Behavior Of Shape Memory Alloys Research Articles

Effect of Nb Content on Behavior of (Ti-Nb) Shape Memory Alloys for Biomedical Applications

Effect of Nb Content on Behavior of (Ti-Nb) Shape Memory Alloys for Biomedical Applications

Experimental-Numerical Assessment of Mechanical Behavior of Laboratory-Made Steel and NiTi Shape Memory Alloy Wire Ropes

The mechanical behaviors of laboratory-fabricated steel and superelastic shape memory alloy (SMA) wire ropes are assessed in this study through a comprehensive approach encompassing both experimental investigations and finite element (FE) numerical simulations. The assessment of steel wire ropes involves experimental scrutiny under sinusoidal cyclic loading and natural earthquake loading conditions. In parallel, SMA wire ropes’ behaviors are analyzed utilizing FE simulations employing the widely acknowledged ABAQUS software version 2020. The validation of all numerical simulations is undertaken against the experimentally observed behaviors. Moreover, full-scale steel wire ropes are subjected to shaking table tests to validate the simulations, facilitating a comparative analysis between the mechanical responses of SMA and steel wire ropes. The findings demonstrate that SMA wire ropes exhibit superelastic behavior akin to SMA wires, with marginal variations in overall response observed across distinct configurations, akin to steel wire ropes. Furthermore, augmenting the helix angle of SMA wire ropes results in reduced stress and increased strain when exposed to the El Centro earthquake scenario. Nevertheless, the mechanical response of SMA wire ropes closely mirrors that of a single wire.

Experiment and simulation study on the functional fatigue of shape memory alloy beam actuators

The functional fatigue behavior of shape memory alloy (SMA) beam actuators is gaining importance as their utilization in engineering applications becomes more widespread. However, research on the functional fatigue behavior of SMA beam actuators under bending conditions is not as extensive as that on SMA wires. In this paper, an experimental study and theoretical analysis of the functional fatigue behavior of SMA beam actuators were conducted. A measuring method for bending deflection and an automatic thermal cyclic test bench was designed. A series of functional fatigue tests were conducted on SMA beam actuators under different bias load conditions and the functional fatigue patterns were obtained. The material damage factor is defined and calculated through specimen tests. By incorporating the damage factor to modify the existing constitutive model, a model considering functional fatigue and tension-compression asymmetry is obtained. Finite element analysis (FEA) is performed based on this modified model to simulate the actuating performance of SMA beam actuators and to compute the deflection at different numbers of cycles. Additionally, a small parameter study on the actuator shape is conducted using FEA. By comparing the FEA results with functional fatigue experimental data, the effectiveness of the modified model is validated, demonstrating its capability to describe functional fatigue and predict actuator lifespan.

Experimental Study of Nitinol Springs: Apparatus and Results

Abstract Background The behavior of shape memory alloys that admit large reversible deformations in response to thermal excitation has been extensively studied in recent years. Yet, the number of works dealing with springs made from these alloys is rather limited in spite of their attractiveness in various applications. Objective To bridge this gap we designed and constructed an experimental system for characterizing the behavior of the springs. It enables precise control of the three state variables: temperature, elongation, and force. Methods Control of the sample temperature is achieved by immersing it in a water-filled thermal bath, where the water temperature is adjusted using a thermoelectric Peltier device. A tension-compression motorized unit sets the spring elongation and a force gauge is used for measuring the force exerted on the spring. The data is continuously monitored and acquired with a self-coded LabVIEW program. An important aspect is the calibration procedure developed for identifying the spring load-free state and ensuring the repetitiveness of the measurements. Results Experiments in which the elongation or the force were measured as a function of the temperature demonstrate the role of the phase transformations. Isothermal experiments enabled to characterize the variations of the force versus the elongation at different temperatures. Conclusions The proposed system facilitates the execution of highly accurate experiments through which the complex history-dependent behavior of shape memory springs can be revealed and studied.

Phase transformation behavior and shape memory effect of nickel-titanium shape memory alloy using the magnetic field assisted wire electrical discharge machining

