Deformation Of Shape Memory Alloys Research Articles

Структурно-имитационная модель деформирования сплавов с памятью формы. Часть 2. Описание эффекта реверсивной памяти формы

Within the framework of a one-dimensional structural-simulation model for deformation of shape memory alloys during thermoelastic phase transformations, an adequate description of the process of changes in strains during direct transformation under the influence of reverse stresses and the effect of reverse shape memory has been performed

Tension induced detwinning of hierarchically twinned non-modulated martensite in Ni-Mn-Ga alloys

Detwinning of martensite accounts for the macroscopic deformation in shape memory alloys, yet the corresponding microstructural and crystallographic evolution remains less understood. Here, the detwinning behaviors of hierarchically twinned non-modulated (NM) martensite in a directionally solidified Ni54Mn24Ga22 alloy under uniaxial tension along the <001>A direction of austenite are systematically investigated. Based on the interrupted in-situ EBSD measurements, it is demonstrated that tensile loading leads to the thickening of favorable variants with the <001>NM parallel to the loading direction through intensive detwinning, which follows the route of detwinning of internal nanotwins as well as collective detwinning of internal nanotwins and inter-plate major-major variant pairs. Besides, the internal detwinning is also coupled with the rigid body rotation owing to the constraints of inter-plate boundaries, resulting in the refinements in the inter-plate correlation as the variation of thickness ratio of nanotwins. Consequently, the hierarchically twinned microstructure finally evolves into a single-variant state with the <001>NM along the loading direction. This work provides clear demonstrations on the detwinning behaviors of hierarchically twinned NM martensite induced by tensile loading, which allows us to gain deep crystallographic insights into the stress-induced rearrangement of martensite variants in association with the macroscopic deformation.

Microscale investigation of phase transformation and plasticity in multi-crystalline shape memory alloy using discrete dislocation–transformation method

Martensitic phase transformation and plasticity are two primary mechanisms of deformation in shape memory alloys (SMAs) and the interaction between them influences the behaviour of SMA during cyclic loading, specifically the pseudoelasticity behaviour and the shape memory effect. This interaction, which occurs in microscale, affects the reversibility and eventually the actuation capacity of SMAs. In order to capture this interaction in microscale, a discrete dislocation–transformation model was developed in Sakhaei et al. (Mech Mater 97:1–18, 2016) and was applied to simulate the single-crystalline NiTi samples under thermo-mechanical loads. In this study, the microscale coupling between phase transformation and plasticity as well as grain size and orientation effects is investigated in multi-crystalline shape memory alloys under thermal and mechanical loading by using the discrete dislocation–transformation framework through the representative numerical simulations. The results illustrated the dependency of dislocation slip and martensitic transformation to crystalline orientations as well as grain size and grain boundary densities in the multi-crystalline SMAs.

Model for the Inelastic Deformation of Shape Memory Alloys

Model for the Inelastic Deformation of Shape Memory Alloys

Актуатор с последовательным соединением стержня из сплава с памятью формы и упругого элемента смещения

One of the most promising applications of shape memory alloys (SMA) is their use for creating working bodies of power exciters (actuators). The working body of the actuator must perform certain movements due to shape memory phenomena (heating, working stroke of the actuator) and the accumulation of direct transformation deformations (cooling, idling of the actuator). It is known that the process of deformation of SMA during cooling occurs only in the presence of mechanical action, while the return to the original form during heating occurs in the absence of a corresponding mechanical action and even in the presence of a sufficiently large counteraction. To ensure the possibility of not only working, but also idling of the actuator, the working body of the SMA is connected to the elastic bias element so that both of these elements are deformed together. In this case, when the SMA element is heated, it is deformed due to the shape memory phenomenon, which leads to the deformation of the bias element associated with the working body and the appearance, both in this element and in the working body, of mechanical stresses that increase with the temperature of the SMA element. It is these stresses, which continue to act during the cooling of the working body, that ensure its deformation in the opposite direction at the stage of cooling of the working body and the corresponding direct transformation into SMA. In this paper, the behavior of an actuator consisting of a SMA rod and an elastic rod (an bias element) connected in series, the total length of which is assumed to be unchanged, is analytically investigated. The influence of the system parameters on the stresses in the SMA rod and the value of the working stroke of the actuator is investigated. The conditions for the implementation of the closed two way shape memory effect in this system are determined.

Joint Model for the Phase-Structural Deformation of Shape Memory Alloys

Joint Model for the Phase-Structural Deformation of Shape Memory Alloys

Модель неупругого деформирования сплавов с памятью формы

Предложен вариант модели фазово-структурного деформирования сплавов с памятью формы, в рамках которого параметром изотропного упрочнения для процесса деформирования по структурному механизму является интенсивность собственной фазово-структурной деформации мартенситной части представительного объема материала. Получено решение задачи об обратном термоупругом фазовом превращении в заневоленном стержне с предварительно заданной растягивающей или сжимающей деформацией. Исследовано влияние на решение значения параметра трансляционного упрочнения.

Ballistic Impact Behaviour of Glass/Epoxy Composite Laminates Embedded with Shape Memory Alloy (SMA) Wires.

