Transformation In Shape Memory Alloys Research Articles

Proposition of R-phase transformation strip in the phase diagram of Ni-Ti shape memory alloy using electromechanical experiments

The shape memory mechanism associated with R-phase transformation was investigated using electromechanical tests on Ni-Ti shape memory alloy thin wires. To provide a more precise insight into the effects of R-phase on phase transformation of shape memory alloys, wires were prepared in three different initial states prior to conducting the experiments: pure martensite, mixture of R-phase and martensite, and pure R-phase. The electromechanical tests were done in two different loading states (under constant and variable stresses) using specially designed and manufactured apparatuses. Tests under different constant stresses were carried out for three cases of pure martensite, a mixture of martensite and rhombohedral, and pure rhombohedral phase, while those under variable stresses were done on pure martensite as well as mixed martensite and rhombohedral. For all the expressed cases, strain–time responses were investigated during electric heating–cooling cycles to interpret possible microstructural changes in the samples. According to the results, a phase diagram containing R-phase transition strip was proposed for Ni-Ti shape memory alloys.

Photostress analysis of stress-induced martensite phase transformation in superelastic NiTi

Phase transformation in shape memory alloys is the most important factor in their unique behavior. In this paper, the formation of stress induced martensite phase transformation in a superelastic NiTi (50.8% Ni) shape memory alloy was investigated by using the photo-stress method. First, the material's fabrication procedure has been described and then the material was studied using the metallurgical tests such as differential scanning calorimetry and X-ray diffraction to characterize the material features and the mechanical tensile test to investigate the superelastic behavior. As a new method in observation of the phase transformation, photo-stress pictures showed the formation of stress induced martensite in a superelastic dog-bone specimen during loading and subsequently it's disappearing during unloading. Finally, finite element analysis was implemented using the constitutive equations derived based on the Boyd-Lagoudas phenomenological model.

Three-dimensional, non-isothermal phase-field modeling of thermally and stress-induced martensitic transformations in shape memory alloys

The transition between austenite and martensite provides shape memory alloys (SMAs) with many unique properties. A three-dimensional, non-isothermal phase-field model was employed to comprehensively investigate the latent heat and elastocaloric effects in SMAs, which are originated from the thermoelastic martensitic transformations (MTs). We analyzed thermally induced MTs upon cooling and heating and stress-induced MTs at different temperatures in Mn–Cu alloys. It was observed that the size of twinned martensitic domain varied with temperature and large internal stresses resulted in thermoelastic equilibrium between austenite and martensite. The evolution of temperature field, caused by the latent heat effect, was obtained and the maximum temperature was found nearby the phase interface during the transformation. Twinned and single-variant martensites formed upon uniaxial tension and compression, respectively. Above the thermodynamic equilibrium temperature between austenite and martensite, with the occurring of reverse MT during unloading, superelasticity was realized. The localized temperature change and global elastocaloric effect during uniaxial loading and unloading were obtained and discussed. This model helps understand and explain the uniformity of temperature, stress and microstructural fields in the phase-field method.

In Situ Neutron Diffraction Study of NiTi–21Pt High-Temperature Shape Memory Alloys

In situ neutron diffraction was used to investigate the microstructural features of stoichiometric and Ti-rich NiTiPt high-temperature shape memory alloys with target compositions of Ni29Ti50Pt21 and Ni28.5Ti50.5Pt21 (in atomic percent), respectively. The alloys’ isothermal and thermomechanical properties (i.e., moduli, thermal expansion, transformation strains, and dimensional stability) were correlated to the lattice strains, volume-averaged elastic moduli, and textures as determined by neutron diffraction. In addition, the unique aspects of this technique when applied to martensitic transformations in shape memory alloys are highlighted throughout the paper.

