-roll bonding (ARB) process, serve as reactive multilayer, capturing energy to optimize manufacturing processes and contribute to TiNi (shape memory alloy) production. This research specifically examines the impact of the number of layers on microstructural and thermal properties after activation through heating. Energy-dispersive spectroscopy (EDS), field emission scanning electronmicroscopy (FESEM), and differential scanning calorimeter (DSC) were employed to analyse samples subjected to various ARB cycles. The findings indicate that in low-cycle ARB processing, the interdiffusion of Ni and Ti under normal conditions initiates the production of a very thin layer of TiNi at 800 °C, requiring extended holding times for increased thickness. Conversely, the formation of intermetallic compounds, particularly TiNi, and diffusion in high-cycle ARB-processed composites are accelerated. According to the DSC results, complete activation and phase transformation, accompanied by the release of energy (43–50 kJ/mol), occur at approximately 500–525°C without the need for any holding time. Consequently, for achieving a uniform TiNi phase, heat treatment of high-cycle ARB-processed composites up to 800°C – a temperature surpassing the activation temperature – without any holding time can be effectively employed.

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

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

Experimental investigation during nano powder added µ-ED milling on nickel-titanium shape memory alloy

Nickel-titanium (NiTi) shape memory alloy (SMA) is one of the smart materials which has a vast application in the aerospace and biomedical industry due to its shape memory effect (SME), strong corrosion and wear resistance. Thus, the present study investigates the influence of process variables like gap voltage (V), powder concentration (PC) and pulse on time (ton) towards the material removal rate (MRR), surface roughness (SR) and micro-hardness (MH) amid graphene nano powder added micro-electrical discharge milling (µ-ED milling) of NiTi SMA. ANOVA analysis has been carried out to determine the % distribution of individual machining parameters towards all responses. The addition of graphene nano particles to the dielectric oil considerably enhanced the MRR, MH, and decreased the SR of the milled micro-channel. Taguchi’s grey relational analysis (GRA) and grey-desirability approach have been applied for multi-response optimisation to find the maximum MRR, MH, and minimum SR. It is discovered that the grey-desirability method improves all the responses. The Field emission scanning electron microscopy (FESEM) micro-graphs confirm defect-free mirror-like surface finish at the optimal grey-desirability setting. The X-ray diffraction (XRD) analysis discovered the formation of oxides and hardened phases in the machined surface. The application of graphene enhanced the recast layer thickness and reduced the indentation depth at subsurface region. The nano indentation test validates the rise in nano hardness and elastic modulus at the grey desirability optimal setting compared to the initial machining condition.

Influence of an Engineered Notch on the Electromagnetic Radiation Performance of NiTi Shape Memory Alloy.

This work explores the influence of a pre-engineered notch on the electromagnetic radiation (EMR) parameters in NiTi shape memory alloy (SMA) during tensile tests. The test data showed that the EMR signal fluctuated between oscillatory and exponential, signifying that the specimen's viscosity damping coefficient changes during strain hardening. The EMR parameters, maximum EMR amplitude, and average EMR energy release rate remained constant initially but rose sharply with the plastic zone radius with progressive loading. It was postulated that new Frank-Read sources permit dislocation multiplication and increase the number of edge dislocations participating in EMR emissions, leading to a rise in the value of EMR parameters. The study of the correlation between EMR emission parameters and the plastic zone radius before the crack tip is a vital crack growth monitoring tool. An analysis of the interrelationship of the EMR energy release rate at fracture with the elastic strain energy release rate would help develop an innovative approach to assess fracture toughness, a critical parameter for the design and safety of metals. The microstructural analysis of tensile fractures and the interrelation between deformation behaviours concerning the EMR parameters offers a novel and real-time approach to improve the extant understanding of the behaviour of metallic materials.