Shape Memory Alloy Phase Research Articles

Effect of laser energy density on the phase transformation behavior and functional properties of NiTi shape memory alloy by selective laser melting

NiTi shape memory alloy (SMA) was prepared via selective laser melting (SLM). Orthogonal tests were conducted with varying laser power and scanning speeds to explore the influence of laser energy density on microstructure, phase transformation behavior, mechanical properties and functional properties (superelasticity and shape memory effect). As energy density increases from 24.3 to 106.3 J/mm³, microstructure transforms from fine to columnar grains. Ni consumption intensifies, boosting transformation temperature and B2 content at room temperature. Post cyclic stretching, B2 content in alloy escalates further at room temperature. Tensile strength peaked at 53.6 J/mm³, while the elongation rate significantly increases with increasing energy density. During cyclic stretching, low or high energy densities in SLM NiTi induce high-density dislocations and stable residual martensite, respectively, significantly inhibiting martensitic transformation and resulting in extremely poor functional properties. The alloy produced at 53.6 J/mm³ boasts exceptional superelasticity due to stress-induced martensitic transformation and phase inversion post-unloading. Following heating past the critical transition, it nearly fully recovers (97.78 % recovery rate). Despite multiple shape memory fatigue tests, it maintains a recovery rate above 90 %. This study provides crucial theoretical insights for optimizing NiTi SMA phase transformation behavior and enhancing the performance.

First-principles study of the martensitic transformation in pressure of the all-d-metal Heusler alloy Fe2NbIr

The martensitic transformation of the new all-d-metal Heusler alloy Fe2NbIr is investigated using first-principles. The structure undergoes softening and hardening under pressure, which corresponds to the two phases of the martensitic phase transformation, i.e. C′ softening of the austenite and C′ hardening of the pre-martensite phase, implying that the pressure may induces to produce a cubic pre-martensite phase. An exciting phenomenon is found in the tetragonal deformation of the pre-martensite phase, where the pre-martensite phase overcome the energy barrier of the jump to produce a tetragonal martensite phase with a much lower energy. It explains the presence of a pre-martensite phase in shape memory alloys, i.e., it provides a conduit for the phase change. This work presents an analysis of the anomalous softening behaviour of Fe2NbIr and the mechanism of the pressure-induced martensitic transformation, and provides a new idea for study of the precursor effect of martensitic phase transformation.

Application and Performance Evaluation of Superelastic Shape Memory Alloys in Building Vibration Isolation Systems

Abstract In this study, the intrinsic model of Shape Memory Alloys (SMA) is employed to facilitate a comprehensive analysis of the phase transition process, encapsulating the evolution of stress and strain across varying temperatures. The martensite volume fraction is quantitatively described using a cosine function, culminating in the formulation of a cosine-type SMA phase transition equation. This model enables the simulation of superelasticity, shape memory effects, and the elastic-thermal behavior of SMA by varying the loading conditions. This approach allows for an assessment of how different material parameters influence SMA properties. Notably, the results highlight that the heating temperature significantly affects the hyperelastic energy dissipation properties of SMA. The energy dissipation coefficient increased by 15.3% when the temperature was increased from 350 to 550 and decreased by 36.3% when the temperature was increased from 550 to 650. The peak values of top acceleration and interstorey displacement of the SMA-isolated structure were 0.826 m/s² and 0.641 mm for EL-centro waves at 7+ seismic intensities. The values of non-isolated were 1.151 m/s², 0.856 mm. 10 ground shaking, the SMA braced structure relative to the steel frame structure on the maximum peak inter-story displacement angle and the structure of the top layer of the permanent displacement, have different degrees of reduction. Therefore, the SMA material possesses good damping performance in the seismic isolation system.

A machine learning approach for investigation of the natural frequency of a nitinol-reinforced composite beam

