Phase Transformation In Shape Memory Alloys Research Articles

Effect of laser energy density on the phase transformation behavior and functional properties of NiTi shape memory alloy by selective laser melting

NiTi shape memory alloy (SMA) was prepared via selective laser melting (SLM). Orthogonal tests were conducted with varying laser power and scanning speeds to explore the influence of laser energy density on microstructure, phase transformation behavior, mechanical properties and functional properties (superelasticity and shape memory effect). As energy density increases from 24.3 to 106.3 J/mm³, microstructure transforms from fine to columnar grains. Ni consumption intensifies, boosting transformation temperature and B2 content at room temperature. Post cyclic stretching, B2 content in alloy escalates further at room temperature. Tensile strength peaked at 53.6 J/mm³, while the elongation rate significantly increases with increasing energy density. During cyclic stretching, low or high energy densities in SLM NiTi induce high-density dislocations and stable residual martensite, respectively, significantly inhibiting martensitic transformation and resulting in extremely poor functional properties. The alloy produced at 53.6 J/mm³ boasts exceptional superelasticity due to stress-induced martensitic transformation and phase inversion post-unloading. Following heating past the critical transition, it nearly fully recovers (97.78 % recovery rate). Despite multiple shape memory fatigue tests, it maintains a recovery rate above 90 %. This study provides crucial theoretical insights for optimizing NiTi SMA phase transformation behavior and enhancing the performance.

Thermomechanical response and elastocaloric effect of shape memory alloy wires

Refrigeration technologies based on the elastocaloric effect of shape memory alloys (SMAs) have attracted much interest in recent years. To seek for schemes that can improve the temperature span and efficiency of elastocaloric devices, this work explores more practical loading conditions (between adiabatic and isothermal) of SMAs. To account for the smoothness of the martensite phase transformation of SMAs, a general polynomial form of hardening-like function is adopted in the phenomenological multi-physics coupling framework. The modeling framework is further calibrated by measurements of the thermomechanical responses of pre-trained Ni–Ti wires. Due to the hardening-like function, the martensitic volume fraction cannot be explicitly expressed. Accordingly, a new numerical scheme is also presented in this work. Moreover, an empirical function of the maximum transformation strain is proposed based on the experimental observation, which improves the capability of the model to capture experimental data. This work aims to provide a paradigm from material characterization to theoretical modeling and numerical simulation, and then to elastocaloric cooling performance analysis of SMAs.

Design and Evaluation of Smart Textile Actuator with Chain Structure.

Textiles composed of fibers can have their mechanical properties adjusted by changing the arrangement of the fibers, such as strength and flexibility. Particularly, in the case of smart textiles incorporating active materials, various deformations could be created based on fiber patterns that determine the directivity of active materials. In this study, we design a smart fiber-based textile actuator with a chain structure and evaluate its actuation characteristics. Smart fiber composed of shape memory alloy (SMA) generates deformation when the electric current is applied, causing the phase transformation of SMA. We fabricated the smart chain column and evaluated its actuating mechanism based on the size of the chain and the number of rows. In addition, a crochet textile actuator was designed using interlooping smart chains and developed into a soft gripper that can grab objects. With experimental verifications, this study provides an investigation of the relationship between the chain actuator's deformation, actuating force, actuator temperature, and strain. The results of this study are expected to be relevant to textile applications, wearable devices, and other technical fields that require coordination with the human body. Additionally, it is expected that it can be utilized to configure a system capable of flexible operation by combining rigid elements such as batteries and sensors with textiles.

On the nonlinear analysis of pre-strained shape memory alloy beams

We present a theoretical and numerical investigation of the transverse dynamic response of monolithic Shape Memory Alloy (SMA) beams under the effect of axial pre-strains. The SMA beam is taken as benchmark structure to evaluate the effect of (axial) pre-strain on either the free or the forced dynamics. The combination of the static stress, produced by the pre-loading conditions, with the dynamic stress, associated with the vibratory response, affects the occurrence of the stress-induced SMA phase transformation hence providing a mechanism to passively tune the dynamic response. Particular attention is given to the effect that different pre-strain levels have on the overall ability to dissipate mechanical energy, that is on the effective damping. The numerical model of the SMA beam accounts for both material and geometric nonlinear behaviors. The nonlinear material behavior is due to the SMA phase-transformation and it is modeled via the one-dimensional improved Brinson’s model (including tension–compression asymmetry), while the geometric nonlinear behavior is due to large displacements occurring during the phase transformation and it is accounted for via von Kármán assumptions. The resulting model is solved numerically via the finite element method and used to evaluate both the free and the forced response of the beam. The free response analysis suggests the existence of optimal pre-strain levels to achieve maximum damping capacity, while the forced response highlights the occurrence of nonlinear dynamic features, such as bifurcation. In both cases, it is found that the level of pre-strain can significantly affect the dynamic response as well as the effective damping. The results presented in this work provide useful guidelines to understand the dynamics of continuous SMA structures under pre-strain and the ability to tune either their resulting dynamics or dissipation properties.

