Polycrystalline Shape Memory Alloys Research Articles

Effect of Cu addition on the physical and mechanical properties of a Ni–Mn–In–Co polycrystalline shape memory alloy

The martensite transformation (MT), magnetocaloric effect (MCE), and mechanical properties of the polycrystalline Ni46Mn36In14Co4 (Co4) alloys with doping of 4 at.% Cu were examined. A large magnetization change (ΔM, 92 emu/g), large magnetic entropy change (ΔSM, 37.3 J kg−1K−1), high refrigerant capacity (RC, 261 J kg−1), relatively high sensitivity to magnetic fields (2.8 K/T), and two-way magnetic-field-induced MT were attained at approximately room temperature. Moreover, the compressive fracture strength and strain of the alloy reached 335 MPa and 7.4 %, respectively. Finally, the mechanism of the excellent combination properties of the Ni42Mn36In14Co4Cu4 (Co4Cu4) alloy was discussed. This study can aid in the development of high-performance magnetic refrigeration and intelligent actuator materials.

Effect of Ni4Ti3 precipitates on the functional properties of NiTi shape memory alloys: A phase field study

Ni4Ti3 precipitates, which generally exist in aged Ni-rich NiTi shape memory alloys (SMAs), can have a profound effect on material properties. However, the fundamental insights into the effects of Ni4Ti3 precipitates on the functional properties, including the superelasticity (SE), elastocaloric effect (eCE), and shape memory effects (SMEs), are not well understood yet, especially those originating from the B2-B19′ martensite transformation (MT). In this work, a phase field model coupling the precipitation of Ni4Ti3 and the B2-B19′ MT is proposed, where the thermo-mechanical coupling effect and grain size effect are considered. The precipitate-dependent SE, eCE, one-way SME (OWSME), and stress-assisted two-way SME (SATWSME) of single-crystal, polycrystalline, and gradient-nanograined NiTi SMAs are simulated. The effects of the precipitate density, grain orientation range (texture), and gradient-distributed precipitate are examined, and the underlying microscopic mechanisms are revealed. The simulation results and new findings not only contribute to a more comprehensive insight into the effect of Ni4Ti3 precipitates on the MT, martensite reorientation, and functional properties of NiTi SMAs but also provide a reference for the development of excellent SMA-based solid refrigerants or SMA smart materials with designable functional properties.

Effect of pore on the deformation behaviors of NiTi shape memory alloys: A crystal-plasticity-based phase field modeling

Porous NiTi shape memory alloys (SMA) have been developed into important biomedical and damping materials. However, the effect of pore on the martensitic microstructure evolution, functional properties, and plastic deformation has not been fully clear yet. In this work, to reveal the functional properties including superelasticity (SE), elastocaloric effect (eCE), one-way shape memory effect (OWSME), and stress-assisted two-way shape memory effect (SATWSME), as well as plastic deformation of porous NiTi SMAs, the polycrystalline SMA system is considered as a composite containing the grain-interior (GI) and grain-boundary (GB) phases, a thermo-mechanically coupled, grain-size-dependent and crystal-plasticity-based phase field model is proposed for the GI phase, and an elasto-viscoplastic constitutive model is established for the GB phase. The effects of porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution, as well as the interaction between pore and grain size, are comprehensively revealed and discussed. The simulations demonstrate that the pores cause a heterogeneous stress field, and the interaction among pores and that between pores and grain boundaries can enhance such heterogeneity, which can promote the initiation of martensitic transformation and reorientation. Moreover, the geometrical constraints caused by the pores can significantly affect the morphology and evolution of martensite microstructures. However, due to the diverse microstructure evolutions during SE, OWSME, and SATWSME, their interactions with the pores are quite different, resulting in various dependencies of each functional property and plastic deformation on the pore, and such dependencies are strongly influenced by porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution. The simulated results in this work are helpful to comprehensively and deeply understand the pore effect and its microscopic mechanism.