Nickel-titanium shape memory alloy (NiTi-SMA) can restore its original shape after deformation, making it show great application in the fields of biomedicine, aerospace, etc. The wire electrical discharge machining (WEDM) has the advantage of machining micro and complex structures, which can greatly promote the application of NiTi-SMA. However, there are issues such as heat affected layer (HAL) on the surface of WEDM, which can affect the functionality of NiTi-SMA. Therefore, when using WEDM to NiTi-SMA, more attention should be paid to the functional changes after machining. This article primarily examine is the impact of peak current and pulse on time on the phase transformation behavior, shape memory effect, and mechanical properties of NiTi-SMA during WEDM, and introduces into the magnetic field assisted (MFA) process to improve the machining performance of NiTi-SMA. The results indicate that the HAL formed on the surface of NiTi-SMA is primarily composed of the irreversible phase transformation of B2 austenite. This layer induces a hardening effect, resulting in an increase in the phase transition temperature and a decrease in the phase transition energy (∆HM-A), as a result, the martensitic phase transformation behavior is suppressed. When the peak current and pulse on time increase, the surface quality of the NiTi-SMA processed decreases. The thickness of the HAL and microhardness increase, causing an increase in the phase transition temperature of the NiTi-SMA processed samples. Additionally, the transformation energy ∆HM-A, shape recovery rate, and mechanical properties decrease, and after recovery, more surface cracks appear and show larger extensions. MFA-WEDM can improve the surface quality of NiTi-SMA processing, reduce the thickness of the HAL, and effectively enhance the phase transformation temperature and energy ∆HM-A of the NiTi-SMA processed samples. With a magnetic field strength of 0.45T, the tensile shape recovery rates of 98%, 94.4%, and 91.0% were achieved at 2%, 5%, and 8% strain, respectively. This represents an improvement of 8.9% compared to the maximum shape recovery rate with conventional WEDM. At the same time, it also inhibits the generation and propagation of surface cracks after deformation recovery. Additionally, MFA-WEDM improves the mechanical properties of NiTi-SMA, and reduces the thickness of the brittle fracture region by 49.7% compared to conventional WEDM. Therefore, MFA-WEDM can effectively enhance the shape memory effect of NiTi-SMA. However, it is important to avoid excessive peak currents and pulse on time to achieve optimal shape memory functionality.

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.

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.

Vibration analysis of active composite structures embedded with long and short shape memory alloy (SMA) fibers

The shape memory effect (SME) is a source of internal compressive stresses that modify the vibration behavior of shape memory alloy (SMA) embedded composite structures. Generally, long fibers are used due to their suitability for activating SMA using resistive heating. Longer SMA fibers embedded in composite structures have their limitations in design and manufacturing. Minimal research work has been done on tuning the vibration behavior of composite structures embedded with short SMA fibers. In this research paper, an analytical model for evaluating the natural frequencies of an SMA embedded composite beam has been proposed. Effects of various design parameters for long (continuous) and short (discontinuous) SMA fibers embedded in composites have been investigated. It was observed that short SMA fibers with a higher aspect ratio can tune their frequencies similarly to long SMA fiber composites and are more effective in thicker beams. Matrix material with negative coefficient thermal expansion is more suitable for embedding SMA for modifying natural frequency in active mode.

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%.

Improving the actuation behavior of nitinol shape memory alloys by nanosecond laser surface texturing

Laser surface texturing of nitinol was performed using a nanosecond fiber laser to modify the surface structure and enhance the laser actuation performance of nitinol. The influence of laser processing parameters on surface morphology and chemical composition was studied in detail. The influence of texturing on the laser actuation behavior was evaluated using a continuous wave fiber laser and a CO2 laser. Compared to the pristine sample, the textured samples demonstrated better displacements and faster actuation. During laser actuation using a fiber laser, the textured sample showed maximum and minimum displacement enhancements of 65% and 15%, respectively. The actuation performance of the textured sample was approximately twice that of the pristine sample during the irradiation with CO2 laser. This paper presents the viability of laser surface texturing to enhance the actuation performance of shape memory alloys, and the results indicate that the power required for actuation can be reduced to avoid the use of bulky power and control units.

Effect of solution treatment on the microstructure, phase transformation behavior and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of the additively manufactured metallic materials. In this work, a ternary (NiTi)91Nb9 (at.%) shape memory alloy was produced by laser powder bed fusion (L-PBF) using pre-alloyed NiTi and elemental Nb powders. The effect of solution treatment on the microstructure, phase transformation behavior and mechanical/functional performances was investigated. The in-situ alloyed (NiTi)91Nb9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix. Upon high-temperature (1273 K) solution treatment, Nb-rich precipitates were precipitated from the supersaturated matrix. The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment, leading to a morphological transition from mesh-like into rod-like and sphere-like. Coarsening of the β-Nb phases occurred with increasing holding time. The martensite transformation temperature increases after solution treatment, mainly attributed to: (i) reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi, and (ii) the Ti expulsion from the β-Nb phases that lowers the ratio Ni/Ti in the B2-NiTi matrix, which resulted from the microstructure changes from non-equilibrium to equilibrium state. The thermal hysteresis of the solutionized alloys is around 145 K after 20% pre-deformation, which is comparable to the conventional NiTiNb alloys. A short-term solution treatment (i.e. at 1 273 K for 30 min) enhances the ductility and strength of the as-printed specimen, with the increase of fracture stress from (613 ± 19) MPa to (781 ± 20) MPa and the increase of fracture strain from (7.6 ± 0.1)% to (9.5 ± 0.4)%. Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates >90%. This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.