Refined constitutive, bridging, and contact laws for including effects of the impact-induced temperature rise in impact responses of composite plates with embedded SMA wires

In the present research, effects of the impact-induced temperature rise on the phase transformation of shape memory alloy (SMA) wires embedded in composite plates and consequently, impact responses of the hybrid plates are investigated, for the first time. In the available approaches, effects of the temperature rise have been studied only for single SMA wires under uniaxial tensile loads whereas the stress and martensite volume fraction vary along the SMA wires embedded in a hybrid composite plate under transverse impact. The impact-induced heat of the SMA wires is determined based on proposing a refined free energy function that includes the latent heat of the phase transformation. Furthermore, Brinson's constitutive law of the superelastic SMAs and Hertz contact law are revised based on the refined phase version of the free energy function and a proper bridging law wherein, distributions of the SMA phases are considered to be both localized and time-dependent. Results of the employed bridging law are more accurate than those of the common homogenization techniques wherein, stresses of the wires cannot be easily distinguished from those of the mixture. The resulting governing equations are solved by the finite element method and the refined nonlinear constitutive laws are solved through a return-mapping Newton-Raphson procedure, within each time step. Results reveal that incorporation of the realistic heat generation phenomenon leads to remarkably higher local stresses; a fact that has to be considered in the design stage.

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.

Elastically driven metamagnetic-like phase transformations of shape memory alloys

A theoretical model of metamagnetic-like (ferromagnetic–paramagnetic and ferromagnetic–antiferromagnetic) phase transitions is developed for the interpretation of experimental results obtained recently for the Ni–Mn–Co–X (X = In, Sn, Ga) shape memory alloys. The conditions of elastically driven (caused by the martensitic transformation of alloy) metamagnetic-like phase transitions are determined. These conditions are: high magnetic susceptibility of paramagnetic/antiferromagnetic phase; large (but real for some alloys) volume change during the martensitic transformation; and large value of volume magnetostriction caused by the metamagnetic-like phase transition. The magnetoelastic mechanism is proposed for the explanation of magnetic field influence on the martensitic transformation. The elastically driven ferromagnetic–paramagnetic phase transition is considered in more detail and the results of corresponding magnetic measurements are described.

Martensitic phase transformations in shape memory alloy: phase field modeling with surface tension effect

The coupled phase-field and elasticity equations are presented for multivariant martensitic phase transformations (PTs) to investigate the effect of surface tension on PTs. The finite element method is utilized to solve the coupled phase-field and elasticity equations for two dimensional problems. Several numerical examples for cubic-to-tetragonal PTs in NiAl shape memory alloy are studied including surface-induced pretransformation and transformation, the stress- and temperature-induced growth of a martensitic nucleus inside austenite, and temperature- and surface-induced martensitic PTs for single and two martensitic variants. The phase and stress fields are obtained and the crucial effect of surface tension on stress and nucleation and propagation of martensitic nanostructures is revealed and discussed. It is found that the surface tension can suppress nucleation so that a larger driving force is needed for PTs to happen. Also, it can significantly change the stress and phase field solution. Moreover, it can suppress the propagation of transformed region.

Magnetically driven magnetostructural transformations of shape memory alloys

A conception of magnetically driven martensitic transformation (MT) has been put forward. The MT driven by the ferromagnetic–antiferromagnetic phase transition has been analysed in detail for the case of weak antiferromagnetic exchange between the magnetic sublattices. It has been shown that in this case the MT is characterized by (i) a strong dependence of the MT temperature on the external magnetic field; (ii) a pronounced decrease in the martensite volume fraction and the macroscopic MT strain under the increasing field; (iii) the ‘arrest’ of the forward MT on the cooling of the alloy in a strong magnetic field; and (iv) a drastic decrease in the entropy difference between the austenitic and martensitic phases on approaching the temperature of arrest of the MT. The assumption about the antiferromagnetic ordering of the martensitic phase is not crucial for the proposed model of magnetically driven MT, and so its applicability to other types of ordering has been discussed. The Ni–Mn–In-based alloys have been considered as a case for study.

Investigation of the energetics of structural transformations in Co-based binary alloys

The energy pathways associated with the martensitic transformation in shape memory alloys (SMA's), though the focus of extensive research over the past decades, are still unclear. In this work we build a framework using an energetics approach to describe the martensitic f.c.c-h.c.p transformation in Co-based alloys and to investigate the effect of alloying on the transformation First principle calculations are used to determine the energy landscape of the transformation using the Shoji - Nishiyama and Wentzcovitch - Lam models in Co - (Al, Fe, Si) binary alloys over a range of composition. The martensitic transformation is an energy barrier crossing event and exhibits concomitant reduction in symmetry. Hence a steepest descent optimization technique is employed to realize the minimum energy path and the transition states of the transformations over the energy landscape. The energy barriers as a function of composition are calculated and compared against available experimental data.

