Transition In Shape Memory Alloy Research Articles

Corrosion resistance of VC-reinforced Fe-based SMA coatings by laser cladding

The corrosion resistance of 42CrMo steel is insufficient to meet the demands in the marine environment, despite its widespread application in bearings and other fields. Herein, Fe-Mn-Si-Cr-Ni-χ(VC) shape memory alloy (SMA) composite coatings were successfully fabricated on the surface of 42CrMo steel using laser cladding technology. The results present that the coatings are mainly composed of γ austenite (fcc) phase and ε martensite (hcp) phase. Based on research findings, the incorporation of VC has been found to promote the γ → ε martensite phase transition in shape memory alloys. Simultaneously, electrochemical tests were conducted on 42CrMo substrate and SMA coating in 3.5 wt% NaCl solution. It was found that the coatings doped with an appropriate amount of VC could effectively enhance its corrosion resistance. When the doping amount of VC reaches 1.5 wt%, the charge transfer resistance (Rct) is significantly higher than that of the 42CrMo substrate and the pure SMA coating, as well as that of the coating doped with 2.0 wt% VC, reaching 21, 2.6, and 3.3 times of them, respectively. The charge transfer resistance Rct of LS1.5 is much higher than that of 42CrMo, LS0, and LS2.0, reaching 21, 2.6, and 3.3 times, respectively. Additionally, the maximal polarization resistance (Rp) of coatings achieves 4.39 times that of 42CrMo. This study points out that VC can efficiently boost the corrosion resistance of SMA composite coatings and may have uncovered a novel approach for improving the surface corrosion resistance of medium carbon steel.

Coupled Thermomechanical Dynamics of Phase Transitions in Shape Memory Alloys and Related Nonlinear Phenomena

The aim of this paper is to study the coupled non-linear dynamics of the behaviour of Shape Memory Alloys (SMA) under harmonic excitation. The thermomechanical model developed by Falk and based on Landau theory is used to describe the coupled thermomechanical behaviour of the SMA mass-damper-bar system. Fully coupled mechanical and thermal field equations are established to reveal the strong coupling phenomena arising from the phase transition. Then, the harmonic balance method is used to obtain the steady-state frequency responses. The effect of thermomechanical coupling on the frequency and time response curves is taken into account by modifying the heat transfer coefficient. The results show that deformation and temperature provide a stable sinusoidal response. The analysis of the results leads to different ways of controlling the nature and extent of the coupled and non-linear response of SMA-based oscillators. Comparison of the analytical results with experimental data from the literature validates the coupled thermomechanical nonlinear model. These results can be used effectively to control the external vibrations of various systems.

Study of microstructural growth under cyclic martensite phase transition in shape memory alloys; A molecular dynamics approach

Shape memory alloys are referred to as a group of alloys that can retrieve the permanent deformation and strain applied to them and eventually return to their original form. So far, various studies have been done to determine the behavior of these alloys under cyclic loading. Most of the studies have mainly been conducted by using the foundations of Continuum Mechanics in order to examine the properties of memory alloys. In this study, instead of using the Continuum Mechanics, a Molecular Dynamics simulation method using Lennard-Jones potential is utilized. The changes in the behavior and properties of memory alloy under cyclic loading are being examined. First, the functional form parameters for the Lennard-Jones potential are solved. Subsequently, these parameters are implemented to evaluate the response to thermal cyclic loading. The results of this study provide a better understanding of the behavior of memory alloys under cyclic loading.

Application of the Kelvin Approach for the Qualitative Estimation of Possibility of Phase Transitions in Shape Memory Alloys

It is proposed to use the concept of eigenmoduli and eigenstates from the linear theory of elasticity to estimate the possibility of phase transitions (martensitic transformations) in alloys with the shape memory effect. For alloys with cubic and hexagonal lattices, the eigenmoduli and eigenstates are given. The elastic strain energy for the cubic and hexagonal phases is written as the sum of six independent terms. It is proposed to compare the elastic strain energies in the cubic and hexagonal phases. If the deformation energy in the hexagonal phase is greater than in the cubic phase, then the alloy may tend to return to its original state with less energy. It is also possible to use the formulas of the tensors closest in Euclidean energy norm to the cubic and hexagonal tensors to compare the energies in different phases. The examples are given for some specific values of the elasticity constants.

