Behavior Of Shape Memory Alloys Research Articles

Correction to: Modeling of Superelastic–plastic Behavior of Porous Shape Memory Alloys Incorporating Void Shape Effects

Correction to: Modeling of Superelastic–plastic Behavior of Porous Shape Memory Alloys Incorporating Void Shape Effects

A Comparative Study on Deformation Behaviour of Superelastic NiTi with Traditional Elastic–Plastic Alloys in Sub-micron Scale

A nanoindentation-based study is conducted to analyse the small-scale deformation behaviour of superelastic NiTi-based shape memory alloy. The mechanism is compared with respect to that for traditional elastic–plastic ferrous and non-ferrous alloys: ferritic stainless steel of grade 409 and Al. To develop a comprehensive insight into the deformation behaviour, various experiments including nanoindentation, optical microscopy, X-ray diffraction, differential scanning calorimetry are performed on all alloy systems. This detailed study demonstrates that structural properties are primarily controlled by the crystal structures of the phases present in the respective alloy systems. Among these three studied alloys, the highest and lowest nano-hardness is realized for the steel and Al, respectively. Intermediate hardness is noted for the NiTi alloy. Nevertheless, the localized deformation characteristics of the superelastic alloy appeared to be entirely different in relation to that for the ferrous and non-ferrous materials. Most importantly, a fourfold increment in the depth recovery is realized for the NiTi alloy in contrast to the traditional metallic system. To understand this exceptional recovery, detailed analysis is performed on the unloading response of all the alloys. The crucial role of reversible stress-induced martensitic transformation in contributing towards the depth recoverability for superelastic NiTi alloy is thereby revealed.

A finite beam element multifunctional fabric modeling framework demonstrated with SMA knitted actuators

Multifunctional textiles have gained recent attention due their intrinsic properties that provide actuator, energy dampening, or sensory capabilities within small form factor textiles without external attachments. Such technologies are specifically appealing for medical and aerospace wearables, where active compression, haptic feedback, or the tracking of bodily functions are important tasks that are ideally conducted in a minimally-intrusive fashion. Current design capabilities of multifunctional textile are limited as present predictive tools lack accuracy and universality. This paper presents a finite beam element modeling tool for shape memory alloy (SMA) knitted architectures. The temperature-dependent variation of material properties within the SMA knitted loop affects the macroscopic force-extension behavior of SMA knitted architectures leading to an actuated and a relaxed knitted architecture response. This difference is exploited as the active property in SMA knitted architectures. The modeling architecture defines interfaces between sub-models organized in modules, specifically the material constitutive module, repetitive unit cell module, manufacturing module, contact module, and a boundary condition module. The SMA knitted architecture is modeled utilizing a 1D SMA constitutive model, quarter loop knit unit cell, a differential geometry-based manufacturing model, while assuming 3D Coulomb friction conditions. Kinematically-suitable boundary conditions are applied and the simulation predictions are compared quantitatively to macroscopic tensile experimental results, as well as qualitatively to microscopic x-ray diffraction phase analysis. The verification against experimental data supports the ability of the modeling tool to accurately predict the SMA knitted architecture thermo-mechanical performance with mean force-extension errors of less than 5%. The modeling tool provides the basis to understand, design, and optimize the lightweight, large force and deformation SMA knitted actuator textiles for novel applications. Additionally, the multifunctional textile modeling tool is implemented based on highly interchangeable sub-models to create synergies and propel the modeling of any multifunctional textile.

Orientation-dependent plastic deformation mechanisms and competition with stress-induced phase transformation in microscale NiTi

Understanding the orientation-dependent deformation behavior of NiTi shape-memory alloys at small length scales is of importance for designing nano- and micro-electromechanical systems. However, a complete understanding of the orientation- and size-dependent competition between the various modes of slip, deformation twinning, and martensitic transformation in NiTi shape-memory alloys is still lacking, especially in micron-scale specimens. In the present study, we perform micro-compression tests on [001]- and [112]-oriented micro-pillars of a solutionized Ti-49.9at.% Ni alloy. Post-mortem TEM analysis of the deformed pillars reveal that the operating plastic deformation modes are {011}<100> slip and {114}<221¯> deformation twinning, which compete with the martensitic transformation, depending on the crystal orientation. Furthermore, in both experiments and molecular dynamics simulations, we consistently find residual B19′ martensite in a herringbone microstructure composed of finely spaced (001)B19′ compound twins instead of the generally assumed [011]B19′ type II twins common in bulk samples, suggesting that the operative martensitic transformation mode may be size-dependent. Schmid factors in compression are calculated for all commonly reported slip, deformation twinning, and martensitic transformation modes as a function of crystallographic orientation, which rationalize the orientation-dependent competition between these deformation modes.

