Transformation In Shape Memory Alloys Research Articles

Shape memory metamaterials with tunable thermo-mechanical response via hetero-epitaxial integration: A molecular dynamics study

We show that nanoscale epitaxial superlattices (SLs) can be used to engineer the energy landscape that governs the martensitic transformation in shape memory alloys and tune their thermo-mechanical response. We demonstrate the approach using large-scale molecular dynamics simulations of a SL material consisting of alternate layers of a shape memory Ni-rich NiAl alloy and NiAl B2 alloy. The non-martensitic NiAl alloy was chosen to reduce the energy barrier that separates the martensite and austenite phases of the SL and its incorporation leads to a reduction in the thermal hysteresis of the transition. This is a desirable feature in applications involving actuation and our approach represents a generally applicable and powerful avenue to engineer desired behavior in mechanically active materials.

Modeling martensitic phase transformations in shape memory alloys with the self-consistent lattice dynamics approach

The unusual properties of shape memory alloys (SMAs) are due to solid-to-solid martensitic transformations (MTs) which correspond to a lattice level instability of the crystal structure. Currently, there exists a shortage of material models of MTs based on the material's atomic composition and crystal structure. The present work develops a model using first-order self-consistent lattice dynamics (SCLD) that captures the qualitative behavior of MTs. The effects of atomic vibrations on the material properties are captured by renormalizing the frequencies of atomic vibration using self-consistent equations. These renormalized frequencies are dependent on both configuration and temperature. The model is easily extended to three-dimensional problems.The model is illustrated for the case of a one dimensional bi-atomic chain. The constant Morse pair potential parameters are chosen to demonstrate the usefulness of the SCLD model. The resulting model is evaluated by generating equilibrium paths with temperature and mechanical load as the loading parameters. In both types of loading, a first-order entropically stabilized MT is predicted indicating that the current model is able to capture the first-order MTs that occur in SMAs.This qualitative prediction of a first-order MT indicates the likely-hood that the SCLD model can be used for the computational design and discovery of SMAs with better properties. Such an undertaking would involve, first, determining the potential parameters of new alloys from first-principles calculations and, second, using these parameter values with the SCLD to evaluate the shape memory behavior of the new previously unstudied materials.

Improved Constitutive Modeling for Phase Transformation of Shape Memory Alloys

Improved Constitutive Modeling for Phase Transformation of Shape Memory Alloys

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 cyclic stability of the elastocaloric effect in sputtered TiNiCu shape memory films

The elastocaloric effect that occurs during the stress-induced martensitic transformation in shape memory alloys is a promising mechanism in view of solid state cooling applications. It also allows for downscaling to feature sizes in the μm range, thus, being attractive for micro-cooling applications using thin film materials. In this study, elastocaloric properties of TiNi and TiNiCu films and their relation to functional fatigue were investigated. Both materials show similar effect sizes, their fatigue behavior is however different. While the temperature change in TiNi degrades by a factor of two within 150 cycles, no significant elastocaloric fatigue was found in TiNiCu.

Dynamic thermo-mechanical coupling and size effects in finite shape memory alloy nanostructures

In this paper, the dynamics of martensitic transformations in shape memory alloy (SMA) constrained finite nanostructures is studied using a phase field model with the Ginzburg–Landau free energy. The nonlinear coupled thermo-mechanical properties in SMAs have been extensively studied in the bulk case. However at the nanoscale the thermal physics has been usually neglected in a study of SMA properties, and most of the model developments have been carried out under the assumption of isothermal phase transformations. The main aim of this paper is to develop a model that couples the thermal physics and the mechanical dynamics to study the influence of such coupling on the mechanical properties of SMA nanostructures. The developed model is solved using the finite element method. Analyzing FePd alloy nanowires, we observe a steeper slope of the stress–strain curve in the coupled thermo-mechanical case due to temperature evolution during the loading–unloading cycle of such nanostructures. We also observe the martensitic suppression phenomenon in constrained nanowires and nanograins on cooling. We have developed a semi-analytical model to predict a critical size at the onset of martensitic suppression. The semi-analytical model predicts a critical size which is in a good agreement with the numerical results for FePd nanograins.

