Transformation In Shape Memory Alloys Research Articles

Uncovering the rate of the martensitic transformation in superheated shape memory alloy wires

The martensitic transformation in shape memory alloys is known to be very fast. However, its rate in bulk materials that serve in common actuator applications is still an open question. In this study, the martensitic transformation is induced by a high-voltage microsecond superheating pulse in a polycrystalline NiTi shape memory alloy wire and its rate is tracked using time resolved X-ray diffraction by synchrotron radiation . The study employs a novel experimental technique that allows for multi-frame time-resolved detection with a temporal resolution below 1 µs, demonstrating an order-of-magnitude improvement compared to previous time-resolved synchrotron studies. Results show that the transformation begins at the moment the wire temperature reaches the transformation temperature and the transformation rate follows the rate of heating, completing most of the process within approximately 1 µs. Yet, part of the transformation continues at much slower time scales of several tens of microseconds.

Austenite Transformation of Shape Memory Alloys in the Ultrasonic Field

Austenite Transformation of Shape Memory Alloys in the Ultrasonic Field

Anomalous high-temperature quasi-linear superelasticity of Ni-Fe-Ga-Co shape memory alloy

The superelasticity of shape memory alloys is associated with the martensitic transformation which has been widely used in engineering applications. Here, we clarify quasi-linear superelasticity in Ni-Fe-Ga-Co shape memory alloy exhibiting a recovery strain of 3.58% at 573 K, which is far above the martensitic transformation temperature. Real-time in-situ neutron diffraction measurement was used to explore the underlying quasi-linear superelasticity mechanism via tracing the structural evolution during cyclic compression loading. Neutron diffraction observation showed that the superelasticity is correlated with stress-induced continuous variation of lattice parameter. The in-situ neutron diffraction provides the direct evidence on the anomalous diffraction peak width broadening, which mainly stems from the spatial heterogeneity in strain. The excessive Co doping is responsible for the decrease in stacking-fault energy leading to an increase in stacking faults, which suppresses the martensitic transformation and triggers the nucleation of the weak first-order phase under the external stress. The study provides insights into the interplay between superelasticity and structural transformation in shape memory alloys, and it is also instructive for understanding the anomalous high-temperature quasi-linear superelasticity in functional materials. • The quasi-linear superelasticy of Ni-Fe-Ga-Co shape memory alloy is caused by the weak first-order transformation. • The quasi-linear superelasticity of Ni-Fe-Ga-Co exhibits a recovery strain of ~3.58% at 573 K, which is rarely reported. • The excessive Co is account for trigger the nucleation of the weak first-order phase under the external stress.

Elastic interaction energy analysis during twin plane formation in the martensitic transformation process of α"-martensite in Ti-Nb-Al

To understand the twin selection mechanism that occurs in the initial stage of the martensitic transformation of shape memory alloys, the stress state during twin plane formation was numerically analyzed by micromechanics. The self-accommodation microstructure of α'' -martensite of the Ti-23Nb-3Al (at%) shape memory alloy, in which the lattice parameter was adjusted to eliminate the formation of internal twinning due to lattice-invariant deformation, was adopted for analysis. The two lattice correspondence variants (CV i and CV j ) formed a twin plane. CV i was assumed to be spheroid, and the elastic strain energy during twin plane formation was evaluated by analyzing the elastic interaction energy when CV j formed around CV i . There are three types of twins ({011} α" compound twin, {111} α" type I twin, and <211> α" type II twin) in the self-accommodation microstructure of α" -martensite. Regardless of the CV i geometry (i.e., aspect ratio (1 to 0.01) and minor axis orientation), the {111} α" type I twin exhibited minimum elastic interaction energy, indicating that minimum elastic strain energy was generated during this twin plane formation. This result is consistent with the experimental results, which showed the selective formation of {111} α" type I twins in the initial stage of martensitic transformation.