Cyclic pseudoelastic training and two-way shape memory behavior of a NiTi alloy with small irrecoverable plastic strains:numerical modeling

Novel applications of shape memory alloys (SMAs) in various fields of medicine and engineering necessitate theoretical models that can capture non-inherent material behaviors such as the two-way shape memory effect (TWSME). Most often, this requires the use of completely different sets of models or parameters to distinguish the “virgin” material exhibiting the fundamental behaviors like superelasticity (also known as pseudoelasticity), pseudoplasticity, and one-way shape memory effect, from the “trained” alloy showing the TWSME. Although using a single model (or set of parameters) can provide much insight on the changes within the material during various loading paths and histories from the initial virgin state to the trained state, it is quite challenging. In this study, an attempt was made to use a developed general multi-mechanism material model and a single set of parameters to predict both the inherent pseudoelastic behavior and non-intrinsic two-way shape memory effect characteristics of a NiTi shape memory alloy. To this end, several simulations were carried out to investigate the effect of pseudoelastic training conditions on the corresponding actuation strain, cyclic stability, and transformation temperatures of the NiTi under zero-load TWSME. Whilst previous studies considered external indicators such as the magnitude of plastic or irrecoverable strain as a prerequisite for TWSME, the present model focused primarily on internal stresses. Based on the magnitude of the zero-load actuation strain (measured from the trained material under no external load), the model results indicated that the magnitude of the pseudoelastic training strain (in comparison to the training temperature, and the number of training cycles) has a pronounced effect on the cyclic TWSME actuation characteristics. Moreover, observations from the internal state (hidden) variables of the model, representing the intrinsic residual stress state in the trained material (under zero observable/external stress) showed the possibility for an SMA to generate TWSME even if there is no significant accumulation of permanent strains.

Modeling of Superelastic–plastic Behavior of Porous Shape Memory Alloys Incorporating Void Shape Effects

A new constitutive model for describing the superelastic–plastic behavior of porous shape memory alloys (SMAs) is proposed. The model incorporates the influences of void shape and hydrostatic pressure as well as the elastic modulus mismatch between austenite and martensite. In addition, the interactions between plastic strain and transformation strain are considered via the plastic back stress. The porous SMAs are considered as two-phase composites with the dense SMA matrix and the second phase representing ellipsoidal voids. Based on Gurson’s formulation, the transformation and plastic flow potentials accounting for the transformation–plasticity coupling are developed. The numerical results present good agreement with available experimental data for various levels of porosity, which proves that the model is capable of capturing stress-induced phase transformation and plastic deformation of porous SMAs. Using the proposed model, the influence of plastic strain on reverse transformation and the effects of porosity and void shape on the pseudoelastic and plastic behavior of porous SMAs are investigated.

The Effects of Chemical Oxidation on Corrosion Behavior of Ni-Ti Alloy

Nickel Titanium intermetallic (NiTi) has been used in several fields (mostly corrosion issues were not a concern) since it was discovered about sixteen years ago. In the last decade, alloys made from NiTi have been widely used in the internal and external biomedical tools and equipment such as nails, fixation plates, bone fracture, self-expanding cardiovascular and urological stents, and orthodontic wires. The effects of chemical oxidation on corrosion behavior of Nickel-Titanium shape memory alloy have been investigated in this study. Technique of powder metallurgy was used to prepare the alloy from the elemental powders of Titanium and Nickel with 600 MPa of compacting pressure. The sintering process was achieved in a 10−4 torr vacuum atmosphere at 950 °C . XRD analysis exhibited that Ni and Ti were fully transformed into NiTi (both monoclinic and cubic phase) and Ni3Ti phase. After sintering, the samples are grinded and polished, then samples were surface modified by chemical oxidation. Optical microscopy, scanning and x-ray diffraction techniques were utilized for characterizing the surface samples. The corrosion rate has been studied in vitro by using polarization curves technique in NaF solution at different concentration (0.5, 1,1.5, 2) wt,%. Results of corrosion test indicated that the corrosion current density after chemical oxidation decreased from 0.303 for bare sample to 0.074 in 0.5% NaF solution, which indicates that TiO2 layer on the NiTi SMA surface are protective after chemical oxidation.

