Shape Memory Alloy Sheet Research Articles

Pseudo-creep in Shape Memory Alloy Wires and Sheets

Interruption of loading during reorientation and isothermal pseudoelasticity in shape memory alloys with a strain arrest (i.e., holding strain constant) results in a time-dependent evolution in stress or with a stress arrest (i.e., holding stress constant) results in a time-dependent evolution in strain. This phenomenon, which we term as pseudo-creep, is similar to what was reported in the literature three decades ago for some traditional metallic materials undergoing plastic deformation. In a previous communication, we reported strain arrest of isothermal pseudoelastic loading, isothermal pseudoelastic unloading, and reorientation in NiTi wires as well as a rate-independent phase diagram. In this paper, we provide experimental results of the pseudo-creep phenomenon during stress arrest of isothermal pseudoelasticity and reorientation in NiTi wires as well as strain arrest of isothermal pseudoelasticity and reorientation in NiTi sheets. Stress arrest in NiTi wires accompanied by strain accumulation or recovery is studied using the technique of multi-video extensometry. The experimental results were used to estimate the amount of mechanical energy needed to evolve the wire from one microstructural state to another during isothermal pseudoelastic deformation and the difference in energies between the initial and the final rest state between which the aforementioned evolution has occurred.

Is it Possible to Use Rolling Methods to Improve Textures on Fe–Mn–Si Shape Memory Alloys?

No uniform rolling deformation produces shear strains that give rise to textural and microstructural heterogeneities in processed metals and alloys. In this work, the authors investigate Fe–30Mn–4Si shape memory alloy sheets rolled in different conditions at 600 °C, in order to determine the process giving rise to the best structure and the strongest {100}<110> shear texture. This crystallographic orientation is the most favorable for the γ → ϵ martensitic transformation, which provides the shape memory effect in these alloys. In the current conditions, the authors find that unidirectional rolling produces a shear texture in sheet's surface layers. The authors compare the texture and microstructure from this process to those obtained from reverse rolling and single‐roller drive rolling.

Development of a shape-memory-alloy micromanipulator based on integrated bimorph microactuators

This paper reports a novel structure of a shape-memory-alloy (SMA) micromanipulator with gripping mechanism. A featured integration of multiple SMA bimorph microactuators has been utilized to form a micromanipulator with three degrees of freedom. The design consists of two links (SMA sheets) and a gripper at the end of the second joint. The overall dimensions of the micromanipulator are 33mm×9mm×3mm. The displacement of each actuator is controlled by a heating circuit that generates a pulse-width modulation signal. Theoretical modeling of SMA actuators is studied and verified with simulation. The SMA micromanipulator is able to move in the x- and y-axis by 7.1mm and 5.2mm, respectively, resulting in a maximum displacement of 8.9mm. The micro-gripper has a maximum opening gap between its fingers of 1.15mm. The micromanipulator has a temporal response of 7.5s and 9s for its x- and y-axis. The maximum actuation force generated by the x- and y-axis was around 100mN and 130mN, respectively. The developed micromanipulator has been successfully used to move a small object.

Observation on rate-dependent cyclic transformation domain of super-elastic NiTi shape memory alloy

Formation and evolution of transformation domains in the super-elastic NiTi shape memory alloy (SMA) subjected to the strain- and stress-controlled cyclic tension-unloading tests were observed by using a non-contact digital image correlation method. The effect of loading rate on the transformation domains of NiTi SMA sheet was investigated. It was found that the number of transformation domains increases monotonically with the increasing loading rate. At relatively low loading rate, the deformation mode changes from a localized one to a homogeneous one with the increasing number of cycles. The residual martensite domains occur in the cyclic tests of the NiTi sheet at a high loading rate due to the hindrance of an increased dislocation density to the reverse transformation from the martensite to austenite phases. Moreover, the strain and temperature distributions obtained in the stable stages of stress-controlled cyclic tests are more uniform than that in the strain-controlled ones. The competition between transformation heat production and heat exchange results in the rate-dependent transformation domain, which reaches an equilibrium state in the stable cycle.

Abnormal grain growth induced by cyclic heat treatment in Fe-Mn-Al-Ni superelastic alloy

Abnormal grain growth (AGG) and related microstructural features were investigated for Fe-Mn-Al-Ni shape memory alloy sheets subjected to thermal cycling through α (bcc)→α+γ (fcc)→α phase transformations by means of microstructural observations including the electron backscatter diffraction (EBSD) technique. In the initial cooling from the α single-phase to the α+γ two-phase region, a high density of subgrain boundaries with small angle deviation of about 1° through 2° is formed around the γ precipitates in the α matrix. The subgrain structure remains in the α single phase even after dissolution of the γ precipitates by heating, and some limited grains grow faster by encroaching on neighboring grains, including the subgrains, resulting in AGG. The maximum grain size after the AGG becomes extremely large when annealing in the α+γ two-phase region is performed at temperatures below 1000°C, and a single crystal over 30mm in length can be obtained by repeating the cyclic heat treatment. It is found that the boundary energy of the subgrains dominantly contributes to the AGG as the driving force.