This paper aims to estimate the enhancement in the energy absorption characteristics of the glass fiber reinforced composites (GFRP) by embedding prestrained pseudo-elastic shape memory alloy (SMA) that was used as a secondary reinforcement. The pseudo-elastic SMA (PE-SMA) embedded were in the form of wires and have an equiatomic composition (i.e., 50%–50%) of nickel (Ni) and titanium (Ti). These specimens are fabricated using a vacuum-assisted resin infusion process. The estimation is done for the GFRP and SMA/GFRP specimens at four different impact velocities (65, 75, 85, and 103 m/s) using a gas-gun impact set-up. At all different impact velocities, the failure modes change as we switch from GFRP to SMA/GFRP specimen. In the SMA/GFRP specimen, the failure mode changed from delamination in the primary region to SMA-pull out and SMA deformation. This leads to an increase in the ballistic limit. It is observed that energy absorbed by SMA/GFRP specimens is higher than the GFRP specimens subjected to the same levels of impact energy. To understand the damping capabilities of SMA embedment, vibration signals are captured, and the damping ratio is calculated. SMA dampens the vibrations imparted by the projectile to the specimen. The damping ratio of the SMA/GFRP specimens is higher than the GFRP specimens. The damping effect is more prominent below the ballistic limit when the projectile got rebounded (65 m/s).

Объединенная модель фазово-структурного деформирования сплавов с памятью формы

A combined model of inelastic deformation of shape-memory alloys for phase and structural transformations is proposed, taking into account the fundamental difference between these two mechanisms and the influence of the first mechanism on the second. In contrast to the known analogs, the model allows for non-monotonic loading processes to exceed the long arc of phase-structural deformation (an analogue of the Odqvist parameter in the theory of plasticity) of the intensity of crystallographic deformation of the phase transition.

Model for the Effect of the Phase Mechanism of Deformation on the Structural Mechanism in Shape Memory Alloys

A model is proposed for the deformation of shape memory alloys to describe the phase and structural mechanisms of changing the inelastic deformation and the influence of the former mechanism on the latter. The constitutive relations derived to describe the phase-transition-induced change in the strain do not use the concept of loading surface, and the model of the structural-transition-induced deformation is an analog of the theory of plasticity with isotropic hardening.

Phenomenological Simulation of the Phase and Structural Deformation in Shape Memory Alloys. One-Dimensional Case

Structural elements made of shape memory alloys (SMAs) undergo cooling/heating and stresses changing in value and direction during operation. As a result, phase and structural transformations, which are accompanied by the shape memory effect, cross hardening and martensitic inelasticity, occur in a material. Moreover, a change in the stresses causes a shift in the phase-transition temperatures, and forward and reverse phase transitions are possible during an isothermal increase or decrease in the load (superelasticity phenomenon). The purpose of this work is to develop a phenomenological model in terms of a general approach to take into account these phenomena, since they substantially affect the state of stress in a construction. This model is based on the relation between forward transformation and martensitic inelasticity diagrams, which implies a general description of the strains of phase and structural transformations. This approach seems to be useful, since both strain components are caused by the formation of oriented martensite. A set of series-connected martensitic structural elements, each of which has a specific structural transformation limit (initial stress), is introduced into consideration. This limit depends on the conditions of appearance of an element during a phase transition and the subsequent deformation history. This approach can take into account the influence of, first, phase deformation and structural deformation on each other and, second, the deformation history on the subsequent transformation. To demonstrate the possibilities of the model, we solve the problem of joint deformation of a stack of SMA rods, which illustrates the evolution of the state of stress in the system during the simultaneous phase and structural transformations induced by an external thermomechanical action.

Maximum deformation of shape memory alloy based adaptive fiber-reinforced plastics

In order to simplify and improve the existing kinematic functional range, the elimination of conventional joints and external drives as well as a reduction in the number of gear elements is required. This aim can be achieved by means of adaptive fiber-reinforced plastics (FRPs) due to their lightweight and intrinsic actuator properties. Hence, this research project presents adaptive FRPs including shape memory alloys (SMAs) that were structurally integrated into reinforcing fabrics using the open reed weaving technology. The functionalized preform for the formation of adaptive FRPs was varied by the number of interlacements with weft yarns, SMA length, and thickness ratio. For each variation, a hinged structure was developed in order to realize greater deformation. The adaptive FRPs were characterized thermo-mechanically. Results revealed that the maximum deformation of adaptive FRPs is proportional to SMA length and thickness ratio but inversely proportional to the number of interlacements.

The influence of thin film adhesives in pullout tests between nickel–titanium shape memory alloy and carbon fiber reinforced polymer matrix composites

Strips of nickel-titanium (NiTi) shape memory alloy (SMA) and carbon fiber-reinforced polymer matrix composite (PMC) were bonded together via thin film adhesives. Tensile and lap shear tests were conducted to confirm the deformation of SMAs and to evaluate the adhesive strength between the SMA and PMC. Scanning electron microscopy (SEM) was used to examine interfacial bonding after failure. ABAQUS models were generated to show the effects of enhanced bond strength and stress distribution. Results revealed that the addition of thin film adhesives increased the average adhesive strength between SMA and PMC while halting shape memory effect within the pullout specimen.

