NiTi SMA Research Articles

Mechanical behavior and fatigue life of micro welded joints obtained by TIG spots in NiTi wires

This work evaluates the effect of welding energy and cycling strain amplitude on fatigue behavior of welded joints obtained by micro Tungsten Inert Gas spots in NiTi SMA superelastic wires. For this purpose, a Dynamic Mechanical Analyzer operating in single cantilever bending mode is employed. The equipment monitors the evolution of the force level to maintain constant the strain amplitude as cycling proceeds. Fatigue lives of as-received and welded NiTi wires are compared for different strain amplitudes applied during mechanical cycling. In the evaluated range of strain amplitude, as-received NiTi wires showed fatigue lives between 103 and 104 cycles. The fatigue life of the welded joints is lower, although it remains in the same range observed for the as-received wires.

Novel insight into the strain-life fatigue properties of pseudoelastic NiTi shape memory alloys

The whole low-to-high cycle fatigue behavior of a pseudoelastic NiTi SMA was analyzed. Global and local strains measurements were carried out by an extensometer and digital image correlation. Significant differences were observed at the two scales, due to the localized nature of stress-induced transformations. Local mechanisms play a crucial role on fatigue damage and have a direct effect on the Z-shaped strain-life curves observed in previous works. The effective nature of this unusual behavior was revealed by systematic analysis of local strain. Finally, strain decomposition was made and data were analyzed by a recent modified Coffin–Manson approach.

Effects of thermal aging on phase transformation and microstructural characteristics of NiTi shape memory alloy

In this study, the effects of aging temperature and time on microstructure and phase transformation behaviors of the NiTi shape memory alloy were investigated with the effects of precipitates. NiTi SMA was synthesized using powder metallurgy process followed by the solution annealing at 850 °C for 1 h. The thermal aging process has been executed on NiTi alloy in the temperature range of 450 °C–750 °C for 1 h, 3 h and 5 h respectively. It was found that the enhancement of transformation temperatures due to the formation of secondary phases during the transformation of cubic to monoclinic. The lenticular structured Ni4Ti3 precipitate was mainly influenced the phase transformation by changing the Ni composition in the NiTi matrix. The significant changes were observed in the austenite and martensite finish temperatures with increasing the aging temperature and time. This study suggests that the NiTi SMA can be implemented in biomedical applications when heat treated at 650 °C for 5 h.

Forming and two-way shape memory effect of NiTi alloy induced by laser shock imprinting

Nickel–Titanium shape memory alloy (NiTi SMA) is an intelligent material that is a widely applied and studied. The two-way shape memory effect (TWSME) of the NiTi SMA has garnered particular attraction in the further miniaturisation and integration of micromechanical systems. To realise the micro-forming and TWSME of the NiTi SMAs at a micron scale, this study reports a method that involves laser shock imprinting (LSI) technology to induce the TWSME. The production efficiency of this method was significantly higher than that of the thermo–mechanical treatment; the proposed method integrated the induction and forming processes. In this study, the precise and rapid forming of microstructures was realised; meanwhile, the TWSME was induced successfully at the micron scale. Moreover, the experimental results indicated that this method can improve the mechanical properties of the formed part. The LSI-induced TWSME exhibited good thermal cycling stability during the thermal cycling attenuation test, the maximum phase transition angle of bending workpiece began to remain stable after the fourth thermal cycle, and the stability value was approximately 19.0% of the initial bending angle. Furthermore, the feasibility of the LSI-induced TWSME was proved by theoretical analysis, and its strengthening mechanism, phase transition process, and thermal cycling stability were discussed.