An analytical study on SMA beam-column actuators for anti-buckling phenomenon

The present paper focuses on studying the anti-buckling behavior of prismatic martensitic shape memory alloy (SMA) beam-columns. It combines analytical and semi-analytical approaches to investigate the process of column straightening for anti-buckling. We try to comprehensively describe this phenomenon and develop a mathematical model to formulate each step of the anti-buckling problem. Due to the complex stress–strain behavior of SMA material, nine different stages of stress-height diagrams may potentially occur during this effect; thus, for facilitating the design process of SMA structures, corresponding forces and moments to each stage, are analytically derived. Our demonstration establishes that the primary cause of beam-column straightening is not the uniformity of stress, but rather the achievement of uniform strain across all fibers of the cross-section. This uniform strain distribution implies that the curvature of the beam-column diminishes to zero.

Topology optimization structure design of shape memory alloy with multiple constraints

As an emerging functional material, shape memory alloy (SMA) exhibits remarkable mechanical properties and finds diverse applications across industries. This paper presents a topology optimization framework based on the bi-directional evolutionary structural optimization (BESO) method for designing SMA structures, which maximizes structural stiffness under multiple constraints of specified volume fraction, displacement, and fundamental frequency. A phenomenological constitutive model is utilized to simulate the mechanical behavior of SMA accurately. The unit virtual load method is employed to determine sensitivities. Several optimized SMA beam structures and simply-supported cube structures are designed under different thermal-mechanical loads, and their displacement, mean compliance, and fundamental frequency are evaluated throughout the optimization process. The results demonstrate that the proposed framework successfully customizes the SMA topology structure with adjustable displacement and fundamental frequency, and the optimized schemes exhibit more considerable deformation and more uniform mechanical properties than their initial counterparts. The proposed framework has higher computational efficiency than the traditional SIMP-based SMA topology optimization design method.

Atomistic investigation of phase transformations in NiTiCu shape memory alloys

NiTiCu shape memory alloys have attracted much attention for their ability to reduce hysteresis, which is required fast responses in actuator and sensor applications, but the correlation between the properties and specific process of phase transformation is unclear. In this work, an atomic-level understanding of the detailed phase transformation behavior of NiTiCu shape-memory alloys is achieved based on a newly developed interatomic potential for the Ni–Ti–Cu ternary system. Free energy calculations based on the thermodynamic integration method faithfully reproduce reported experimental trends, such as the compositional dependency of different phase transformation paths. Further, large-scale molecular dynamics simulations performed on nanocrystalline alloys provide visual evidence of the detailed phase transformation process involving different martensitic transformation paths. Our results suggest that although both the composition and grain size can be adjusted to realize slim hysteresis, the effect of increasing the Cu amount is only significant if changes in the phase transformation modes are involved at relatively low Cu contents. When the material exhibits only a single phase transformation mode at relatively high Cu contents, the additional Cu is not highly effective in reducing the hysteresis.

Gd Effect on Micro-Crystal Structure and Thermomagnetic Behavior of NiMnSn Magnetic Shape Memory Alloy

In this study, the rare earth Gadolinium (Gd) element was added to the NiMnSn alloy, which is an alternative to the NiMnGa alloy group, with the increasing popularity of magnetic shape memory alloys. Since rare earth elements have strategic importance for our country in recent years, Gd element has been preferred in this study. X-rays and SEM-EDX analysis were performed to determine the morphological properties of the crystal structure and microstructure of the alloys. Magnetic measurements of the alloys were made with the physical property measuring device and it was determined that the magnetization values decreased with the addition of Gd.

Effect of pore on the deformation behaviors of NiTi shape memory alloys: A crystal-plasticity-based phase field modeling

Porous NiTi shape memory alloys (SMA) have been developed into important biomedical and damping materials. However, the effect of pore on the martensitic microstructure evolution, functional properties, and plastic deformation has not been fully clear yet. In this work, to reveal the functional properties including superelasticity (SE), elastocaloric effect (eCE), one-way shape memory effect (OWSME), and stress-assisted two-way shape memory effect (SATWSME), as well as plastic deformation of porous NiTi SMAs, the polycrystalline SMA system is considered as a composite containing the grain-interior (GI) and grain-boundary (GB) phases, a thermo-mechanically coupled, grain-size-dependent and crystal-plasticity-based phase field model is proposed for the GI phase, and an elasto-viscoplastic constitutive model is established for the GB phase. The effects of porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution, as well as the interaction between pore and grain size, are comprehensively revealed and discussed. The simulations demonstrate that the pores cause a heterogeneous stress field, and the interaction among pores and that between pores and grain boundaries can enhance such heterogeneity, which can promote the initiation of martensitic transformation and reorientation. Moreover, the geometrical constraints caused by the pores can significantly affect the morphology and evolution of martensite microstructures. However, due to the diverse microstructure evolutions during SE, OWSME, and SATWSME, their interactions with the pores are quite different, resulting in various dependencies of each functional property and plastic deformation on the pore, and such dependencies are strongly influenced by porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution. The simulated results in this work are helpful to comprehensively and deeply understand the pore effect and its microscopic mechanism.