Many applications are using composites to improve performance and reduce weight, but it is essential to know the different properties of the composite before manufacturing. Properties like natural frequency and elastic modulus are also crucial in many applications. The use of shape memory alloys (SMA) composite has increased in the last few years due to various advantages of the shape memory alloys, like a shift in natural frequency and elastic modulus during phase transformation. Hence it is essential to know the composite’s natural frequency and elastic modulus before constructing it. Although experimental and numerical methods for calculating natural frequency exist, they are time-consuming and infrastructure-dependent. This paper explores relationships between SMA composite construction parameters and natural frequency to predict it better. Nitinol-reinforced silicon rubber composite beams are investigated with various parametric combinations using an orthogonal array. Different machine-learning techniques are applied for natural frequency prediction after training models on numerical results from varied construction combinations. The study identifies the best-performing algorithm and provides tuning recommendations. Linear regression model, Ridge regression model, and Decision Tree regression are the best-performing algorithms for the dataset in this paper. A weighted sum method finds optimal construction parameters for maximum natural frequency. These models can predict natural frequency before construction and the shift during SMA phase transformation. The research aids in designing SMA-reinforced beams by identifying optimal parameters like volume fraction, location, and activation pattern, targeting maximum natural frequency. The composite studied in this research shows a maximum natural frequency of 19.58 Hz for a 3.53% volume fraction of SMA, 3 mm distance of reinforcement, all wires activated, and austenite temperature.

On the nonlinear analysis of pre-strained shape memory alloy beams

We present a theoretical and numerical investigation of the transverse dynamic response of monolithic Shape Memory Alloy (SMA) beams under the effect of axial pre-strains. The SMA beam is taken as benchmark structure to evaluate the effect of (axial) pre-strain on either the free or the forced dynamics. The combination of the static stress, produced by the pre-loading conditions, with the dynamic stress, associated with the vibratory response, affects the occurrence of the stress-induced SMA phase transformation hence providing a mechanism to passively tune the dynamic response. Particular attention is given to the effect that different pre-strain levels have on the overall ability to dissipate mechanical energy, that is on the effective damping. The numerical model of the SMA beam accounts for both material and geometric nonlinear behaviors. The nonlinear material behavior is due to the SMA phase-transformation and it is modeled via the one-dimensional improved Brinson’s model (including tension–compression asymmetry), while the geometric nonlinear behavior is due to large displacements occurring during the phase transformation and it is accounted for via von Kármán assumptions. The resulting model is solved numerically via the finite element method and used to evaluate both the free and the forced response of the beam. The free response analysis suggests the existence of optimal pre-strain levels to achieve maximum damping capacity, while the forced response highlights the occurrence of nonlinear dynamic features, such as bifurcation. In both cases, it is found that the level of pre-strain can significantly affect the dynamic response as well as the effective damping. The results presented in this work provide useful guidelines to understand the dynamics of continuous SMA structures under pre-strain and the ability to tune either their resulting dynamics or dissipation properties.

Influence of amorphous phase on crack initiation and propagation behaviours in NiTi shape memory alloy based on extended finite element simulation

Local amorphous phases often appear in NiTi shape memory alloys (SMAs) which undergo a large degree of cold plastic deformation. Fracture damage is a common failure mode of NiTi SMAs in engineering application and it is usually caused by the crack initiation and propagation. The crack initiation and propagation behaviours in NiTi SMA parts containing amorphous phases have not been reported so far. In the present study, extended finite elements method (XFEM) was applied to investigate the influence of amorphous phase on crack initiation and propagation behaviours in a notched NiTi SMA specimen processed by cold plastic deformation. The results show that the amorphous zone has a significant influence on both the crack initiation and the crack propagation. Crack is easier induced if there exists amorphous phase in the NiTi SMA but it is difficult for the crack to pass through the amorphous zone.

A Novel Intelligent Fan Clutch for Large Hybrid Vehicles

To solve the problems of complex structure, poor reliability, and low intelligence of existing fan clutches for large hybrid vehicles, this paper proposes a new adaptive shape memory alloy intelligent fan clutch for large hybrid vehicle motor cooling. Based on the pure shear shape memory alloy thermodynamic effects, the relationship between shape memory alloy spring recovery force and temperature has been established; based on the shape memory alloy spring thermal drive characteristics and clutch construction dimensions, clutch torque transmission equations have been established. The shape memory alloy fan clutch transmission characteristics were quantitatively analyzed in terms of temperature, torque, rotational speed, and slip rate. The results show that the shape memory alloy fan clutch model based on the finite element method (FEM) and the established transmission model can accurately describe the mechanical characteristics of the shape memory alloy phase change process and the clutch torque transmission characteristics. When the clutch input speed is 3000 rad/min and the temperature is 100 °C, the output torque is 19.04 N·m, the speed is 2877.2 rad/min, and the slip rate is 4.3%. Due to the shape memory effect of shape memory alloy, the clutch can intelligently adjust the fan speed by sensing the ambient temperature. A fan clutch can satisfy the heat dissipation requirement of a large hybrid vehicle’s transmission system under complicated road conditions.