Modeling and design optimization of shape memory alloy-enabled building skins for adaptive ventilation

This paper presents a modeling and design study of shape memory alloy (SMA)-based skins that enable adaptive ventilation in buildings. The skins are formed by panels with attached SMA wires and can be periodically arranged to form a building envelope. When the environmental temperature is below the SMA austenitic transformation temperatures, the SMA wires are relaxed and the panel forms a closed surface, preventing circulation of exterior airflow into the building. When the environmental temperature exceeds such SMA transformation temperatures, the SMA wires contract and bend the panel to create an opening that enables circulation of airflow into the building. The response of the adaptive skins is evaluated through a thermomechanically coupled finite element model. The applied thermal loads are determined from published field measurements. Metallic materials and fiber-reinforced composites are explored as candidates for the panel. Genetic algorithms are employed for optimization, where the objective is that the panel exhibits a large opening when the environmental temperature exceeds a prescribed ideal temperature and stays closed otherwise. Optimization results show that two-layered composite panels with ply angles of 45°/−45° and thicknesses of 0.1/0.25 mm (layers ordered from the interior to the exterior side) display the most favorable performance.

Synergistic Size–Surface Effects on Martensitic Transformation of Shape Memory Alloy Nanorods for Micro/Nanoelectro-Mechanical Systems

Martensitic transformation (MT) of shape memory alloys (SMAs) deserves promising applications in various smart devices including micro/nanoelectro-mechanical systems owing to the material’s large reversible deformation from cyclic MT. However, such systems always suffer from critical fatigue problems under cyclic loadings with microdamage accumulations closely related to hysteresis energy and residual strain. Here, the synergistic effects of the sample size and surface composition on the MT of SMA nanorods are systematically investigated by combining global responses and atomic evolutions based on molecular dynamics. It is found that the surface energy plays a resisting role in MT and the resistance increases with decreasing sample size, which in turn increases the critical stress, hysteresis, and residual strain. Intriguingly, the nucleated austenite–martensite transition zone during MT can be influenced by surface composition, and an ultralow hysteresis and residual strain with little size-dependence can be achieved in the nanorod with the surface composed of alternate Ni and Ti atoms. The low energy density of the transition zone indicates better microscopic lattice compatibility and smaller dissipations during MT. This reveals that the surface modification is promising for modulating the size effect and improving the reversibility and the fatigue resistance of the superelastic SMA nanostructures.

Latent heat storage of Nickel Titanium shape memory alloys and its application to solid-state heat storage device

Nickel Titanium shape memory alloy was proposed for heat storage material. When the shape memory alloy is in contact with a heat source, the heat flow from the heat source to the alloy was enhanced by the latent heat associated with the reverse martensitic transformation of shape memory alloy, and vice versa. This study performed both theoretical and computational analyses. First, the enthalpy curve in the temperature range of martensitic transformation was determined from the DSC curves obtained in experiment, which can describe the emission/absorption of latent heat during the martensitic transformation as the function of temperature. Next, a metal matrix composite of NiTi shape memory wires embedded in aluminum block was designed and the heat transfer with a heat source was observed by three-dimension finite element heat conduction analysis with ANSYS. We focused on the effects of the configuration of SMA wires and the heat resistance between the wire and aluminum matrix on the performance of the heat storage.

Continuous Heating Dissolution and Continuous Cooling Precipitation Diagrams of a Nickel-Titanium Shape Memory Alloy

Binary NiTi alloys are the most common shape memory alloys in medical applications, combining good mechanical properties and high biocompatibility. In NiTi alloys, the shape memory effect is caused by the transformation of an austenite phase to a martensite phase and the reverse process. Transformation temperatures are strongly influenced by the exact chemical composition of the NiTi phase and the presence of precipitates in the microstructure induced by thermo-mechanical treatment, especially solution annealing and ageing. Isothermal time–temperature precipitation diagrams can be found in the literature. Cooling is frequently not considered, as water quenching is typically assumed to be sufficient. To the best of our knowledge, continuous heating dissolution (CHD) and continuous cooling precipitation (CCP) diagrams do not exist. Differential scanning calorimetry (DSC) is a common method to analyse the austenite/martensite transformation in shape memory alloys, but it has not yet been used to analyse precipitation processes during continuous temperature changes. We have enabled DSC to analyse dissolution and precipitation processes in situ during heating as well as during cooling from the solution annealing temperature. Results are presented as CHD and CCP diagrams, including information from microstructure analysis and the associated changes in the austenite/martensite transformation temperatures.

Modeling the Stress-Induced Transformation Behavior of Shape Memory Alloys under Multiaxial Loading Conditions

A large number of criteria to model the onset of plasticity for ductile metals have been proposed by researchers in the last century. Strangely, very few researchers have tried to model the stress-induced crystalline phase transformation of Shape Memory Alloys (SMAs) according to yield criteria. This paper focuses on the question: is a yield criterion originally proposed for describing the plastic behavior of metals suitable to model the “pseudoelastic” behavior of SMAs? To answer this question, two yield criteria originally proposed by the present author are used to predict the initial surface of transformation onset of two different SMAs: Cu-Al-Be and Ni-Ti alloy. The predicted initial transformation onset surfaces of the two SMAs are compared with experimental results and existing theories reported in the literature and some significant conclusions and recommendations are given.