Modeling and analysis of laminated shape memory alloy composite plates

In the present work, static deformation, dynamic characteristic, and time response of laminated composite plates with surface-bonded shape-memory alloy (SMA) sheets are modeled. A modified higher order shear deformation theory with Ritz method is utilized. The energy balance equations are used to formulate the nonlinear heat transfer governing equations. The time response of the SMA/elastomer plate and the deflection as well as dynamic characteristics before and after activation of the SMA is computed. A Mathematica code is developed to analyze different plate problems. Parametric studies are performed for proposed structure system to demonstrate the effect of thickness ratio, aspect ratio, material properties, thermal expansion coefficient, electric input power, and thickness of SMA sheet on the transverse deflections, natural frequencies, and response time of the SMA layer. The obtained results are compared to the available studies solved by different theories.

Investigation on the Laser Based Actuation of Single way Trained SMA Sheet and their Application for the Development of Micro Positioning Stage

Shape memory alloy (SMA) is an intermetallic compound able to recover, in a predetermined shape during a thermal cycle while generating mechanical work. The primary objective of this research is to develop a shape memory alloy based micro-positioning stage. This paper reports about the training, actuation method of NiTi shape Memory alloy thin sheets and its displacement measurement through Michelson Interferometer. The SMA based Tripod was fabricated the actuation was done using an electrical biasing with 5 A(1.2 Volt) where a displacement of around 6.10 mm was achieved. The actuation studies were also performed using a laser with a optimum fluence of 100 mJ/cm2 that is of upto7.25 mm achieved.

Wavelets Image Analysis for Friction Stir Processed TiNi Functional Behavior Characterization

A key topic regarding Ti Ni Shape Memory materials concerns the possibility to attain welded junctions that preserves the shape memory properties of material. Other research topic for SMAs regards the retention of the shape memory effect cyclic stability; in fact, good shape memory properties frequently decrease during SME cycling of material. A method able to improve the cyclic stability of TiNi shape memory effect is the grain refinement. Considering these above mentioned research topics, a solid state welding process, as the Friction Stir Welding, is thus attractive for SMA joining and it exhibits potentials for achieving welded joints affected by microstructural changes that preserve the shape memory properties and retain, furthermore, the cyclic stability of SME. The basic objective of this study was to investigate the feasibility of friction stir welding process to join TiNi shape memory alloy sheets flat memorized, preserving the TiNi shape memory behaviour. This aim has been pursued determining the influence of the thermomechanical modifications induced by Friction Stir Processing of TiNi sheets on the functional properties of material. Optical microscopic investigations of Friction Stir Processed material cross sections have been used to highlight the modified microstructure of processed zone. A proper image processing procedure has been performed in order to quantify the amount of martensitic phase and to detect its morphology modification along the processed region. Particularly each micrographic image at first has been denoised using the 2D Wavelet transform technique and successively a texture segmentation procedure allows evaluating the amount of the martensite and austenite phases and classifying the morphological changes of martensitic regions. The austenitic and martensitic transformation temperatures of material were investigated using a stress applied characterization method suitably set up to perform the whole stress-temperature material characterization.

Study on Measurement of Group Velocity of Lamb Waves in Shape Memory Alloy Sheets

This paper describes the nondestructive evaluation of microstructure using laser-excited Lamb waves to detect the phase transformation in NiTi shape memory alloy sheets. Lamb waves were applied in the NiTi sheet using a pulse laser beam. Piezoelectricity transducers were used to receive the Lamb waves, the group velocities of which were measured using a time-frequency analysis method at different temperatures. Results show that a marked variation in the group velocity occurs during the phase transformation in the NiTi alloy. The dependence of group velocity on temperature provides a effective means of inspecting microstructure transformation in NiTi alloys.

Design of a constant-force component using superelastic SMA

Soft tissue injuries remain as problems in the application of clamping devices due to overload. Clamping devices with constant force or pressure is desired to solve the problem. This paper proposed a C-shaped component using superelastic shape memory alloy (SMA) sheet to realize constant force clamping. Optimization design of the component was carried out with computational simulation. Numerical results show that approximately constant force can be obtained by varying the initial shape, the end width and the end thickness of the SMA sheet.