A flexible and smart shape memory alloy composite sheet based on efficient and bidirectional thermal management

ABSTRACT Leveraging advances in shape memory alloys (SMAs) and flexible thermoelectric devices (f-TEDs), this paper presents a structural and functional integrity composite sheet to address the inefficient and bulky activation of 2-D SMAs. A series of experimental tests were performed to reveal the generation, change, and transfer mechanisms of different types of electrically-induced heat in the f-TED, as well as the temperature-induced SMA phase transformation behaviors. The results show that the composite sheet exhibited good bidirectional thermal management capacity. The austenite deformation can be completed within 10s at an operating current of 2 A. The cooling recovery, in particular, performs much better than in conventional modes (the temperature declines exponentially with time). Finally, through two functional prototypes, a light switch and a flexible gripper, the application potential of the proposed composite was further experimentally demonstrated.

Ductile CuAlMn shape memory alloys with higher strength by Fe alloying and grain boundary engineering

A higher yield strength of parent phase in shape memory alloys is necessary for achieving a higher recovery stress. Fe alloying combined with grain boundary engineering (GBE) were investigated to improve the yield strength of ductile Cu17Al11Mn alloy. Results showed that the precipitation of low strength ductile α phase through the GBE remarkably enhanced the ductility while not obviously reducing the yield strength in the step-quenched Cu17Al11Mn4.5Fe alloy due to the precipitation of nanoscale κ phase inside them. Compared to the Cu17Al11Mn alloy, a 200 MPa higher yield strength and better ductility could be realized in the Cu17Al11Mn4.5Fe alloy step-quenched into two-phase region between 993 K and 1003 K. The recovery strains of ∼6.5% could be guaranteed in the step-quenched Cu17Al11Mn4.5Fe alloy. The Fe alloying combined with the GBE through the step-quenching will be a direction for developing ductile CuAlMn based alloys with both high strength and recovery strain.

DYNAMIC PROPERTIES OF POST-BUCKLED SHAPE MEMORY ALLOY AND ITS APPLICATION TO A BASE ISOLATOR FOR VERTICAL VIBRATION

In recent years, high performance and compact vibration isolators have been in demand to support the advancement of precision technology. Among various vibration isolators, a passive vibration isolator is the most commonly adopted form due to its simplicity, stability and low cost. Previously, the authors have proposed a simple and compact passive vibration isolator for the vertical direction using a post-buckled shape memory alloy (SMA) beam. This isolator achieved a low natural frequency assuring high static stiffness by utilizing the negative tangent stiffness of a post-buckled SMA beam. It is expected that the appearance of the negative stiffness is related to the phase transformation of a post-buckled SMA beam, however, the fundamental principle of the appearance and the response for a reciprocating motion (vibration) have not been clarified. In this study, to clarify these characteristics, the restoring force of an SMA beam subjected to reciprocating motion in its post-buckled state was measured experimentally and predicted by Finite Element Analysis in which the phase transformation of the post-buckled SMA was considered. As the result, it was found that the restoring force converged to a certain force when SMA was subjected to reciprocating motion. It was also found that the negative tangent stiffness arose when the phase of SMA transforms from Austenite phase to Martensite phase in compression process. Therefore, SMA with small hysteretic property in the stress-strain curve is appropriate for an effective isolation element. In future research, it will be necessary to validate the performance of the isolator considering the phase transformation.

Introduction and comprehensive theoretical framework of analysis of Smart Plate retrofitted reinforced concrete members (SPRRCs)