Transformation Sequence Rule of Martensite Plates and Temperature Memory Effect in Shape Memory Alloys

In thermoelastic martensitic transformation, it is well established that the first martensite plate appearing upon cooling becomes the final one during reverse transformation to austenite upon heating. The results obtained from this work show that the transformation sequence of the martensite appears to be random. Newly formed martensite plates can modify the elastic strain energy level stored in the already existing martensite. Additionally, the elastic strain energy stored in newly formed martensite is not necessarily to be higher than the remaining martensite. The obtained results may assist in understanding phenomena related to partial transformation of shape memory alloys, such as temperature memory effect.

Multiple bifurcations and local energy minimizers in thermoelastic martensitic transformations

Thermoelastic martensitic transformations in shape memory alloys can be modeled on the basis of nonlinear elastic theory. Microstructures of fine phase mixtures are local energy minimizers of the total energy. Using a one-dimensional effective model, we have shown that such microstructures are inhomogeneous solutions of the nonlinear Euler–Lagrange equation and can appear upon loading or unloading to certain critical conditions, the bifurcation conditions. A hybrid numerical method is utilized to calculate the inhomogeneous solutions with a large number of interfaces. The characteristics of the solutions are clarified by three parameters: the number of interfaces, the interface thickness, and the oscillating amplitude. Approximated analytical expressions are obtained for the interface and inhomogeneity energies through the numerical solutions.

Surface Energies Arising in Microscopic Modeling of Martensitic Transformations in Shape-Memory Alloys

In this study we construct and analyze a two-well Hamiltonian on a 2D atomic lattice.The two wells of the Hamiltonian are prescribed by two rank-one connected martensitic twins,respectively. By constraining the deformed con gurations to special 1D atomic chains withposition-dependent elongation vectors for the vertical direction, we show that the structure ofground states under appropriate boundary conditions is close to the macroscopically expectedtwinned con gurations with additional boundary layers localized near the twinning interfaces.In addition, we proceed to a continuum limit, show asymptotic piecewise rigidity of minimizingsequences and rigorously derive the corresponding limiting form of the surface energy.

Elastocaloric cooling potential of NiTi, Ni2FeGa, and CoNiAl

Solid state elastocaloric cooling, the endothermic reversible martensitic phase transformation in shape memory alloys, has the potential to replace vapor compression refrigeration. NiTi, Ni2FeGa, and CoNiAl shape memory alloys were experimentally investigated to measure the magnitude of temperature change using thermography during uniaxial tensile experiments. Consecutive tensile cycles were also performed, and they revealed a symmetric temperature profile between the two cycles. The unique, dual camera technique of digital image correlation and thermography was utilized to track the transformation bands and temperature gradients to gain insight about the unloading, endothermic process. Fatigue implications, elevated temperature environments, and the theoretical maximum temperature based on entropy change were discussed.

Analysis of localization phenomena in Shape Memory Alloys bars by a variational approach

Localization of the phase transformations in Shape Memory Alloys (SMA) wires are well known. Several experimental and theoretical studies appeared in the last years. In this work the problem is addressed by means of a variational approach within the framework of the modeling of rate-independent materials by the specification of a non-local free energy and a dissipation function, focusing attention on the basic case of isothermal conditions. General expressions are given for a rather broad class of models, whereas a simple model is studied in detail. A full stability analysis of both homogeneous and non-homogeneous solutions is carried out analytically, showing that stable non-homogeneous solutions have necessarily to occur if the bar is longer than an internal length determined by the constitutive parameters. The analysis also shows that snap-back phenomena may occur both in the nucleation and the coalescence phase, depending on another material length which is also function of the number of transformation fronts. This helps to explain why the second stress drop associated to coalescence is much more difficult to observe experimentally. Closed form expressions are given for the phase fraction profiles of both single and multiple localizations as well as nucleation and propagation stresses. A comparison between the prediction of the model with experimental data finally shows a good agreement both in terms of global response and in the spatio-temporal evolution of the transformation domains.