Modeling the transient behavior of the NiTi shape memory actuator using finite element analysis: Parametric study of the rate effects

Finite element simulation is presented to study the rate effects of the shape memory response of bent NiTi wire “C-shaped” exposure to load. A three-dimensional constitutive model for shape memory alloys proposed by Auricchio is implemented in the built-in library of Ansys Workbench 18.2 to capture the shape memory behavior of three different NiTi alloys that are subjected to different thermomechanical treatments. The quasi-static (slow) loading rate and the dynamic (fast) loading rate have been applied isothermally or at adiabatic conditions. Other models assumed the classical shape memory stress-strain behavior, but the realistic behavior of the NiTi SMA is different. The NiTi SMA is influenced by its microstructural changes, such as internal defects, grain size, chemical compositions, content of dislocations and density; all these effects should be added when modeling, which has been considered in this study. The results showed a significant effect of the dynamic loading of NiTi on decreasing the recoverable strain. As a result, it could be expected that the actuator lifetime could be reduced when a rapid, as opposed to a slow, loading rate is adopted. Inhomogeneity of stresses and deformations after loading can cause irreversible plastic deformation. The complete deformation recovery is noticed in the reorientation scenario, unlike in the SIM scenario, which exhibits unrecoverable deformation. Experimental validation is done by comparing the macroscopic displacement of the bent NiTi actuator with the model results, which shows good agreement.

Neutron diffraction study of temperature-dependent elasticity of B19′ NiTi---Elinvar effect and elastic softening

The temperature-dependent elasticity of the B19′ NiTi is unknown today. To gain insights into the lattice-level temperature-dependent elasticity of the B19′ crystal, we present results of in-situ neutron diffraction experiments performed on polycrystalline martensitic specimens in the temperature range of 300 down to 50 K. The experimental results are compared with the density functional theory molecular dynamics (DFT-MD) and Quasi Harmonic Approximation (QHA) calculations. The results confirm that the temperature-dependent Young's modulus (TDYM) of the B19′ crystal is strongly anisotropic. For different crystallographic orientations, the change in Young's modulus over the temperature range of 300–50 K (ΔE(hkl)=E(hkl)50K−E(hkl)300K), ranges from ΔE(102¯) = 2.8 ± 3.5 GPa (extremely weak dependence) to ΔE(103) = 59.6 ± 9.1 GPa (strong dependence). Moreover, it is found that the orientation-specific TDYM and thermal expansion (TE) of the B19′ crystal are correlated. The crystallographic orientations with weak and negative TE responses exhibit a weaker TDYM than the orientations with positive TE. The DFT-MD and QHA results capture qualitatively the above experimental observations and further show that there are orientations in a B19′ crystal exhibiting elastic softening (ΔE(hkl)<0) and ideally no change in Young's modulus (ΔE(hkl)= 0) with cooling. This is found to originate from the strong negative temperature dependence of c35 stiffness constant. The experimental results along with the first-principles calculations confirm that the Elinvar and Invar are two confluent properties in NiTi SMAs and can be tailored by texturing B19′ crystallographic orientations.

Characterization of NiTi SMA in its unusual behaviour in wear tests

Abstract The manuscript presents experimental investigations of an unusual behaviour of shape memory alloys (NiTi) having different characteristic temperatures in wear test. The studies are focused on the different wear mechanism at varying loads and sliding times and include a description of the phenomena accompanying the wear process. The ball-on-plate reciprocating sliding wear tests were conducted on NiTi shape memory alloys against a sapphire ball. We show that the wear resistance of NiTi is affected by its specific stress-strain characteristic. The understanding of the tribological behaviour of NiTi, a knowledge of the course of wear and an examination of its mechanisms can enable an effective prevention of the destruction of devices components and prolong their safe working life.

An Investigation on Chemical Machining of NiTi SMA Prepared by Powder Metallurgy

Chemical machining is very important in solving problems constantly arising due to the requirement of high surface finish along with high productivity through machining of metals and alloys. Such requirement appears in machining of NiTi shape memory alloy (55wt%Ni and 45wt% Ti) which has widespread engineering applications. The present work aimed at utilizing of Taguchi design of experiments with the method of desirability function to optimize the chemical machining parameters (machining temperature and machining time) of NiTi alloy prepared by powder metallurgy technique. Taguchi experimental design concept, L25 (2×5) orthogonal array was used to determine the S/N ratio. Based on the analyses of multiple regression method, mathematical predictive models for the studied parameters had designed and validated. To achieve the objectives of the present work Datafit ver9, and Mini Tab14 softwares had employed. The powders with the required percentages were mixed for 5hours, compacted at pressure of 900MPa, and sintered in two stages, at 500°C for 2 hours and then at 950°C for 6hours under vacuum conditions (10-4 torr ). The X-ray diffraction test showed that the prepared sample are consisting of three phases (NiTi monoclinic phase, NiTi cubic phase, and Ni3Ti hexagonal phase). Machining temperature appears as the most significant factor, which controls both surface roughness and metal removal rate. Two mathematical models were developed to predict the metal removal rate and the surface roughness for any collection of machining temperature, and machining time. A minimum surface roughness of 0.666μm with a maximum metal removal rate of 0.0041g/min can be associated with machining temperature of 650C and machining time of 2min in solution consisting of 6%HCl, 19%HNO3, and 75%H2O.