Solutions to a phase‐field model for martensitic phase transformations driven by configurational forces

AbstractWe study the existence of weak solutions to an initial‐boundary value problem for a new phase‐field model, which consists of a degenerate parabolic equation coupled to linear elasticity equations. This model is used to describe the evolution of interfaces in elastically deformable solid materials which moves by configurational forces, such as martensitic phase transformations in shape memory alloys. The existence proof is valid only for one‐dimensional case.

Phase transformation in shape memory alloys: a numerical approach for thermomechanical modeling via peridynamics

The work is devoted to the numerical aspects of the modeling tool elaborated to simulate the phenomenon of solid phase transformation in shape memory alloys. Particularly, a nonlocal approach, namely the bond based variant of peridynamics, is of concern to handle material model nonlinearities conveniently. The proposed model considers thermomechanical coupling which governs kinetics of the process of phase change. The phenomenological peridynamic model of a shape memory alloy is based on the Gibbs free energy concept and thermoelasticity. The work focuses on the superelasticity effect which can be observed when a tested material is subjected to a mechanical load. As a demanded application scope for the proposed smart material model, its scheduled future use in the study of operational conditions for a gas foil bearing is considered. The motivation of the work has primarily originated from the perspective of more accurate, i.e., more physical, modeling of the structural components which employ shape memory alloys to stabilize the bearing operation. The authors conduct a preliminary investigation regarding the properties of the newly proposed numerical multiphysics approach. Specifically, the scope of the work covers description of the developed computational framework as well as detailed derivation of its stability criteria. Exemplary numerical results complement the paper providing with determination of the stress–strain relation and adequate parametric study.

Two methods for calculating the stress-strain state of shape memory alloy constructions taking into account tension-compression asymmetry

Two methods for calculating the phase-structural deformations of shape memory alloy (SMA) structures under complex stress conditions are considered. They both are based on the one-dimensional phenomenological model, which is built upon the relationship between the direct transformation and martensitic inelasticity diagrams, which makes it possible to uniformly describe strains in the phase and structural transformations, since both of the strain components are associated with the formation of oriented martensite. The ability of the model to describe a number of basic macromechanical effects caused by martensitic transformations in SMA was shown in our previous work. After the generalization to the case of a complex stress state it can successfully be used for solving certain engineering problems. The generalization of the model can be accomplished in two ways. The first method involves the construction of three-dimensional constitutive relations, proceeding from the previously developed one-dimensional relations and some simplifying hypotheses, and the numerical implementation of these relations by the finite element method. The second is the structural method, applicable to structures, in which the stress-strain state is described by one kinematic and one force parameter. This method suggests the use of structural diagrams of direct transformation and martensitic inelasticity, which are similar to the corresponding material diagrams, but establish the dependence of the phase-structural component of the kinematic parameter on the force parameter (not the dependence of phase-structural strains on the stress). Although the structural method is associated with the necessity to experimentally determine the structural diagrams, it has the advantage of significantly reducing the computational costs. Additionally, the article presents a comparison of two methods for describing the tension-compression asymmetry, and also develops a method taking finite deformations into account.

TEM analysis of deformation bands created by tensile deformation of superelastic NiTi wires

Abstract Deformation processes derived from martensitic transformation in shape memory alloys are theoretically fully recoverable in a complete thermomechanical loading cycle across transformation range and do not leave any lattice defects in the microstructure. In reality, this is rarely the case in NiTi, since plastic deformation tends to accompany the martensitic transformation, particularly if it proceeds under large stress. Lattice defects observed in the microstructure of deformed NiTi wires (slip dislocations and deformation bands) attract the attention of researchers, since they are linked to unrecovered strains and play significant role in functional fatigue, shape setting or two-way shape memory effect. In this work, we present an experimental approach allowing for analysis of deformation bands in deformed NiTi consisting in: i) preparation of superelastic NiTi wires with recrystallized, small grained microstructure, ii) subjecting these wires to desired tensile test (e.g. superelastic or shape memory cycle) and iii) characterizing the deformation bands in TEM using selected area electron diffraction combined with dark field imaging following simple rules described in this work. If the deformed microstructure becomes too complex due to the high density and small size of deformation bands, ASTAR orientation mapping can be beneficially applied to reveal the refinement of the microstructure through the introduction of deformation bands.