Temperature-resistance sensor arrays for combinatorial study of phase transitions in shape memory alloys and metallic glasses

We have developed sensor arrays using a simple and inexpensive fabrication process to measure the electrical resistance of thin-film materials as a function of temperature and composition. The sensors are capable of characterizing materials from liquid‑nitrogen temperature to approximately 900 K and are not limited by low sample conductivity. We demonstrate the sensitivity and accuracy of the technique by analyzing the phase evolution in Ti-Ni-Cu shape memory alloys and PdSi-based metallic glasses. For the latter, we find that the change in resistance on crystallization correlates with glass-forming ability. This observation may be useful in identifying good glass formers using high-throughput techniques.

Exploiting heat treatment effects on SMAs macro and microscopic properties in developing fire protection devices

Ni-Ti shape memory alloys (SMAs) are intelligent alloys which demonstrate unique properties, such as shape memory effect, two-way shape memory effect, super-elasticity and vibration damping which, accompanied by good processability, excellent corrosion resistance and biocompatibility as well as fair wear resistance and cyclic stability, enabled the development of important industrial applications (such as sensors, actuators, fasteners, couplings and valves), medical applications (such as stents, bone implants, orthodontic archwires, minimal invasive surgical equipment) as well as environmental health and safety devices (anti-seismic dampers, fire safety devices). The phase transitions in Ni-Ti SMAs are strongly influenced by processing methods, chemical compositions and thermomechanical history. This paper presents a study of the effects of heat treatment on the mechanical and thermal properties of commercial Ni-Ti shape memory alloy (SMA). The experimental work involved subjecting a SMA rod to heat-treatment consisting in heating up to 500°C, 10 minutes-maintaining and water quenching. Mechanical properties were highlighted by microhardness tests while thermal characteristics were emphasized by differential scanning calorimetry (DSC). The presence of chemical composition fluctuations was checked by X-ray energy dispersive spectroscopy performed with an EDAX Bruker analyzer.

One dimensional constitutive model with transformation surfaces for phase transition in shape memory alloys considering the effect of loading history

Existing constitutive models for Shape Memory Alloys (SMAs) assume that both forward and reverse transformations occur when the thermodynamic driving force reaches a specific amount regardless of loading history. In this article, these assumptions are examined, and some cases are introduced where these models predict contradictory results. The effects of initial martensitic volume fraction on both forward and reverse transformations are shown by carrying out simple tensile tests on SMA wires. In line with these experiments, a one-dimensional constitutive model with new transformation conditions is proposed phenomenologically in order to model pseudoelastic behavior. The constitutive model is proved to be consistent with the theory of continuum mechanics. New transformation surfaces are introduced to govern transformations start conditions, rather than using preexisting common phase diagrams. As a result, history-dependent transformation start temperatures are determined. The obtained experimental stress-strain diagrams, available DSC test results, and experimental strain-temperature responses are used to validate the proposed model. It is shown that loading history affects transformation start conditions.

Existence of Solutions for a Mathematical Model Related to Solid–Solid Phase Transitions in Shape Memory Alloys

We consider a strongly nonlinear PDE system describing solid–solid phase transitions in shape memory alloys. The system accounts for the evolution of an order parameter χ (related to different symmetries of the crystal lattice in the phase configurations), of the stress (and the displacement u), and of the absolute temperature ϑ. The resulting equations present several technical difficulties to be tackled; in particular, we emphasize the presence of nonlinear coupling terms, higher order dissipative contributions, possibly multivalued operators. As for the evolution of temperature, a highly nonlinear parabolic equation has to be solved for a right hand side that is controlled only in L 1. We prove the existence of a solution for a regularized version by use of a time discretization technique. Then, we perform suitable a priori estimates which allow us pass to the limit and find a weak global-in-time solution to the system.