Feasibility of Shape Memory Alloy Wire Actuation for an Active Steerable Cannula

Needle insertion is used in many diagnostic and therapeutic percutaneous medical procedures such as brachytherapy, thermal ablations, and breast biopsy. Insufficient accuracy using conventional surgical cannulas motivated researchers to provide actuation forces to the cannula's body for compensating the possible errors of surgeons/physicians. In this study, we present the feasibility of using shape memory alloy (SMA) wires as actuators for an active steerable surgical cannula. A three-dimensional (3D) finite element (FE) model of the active steerable cannula was developed to demonstrate the feasibility of using SMA wires as actuators to bend the surgical cannula. The material characteristics of SMAs were simulated by defining multilinear elastic isothermal stress–strain curves that were generated through a matlab code based on the Brinson model. Rigorous experiments with SMA wires were done to determine the material properties as well as to show the capability of the code to predict a stabilized SMA transformation behavior with sufficient accuracy. In the FE simulation, birth and death method was used to achieve the prestrain condition on SMA wire prior to actuation. This numerical simulation was validated with cannula deflection experiments with developed prototypes of the active cannula. Several design parameters affecting the cannula's deflection such as the cannula's Young's modulus, the SMA's prestrain, and its offset from the neutral axis of the cannula were studied using the FE model. Real-time experiments with different prototypes showed that the quickest response and the maximum deflection were achieved by the cannula with two sections of actuation compared to a single section of actuation. The numerical and experimental studies showed that a highly maneuverable active cannulas can be achieved using the actuation of multiple SMA wires in series.

3D coupled thermo-mechanical phase-field modeling of shape memory alloy dynamics via isogeometric analysis

The paper focuses on numerical simulation of the 3D phase-field (PF) equations for modeling cubic-to-tetragonal martensitic transformations in shape memory alloys (SMAs), their complex microstructures and thermo-mechanical behavior. The straightforward solution to the fourth-order diffuse interface 3D PF equations, based on the Landau–Ginzburg potential, is numerically solved using an isogeometric analysis. We present microstructure evolution in different geometries of SMA nanostructures under temperature-induced phase transformations to illustrate the geometrical flexibility, accuracy and robustness of our approach. The simulations successfully capture the dynamic thermo-mechanical behavior of SMAs observed experimentally.

Shape memory alloy nanostructures with coupled dynamic thermo-mechanical effects

Employing the Ginzburg–Landau phase-field theory, a new coupled dynamic thermo-mechanical 3D model has been proposed for modeling the cubic-to-tetragonal martensitic transformations in shape memory alloy (SMA) nanostructures. The stress-induced phase transformations and thermo-mechanical behavior of nanostructured SMAs have been investigated. The mechanical and thermal hysteresis phenomena, local non-uniform phase transformations and corresponding non-uniform temperatures and deformations’ distributions are captured successfully using the developed model. The predicted microstructure evolution qualitatively matches with the experimental observations. The developed coupled dynamic model has provided a better understanding of underlying martensitic transformation mechanisms in SMAs, as well as their effect on the thermo-mechanical behavior of nanostructures.

Temperature dependent local phase transformation in shape memory alloys by nanoindentation

A novel approach, based on nanoindentation experiments, for analysing temperature dependent local phase transformations in shape memory alloys is proposed. The method was used to analyse cracked specimens and the effects of temperature on local crack-tip transformations were analysed by isothermal nanoindentation tests. Furthermore, experimental results were compared with predictions of a recent analytical model and good agreements were observed. It was demonstrated that the proposed approach can be advantageously used instead of more complex microstructural investigations, like X-ray microdiffraction.

<i>In Situ</i> Optical Microscopic Examination Techniques of Thermally Induced Displacive Transformations

The mechanical (reversible deformation, stress-strain diagrams, etc.) and thermal (transformation temperatures, hysteresis) characteristics of the thermoelastic martensitic transformations are in the focus of many manuscripts, however, other aspects of the transformations are given less attention. The relief formation accompanied with displacive transformations ensures the possibility of the direct observation of the mechanism and physical metallurgical characteristics of the martensite↔austenite transformations. The authors of the present manuscript applied the in situ optical microscopy method successfully using self-developed examination techniques and self-made heating stages to characterize the thermally induced displacive transformations in shape memory alloys (SMAs) and TWIP steels.