Модель влияния фазового механизма деформирования на структурный в сплавах с памятью формы

Модель влияния фазового механизма деформирования на структурный в сплавах с памятью формы

A new microplane model for non-proportionally multiaxial deformation of shape memory alloys addressing both the martensite transformation and reorientation

A new microplane model is established to predict the non-proportionally multiaxial deformation of pseudoelastic shape memory alloys (SMAs) by considering martensite transformation and reorientation. In each microplane, various martensite variants with certain shear directions are introduced. Adopting the static constraint formulation, the shear and normal stresses in each microplane are obtained through a macroscopic stress tensor. Thermodynamically-consistent kinetic equations of internal variables are derived based on the Clausius's dissipative inequality. Employing the principle of complementary virtual work, a macroscopic strain tensor is obtained by integrating the strains on the microplanes with all orientations. Comparisons between the simulated results and corresponding experimental ones show that the non-proportionally multiaxial deformations of pseudoelastic SMAs can be reasonably predicted. Moreover, the effects of the martensite reorientation and the number of martensite variants on the macroscopic stress-strain responses of pseudoelastic SMAs are discussed.

Экспериментальная идентификация модели нелинейного деформирования сплавов с памятью формы при фазовых и структурных превращениях

Экспериментальная идентификация модели нелинейного деформирования сплавов с памятью формы при фазовых и структурных превращениях

A novel variable-stiffness flexible manipulator actuated by shape memory alloy for minimally invasive surgery.

This article presents a novel variable-stiffness flexible manipulator for minimally invasive surgery. Each module of the proposed manipulator contains a variable-stiffness mechanism actuated by proactive deformation of shape memory alloy. Due to low driving current, apparent mechanical deformation, suitable phase transformation temperature and biocompatibility of shape memory alloy wire actuation, it is well suited for the manipulator applied in minimally invasive surgery, where variable stiffness is urgently required. In this article, the conceptual design, elastic modulus model, thermo-electric model, stiffness controlling method and finite element method simulation for a single module of the proposed variable-stiffness flexible manipulator are presented. Moreover, the memory shape setting experiment of shape memory alloy wire and fabrication of the single module are carried out. Finally, stiffness characterizations of the mechanism and the single module are studied separately, theoretically and experimentally.

Microstructural Model for the Deformation of Shape Memory Alloys

A model is proposed to describe the phase-structural deformation of shape memory alloys with allowance for the nonuniform strain hardening of the martensite part of representative volume. A scheme is developed to determine the volume fraction of martensite undergoing structural transformation during proportional nonreversible loading. The problem of reactive-stress generation in experiments on orientational transformation with constrained deformation after unloading is resolved.

Recent Progress on Modeling Slip Deformation in Shape Memory Alloys

This paper presents an overview of slip deformation in shape memory alloys. The performance of shape memory alloys depends on their slip resistance often quantified through the Critical Resolved Shear Stress (CRSS) or the flow stress. We highlight previous studies that identify the active slip systems and then proceed to show how non-Schmid effects can be dominant in shape memory slip behavior. The work is mostly derived from our recent studies while we highlight key earlier works on slip deformation. We finally discuss the implications of understanding the role of slip on curtailing the transformation strains and also the temperature range over which superelasticity prevails.

Large Anhysteretic Deformation of Shape Memory Alloys at Postcritical Temperatures and Stresses

Magnetic and nonmagnetic shape memory alloys (SMAs) exhibit thermoelastic martensitic transformations (MTs) which are hysteretic due to their first‐order nature. According to the thermodynamic Landau theory of phase transitions, which assumes ideal thermoelastic equilibrium at each point of the MT interval, the hysteresis is explained by the different limits of stability for austenite and martensite in the phase diagram. No interactions on the phase boundaries are taken into account. In the real alloys, the hysteresis of MT is related not only to the stability intervals of two phases but also to the processes of nucleation and growth of the resultant phase inside the parent phase. In turn, the features of these processes are related to the heights of energy barriers caused by the incompatibility of austenitic and martensitic lattices, crystal defects and some other physical factors. However, the defects, normally, play a minor role in the width of MT hysteresis if compared to the thermodynamic and crystallographic factors. A reduction of hysteresis of MT in SMAs, being crucial for technology, presents a challenging problem for science. A decrease of hysteresis width of MT was observed recently for the single crystals of ferromagnetic SMAs such Ni–Fe(Co)–Ga and Fe–Pd on approaching of their transformation paths to the critical point in stress–temperature phase diagram. Moreover, the superelastic and shape memory properties characterized by the nearly‐zero hysteresis width were observed in the postcritical transformational regime. Here we show that both the Landau‐type theory of ferroelastic phase transitions and neutron diffraction experiments carried out under axial compression describe the essential features of these properties. We also interpret the experimentally observed anhysteretic phenomena in Ni–Mn–Ga thin films and nanobeam actuators in terms of their postcritical state.