Effect of ternary additions of Cu and Fe on the hot deformation behavior of NiTi shape memory alloy - A study using processing maps

The addition of ternary alloying elements Cu and Fe on the hot workability of binary NiTi SMA is assessed using the processing maps approach. Uniaxial compression test data obtained in the temperature, T, range 800–1050 °C and strain rate, ε˙, of 10−3 – 10+1 s−1 were utilized to construct power dissipation, instability and strain rate sensitivity maps at true strain, ε = 0.5. These maps and complementary microstructural analyses of the deformed specimens were used to identify various deformation mechanisms operating in different T-ε˙ regimes. Based on the processing maps, three stable domains for processing were identified with Domain-2 occurring at higher T and intermediate ε˙ with power dissipation efficiency of 30–45% being the optimum for thermo-mechanical processing. The addition of Cu in NiTi SMA improves its hot workability through enlarging stable domains for processing whereas Fe addition shrinks it. Microstructural analysis indicates occurrence of dynamic recrystallization (DRX) in NiTi, NiTiFe and dynamic recovery (DRV) in NiTiCu as the dominating deformation mechanism in Domain-2. Sluggish DRX kinetics in NiTiCu SMA in Domain-2 is related to its higher deformation resistance and higher critical stress required for the onset of DRX vis-à-vis NiTi, NiTiFe SMAs. Instability maps corroborated through microstructural analysis showed instability in the form of flow localization.

Effect of Stress Aging Induced Precipitates on Corrosion Behavior of NiTi Shape Memory Alloys

In this study, the influences of the stress aging process on the electrochemical behaviors toward evaluating corrosion resistance of NiTi shape memory alloys in the in-vitro condition have been investigated. For this approach, the samples have been manufactured by introducing multiple precipitation morphology in the alloy structure via applying different levels of stresses during the aging process. The samples were characterized using multiply electron microscopy, electrochemical methods, X-ray diffraction, and differential scanning calorimetry. Results show that by prolonging aging time from 1 to 5 h and increasing the stress aging level (15–60-150 MPa) the corrosion resistance improves, which is implied a better formation of a protective layer. It seems that homogeneous precipitation of Ni-rich phases under the stress aging process improves the corrosion resistance of the alloy.

A thermomechanical constitutive model for porous SMAs accounting for the influence of void evolution

A new thermomechanical constitutive model is proposed for shape memory alloys (SMAs) with randomly distributed microscopic porosity. The influence of the microscopic voids on SMA behavior is accounted for by means of two scalar variables representing damage due to porosity and inelastic dilatation due to the expansion or shrinking of voids. Starting with a newly formulated expression of the Helmholtz free energy density, a general stress-strain relation is derived for porous SMAs. The relation features internal dissipative variables for which thermodynamically consistent flow rules are proposed. In particular, loading functions are obtained for variables representing the volume fraction of martensite, inelastic strain due to martensite detwinning and reorientation, as well as damage and inelastic dilatation caused by the presence and evolution of voids. Appropriate loading conditions and flow rules are then introduced in a way that satisfies the principles of thermodynamics. The model is numerically integrated using a multi-surface return mapping algorithm implemented in a finite element software. The integration procedure, including the formulation of the required time-discrete equations and the derivation of the material Jacobian are presented in detail. The developed model is validated by two sets of experimental results and one modeling work to good agreement. The model is then used for simulating reference stress-strain data taken from the literature, in addition to test cases showcasing the ability of the model to simulate complex structures in presence of non-proportional loading and strong stress gradients.

Finite element and experimental structural analysis of endodontic rotary file made of Cu-based single crystal SMA considering a micromechanical behavior model

Shape Memory Alloys (SMAs) are widely used in endodontics as instruments for root canal preparation namely endodontic rotary files. Despite their high performances, compared to stainless steel instruments, it is still possible to enhance their cutting efficiency by acting on their shape and material properties. As an example, increasing the exhibited reversible martensitic transformation strain makes them more flexible without decreasing their mechanical strength. It becomes possible with single crystal SMAs. Cu-based (Cu-Al-Be, Cu-Zn-Al, Cu-Al-Mn) single crystal SMAs start emerging and can reach about 12% of martensitic transformation strain, in addition to their interesting antimicrobial properties. This study investigates, both numerically and experimentally, the development of endodontic files made of Cu-based single crystal SMAs. The numerical analysis is carried out by the finite element method. The geometry of the instrument is parameterized. Depending on the applied boundary conditions, bending, torsion, or combined bending-torsion loadings are represented. A micromechanical constitutive law is implemented in Abaqus via the UMAT subroutine to describe the thermomechanical behavior of the Cu-based single crystal SMA. Following the thus obtained numerical results, Coltene-Micromega company has manufactured endodontic file prototypes made of Cu-Al-Be single crystal SMA. A specific setup applying the same boundary conditions of torsion-bending loading is used to characterize these SMA file prototypes. The good agreement between experimental and numerical responses, for a combined bending-torsion loading, proves the relevance of the proposed approach and the pertinence of considering Cu-based SMAs for endodontic file applications.