A Physics-Based Hybrid Dynamical Model of Hysteresis in Polycrystalline Shape Memory Alloy Wire Transducers

Shape memory alloys (SMAs) are a class of smart materials that exhibit a macroscopic contraction of up to 5% when heated via an electric current. This effect can be exploited for the development of novel unconventional actuators. Despite having many features such as compactness, lightweight, and high energy density, commercial SMA wires are characterized by a highly nonlinear behavior, which manifests itself as a load-, temperature-, and rate-dependent hysteresis exhibiting a complex shape and minor loops. Accurate modeling and compensation of such hysteresis are fundamental for the development of high-performance SMA applications. In this work, we propose a new dynamical model to describe the complex hysteresis of polycrystalline SMA wires. The approach is based on a reformulation of the Müller–Achenbach–Seelecke model for uniaxial SMA wires within a hybrid dynamical framework. In this way, we can significantly reduce the numerical complexity and computation time without losing accuracy and physical interpretability. After describing the model, an extensive experimental validation campaign is carried out on a 75- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m diameter SMA wire specimen. The new hybrid model will pave the development of hybrid controllers and observers for SMA actuators.

Experimental investigation and multi-objective optimization of eco-friendly near-dry electrical discharge machining of shape memory alloy using Cu/SiC/Gr composite electrode.

The near-dry electrical discharge machining processes have been conducted using air-mist or gas mist as a dielectric fluid to minimize the environmental impacts. In this article, near-dry electrical discharge machining (NDEDM) experiments have been performed to improve machining performance using an oxygen-mist dielectric fluid, a copper composite electrode, and Cu-Al-Be polycrystalline shape memory alloy (SMA) work materials. The copper composite electrode is made up of 12 wt% silicon carbide and 9 wt% graphite particles. The oxygen-mist pressure (Op), pulse on time (Ton), spark current (Ip), gap voltage (Gv), and flow rate of mixed water (Fr) were used as process parameters, and the material removal rate (MRR), tool wear rate (TWR), and surface roughness (SR) were used as performance characteristics. The global optimal alternative solution has been predicted by the PROMETHEE-II (Preference Ranking Organization METhod for Enrichment Evaluations-II) optimization technique. The best combinations of process parameters have been used to examine the microstructure of composite tools and SMA-machined surfaces by scanning electron microscopy (SEM) analysis. The best global optimum settings (oP: 9bar, Ip: 60µs, Ip: 12 A, Gv: 40V, and Fr: 12ml/min) are predicted to attain optimum machining performance (MRR: 39.049g/min, TWR: 1.586g/min, and SR: 1.78µm). The tool wear rate of the NDEDM process has been significantly reduced by the copper composite electrode due to increasing microhardness, wear resistance, and melting point. When compared to the pure copper electrode tool, the MRR of NDEDM is improved to 21.91%, while the TWR and SR are reduced to 46.66% and 35.02%, respectively.

A micromechanically consistent energy estimate for polycrystalline shape-memory alloys, II: Application to Lüders-type strain localization

We make use of a micromechanical energy estimate proposed in the companion paper (Peigney, 2023) to study strain Lüders-like localization in polycrystalline NiTi specimen under tension. Under the assumption that only the most favorably oriented martensitic variant is active in each grain, it is shown that the homogeneous equilibrium problem can be solved analytically for fiber textures (as commonly found in drawn wires or tubes) and more generally for transversely isotropic textures. This leads to an analytical condition for strain localization that admits an explicit expression in terms of the elastic moduli, the lattice parameters and simple statistical information on the texture. The obtained condition allows one to understand what is distinctive about NiTi compared to other shape memory alloys and why strain localization does not appear in compression. In the cases when strain localization is predicted, several quantities of interest such as the strain gap between initiation of phase transformation and localization, the stress drop at the onset of localization and the inclination of the bands can be derived. The results obtained from the presented analytical solution are compared with numerical simulations performed by taking several active variants in each grain into account and considering imperfect transversely isotropic textures. It is found that the presented analytical solution gives a fairly accurate estimate of the response of 100 (and more)-grain aggregates. Taking several active variants per crystalline orientation does not impact the localization condition significantly but gives values of the strain gaps between initiation and localization that are closer to the experimental values reported in the literature.

A micromechanically consistent energy estimate for polycrystalline shape-memory alloys. I - General formulation

We present an estimate for the effective energy of polycrystalline shape memory alloys that takes both intra-grain compatibility conditions and inter-grain constraints into account. That estimate is constructed using ideas and results from homogenization theory and from the translation method. The proposed estimate depends on the texture through 2-point statistics and is proved to satisfy known rigorous bounds that depend on the same microstructural information. Several examples are provided to give some insight on the proposed estimate and illustrate how it behaves relative to the bounds. The presented examples range from strain-driven uniaxial tension in the superelastic regime (high temperature) to the estimation of recoverable strains in the shape memory regime (low temperature).