NiTi SMA Parts Production with Different Porosity Ratios

NiTi SMA Parts Production with Different Porosity Ratios

Influence of microstructure on mechanical and magnetic properties of an Fe-Ni-Co-Al-Ta-B shape memory alloy

Ni-Ti shape memory alloys are the only shape memory alloys that are commercially available on the market and are also used in diverse medical and engineering applications. However, they still have some limitations associated with them, such as the use of expensive raw materials, sophisticated equipment for production and lack of easy cold formability when compared with iron-based and copper-based SMAs. Researchers have therefore been looking for viable alternatives to Ni-Ti SMAs, such as iron- and copper-based SMAs, that are cheaper and easier to produce and process. Fe-28Ni-17Co-11.5Al-2.5Ta-0.05B (at%) (abbreviated NCATB) shape memory alloy shows huge superelasticity (>13%) and high tensile strength at room temperature. NCATB alloy has therefore attracted the attention of researchers and engineers to exploit its advantages in different fields. In the current work, an NCATB shape memory alloy was prepared by two different techniques, namely vacuum induction melting under an argon atmosphere and vacuum arc remelting followed by suction casting technique. The cast alloys were subsequently thermally treated and characterized by optical microscopy (OM), differential scanning calorimetry (DSC), x-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). Vickers hardness test and carbon and sulphur analyses were also carried out on the alloys. The major objective of the present work is to evaluate the influence of aging on microstructure and, in turn, on mechanical and magnetic properties of an NCATB shape memory alloy. Aging at 600 °C up to 48 h was found to be the optimum heat treatment to achieve enhanced mechanical properties in both NCATB1 and NCATB2 alloys. Ferromagnetic (Ni,Fe,Co)3(Al,Ta) γ′ ordered precipitate particles and paramagnetic NiAl (β) phases were found to be critical for using the alloy for magnetic applications.

Torsional whole-life transformation ratchetting under pure-torsional and non-proportional multiaxial cyclic loadings of NiTi SMA at human-body temperature: Experimental observations and life-prediction model

Torsional whole-life transformation ratchetting is investigated under pure-torsional and non-proportional multiaxial loadings of NiTi SMA micro-tubes at human-body temperature (310 K), where three paths of torsional loadings and five paths of multiaxial ones are considered. It is observed that the evolution of the torsional whole-life transformation ratchetting depends strongly on the loading paths and stress levels, and the fatigue lives of pure-torsional loadings decrease faster than that of uniaxial and multiaxial ones with the increase of peak stress. Based on the experimental investigations, a life-prediction model which depends on the applied stress levels is proposed for NiTi SMA micro-tubes, where the martensite transformation and reorientation of NiTi SMAs are considered. The predicted fatigue lives under uniaxial, torsional and non-proportional multiaxial loadings are mostly located within the triple error band.

Chilled Air System and Size Effect in Micro-milling of Nickel\u2212Titanium Shape Memory Alloys

Although Nickel-Titanium Shape Memory Alloys (NiTi SMAs) are used in a variety of applications due to their shape memory and superelasticity properties, their features of high ductility, temperature sensitivity, and strong work hardening render these materials difficult to machine. The viability of a new approach in improving the machinability through temperature control using chilled air system application was investigated. Differential scanning calorimetry was used to characterise material response to thermal loads. Microstructure phase identification was evaluated with X-ray diffraction. Micro-milling tests were performed using chilled air system and benchmarked to dry cutting and the use of minimum quantity lubricant (MQL). To augment lubrication, chilled air was also applied concurrently with MQL. Results indicated that the application of chilled air reduced cutting temperature and minimised burr height, while their simultaneous application with MQL further improved the machinability. Further investigation was conducted to explore the influence of the ploughing mechanism on machining performance and product quality. The results pointed to higher feed per tooth producing better outcomes. This paper puts forward a new hypothesis that the machinability could be improved by inhibiting or locking in phase transformation through temperature control, and optimising chip thickness, one of the principal parameters of size effect.