Constitutive modeling of rolled shape memory alloy sheets taking into account pre-texture and anisotropic hardening

The drawing or rolling process endows polycrystal shape memory alloy with a crystallographic texture, which can result in macroscopic anisotropy. The main purpose of this work is to develop a constitutive model to predict the thermomechanical behavior of shape memory alloy sheets, which accounts for the crystallographic texture. The total macroscopic strain is decomposed into elastic strain and macro-transformation strain under isothermal condition. Considering the transformation strain in local grains and the orientation distribution function of crystallographic texture, the macro-transformation strain and the effective elastic modulus of textured polycrystal shape memory alloy are developed by using tensor expressions. The kinetic equation is established to calculate the volume fraction of the martensite transformation under given stress. Furthermore, the Hill's quadratic model is developed for anisotropic transformation hardening of textured SMA sheets. All the calculation results are in good agreement with experimental data, which show that the present model can accurately describe the macro-anisotropic behaviors of textured shape memory alloy sheets.

Microstructure and mechanical behaviors of electron beam welded NiTi shape memory alloys

The vacuum electron beam welding (EBW) technique was employed to weld Ni50.8Ti49.2 shape memory alloy sheets, and the microstructure, transformation behaviors and mechanical behaviors of the welding joints were investigated systematically. The microstructure observation showed that the weld seam was composed of coarse columnar crystals at the center and relatively fine columnar crystals near the fusion line. The abnormal high intensity of B2200 peak in XRD patterns and preferred orientation in EBSD indicated that the grains in the weld seam have grown preferentially along the 〈100〉 crystal orientation. Differential scanning calorimetry (DSC) curves exhibited an increase of the martensite start temperature (Ms) of the weld seam, which led to the mixed microstructure of martensite and austenite at room temperature. As a result, the ultimate tensile strength of the welding joint was 85% as high as that of the base metal at room temperature, while it could reach 93% at 223K when both the weld seam and the base metal were in pure martensitic state.

Active control of a smart composite with shape memory alloy sheet using a plastic optical fiber sensor

This paper investigates the morphing characteristics of a fiber metal laminate (FML) based on shape memory alloy (SMA) skin and a carbon fiber reinforced epoxy composite. Prior to manufacture, the SMA surface layer was trained by heating them to 500°C in a furnace and quenching rapidly in a water bath. The SMA skin was then bonded to the composite and a thin silicon heater was attached to the SMA skin. During activation, the deflection of the beam was detected via a plastic optical fiber (POF) sensor bonded to the upper surface of the smart FML. The output from the POF sensor was monitored using a photodetector which in turn was connected to a control unit. This closed-loop arrangement enables the operator to introduce the prescribed set-point deflection values of the beam. The desired deflections of the smart FML are achieved by the control unit switching on, and subsequently switching off, the silicon heater using the POF to provide the feedback signal.Tests have shown that this smart FML is capable of accurately achieving prescribed values of beam deflection based on the POF sensors signal. The repeatability of the system has been demonstrated by systematically increasing and then decreasing the prescribed deflection and monitoring the subsequent response. In addition, the structure has been shown to be capable of accommodating unwanted disturbances, such as those associated with external loading regimes. During initial studies, using an on–off control strategy, the beam tended to overshoot the required deflection before stabilizing at the set-point value some tens of seconds later. This effect was reduced through the tuning of a proportional-integrative-derivative (PID) control setting on the control unit. It is believed that the findings outlined in this paper demonstrate the potential benefits offered by smart hybrid structures.

Wafer-level integration of NiTi shape memory alloy on silicon using Au–Si eutectic bonding

This paper reports on the wafer level integration of NiTi shape memory alloy (SMA) sheets with silicon substrates through Au–Si eutectic bonding. Different bond parameters, such as Au layer thicknesses and substrate surface treatments were evaluated. The amount of gold in the bond interface is the most important parameter to achieve a high bond yield; the amount can be determined by the barrier layers between the Au and Si or by the amount of Au deposition. Deposition of a gold layer of more than 1 μm thickness before bonding gives the most promising results. Through patterning of the SMA sheet and by limiting bonding to small areas, stresses created by the thermal mismatch between Si and NiTi are reduced. With a gold layer of 1 μm thickness and bond areas between 200 × 200 and 800 × 800 μm2 a high bond strength and a yield above 90% is demonstrated.