Experimental study on Fe-SMA-to-steel adhesively bonded interfaces using DIC

This study aimed to reveal the interface behavior of iron-based shape memory alloy (Fe-SMA) strips bonded to metallic substrates in order to ensure the integrity of such joints. A series of single lap-shear tests was performed on Fe-SMA-to-steel adhesively bonded joints. The test results reveal that the bond strength can reach over 70% of the tensile strength of the Fe-SMA and the cohesive failure mode can be realized. It is identified that three energy dissipation mechanisms occurred during the debonding process, indicating that the experimentally obtained bond-slip relations represent more than the energy dissipation by the failure of the adhesive.

Influence of Ni4Ti3 precipitation on martensitic transformations in NiTi shape memory alloy: R phase transformation

Precipitation of Ni4Ti3 is effective in tuning martensitic transformations (MTs) in NiTi shape memory alloys (SMAs). However, several fundamental issues concerning the influence of Ni4Ti3 on MTs remain unclear. In this study, the microstructure evolution process during precipitation and its influence on B2→R MT are investigated using computer simulations based on the phase field method. In particular, the roles played by Ni concentration gradient in the B2 matrix and internal coherency stress fields associated with different precipitate microstructures are analyzed in detail. It is found that Ni concentration gradient in the B2 matrix created by Ni4Ti3 precipitates alters significantly local martensitic start temperature and the overall MT behavior, while the internal coherency stress field associated with the Ni4Ti3 precipitates dictates the structure of martensitic domains via variant selection. The latter effect makes it promising to develop Invar alloys via stress-ageing to tailor the precipitate microstructure. These findings provide fundamental insights into the mechanism controlling the MT behavior in precipitate-bearing SMAs and provide guidance for the design of thermomechanical treatment for desired precipitate microstructures and the corresponding MT behavior.

Deformation Behavior, Structure, and Properties of an Aging Ti-Ni Shape Memory Alloy after Compression Deformation in a Wide Temperature Range

The deformation behavior, microstructure, phase composition, and mechanical and functional properties of Ti-50.9 at.%Ni shape memory alloy during uniaxial compression in the temperature range from 25°C to 1000°C have been analyzed and are discussed herein. It was found that the deformation temperature of 300°C marked a boundary for the transition from the low- to high-temperature type of flow curves; achievement of the steady-state deformation stage was observed across a wide range of deformation temperatures. Following comprehensive analysis of the obtained data, the temperature ranges of the dynamic processes of recovery, polygonization, and recrystallization of the Ti-50.9 at.%Ni alloy were determined. Deformation in the range of dynamic polygonization is accompanied by not only the formation of B2-austenite and R-phase, but also the precipitation of fine Ti3Ni4 particles during deformational aging. The highest shape recovery characteristics were obtained after deformation of the Ti-50.9 at.%Ni alloy in the temperature range from 300°C to 600°C.

Effect of Hatch Spacing on Phase-Transformation Behavior and Mechanical Properties of NiTi Shape Memory Alloy Fabricated Using Selective Laser Melting

Effect of Hatch Spacing on Phase-Transformation Behavior and Mechanical Properties of NiTi Shape Memory Alloy Fabricated Using Selective Laser Melting

Influence of alloying elements on the thermal behavior of NiTi shape memory alloys

NiTi based shape memory alloys are one of the most intensely studied alloys from its class. Therefore, diverse commercial applications have been developed due to certain properties such as: shape memory effect, superelasticity and corrosion resistance. Currently, the main applications of NiTi alloys are automotive manufacturing and aerospace actuators, biomedical devices or pipe couplings. In recent years, NiTi shape memory alloys have been alloyed with a third element in order to improve the above-mentioned properties. In order to investigate the influence of the alloying elements on the thermal behavior of NiTi alloys, the addition of the third alloying element (Ta and Nb) is under investigation in the present study. The thermal behavior of the three alloys (NiTi, NiTiNb and NiTiTa) was studied by differential scanning calorimetry. Following the experiments, it was observed that the addition of the third alloying element influences the critical transformation temperatures.