Effect of thermomechanical coupling on stress-induced martensitic transformation around the crack tip of edge cracked shape memory alloy

In this work, the effect of thermomechanical coupling on stress-induced martensitic phase transformation around the crack tip of a pseudoelastic NiTi SMA compact tension (CT) specimen is studied experimentally and numerically. Six CT specimens are tested under different loading rates and the temperature distributions around the crack tip are measured using an infrared camera. For the numerical evaluation of temperature field and phase transformation, extended thermo-mechanically-coupled ZM (Zaki–Moumni) model is implemented into ABAQUS. The results of temperature field show that experimental and simulation results of temperature show a good agreement, and the maximum temperature at the crack tip increases with loading frequency. Furthermore, numerical results show that the phase transformation region size decreases when the loading frequency increases.

On the Fatigue Behavior of Nanocrystalline NiTi Shape Memory Alloys: A Review

This paper presents a review of the research on the fatigue performance of superelastic polycrystalline nanocrystalline NiTi shape memory alloys (nc NiTi SMAs). A brief introduction to some focal definitions and basic concepts of fatigue measurement and response in NiTi SMAs were given. Results found in the literature on fatigue in nc NiTi SMAs are discussed. Mechanical behavior and energy dissipation capacity of nc NiTi SMAs are explored and collectively compared with coarse-grained NiTi SMAs, along with an assessment of the influence of grain size refinement and thermomechanical treatments. Several conclusions and suggestions are made, including that nc NiTi SMAs exhibit larger functional fatigue resistance, lower crack tolerance, higher superelasticity, and smaller hysteresis loss.

Low-to-high cycle fatigue properties of a NiTi shape memory alloy

Low-to-high cycle fatigue behavior of a pseudoelastic NiTi SMA was analyzed. The evolution of both global and local strain were captured during fatigue tests. Local strains were measured in-situ by the digital image correlation (DIC) technique. Significant differences were observed at the two scales, due to the localized nature of stress-induced transformations. Strain-life fatigue curves obtained from global and local strain measurements were compared. It was demonstrated that local phenomena play a very important role on the fatigue properties of pseudoelastic SMAs.

Reinforced concrete beam with pseudoelastic SMA hybrid with steel structure for seismic mitigation: A critical review of their prospective, influence factors, drawback and advantages

This paper presents a review paper of concrete beam reinforced with SMA hybrid with steel. Existing literature presented a number of analytical and experimental studies on the use of SMAs using pseudoplastic to be embedded in reinforced concrete beam for crack recovery. However, little research has been carried out on the use of pseudoelastic SMA as reinforcement as partial replacement in reinforced concrete beam. Hence the feasibility of pseudoelastic SMAs under austenitic temperature window and several components of RC building structures to exhibit stable superelastic under austenite crystallization were investigate. This paper is to review the feasibility study NiTi SMA as reinforcement rebar for seismic mitigation. This research investigates the application of concrete beams reinforced with pseudoelastic SMA longitudinal bars to evaluate the applicability of SMAs as alternative reinforcement, and to provide benchmark tests against the microstructure investigation to the performance of pseudoelasticNiTi used Malaysian ambient temperature for seismic mitigation design. The feasibility study of the superelasticNiTi SMA reinforcement bar for hybrid reinforcing of concrete beam also reviewed The factors influence the design and performance parameter of SMA in reinforced concrete beam, Comparison of NiTi, Steel and other Alloys, advantages and disadvantages, drawbacks and restraint of using pseudoelastic SMAs were also were studied.

Phase Transformation, Twinning, and Detwinning of NiTi Shape-Memory Alloy Subject to a Shock Wave Based on Molecular-Dynamics Simulation.