Investigation and Validation of a Shape Memory Alloy Material Model Using Interactive Fibre Rubber Composites.

The growing demand for intelligent systems with improved human-machine interactions has created an opportunity to develop adaptive bending structures. Interactive fibre rubber composites (IFRCs) are created using smart materials as actuators to obtain any desired application using fibre-reinforced elastomer. Shape memory alloys (SMAs) play a prominent role in the smart material family and are being used for various applications. Their diverse applications are intended for commercial and research purposes, and the need to model and analyse these application-based structures to achieve their maximum potential is of utmost importance. Many material models have been developed to characterise the behaviour of SMAs. However, there are very few commercially developed finite element models that can predict their behaviour. One such model is the Souza and Auricchio (SA) SMA material model incorporated in ANSYS, with the ability to solve for both shape memory effect (SME) and superelasticity (SE) but with a limitation of considering pre-stretch for irregularly shaped geometries. In order to address this gap, Woodworth and Kaliske (WK) developed a phenomenological constitutive SMA material model, offering the flexibility to apply pre-stretches for SMA wires with irregular profiles. This study investigates the WK SMA material model, utilizing deformations observed in IFRC structures as a reference and validating them against simulated models using the SA SMA material model. This validation process is crucial in ensuring the reliability and accuracy of the WK model, thus enhancing confidence in its application for predictive analysis in SMA-based systems.

Research on elasticity behavior of Ni-Ti shape memory alloys using the reproducing kernel particle method

The unique superelastic properties of the Ni-Ti shape memory alloys (SMAs) have been found extensive practical application in many engineering fields, but the Ni-Ti SMAs exhibits elastic behavior in small load, so the elastic behavior of the Ni-Ti SMAs is studied by using meshless method of reproducing kernel particle method (RKPM) in this paper. The displacement boundary conditions are applied by utilizing the penalty function method, and the impact of penalty factor and parameter of influence domain on errors are discussed, and computational stability is analyzed. At last, the correctness of the RKPM in studying the elastic behavior of the Ni-Ti SMAs is evidenced by using three numerical examples.

Mechanical behavior of Ni[formula omitted]Ti[formula omitted] shape memory alloys with void defects

Superelasticity of NixTi100−x B2 shape memory alloys (SMAs) with different atomic compositions and void defects are investigated using molecular dynamics simulations. In the first part of this study, the effect of species content on the martensitic transformation is explored, with x ranging from 48% to 52%. All samples displayed similar behavior under compression, where transformation from B2 to B19’ was observed. On the other hand, under tension, both Ni48Ti52 and Ni50Ti50 displayed single-step transformation from B2 to L10, while the Ni-rich sample exhibited a simultaneous double-step transformation from B2 to B19’ to L10. The mechanical performance of the samples with voids under compression was also explored. The void surface was found to act as a barrier, hindering the phase transformation to B19’ structure and its elastic response, ultimately reducing the maximum stress prior to fracture. Statistical methods established connections between various properties and offered mathematical models to correlate the initial setup variables Ni content and void radius, with properties derived from deformation tests. Consequently, this study provides both an atomic description of the deformation behavior of NiTi SMAs and a statistical framework for establishing the relationships between materials properties.

Enhancement of the three-point bending behavior of a coarse-grained NiTi shape memory alloy with surface heterogeneous gradient microstructure and compressive residual stress

NiTi shape memory alloys offen subject to bending deformation as common functional materials. Laser shock peening (LSP) was utilized to fabricate a gradient layer to improve the bending mechanical behavior of a coarse-grained NiTi plate. A surface gradient layer with a thickness of approximately 100 µm was created in a 1.38 mm thick coarse-grained NiTi plate. The LSPed surface contained nanocrystals, dislocations, and amorphous, while the interior layer contained a substantial number of deformation twins, dislocations, and precipitates. The nanohardness of the surface treated with LSP can reach 10 GPa. The results of a three-point bending experiment indicate that LSP treatment increases the bending resistance of the NiTi plate. This study demonstrated that laser shock peening is an effective method for increasing the deformation resistance of a coarse-grained NiTi by means of surface gradient structures, residual stress, and precipitates.