Several reasons result in the need for retrofitting of existing reinforced concrete structures, and extensive effort is devoted to invent and implement different methods for strengthening RC members. One of the leading rehabilitation techniques is attaching FRP strips, but FRP materials’ low ductility has caused several issues such as FRP-structure debonding, boundary layer cracking, and brittle behaviour of the strengthened RC members. To overcome these drawbacks, the Smart Plate (SP) which is a ductile retrofitting cover, is proposed for the first time. This cover is formed by embedding Shape Memory Alloy (SMA) wires in an elastomeric substrate. The SMAs exhibit certain properties, including superelasticity, shape memory effect, and temperature-dependent behaviour, while the elastomer can withstand severe stretches. To investigate SP-retrofitted RC beams (SPRRC) characteristics, at first, a computer program has been developed based on the fiber analytical method to analyse the strengthened concrete members. The program is verified against experimental and finite element method (FEM) data for ordinary, FRPRC, SMARC and SPRRCs members. Following verification, the program and FEM are implemented to conduct a comprehensive analysis of the influential parameters affecting the behaviour of SPRRCs including SMA percentage and working temperature. As a result, the SPRRCs show improved stiffness and strength compared to ordinary beams. They also behave in a ductile way contrary to FRP-strengthened beams. The FMA program and the FEM simulations both indicate that the SMA phase transition temperature significantly alters the performance of SPRRCs. Moreover, FMA and FEM outcomes approve that the smart plate is less likely to debond from the strengthened structural member, which is a remarkable advantage. Additionally, a parametric study is conducted, and a characteristic moment–curvature diagram is proposed, which approves the merits of using SP as an alternative retrofitting cover. These advantages are the significant enhancement in moment capacity, yielding moment, ultimate curvature, and ductility.

Shape Memory Alloy-Based Frequency Reconfigurable Ultrawideband Antenna for Cognitive Radio Systems

The present work demonstrates a frequency reconfigurable planar antenna with narrowband and ultra-wideband (UWB) characteristics for cognitive radio systems. The designed antenna has a dimension of $60\times 58\times1.6$ mm3 with a ground plane made of nickel–titanium (Ni–Ti) shape memory alloy (SMA). The antenna operates at a flat condition with frequency 1.46 GHz. Bending the SMA ground plane at an angle of 200° with a transformation temperature of 66 °C. The same antenna operates over 3.4–10.2-GHz UWB range. This is due to the SMA phase transformation between martensite and austenite via resistive heating/cooling. The insertion of rectangular slits in the antenna creates dual band-notched characteristics 4.7–5.1 and 5.3–6.0 GHz with VSWR $S_{{11 {}}} dB. This creates a different working frequency leading to frequency reconfiguration. Details about the proposed antenna have been discussed along with measured and simulation results.

Effect of shape memory alloys on the mechanical properties of metallic glasses: A molecular dynamics study

The strength-plasticity trade-off of metallic glass (MG) has not still been effectively overcome. The introduction of shape memory alloy (SMA) is an effective way to improve the mechanical properties of MG. Here, the deformation behavior of amorphous/SMA Cu64Zr36/B2-CuZr nanomultilayers (ASNMs) under tension loading is investigated by molecular dynamics (MD) simulation method. The results show that the peak stresses and flow stress of the ASNMs are greater than those of the monolithic MG regardless of SMA volume fraction. The martensitic transformation (MT) in the SMA phase limits the propagation of shear bands (SBs), avoids a runaway instability, and simultaneously induces plastic strain strengthening. The results also indicate that the plastic deformation mode of ASNMs changes from the interaction of multiple SBs dominated to finally brittle fracture caused by nano-pores aggregation with the increase of SMA volume fraction. This means that the plasticity and strength of ASNMs can be significantly improved by adjusting the volume fraction of SMA. The fruits stem from this paper may provide a valuable guidance and theory route for the design of high-performance MGs.

Microstructure and Phase Transition Characteristics of NiTi Shape Memory Alloy

Shape memory alloy (SMA) with shape memory effect and superelasticity has had an increasing interest for researchers of mechanics of materials in recent decades. With the deepening of theoretical research, the applications of SMA have also made considerable progress. SMA has a unique solid phase transformation characteristic that is thermoelastic martensitic transformation. The high temperature phase of SMA is austenite, while the low temperature phase of SMA is martensite. Because of the biocompatibility and the corrosion resistance, NiTi shape memory alloy has been widely used in biomedical and aerospace. However, the NiTi binary alloy can only be applied to the ambient temperature, because the phase transformation temperature is lower than 100 centigrade. In order to apply the NiTi based shape memory alloy in the wider field, it is necessary to control the phase transformation temperature, thermal hysteresis latent heat of phase change and other phase transformation behavior and mechanical property effectively and accurately. Thus, it is important to add a third or fourth element to NiTi alloy to form a new type of NiTi based shape memory alloy. The results show that NiTi alloy contains many precipitates such as NiTi2, Ni3Ti2, Ni4Ti3. Adding the third or even the fourth element to the NiTi binary alloy can effectively adjust the phase transition and related mechanical properties of NiTi alloy. In this paper, the research progress of NiTi binary alloys is summarized, the microstructure, types and structures of properties, types and properties of phase transitions of NiTi alloys are introduced respectively, the problems faced by NiTi alloys are analyzed and their prospects are prospected, so as to provide reference for the development of shape memory alloys with better properties.