Shape memory materials as an object of solid state mechanics: Experimental study, constitutive relations, solution of boundary value problems

The paper reports on new results of experimental research and simulation of thermomechanical phenomena characteristic of shape memory alloys. Experimental data are presented on the absence of work hardening in strain accumulation attendant on direct martensite transformation in shape memory alloys, cross hardening involved in martensite inelasticity and associated with preliminary direct marten-site transformation, rheonomic properties of shape memory alloys and stability of titanium nickelide rods loaded in the mode of martensite inelasticity. Some of the basic peculiarities of the processes occurring in shape memory alloys at different structural levels are briefly reviewed. A model that takes into account the above peculiarities and describes nonlinear scleronomic deformation of shape memory alloys in phase and structural transformation is set forth. Simplest variants of description of rheonomic properties of shape memory alloys are proposed. Different classes of boundary value and initial-boundary value problems of thermomechanics for shape memory alloys are considered.

Pore control in SMA NiTi scaffolds via space holder usage

Porous NiTi shape memory alloy (SMA) was fabricated by sintering of compressed constituent elements pre-mixed with NaCl or urea spacer holders. Effect of spacer to metal volume-ratio (rS) on shape, size, distribution and openness of the voids was probed by optical metallography, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Differential scanning calorimetry (DSC) was used to determine the SMA transformation temperatures. Controllable void geometry helping osteoblast proliferation and bone cell growth was gained by addition of the spacers. At rS=0.7, percentage of the open pores reached 52% while at rS=1.43, interconnected pores with 200 to 500μm diameter were produced. Although trace of microstructural chlorine was observed in NaCl treated samples, no remains were detectable in the urea made specimens. The latter had a greater advantage of being more homogeneous as well.

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.

Thermal arrest analysis of thermoelastic martensitic transformations in shape memory alloys

Abstract

Taking account of the martensite inelasticity in the reverse phase transformation in shape memory alloys

The model of nonlinear deformation of shape memory alloys (SMA) is generalized to the case in which the possible structural transition in the reverse martensitic transformation is taken into account. The statement of active thermomechanical processes of proportional variation in the stress deviatoric components is justified. The problem of buckling on an SMA bar due to the reverse martensitic transformation is solved. It is shown that taking account of the structural transition under buckling in the process of reverse transformation significantly changes the solution of this problem.

Modelling of localization and propagation of phase transformation in superelastic SMA by a gradient nonlocal approach

In this work, a nonlocal phenomenological behavior model is proposed in order to describe the localization and propagation of stress-induced martensite transformation in shape memory alloy (SMA) wires and thin films. It is a nonlocal extension of an existing local model that was derived from a micromechanical-inspired Gibbs free energy expression. The proposed model uses, besides the local field of the internal variable, namely the martensite volume fraction, a nonlocal counterpart. This latter acts as an additional degree of freedom, which is determined by solving an additional partial differential equation (PDE), derived so as to be equivalent to the integral definition of a nonlocal quantity. This PDE involves an internal length parameter, dictating the global scale at which the nonlocal interactions of the underlying micromechanisms are manifested during phase transformation. Moreover, to account for the unstable softening behavior, the transformation yield force parameter is considered as a gradually decreasing function of the martensite fraction. Possible material and geometric imperfections that are responsible for localization initiation are also considered in this analysis. The obtained constitutive equations are implemented in the Abaqus® finite element code in one and two dimensions. This requires the development of specific finite elements having the nonlocal volume fraction variable as an additional degree of freedom. This implementation is achieved through the UEL user’s subroutine. The effect of martensitic localization on the superelastic global behavior of SMA wire and holed thin plate, subjected to tension loading, is analyzed. Numerical results show that the developed tool correctly captures the commonly observed unstable superelastic behavior characterized by nucleation and propagation of martensitic phase. In particular, they show the influence of the internal length parameter, appearing in the nonlocal model, on the size of the localization area and the stress nucleation peak.

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.

Full discretisation of second-order nonlinear evolution equations: strong convergence and applications

Abstract Recent results on convergence of fully discrete approximations combining the Galerkin method with the explicit-implicit Euler scheme are extended to strong convergence under additional monotonicity assumptions. It is shown that these abstract results, formulated in the setting of evolution equations, apply, for example, to the partial differential equation for vibrating membrane with nonlinear damping and to another partial differential equation that is similar to one of the equations used to describe martensitic transformations in shape-memory alloys. Numerical experiments are performed for the vibrating membrane equation with nonlinear damping which support the convergence results.