Numerical study of a double-effect elastocaloric cooling system powered by low-grade heat

• A double-effect heat-driven elastocaloric cooling cycle was proposed. • The double-effect eC cycle can harvest low-grade heat at 120 to 160 °C. • Driving temperature difference is used to design the transition temperature of SMAs. • COP of the double-effect eC cycle is three times better than single-effect eC cycle. Elastocaloric cooling technology is a novel solid-state cooling technology based on the latent heat associated with martensitic phase transformation in shape-memory alloys. Heat-driven elastocaloric cooling cycle could harvest low-grade thermal energy other than consuming electric power, and was proposed as a new direction in this field. To improve the heat-to-cooling efficiency of heat-driven elastocaloric cooling systems, a double-effect elastocaloric cooling cycle with two beds of actuator shape-memory alloys and two beds of refrigerant super-elastic alloys is proposed in this study. The double-effect cycle utilizes the input heat two times and thus provides improved efficiency. Numerical simulations are carried out to study the characteristics of the phase transformation in both the actuators and refrigerants. The impact of phase transition temperatures of the actuator alloys, the heat source temperature, and the operating parameters on the phase transformation and cooling performance of the system is investigated. If the phase transition temperatures are properly designed, the double-effect heat-driven elastocaloric cooling system is three times more efficient than the baseline single-effect heat-driven elastocaloric cooling system.

Shape Memory Alloy-Based Frequency Reconfigurable Ultrawideband Antenna for Cognitive Radio Systems

The present work demonstrates a frequency reconfigurable planar antenna with narrowband and ultra-wideband (UWB) characteristics for cognitive radio systems. The designed antenna has a dimension of $60\times 58\times1.6$ mm3 with a ground plane made of nickel–titanium (Ni–Ti) shape memory alloy (SMA). The antenna operates at a flat condition with frequency 1.46 GHz. Bending the SMA ground plane at an angle of 200° with a transformation temperature of 66 °C. The same antenna operates over 3.4–10.2-GHz UWB range. This is due to the SMA phase transformation between martensite and austenite via resistive heating/cooling. The insertion of rectangular slits in the antenna creates dual band-notched characteristics 4.7–5.1 and 5.3–6.0 GHz with VSWR $S_{{11 {}}} dB. This creates a different working frequency leading to frequency reconfiguration. Details about the proposed antenna have been discussed along with measured and simulation results.

Convex integration solutions for the geometrically nonlinear two-well problem with higher Sobolev regularity

In this paper, we discuss higher Sobolev regularity of convex integration solutions for the geometrically nonlinear two-well problem. More precisely, we construct solutions to the differential inclusion [Formula: see text] subject to suitable affine boundary conditions for [Formula: see text] with [Formula: see text] such that the associated deformation gradients [Formula: see text] enjoy higher Sobolev regularity. This provides the first result in the modelling of phase transformations in shape-memory alloys where [Formula: see text], and where the energy minimisers constructed by convex integration satisfy higher Sobolev regularity. We show that in spite of additional difficulties arising from the treatment of the nonlinear matrix space geometry, it is possible to deal with the geometrically nonlinear two-well problem within the framework outlined in [A. Rüland, C. Zillinger and B. Zwicknagl, Higher Sobolev regularity of convex integration solutions in elasticity: The Dirichlet problem with affine data in int[Formula: see text], SIAM J. Math. Anal. 50 (2018) 3791–3841]. Physically, our investigation of convex integration solutions at higher Sobolev regularity is motivated by viewing regularity as a possible selection mechanism of microstructures.