A 3D finite-strain beam model for thermo-mechanical deformations of 2D shape memory alloys in 3D space

In this paper, a robust constitutive model adapted to a 3D beam theory in the finite strain regime is proposed to simulate thermo-mechanical behaviors of shape memory alloys (SMAs) under triaxial normal-shear loadings. For the first time, four internal parameters are considered to capture martensitic transformation, orientation/reorientation of martensite variants, normal-shear deformation coupling, and asymmetric/anisotropic strain generation in polycrystalline SMAs. Solution algorithms of the model are presented for stress- and strain-control cases based on the elastic-predictor inelastic-corrector return mapping process. To develop the Cauchy-Green deformation tensor, a 3D exact displacement field is proposed based on the centroid movements and rotations of the cross-section. A finite element formulation along with the Newton-Raphson and Riks techniques is also established to trace the non-linear equilibrium path of 3D SMA beam structures in the large defamation regime. Higher-order shape functions are also considered to avoid shear and membrane locking issues. To explore and demonstrate the capabilities of the proposed model, the results of the triaxial and 3D SMA constitutive models are compared with existing experimental results on uniaxial tension, and combined tension-torsion tests. Afterwards, the results of 3D solid element and proposed 3D beam element are compared with experimental results of deep stretch SMA helical springs under three loading/unloading cycles. An excellent qualitative and quantitative correlation is observed between numerical and experimental results verifying the model accuracy and the solution procedure. The combination of the proposed triaxial SMA constitutive model with 3D beam theory leads to a significant reduction in computational time and storage requirements without any accuracy loss. Extra simulations are then conducted for a simplified model of a 3D biomedical vascular SMA stent under various loading/unloading conditions. The simulations reveal that the new 3D finite-strain SMA-beam element is well appropriate for modeling of 3D spatial lattice-based stents. Due to simplicity and accuracy, the 3D SMA beam model could serve in future efforts for a computationally efficient modeling of large defamations of complicated 3D SMA structures with 2D members (e.g., SMA lattices). • A robust constitutive model is developed to simulate behaviors of SMAs under triaxial normal-shears loadings. • The solution algorithms combined is extended to trace the non-linear equilibrium path of 3D SMA beam structures. • The results of present 3D Beam model has been compared with existing experimental results as well as 3D solid elements. • A good qualitative and quantitative correlation is observed between numerical and experimental results. • The proposed triaxial SMA beam model leads to a significant reduction in computational time without loss inaccuracy.

Multi-scale in situ mechanical investigation of the superelastic behavior of a Cu–Al–Be polycrystalline shape memory alloy

The superelastic behavior of a Cu–Al–Be alloy was studied in situ during tensile tests combining two high-energy synchrotron techniques. The initial microstructure was reconstructed using diffraction contrast tomography; the elastic strain and stress tensors of each individual grain were then determined using a 3D X-ray diffraction microscopy technique. The alloy was heat-treated until coarse grains formed, and the probed volume fraction was limited to ∼ 200 grains. The mean grain size, as estimated from both techniques, agreed well with that determined by optical microscopy, i.e., approximately 130 µm. During the early stage of the martensitic transformation (MT), 187 grains with strong stress heterogeneities were detected in the elastic domain; in particular, the stress values along the tensile direction varied by a factor of three between different grains. The reconstructed 3D microstructure served as input data for finite element modeling, wherein a micromechanical approach factoring in the martensitic transformation was used to simulate the in situ tensile tests. The coupled numerical and experimental tensors of the 187 grains confirmed strong stress heterogeneities between them. The influence of the position and crystallographic orientation of the neighboring grains were also examined.