Phase transformation in free-standing SMA nanowires

The primary focus of this work is on determining if the phase transformation of shape memory alloy (SMA) nanowires exhibits a critical size below which the phase transformation is inhibited. The SMA nanowires are fabricated through the use of the mechanical pressure injection method. The mechanical pressure injection method is a template-assisted nanowire fabrication method in which an anodized aluminum oxide (AAO) template is impregnated with liquid metal. The fabrication of SMA nanowires with different diameters is accomplished through the fabrication of AAO templates of varying diameters. The phase transformation behavior of the fabricated SMA nanowires is characterized using transmission electron microscopy. By analysis of the fabricated SMA nanowires, it is found that the phase transformation of SMA nanowires is not affected for nanowires ranging in diameter from 650 to 10 nm.

Three dimensional large deformation analysis of phase transformation in shape memory alloys

Shape memory alloys (SMAs) have been explored as smart materials and used as dampers, actuator elements, and smart sensors. An important character of SMAs is its ability to recover all of its large deformations in mechanical loading-unloading cycles without showing permanent deformation. This paper presents a stress-induced phenomenological constitutive equation for SMAs, which can be used to describe the superelastic hysteresis loops and phase transformation between Martensite and Austenite. The Martensite fraction of SMAs is assumed to be dependent on deviatoric stress tensor. Therefore, phase transformation of SMAs is volume preserving during the phase transformation. The model is implemented in large deformation finite element code and cast in the updated Lagrangian scheme. In order to use the Cauchy stress and the linear strain in constitutive laws, a frame indifferent stress objective rate has to be used. In this paper, the Jaumann stress rate is used. Results of the numerical experiments conducted in this study show that the superelastic hysteresis loops arising with the phase transformation can be effectively captured.

Probing the Reverse Martensitic Transformation in Constrained Shape Memory Alloys via Electrical Resistance

Variation of electrical resistance and resistivity is used in order to monitor the reverse martensitic transformation in shape memory alloy (SMA) wires and polymer matrix-SMA composites, under constrained conditions. Results indicate that the characteristic temperatures shift towards higher values with increasing pre-strain level. Structural changes and phase transformations can be followed via electrical resistivity. SMA composites exhibit a behavior analogous to SMA wires, although the temperature range of the reverse transformation appears to be more extensive and a higher data scattering is present, especially, in the low temperature region.

Austenite–martensite interface in shape memory alloys

A two-scale phase field simulation is developed for austenite–martensite interface to understand the effects of crystalline symmetry and geometric compatibilities on the reversibility of structural phase transformations in shape memory alloys. It is observed that when the middle eigenvalue of martensite transformation strain is equal to zero, an exact austenite–martensite interface is formed with negligible elastic energy. On the other hand, when the middle eigenvalue is different from 0, an inexact interface between austenite and martensitic twin is formed, and the corresponding elastic energy increases with the increased magnitude of the middle eigenvalue, resulting in substantially higher energy barrier for austenite–martensite transformation, and thus higher thermal hysteresis in shape memory alloys.

Evolution of the martensitic transformation in shape memory alloys under high strain rates

The specific properties of the shape memory alloys are mainly due to the martensitic transformation occuring in the material when mechanical or thermal loadings are applied. Here, the effect of strain rate on the transformation on an NiTi SMA is studied in tension. Different tests were performed at different strain rates in the range of 0,0001 /s to 15 /s. Two distinct methods were used to measure the extension rate of the martensitic phase region in the specimen: digital image correlation technique and infrared thermography (IR during quasi-static tensile tests only). For the dynamic tensile tests, a Split Hopkinson Tensile Bar set-up was used with a fast camera recording at 45’000 fps. A superimposition of DIC and IR measurements in time and space can be done during quasi-static tests and results show that the temperature peak, as expected, follows the transformation front. As a consequence of the former validation of the DIC procedure, the velocity of the transformation front at high strain rate is deduced from space-time figures. As a conclusion, in the range of strain rates investigated in this paper, no strain rate sensitivity is observed for dynamics of extension of the transformation region.