Thermomechanical modeling of nonlinear internal hysteresis due to incomplete phase transformation in pseudoelastic shape memory alloys

This paper presents a thermomechanical model for pseudoelastic shape memory alloys (SMAs) accounting for internal hysteresis effect due to incomplete phase transformation. The model is developed within the finite-strain framework, wherein the deformation gradient is multiplicatively decomposed into thermal dilation, rigid body rotation, elastic and transformation parts. Helmholtz free energy density comprises three components: the reversible thermodynamic process , the irreversible thermodynamic process and the physical constraints of both. In order to capture the multiple internal hysteresis loops in SMA, two internal variables representing the transition points of the forward and reverse phase transformation, $$\phi _s^f$$ and $$\phi _s^r$$ , are introduced to describe the incomplete phase transformation process. Evolution equations of the internal variables are derived and linked to the phase transformation. Numerical implementation of the model features an Euler discretization and a cutting-plane algorithm. After validation of the model against the experimental data, numerical examples are presented, involving a SMA-based vibration system and a crack SMA specimen subjected to partial loading–unloading case. Simulation results well demonstrate the internal hysteresis and free vibration behavior of SMA.

Numerical investigation of reinforced-concrete beam-column joints retrofitted using external superelastic shape memory alloy bars

&lt;abstract&gt; &lt;p&gt;The unique properties of Shape Memory Alloys (SMAs) have motivated researchers to use them as primary reinforcement in reinforced concrete (RC) structures. In this study, the applicability of using external unbonded SMA bars to retrofit RC beam-column joints (BCJs) is investigated. A three-dimensional finite element model, which simulates the suggested retrofitting technique, is first developed, and validated using ABAQUS software. The model is then further simplified and utilized to conduct a parametric study to investigate the behaviour of SMA retrofitted RC BCJs. Results of the parametric study are used to perform multiple linear regression analysis. Simple equations, which can be used to calculate the length and amount of SMA bars required to retrofit a RC BCJ, are then developed.&lt;/p&gt; &lt;/abstract&gt;

Compression after high-velocity impact behavior of pseudo-elastic shape memory alloy embedded glass/epoxy composite laminates

The compression after high-velocity impact (CAI) behavior of pseudo-elastic shape memory alloy (PE-SMA) embedded glass/epoxy composite laminates manufactured via vacuum-assisted resin infusion (VARI) technique was investigated experimentally. High-velocity impact tests were performed on the glass/epoxy composites using an in-house build gas-gun set-up at four different velocities (70, 85, 95, and 105 m/s). To elucidate, the series of damage events undergone on the composites under the high-velocity impact, a high-speed camera was used. Quasi-static compression-after-impact (CAI) tests were performed to evaluate the damage tolerance of different composites subjected to high-velocity impact. Real-time damage progression during the CAI as well as the extent of damage was characterized by using a 2D Digital Image Correlation (DIC) technique. Three different types of embedment configurations of PE-SMA wire, namely, with length variation (35, 70, and 150 mm) and also with and without anchors, were used. The impact test results show that the ballistic limit of anchored PE-SMA composites with an embedment length of 35 mm is 72.72% higher than that of the homogeneous glass/epoxy composites. CAI results indicate that the PE-SMA based composites have better damage tolerance with respect to the homogeneous ones at 70 m/s. In contrast, at higher velocities, the delamination area decreases in the homogenous glass/epoxy composites due to more localized damage and present a higher compressive strength.

Experimental behavior of iron-based shape memory alloys under cyclic loading histories

The present paper investigates the behavior of iron-based shape memory alloys (Fe-SMAs) subjected to cyclic inelastic straining by means of uniaxial coupon experiments. The tests feature round bar coupons subjected to a broad range of uniaxial cyclic strain histories representative of earthquake loading. The experimental results suggest that the Fe-SMA under investigation exhibits an asymmetric stress-strain relation, with limited superelastic behavior. It was found that the post-yield/phase transformation behavior of the Fe-SMA alloy is both strain-rate and temperature-dependent. Quantitative comparisons with structural steels subjected to nominally identical cyclic strain histories indicate that, although the studied Fe-SMA has a similar energy dissipation per loading excursion with respect to conventional S355J2+N, the Fe-SMA’s hardening response is appreciably higher, leading to comparatively larger elastic strain energies being stored.