A variable‐stiffness continuum manipulators by an SMA‐based sheath in minimally invasive surgery

Due to its unique dexterity and safety, continuum manipulators have been widely used in small constrained environment. However, its flexibility also brings negative effects such as poor stiffness and low strength. This paper presents a novel variable-stiffness sheath for continuum manipulators applied in minimally invasive surgery. We present a variable-stiffness sheath based on shape memory alloy (SMA) for each module of the continuum manipulators. The stiffness of the sheath can be continuously adjusted depending on the voltage between both ends of the sheath, along with the phase transformation of SMA between austenite and martensite. The experimental results demonstrate that the stiffness of the sheath and single module are able to increase up to 223.1% and 139.2%, separately, which verify the correctness of the proposed variable stiffness method. The robot integrated with the variable-stiffness sheath is demonstrated to possess a fine capacity of variable stiffness.

Hybrid Shape Memory Alloy-Based Nanomechanical Resonators for Ultrathin Film Elastic Properties Determination and Heavy Mass Spectrometry.

Micro-/nanomechanical resonators are often used in material science to measure the elastic properties of ultrathin films or mass spectrometry to estimate the mass of various chemical and biological molecules. Measurements with these sensors utilize changes in the resonant frequency of the resonator exposed to an investigated quantity. Their sensitivities are, therefore, determined by the resonant frequency. The higher resonant frequency and, correspondingly, higher quality factor (Q-factor) yield higher sensitivity. In solution, the resonant frequency (Q-factor) decreases causing a significant lowering of the achievable sensitivity. Hence, the nanomechanical resonator-based sensors mainly operate in a vacuum. Identification by nanomechanical resonator also requires an additional reference measurement on the identical unloaded resonator making experiments, due to limiting achievable accuracies in current nanofabrication processes, yet challenging. In addition, the mass spectrometry by nanomechanical resonator can be routinely performed for light analytes (i.e., analyte is modelled as a point particle). For heavy analytes such as bacteria clumps neglecting their stiffness result in a significant underestimation of determined mass values. In this work, we demonstrate the extraordinary capability of hybrid shape memory alloy (SMA)-based nanomechanical resonators to i) notably tune the resonant frequencies and improve Q-factor of the resonator immersed in fluid, ii) determine the Young’s (shear) modulus of prepared ultrathin film only from frequency response of the resonator with sputtered film, and iii) perform heavy analyte mass spectrometry by monitoring shift in frequency of just a single vibrational mode. The procedures required to estimate the Young’s (shear) modulus of ultrathin film and the heavy analyte mass from observed changes in the resonant frequency caused by a phase transformation in SMA are developed and, afterward, validated using numerical simulations. The present results demonstrate the outstanding potential and capability of high frequency operating hybrid SMA-based nanomechanical resonators in sensing applications that can be rarely achieved by current nanomechanical resonator-based sensors.

Multiscale TRIP-based investigation of low-cycle fatigue of polycrystalline NiTi shape memory alloys

In this paper, a multiscale investigation of fatigue crack initiation in shape memory alloys (SMAs) based on Transformation Induced Plasticity (TRIP) is presented. A mechanism for fatigue crack initiation during cyclic stress-induced phase transformation along with theoretical model is proposed. To validate the TRIP-based model, quasi-static tests at different ambient temperatures, 40∘C, 52∘C and 65∘C, and strain and stress controlled low-cycle fatigue tests at different frequencies ranging from 0.16 Hz to 5 Hz on pseudoelastic NiTi wires are carried out. The results show that, (i) TRIP appearing on phase transformation interfaces is the key factor that drives the fatigue crack initiation during cyclic stress-induced phase transformation in SMAs; (ii) maximum temperature during phase transformation is a relevant indicator to predict low-cycle fatigue of SMAs and, (iii) within the range of pseudoelasticity and below the plastic yield, low-cycle fatigue of SMAs is not directly correlated with the mechanical loads applied at macro-scale, in the sense that, if the maximum temperature reached during loading cycles is kept constant, the fatigue lifetime remains unchanged whatever the amplitude of the mechanical loading is. Based on the findings, a new criterion for pseudoelastic low-cycle fatigue of SMAs as well as fatigue-isolines diagram are proposed and validated experimentally.

The feasibility design and modeling for thermo-mechanical ocean energy harvester using shape memory alloys

In this paper a feasibility design of thermal energy harvester is proposed base on martensitic phase transformations in shape memory alloys (SMAs). Energy conversion from the thermal form to mechanical form via the martensite phase transformation in SMAs spring is modeled and analyzed. The phase transformation is induced thermally by the sea water temperature difference. In the proposed design, a macroscopic differential model of the two-way shape memory effects in one-dimensional SMA structure is employed to estimate the restoring force of the SMA spring. Power output is measured and found reasonable enough to be used. Thus the feasibility of the proposed design for ocean thermal energy harvesting is validated.