Martensitic transformation, reverse martensitic transformation, twinning, and detwinning of equiatomic nickel–titanium shape-memory alloy (NiTi SMA) under the action of a shock wave are studied using a molecular-dynamics simulation. In the loading process of a shock wave, B2 austenite is transformed into B19′ martensite, whereas in the unloading process of the shock wave, B19′ martensite is transformed into B2 austenite. With repeated loading and unloading of the shock wave, martensitic transformation occurs along with twinning, but reverse martensitic transformation appears along with detwinning. The mechanisms for the twinning and detwinning of NiTi SMA subjected to a shock wave are revealed in order to lay the theoretical foundation to investigate the shape-memory effect and superelasticity.

On the printability and transformation behavior of nickel-titanium shape memory alloys fabricated using laser powder-bed fusion additive manufacturing

Laser powder-bed fusion (L-PBF) additive manufacturing is regarded as an attractive alternative for producing parts with complex geometries for nickel-titanium shape memory alloys (NiTi SMAs). These alloys are known to pose challenges when processed using traditional subtractive or formative manufacturing technologies. Although L-PBF of NiTi has been investigated in some previous research efforts, very little emphasis has been placed on the manufacturability (or printability) of NiTi, where we use printability to refer to the capability of producing parts free of macroscopic defects.The current study elucidates challenges related to the printability of NiTi SMAs using L-PBF, and its interaction with their phase transformation behavior, responsible for their functional properties. More specifically, we conduct experiments and employ machine learning classification techniques to identify an adequate design parameter and an empirical rule for determining the printability of NiTi. Our results indicate that the linear energy density EL is a better design parameter for identifying satisfactory printability, while volumetric energy density, EV, is more relevant in controlling the transformation behavior of the processed material.

Progress in biocompatibility and surface modification of nickel titanium shape memory alloys

To summarize the research progress of biocompatibility and surface modification of nickel titanium shape memory alloys (Ni-Ti SMA). The relative researches about Ni-Ti SMA at home and abroad were reviewed, collated, analyzed, and summarized. At present, Ni-Ti SMA as an internal fixation material has been widely used in clinic. It has the following advantages: the super elasticity, the shape memory characteristic, the good wear resistance, and the strong corrosion resistance. It also can effectively avoid the internal fixator rupture caused by stress shielding. After surface modification, the biocompatibility of Ni-Ti SMA has been improved. The Ni-Ti SMA is the most promising alloy material for the long-term internal fixator because of its excellent material properties.

A micromechanical model for the grain size dependent super-elasticity degeneration of NiTi shape memory alloys

Recent experiments show that super-elasticity degeneration occurs during the cyclic deformation of polycrystalline NiTi SMAs and the extent of such degeneration depends strongly on the grain size of the polycrystalline aggregate. Thus, in this work, a micromechanical model is constructed to describe the grain size dependent super-elasticity degeneration of polycrystalline NiTi SMAs. The polycrystalline aggregate is modeled as a composite material, i.e., each grain-interior (GI) phase is assumed to be a spherical inclusion embedded in a matrix of grain-boundary (GB) phase. For GI phase, two inelastic deformation processes, i.e., martensite transformation and transformation induced plasticity, and their interaction are considered, simultaneously; for the GB phase, its plastic deformation is described by a unified visco-plastic model. To describe the interaction between the inclusion and matrix and calculate the macroscopic overall performances of the modeled composite material, a modified incremental Mori–Tanaka's homogenization method is proposed by introducing the Eshelby's tensor of a spherical inclusion embedded in a finite spherical domain and the affine tangent modulus and affine strain increment. Numerical algorithm is developed to implement the proposed micromechanical model. A new and simple affine linearization method is proposed to linearize the constitutive equations of GI and GB phases. The proposed micromechanical model is verified by comparing the simulated results with the corresponding experimental ones. It is shown that the grain size dependent super-elasticity degeneration of NiTi SMAs can be well captured. Moreover, the calculated results indicate that when the average grain size is large (>160 nm), the super-elasticity degeneration originates from the interaction between the martensite transformation and dislocation slipping in GI phase; while, when the average grain size is small (<160 nm), the super-elasticity degeneration is dominated by the plastic deformation of GB phase and the stress redistribution between GI and GB phase.