Correction: Corrigendum: Nature of hardness evolution in nanocrystalline NiTi shape memory alloys during solid-state phase transition

Due to a distinct nature of thermomechanical smart materials' reaction to applied loads, a revolutionary approach is needed to measure the hardness and to understand its size effect for pseudoelastic NiTi shape memory alloys (SMAs) during the solid-state phase transition. Spherical hardness is increased with depths during the phase transition in NiTi SMAs. This behaviour is contrary to the decrease in the hardness of NiTi SMAs with depths using sharp tips and the depth-insensitive hardness of traditional metallic alloys using spherical tips. In contrast with the common dislocation theory for the hardness measurement, the nature of NiTi SMAs' hardness is explained by the balance between the interface and the bulk energy of phase transformed SMAs. Contrary to the energy balance in the indentation zone using sharp tips, the interface energy was numerically shown to be less dominant than the bulk energy of the phase transition zone using spherical tips.

Nature of hardness evolution in nanocrystalline NiTi shape memory alloys during solid-state phase transition

Due to a distinct nature of thermomechanical smart materials' reaction to applied loads, a revolutionary approach is needed to measure the hardness and to understand its size effect for pseudoelastic NiTi shape memory alloys (SMAs) during the solid-state phase transition. Spherical hardness is increased with depths during the phase transition in NiTi SMAs. This behaviour is contrary to the decrease in the hardness of NiTi SMAs with depths using sharp tips and the depth-insensitive hardness of traditional metallic alloys using spherical tips. In contrast with the common dislocation theory for the hardness measurement, the nature of NiTi SMAs' hardness is explained by the balance between the interface and the bulk energy of phase transformed SMAs. Contrary to the energy balance in the indentation zone using sharp tips, the interface energy was numerically shown to be less dominant than the bulk energy of the phase transition zone using spherical tips.

Stress hysteresis and temperature dependence of phase transition stress in nanostructured NiTi—Effects of grain size

Stress hysteresis (H) and temperature dependence of phase transition stress (dσ/dT) are the two signatures of first-order phase transition in shape memory alloys. We studied the effects of grain size on these two properties in polycrystalline superelastic NiTi with the average grain size from 10 nm to 1500 nm. We identified a critical grain size (∼60 nm) below which both H and dσ/dT rapidly decrease, leading to vanishing hysteresis and breakdown of Clausius-Clapeyron equation. The physics behind such grain size effects are the dominance of interfacial energy in the energetics of the polycrystal and the lack of two-phase coexistence at nano-scales.

Preface

This issue is focussed on the modeling, analysis and simulation of fast reaction-slow transport scenarios as well as corresponding fast-reaction limits. Within this framework, internal sharp and thin reaction layers form and travel through the spatial domain often producing unexpected effects. Such situations appear in a variety of significant applications; for example flame propagation in combustion, segregation and aggregation of biological individuals, chemical attack on reactive porous materials (such as concrete or natural stone), dissolution and precipitation reactions in minerals, tumor growth, grain boundary motion, and temperature-induced phase transitions in shape-memory alloys represent typical cases in which the fast process is localized within a <i> a priori</i> unknown internal active layer. <br><br> For more information please click the “Full Text” above.

A non isothermal Ginzburg-Landau model for phase transitions in shape memory alloys

A thermodynamical model for martensitic phase transitions in shape memory alloys is formulated in this paper in the framework of the Ginzburg-Landau approach to phase transitions. A single order parameter is chosen to represent the austenite parent phase and two mirror related martensite variants. A free energy previously proposed in the literature (Levitas et al. in Phys. Rev. B 66:134206, 2002; Phys. Rev. B 66:134207, 2002; Phys. Rev. B 68:134201, 2003) is employed, in its simplest form, as the main constitutive content of the model. In this paper we treat time-dependent Ginzburg-Landau equation as a balance law on the structure order and we couple it to a energy balance equation, thus allowing to account of heat transfer processes. We obtain a coupled thermo-mechanical problem whose consistency with the Second Law is verified.

Hysteresis and phase transitions for one-dimensional and three-dimensional models in shape memory alloys

By means of the Ginzburg–Landau theory of phase transitions, we study a nonisothermal model to characterize the austenite-martensite transition in shape memory alloys. In the first part of this paper, the one-dimensional model proposed by Berti et al. [“Phase transitions in shape memory alloys: A non-isothermal Ginzburg-Landau model,” Physica D 239, 95 (2010)] is modified by varying the expression of the free energy. In this way, the description of the phenomenon of hysteresis, typical of these materials, is improved and the related stress-strain curves are recovered. Then, a generalization of this model to the three-dimensional case is proposed and its consistency with the principles of thermodynamics is proven. Unlike other three-dimensional models, the transition is characterized by a scalar valued order parameter φ and the Ginzburg–Landau equation, ruling the evolution of φ, allows us to prove a maximum principle, ensuring the boundedness of φ itself.