A macroscopic strain-space model of anisotropic, cyclic plasticity with hardening

We present a strain-based formulation of macroscopic plastic deformation that accounts for anisotropy in both the initial yielding and subsequent hardening. Both isotropic and kinematic hardening are considered. Thus the model captures the evolution in the deformation response on cyclic loading including the Bauschinger effect, and the accumulation of plastic strain (ratcheting response). The model is formulated in the strain space and implemented using an implicit time integration scheme. The capabilities of the model are demonstrated through material parameter studies and by calibrating the model to experimental data for uniaxial tensile loading of Inconel alloy and anisotropic yielding of rolled aluminum alloy sheets. We also show that this strain-based approach is equivalent to the more traditional stress-based approach. The strain-based formulation of this model makes it attractive for coupling with other strain-based phenomena like phase-transformations in shape-memory alloys and failure.

Design of shape memory alloy actuated intelligent parabolic antenna for space applications

The deployment of large flexible antennas is becoming critical for space applications today. Such antenna systems can be reconfigured in space for variable antenna footprint, and hence can be utilized for signal transmission to different geographic locations. Due to quasi-static shape change requirements, coupled with the demand of large deflection, shape memory alloy (SMA) based actuators are uniquely suitable for this system. In this paper, we discuss the design and development of a reconfigurable parabolic antenna structure. The reflector skin of the antenna is vacuum formed using a metalized polycarbonate shell. Two different strategies are chosen for the antenna actuation. Initially, an SMA wire based offset network is formed on the back side of the reflector. A computational model is developed using equivalent coefficient of thermal expansion (ECTE) for the SMA wire. Subsequently, the interaction between the antenna and SMA wire is modeled as a constrained recovery system, using a 1D modified Brinson model. Joule effect based SMA phase transformation is considered for the relationship between input voltage and temperature at the SMA wire. The antenna is modeled using ABAQUS based finite element methodology. The deflection found through the computational model is compared with that measured in experiment. Subsequently, a point-wise actuation system is developed for higher deflection. For power-minimization, an auto-locking device is developed. The performance of the new configuration is compared with the offset-network configuration. It is envisaged that the study will provide a comprehensive procedure for the design of intelligent flexible structures especially suitable for space applications.

Modeling of latent heat effects on phase transformation in shape memory alloy thin structures

• FE analysis of coupling between temperature heterogeneity induced by latent heat and phase transformation in SMA. This paper presents a fully coupled thermo-mechanical constitutive model for shape memory alloys taking into account latent heat effects during forward and reverse martensitic phase transformations. This model is obtained as an extension of an original macroscopic model, derived from a micromechanical-inspired Gibbs free energy expression. The proposed formulation considers beside the constitutive equations describing the thermo-mechanical behavior of Shape Memory Alloys (SMAs), the heat equation with additional internal source terms related to the latent heat generated during phase transformation. Hence, the thermo-mechanical problem to be solved consists of the mechanical equilibrium and the established heat equations. A 2D specific plane stress continuum and isoparametric finite element with three degrees of freedom (dof) per node corresponding to in-plane displacements and temperature is developed to solve the derived thermomechanical problem. The developed 2D finite element is implemented in the Abaqus ® finite element code through the UEL user’s subroutine. The numerical simulations performed by using this new finite element show the delaying effect of the transformation latent heat on the forward and reverse phase transformations in SMA thin structures and the possible heterogeneous character of the phase transformation in this case. These effects are even more important as the strain rate is high.

INVESTIGATION OF SHAPE MEMORY ALLOY SPRING ELASTIC COEFFICIENT BASED ON VARYING APPLIED CURRENTS IN A CARDIAC ASSIST DEVICE

This paper analyses the mechanical properties of shape memory alloy (SMA) springs based on different electric currents applied in a cardiac assist device (CAD). Experimental results show that when the input drive current is constant, the SMA spring is equivalent to a tension spring with determined elastic coefficient that increases with the current. Based on our experiments, the theoretical maximum recovery force produced by SMA can be obtained through this input current. The phase transformation of SMA from austenite to martensite is able to be controlled by the drive current instead of the surface temperature of SMA. In addition, this experiment designed a cardiac supporting device composed of eight SMA springs, and used a saline bag to simulate human heart. The peak pressure inside the saline bag produced by this device was 17.4% of the normal heart systolic pressure. Our results can provide further support for the research of advanced CAD.