Mechanical properties and scaling laws of polycrystalline CuZr shape memory alloy

Thanks to their excellent properties such as superelasticity, high hardness, and shape memory effect, polycrystalline shape memory alloys (SMAs) have extensive applications in various engineering fields including automobile, functional materials, and aerospace. Using molecular dynamics simulations, the present paper aims to a systematic study of the fundamental tensile behavior in the nanoscale of polycrystalline B2-CuZr SMAs with mean grain sizes in the range of 6–25 nm. Effects of mean grain size, temperature, and tensile rate on mechanical properties are considered. Our results show that along with the increase in mean grain size came increases in Young's modulus, yield strength, flow stress, and ultimate tensile strength. The development of amorphous regions in the grain cores is the major deformation mode in polycrystalline CuZr SMAs with larger grain sizes, while the grain boundary sliding and grain rotation for smaller grain sizes. Besides, an increased temperature results in mechanical performance degradation and the temperature sensitivity of mechanical properties does not depend on the mean grain size. Our work would lay the groundwork for the optimization of the mechanical properties of polycrystalline SMAs as well as serving as a useful theoretical guideline for their practical engineering applications.

A phenomenological model for thermally-induced hysteresis in polycrystalline shape memory alloys with internal loops

In the current study, a 1-D phenomenological model is constructed to capture the temperature-induced hysteretic response in polycrystalline shape memory alloys (SMAs). The martensitic and austenitic transformations are regarded as the first-order transitions. A differential single-crystal model is formulated on the basis of Landau theory. It is assumed that the transformation temperatures follow the normal distribution among the grains due to the anisotropic stress field developed in the material. The polycrystalline hysteretic response is expressed as the integration of single-crystal responses. Besides, the prediction strategy for incomplete transitions is presented, and the first-order reversal curves are obtained via density reassignment. The proposed model is numerically implemented for validation. Comparisons between the modeling results and the experimental ones demonstrate the capability of the proposed model in addressing the hysteresis in thermally-induced phase transformations.

Shape memory effect in metallic glasses

Shape memory effect in metallic glasses

A micromechanics-based constitutive model for nanocrystalline shape memory alloys incorporating grain size effects

A micromechanics-based model is developed to capture the grain-size dependent superelasticity of nanocrystalline shape memory alloys (SMAs). Grain-size effects are incorporated in the proposed model through definition of dissipative length scale and energetic length scale parameters. In this paper, nanocrystalline SMAs are considered as two-phase composites consisting of the grain-core phase and the grain-boundary phase. Based on the Gibbs free energy including the spatial gradient of the martensite volume fraction, a new transformation function determining the evolution law for transformation strain is derived. Using micromechanical averaging techniques, the grain-size-dependent superelastic behavior of nanocrystalline SMAs can be described. The internal length scales are calibrated using experimental results from published literature. In addition, model validation is performed by comparing the model predictions with the corresponding experimental data on nanostructured NiTi polycrystalline SMA. Finally, effects of the internal length scales on the critical stresses for forward and reverse transformations, the hysteresis loop area (transformation dissipation energy), and the strain hardening are investigated.

Martensitic transformation in a two‐dimensional polycrystalline shape memory alloys using a multi‐phase‐field elasticity model based on pairwise rank‐one convexified energies at small strain

AbstractFor modeling the solid‐solid transformation using the phase‐field method, we need to describe the phase‐fields strain and stress fields in the interfacial regions which are necessary to define the mechanical driving force density of the phase‐fields motion. Seeking quantitative modeling, we introduce briefly a multi‐phase‐field elasticity model based on the rank‐one convexification of the pairwise energies which satisfies the pairwise interfacial static equilibrium and kinematic compatibility conditions. As an application, we simulate the martensitic transformation in a two‐dimensional polycrystalline SMAs at small strains. The finial evaluated microstructure consists of stress‐free laminates meeting the theoretical expectations.

An experimentally-fitted thermodynamical constitutive model for polycrystalline shape memory alloys

&lt;p style='text-indent:20px;'&gt;A phenomenological model for polycrystalline NiTi shape-memory alloys with a refined dissipation function is here enhanced by a thermomechanical coupling and rigorously analyzed as far as existence of weak solutions and numerical stability and convergence of the numerical approximation performed by a staggered time discretization. Moreover, the model is verified on one-dimensional computational simulations compared with real laboratory experiments on a NiTi wire.&lt;/p&gt;

Thermo-mechanical Characterization of an Aged Ni45.3Ti39.7Hf10Pd5 Shape Memory Alloy

Thermo-mechanical characterization of a Ni45.3Ti39.7Hf10Pd5 polycrystalline shape memory alloy after aging at 550°C for 3 h was conducted. Thermal cycling under constant stress, superelasticity and loading till failure tests were employed to reveal the shape memory and material properties. Recoverable shape memory strain of 3.8% was achieved under compressive stress of 500 MPa. The sample showed almost perfect superelasticity above 70°C with total recoverable strain of about 4%, but failed at 2340 MPa with compressive strain > 13% at 35°C, and aging was not found to significantly affect its shape memory behavior because of the low volume fraction of precipitates.