Numerical and Experimental Characterizations of Damping Properties of SMAs Composite for Vibration Control Systems

Shape memory alloys (SMAs) are very interesting smart materials not only for their shape memory and superelastic effects but also because of their significant intrinsic damping capacity. The latter is exhibited upon martensitic transformations and especially in martensitic state. The combination of these SMA properties with the mechanical and the lightweight of fiberglass-reinforced polymer (FGRP) is a promising solution for manufacturing of innovative composites for vibration suppression in structural applications. CuZnAl sheets, after laser patterning, were embedded in a laminated composite between a thick FGRP core and two thin outer layers with the aim of maximizing the damping capacity of the beam for passive vibration suppression. The selected SMA Cu66Zn24Al10 at.% was prepared by vacuum induction melting; the ingot was subsequently hot-and-cold rolled down to 0.2 mm thickness tape. The choice of a copper alloy is related to some advantages in comparison with NiTiCu SMA alloys, which was tested for the similar presented application in a previous study: lower cost, higher storage modulus and consequently higher damping properties in martensitic state. The patterning of the SMA sheets was performed by means of a pulsed fiber laser. After the laser processing, the SMA sheets were heat treated to obtain the desired martensitic state at room temperature. The transformation temperatures were measured by differential scanning calorimetry (DSC). The damping properties were determined, at room temperature, on full-scale sheet, using a universal testing machine (MTS), with cyclic tensile tests at different deformation amplitudes. Damping properties were also determined as a function of the temperature on miniature samples with a dynamical mechanical analyzer (DMA). Numerical modeling of the laminated composite, done with finite element method analysis and modal strain energy approaches, was performed to estimate the corresponding total damping capacity and then compared to experimental results.

A bidirectional shape memory alloy folding actuator

This paper presents a low-profile bidirectional folding actuator based on annealed shape memory alloy sheets applicable for meso- and microscale systems. Despite the advantages of shape memory alloys—high strain, silent operation, and mechanical simplicity—their application is often limited to unidirectional operation. We present a bidirectional folding actuator that produces two opposing 180° motions. A laser-patterned nickel alloy (Inconel 600) heater localizes actuation to the folding sections. The actuator has a thin ( < 1 mm) profile, making it appropriate for use in robotic origami. Various design parameters and fabrication variants are described and experimentally explored in the actuator prototype.

FLEXURAL VIBRATION SUPPRESSION OF GLASS FIBER/CuZnAl SMA COMPOSITE

This work proposes the functional characterization of a composite material, suitable for passive suppression of flexural vibration of beams and shells. Two patterned thin sheets of CuZnAl Shape Memory Alloy (SMA) are embedded into a layered beam of glass fiber. The composite combines the density and stiffness of the glass fiber with high damping properties of SMA in martensitic state. Properly shaped patterning of the SMA sheets, for improving adhesion between the SMA and glass fiber, is performed by means of laser technology. The effect of the laser micromachining on transformation temperatures and internal friction properties of the SMA elements are analyzed. Finally, measurements of the structural damping of the layered glass fiber/SMA composite are reported and the flexural vibration suppression, due to the embedded CuZnAl sheets, is shown.

Miniaturized shape memory alloy pumps for stepping microfluidic transport

A shape memory alloy (SMA) actuator and a polydimethylsiloxane (PDMS) elastic tube were used to realize a simple pump. A liquid plug in a flow channel could be moved forward and backward reversibly by exploiting the shape memory properties of the actuator. Two SMA sheets were used to create a peristaltic pump capable of moving a solution forward and backward over a long distance. The stepping transport caused by applying pulses of electric power facilitated the adjustment of the plug's position. Manipulations such as volume measurement and the merging of two plugs were demonstrated using a microfluidic device with a T-junction.

Microstructure and properties of laser micro welded joint of TiNi shape memory alloy

Butt welding of 0.2 mm-thick TiNi shape memory alloy sheet (SMA) was carried out using impulse laser, and tensile strength, fracture morphology, microstructure and phase change behaviour of welded joint were studied. The results show that using impulse laser can realize good butt welding of TiNi SMA sheet, tensile strength of welded joint is 683 MPa, which achieves 97% of that of cold rolled base metal, and the fracture mode of welded joint is ductile type as well as base metal. The welded joint can be divided into four zones according to grain size and microstructure. The microstructures of welded seam center zone are fine equiaxed crystals and the microstructures of both lower surface and upper surface edge zones are columnar crystals. When welded joint is vacuum annealed after welding, the phase transformation process is basically similar to the annealed base metal.

YAG Laser Welding of Shape Memory Alloy

With high energy YAG Laser beam method, the weld ability of the TiNi shape memory alloy sheet was studied.The mechanical properties of both reference and laser welded samples were tested by stress–strain measurements. A systematic comparison of the results was carried out. Moreover, differential scanning calorimeter (DSC) investigations on samples taken from the heat affected zone and hardness measurements allowed further clarification that the modification was induced by the welding procedure. The crystal grain in welded joint is large , and new phases such as Ni3Ti separate out.The results obtained on laser welded samples indicate the recovery mechanisms are little weakly modified by the presence of the welding zone.