On anomalous depth-dependency of the hardness of NiTi shape memory alloys in spherical nanoindentation

Abstract

Statistical physics concepts for the explanation of effects observed in martensitic phase transformations

Structural solid-to-solid transformations play a key role for the behaviour of several materials, e.g., shape memory alloys, steels, polymers and ceramics. A novel theoretical approach modelling martensitic phase transformation is demonstrated in the present study. The generally formulated model is based on the block-spin approach and on renormalization in statistical mechanics and is applied to a representative volume element (resp. representative mole element) which is assumed to be in a local thermodynamic equilibrium. The neighbouring representative volume elements are in a generally different thermodynamic equilibrium. This leads to fluxes between those elements. Using fundamental physical properties of a shape memory alloy (SMA) single crystal as input data the model predicts the order parameter ‘total strain’, the martensitic phase fraction and the stress-assisted transformation accompanied by pseudo-elasticity without the requirement of evolution equations for internal variables and assumptions on the mathematical structure of the classical free energy. In order to demonstrate the novel approach the first computations are carried out for a simple one-dimensional case, which can be generalized to the two- and three-dimensional case. Results for total strain and martensitic phase fraction are in good qualitative agreement with well known experimental data according to their macroscopic strain rearrangement when phase transformation occurs. Further a material softening effect during phase transformation in SMAs is predicted by the statistical physics approach. Formulas are presented for the relevant quantities such as volume fraction, total strain, transformation strain, rates of the volume fractions and of the strains.

High-Energy Synchrotron X-Ray Diffraction for In Situ Study of Phase Transformation in Shape-Memory Alloys

This overview highlights very recent progress on the application of high-energy x-ray diffraction for in situ study of the phase transformation of shape-memory alloys. The advantages of the synchrotron-based high-energy x-ray diffraction method and the experimental setup for exploring the phase-transition behavior of single crystals or textured polycrystalline materials under multiple external fields are described. Experimental investigations on the influence of external stress, magnetic, and thermal fields on the phase-transformation behaviors of thermal and ferromagnetic shape-memory alloys, and nanowire-reinforced shape-memory composites are also summarized. Special attention is given to recent scientific issues related to the microscopic “memory” of martensite variants, transition kinetics, magnetic field-induced selection of variants, magnetic field-driven phase transition, and superelasticity.

A statistical mechanics approach describing martensitic phase transformation

A novel theoretical approach modeling martensitic phase transformation is demonstrated in the present study. The generally formulated model is based on the block-spin-approach and on renormalization in statistical mechanics and is applied to a representative volume element which is assumed to be in a local thermodynamic equilibrium. Using fundamental physical properties of a shape memory alloy (SMA) single crystal as input data the model predicts the order parameter “total strain”, the martensitic phase fraction and the stress assisted transformation accompanied by pseudoelasticity without the requirement of evolution equations for internal variables and assumptions on the mathematical structure of the classical free energy. In order to demonstrate the novel approach the first computations are carried out for a simple one-dimensional case. Results for total strain and martensitic phase fraction are in good qualitative agreement with well known experimental data according to their macroscopic strain rearrangement when phase transformation occurs. Further a material softening effect during phase transformation in SMAs is predicted by the statistical physics approach.

Nonlinear Finite Element Analysis of the Bending of Shape Memory Alloy Hybrid Epoxy Beams

On the basis of considering the thermo-viscoelasticity of the epoxy matrix, the geometrically nonlinear finite element formulation is developed to simulate the moderate deflection bending of the shape memory alloy (SMA) hybrid beam upon actuation of SMA. It is found that the midpoint deflection of the epoxy beam increases with increasing temperature when temperature is lower than the austenite start temperature of SMA. Once the austenite start temperature of SMA is reached, the midpoint deflection of the epoxy beam is rapidly decreased to the initial state with the increasing of temperature. And then it increases slowly with increasing temperature after the austenite phase transformation of SMA is finished. The results based on the geometrically nonlinear finite element analysis are also compared with those from the geometrically linear finite element analysis, which shows that they agree well with each other when the bending deflection of the epoxy beam is small, while significant discrepancies occur between them when the deflection is considerably large.