On the elastocaloric effect in CuAlBe shape memory alloys: A quantitative phase-field modeling approach

Abstract The reversible stress-induced phase transformation in shape memory alloys (SMAs) is a dissipative process during which heat is absorbed or released. The inherent temperature variations inside the material has an elastocaloric effect (eCE) with appealing applications in solid-state cooling technology such as compact and efficient on-board refrigeration system for eletronic devices. In this manuscript, we conduct the first study of eCE of CuAlBe SMAs utilizing phase-field modeling. For an applied stress of 500 MPa, the results for polycrystalline Cu-Al11-2Be (at. %) show a minimum adiabatic unloading temperature change of −10 K over a pseudoelastic window of 40 K. In the absence of plastic deformation, the material demonstrates good reproducibility of the eCE over a few loading–unloading cycles. The presence of plastic deformation is found to cause functional fatigue that deteriorates the cooling capacity; however, the coefficient of performance only decreases from 9.04 to 8.03, which is still a very good value. These results place CuAlBe as a frontrunner SMA for solid-state cooling compared to the expensive NiTi.

A Benchmark Study for Phase Transformation of Shape Memory Alloy Based Split Ring Resonator Structure Under a Heating and Cooling Thermal Cycle

In this benchmark study, the integration of shape memory alloy thin film on to a split ring resonator is investigated. Split ring resonators are artificially produced structures which are used to generate magnetic response. One of the applications that can be met in the literature is the solar panel ones where they are benefitted for the electric generation from the resonance of it. Unfortunately, this application is very sensitive to the shape, geometry and dimensions which can be affected from ambient conditions easily. At this particular stage, to compensate the dimensional inconsistencies by shape memory alloy thin film is chosen in an ongoing project. Due to the nature of the shape memory alloy, not only the material properties but also the geometry of the structure can also change. By taking the advantage of this unique feature, the applicability of coupling shape memory alloy thin film by split ring resonator is investigated. And as an initial step of this research, the phase transformation of shape memory alloy thin film is investigated with regard to varying ambient conditions which are simulated by the change of convection conditions and exposure time. The temperature evolution of two significant point is tracked under these conditions and furthermore the change of martensite volume fraction is examined which will enlighten the path for the further steps on this investigation.

Weak solutions and simulations to a new phase-field model with periodic boundary

ABSTRACT We prove the existence and investigate the large-time behavior of weak solutions for a new phase-field model with periodic boundary conditions, which is suitable for structural transformation driven by configurational forces, such as martensitic transformation in shape memory alloys. Furthermore, the evolution of martensitic phase transition in MnNi alloys is simulated by employing this model.

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.

Elastocaloric Cooling: State-of-the-art and Future Challenges in Designing Regenerative Elastocaloric Devices

The elastocaloric cooling, utilizing latent heat associated with martensitic transformation in shape-memory alloys, is being considered in the recent years as one of the most promising alternatives to vapour compression cooling technology. It can be more efficient and completely harmless to the environment and people. In the first part of this work, the basics of the elastocaloric effect (eCE) and the state-of-the-art in the field of elastocaloric materials and devices are presented. In the second part, we are addressing crucial challenges in designing active elastocaloric regenerators, which are currently showing the largest potential for utilization of eCE in practical devices. Another key component of elastocaloric technology is a driver mechanism that needs to provide loading for active elastocaloric regenerators in an efficient way and recover the released energy during their unloading. Different driver mechanisms are reviewed and the work recovery potential is discussed in the third part of this work.

A thermomechanical modelling framework for selective laser melting based on phase transformations

AbstractSLM (selective laser melting) is one of many processes belonging to the additive manufacturing sector, where a laser beam completely melts the particles of the manufactured part layer by layer. In contrast to common models, this framework is based on the minimisation of the free energy density where the phase transformation from powder to molten pool to re‐solidified material is modelled explicitly. The goal is to develop a more sophisticated model which is based on the material behaviour itself rather than on the use of temperature dependent material parameters. The following model is adapted from phase transformations in shape memory alloys, cf. [1]. With this at hand, more insight into the process is gained and the deformation as well as the residual stresses are predictable.