Non-entropic and reversible long-range deformation of an encapsulating bioelastomer

Encapsulation is a widespread biological process particularly in the formation of protective egg cases of oviparous animals. The egg capsule wall of the channelled whelk Busycon canaliculum is an effective shock absorber with high reversible extensibility and a stiffness that changes significantly during extension. Here we show that post-stretch recovery in egg capsules is not driven by entropic forces as it is in rubber. Indeed, at fixed strain, force decreases linearly with increasing temperature, whereas in rubber elasticity the force increases. Instead, capsule wall recovery is associated with the internal energy arising from the facile and reversible structural alpha-helix <--> beta-sheet transition of egg capsule proteins during extension. This behaviour is extraordinary in the magnitude of energy dissipated and speed of recovery and is reminiscent of strain-induced crystallization in some polymeric fibres and of superelastic deformations associated with diffusionless phase transitions in shape-memory alloys.

Stationary solutions to the Falk system on shape memory alloys

AbstractWe study the Falk model system describing martensitic phase transitions in shape memory alloys. Its physically closed stationary state is formulated as a nonlinear eigenvalue problem with a non‐local term. Then, some results on existence, stability, and bifurcation of the solution are proven. In particular, we prove the existence of dynamically stable nontrivial stationary solutions. Copyright © 2009 John Wiley &amp; Sons, Ltd.

THE USE OF SHAPE-MEMORY ALLOYS FOR MECHANICAL REFRIGERATION

This letter reports on stress–strain experiments on a Cu – Zn – Al single crystal performed using a purpose-built tensile device which enables the load applied to the specimen to be controlled while elongation is continuously monitored. From the measured isothermal tensile curves, the stress-induced entropy changes are obtained at different temperatures. These data quantify the elastocaloric effect associated with the martensitic transition in shape-memory alloys. The large temperature changes estimated for this effect, suggest the possibility of using shape-memory alloys as mechanical refrigerators.

Global attractor for a nonlinear thermoviscoelastic system in shape memory alloys

This paper is concerned with the existence of a global attractor for a semi-flow governed by the weak solutions to a nonlinear one-dimensional thermoviscoelastic system with clamped boundary conditions in shape memory alloys. The constitutive assumptions for the Helmholtz free energy include the model for the study of martensitic phase transitions in shape memory alloys. To describe physically phase transitions between different configurations of crystal lattices, we work in a framework in which the strain u belongs to L∞. New approaches are introduced and more delicate estimates are derived to establish the crucial L∞ -estimate of strain u in the course of showing the compactness of the orbit of the semi-flow and existence of an absorbing set.

Nanoscopic size effects on martensitic transformations in shape memory alloys

The effect of the grain size and transverse film size in nano-and micrometer ranges on the parameters of martensitic transitions in shape memory alloys is theoretically considered in the framework of the theory of diffuse martensitic transitions. A quantitative analysis of the size effects is performed including not only the thermodynamic aspect of the martensitic transformation but also its kinetic aspect, which is particularly sensitive to structural and size factors. This complex approach makes it possible to explain the following three basic facts associated with the influence of a decreased grain size or transverse film size on the parameters of the martensitic transition in shape memory alloys: a decrease in the critical (characteristic) transition temperature, an increase in the transition temperature smearing, and the existence of a critical grain size or film thickness below which the martensitic transformation in alloys is blocked.

Elastocaloric Effect Associated with the Martensitic Transition in Shape-Memory Alloys

The elastocaloric effect in the vicinity of the martensitic transition of a Cu-Zn-Al single crystal has been studied by inducing the transition by strain or stress measurements. While transition trajectories show significant differences, the entropy change associated with the whole transformation (DeltaS_(t)) is coincident in both kinds of experiments since entropy production is small compared to DeltaS_(t). The values agree with estimations based on the Clausius-Clapeyron equation. The possibility of using these materials for mechanical refrigeration is also discussed.