ACCOUNTING STRAIN INCOMPATIBILITY IN THE NANOSTRUCTURAL MODEL OF SHAPE MEMORY ALLOY MECHANICAL BEHAVIOR

Within the framework of the previously developed nanostructural model of deformation of polycrystalline shape memory alloys (SMAs), a refined expression for the thermodynamic potential is introduced by adding terms that take into account the incompatibility caused by phase-structural deformation of martensite. The terms of two types were used to take into account the strain incompatibility both between martensite formations and the austenite matrix and between differently oriented martensite formations within the martensite phase. It is shown that taking into account the strain incompatibility affects the temperature intervals of phase transitions. Based on the experimental data, the material functions included in the above-mentioned additional terms are determined.

An Asymmetric Elasto-Plastic Phase-Field Model for Shape Memory Effect, Pseudoelasticity and Thermomechanical Training in Polycrystalline Shape Memory Alloys

We propose an elasto-plastic phase-field model (PFM) to conduct the first microscopic computational study of shape memory effect (SME), pseudoelasticity, stress assisted two-way memory effect (SATWME), and thermomechanical training of CuAlBe shape memory alloy (SMA). This non-isothermal PFM model considers the effects of temperature dependent properties, latent heat, grain boundaries, and asymmetric transformation and plasticity. PFM simulations demonstrate the capacity of our model to capture the thermomechanical and purely mechanical shape recovery in the SMA. When considering transforming grain boundaries, grain refinement generates a higher transformation stress, a steeper transformation hardening, and smaller hysteresis loops for both SME and pseudoelasticity. The results also point out slightly higher transformation stress when geometrical grain boundaries are used. The simulations of SATWME highlight an augmentation of the residual martensite as the hold stress increases, which is consistent with experimental observations. This is the first PFM that can mimic the thermal training within several SATWME cycles, showing an asymptotic increase of plastic strain and the related retained transformation strain until the fourth cycle, and their stabilization thereafter. The activation of plasticity occurs always after initiation of phase transformation. Although plasticity results in more stress relaxation, it deteriorates the shape recovery for both SME and pseudoelasticity by hindering the reverse transformation. Comparison between tension and compression demonstrates the capacity of this PFM to account for, for the first time, the nonsymmetrical transformation and plastic responses of SMAs.

Comparisson of Recrystallization Kinetics and Grain Growth in Polycrystalline Shape Memory Alloys

Comparisson of Recrystallization Kinetics and Grain Growth in Polycrystalline Shape Memory Alloys

Anisotropy and Clausius-Clapeyron relation for forward and reverse stress-induced martensitic transformations in polycrystalline NiTi thin walled tubes

Inside the Clausius-Clapeyron regime, transformation stresses during superelastic tensile tests of polycrystalline shape memory alloys are linearly dependent on temperature, with coefficients being the slopes of the forward and reverse transformation lines. In this work, experiments are performed to investigate the anisotropy of the slopes of the forward and reverse transformation stress-temperature lines in a NiTi superelastic thin walled tube. The classical Clausius-Clapeyron relation is widely used to model these slopes, although, in a strict sense, this relation is defined at thermodynamic equilibrium. Experimental results disagree with the widely used classical Clausius-Clapeyron relation in two points: (i) that there should be no difference between slopes for forward and reverse transformations and (ii) that the products of the slopes by the transformation strains should not depend on orientation, since the remaining terms (mass density and entropy change) are not orientation dependent. A modified “Clausius-Clapeyron” relation is then proposed, better suited to model the anisotropy of the slopes of stress-temperature transformation lines of forward and reverse in superelastic NiTi. This modification is based on a unified thermodynamic theory of thermoelastic martensitic transformation in which irreversible energies are accounted as a sum of stored elastic energy and dissipated energy. The modified “Clausius-Clapeyron” relation is obtained by expressing that this irreversible energy is temperature dependent and that this temperature dependence is dependent on the orientation.