Intergrated Shape Memory Alloys Soft Actuators with Periodic and Inhomogeneous Deformations by Modulating Elastic Tendon Structures

The soft actuators similar to the periodic motion of biological organisms are accomplished based on assembling multiple unidirectional memory effect shape memory alloys (SMAs) components or introducing biasing elements, which give rise to the complex structure and difficult control of soft actuators. Herein, inspired by the power‐amplified biological systems, beneficial with the advantages of 3D integrated molding technology, an integrated SMA‐polydimethylsiloxane (PDMS) composite structure (SPCS) is proposed, which can achieve periodic heterogeneous actuation only by controlling the SMA phase transformation and the PDMS strain potential energy distribution states. To test the feasibility of the mechanism, a theoretical model of SPCS deformation is conducted. The results of numerical feasibility analysis show that the factors affecting SPCS deformation mainly involve the excitation current strength of SMA, PDMS structure thickness and its distribution state. The experimental results show that the current intensity mainly affects the deformation rate of SPCS, and the thickness of PDMS is not only the key to realize the periodic deformation of SPCS but also the orderly arrangement of PDMS structure thickness is helpful for SPCS to achieve periodic heterogeneous deformation. These demonstrate that the proposed mechanism can inspire the design of soft actuators, smart wearable equipment, and medical devices.

A new variable speed phase transformation constitutive model of shape memory alloys

Due to their unique shape memory effect, superelastic effect, as well as the excellent physical and chemical properties and biocompatibility, shape memory alloys (SMAs) have been widely used in aerospace, weapons and industrial automation. It is of great engineering significance and academic value to establish a constitutive model for accurately and effectively describing the thermomechanical behavior of SMAs. In this paper, a concept of crystal variable speed coefficient is introduced. A new variable speed constitutive model of SMAs considering rate distribution of SMA phase transformation is proposed. In order to verify the accuracy of the constitutive model proposed in this work, thermomechanical properties of the SMA wire were tested experimentally first, and both a new method for obtaining the starting and ending points of phase transformations and the conventional method have been used in this process. Then, numerical simulations were performed by using the variable speed constitutive model and the conventional Brinson model, respectively. The results were compared which indicates that the variable speed model is in better agreement with the experimental data. Therefore, the proposed model in this paper can predict the thermomechanical response of SMAs more accurately and effectively.

A theoretical and experimental investigation on the stress distribution in the interface of pre-strained SMA wire/polymer composites

The major concern in using the shape memory alloy (SMA) wires embedded in a polymer matrix is the possibility of debonding between SMA and the surrounding polymer due to the SMA phase transition recovery stress. Hence, it is important to examine the distribution of stresses in the SMA wire and the surrounding matrix. In the present study, using the principle of the minimum complementary energy a new analytical model was developed to determine the axial, radial, and shear stresses along the SMA/polymer interface. By considering the Poisson's effect and shear stress in the present analytical model, while ignored in the classical shear-lag model, more accurate results were obtained. Furthermore, by considering the effects of the SMA wire pre-strain, actuating temperature, recovery stress and thermal expansion, a novel relationship was also presented for calculating the maximum interfacial shear stress. To show the effect of the SMA wire pre-strain on the distribution of stresses throughout the SMA/epoxy interface, pull-out tests were conducted at different pre-strain levels in the SAM wire. Results showed applying the pre-strain in the SMA wire followed by a thermal activation enhanced the debonding load between the SMA wire and epoxy matrix and caused a significant variation in the stress distribution.

A micromechanical constitutive model for unusual temperature-dependent deformation of Mg–NiTi composites

Based on the mean-field approach, a micromechanical constitutive model is established to describe the unusual temperature-dependent deformation of Mg–NiTi composites. Firstly, the constitutive equations of two phases are proposed: for NiTi shape memory alloy (SMA), a simplified Hartl–Lagoudas model (Hartl and Lagoudas, 2009) which considering the martensite transformation and plastic deformation is adopted; while, for magnesium (Mg), an elastic-plastic model including a new nonlinear plastic hardening law is employed. The dependence of elastic moduli and yield surfaces of two phases at ambient temperature is addressed. Then, a non-isothermal incremental Mori–Tanaka homogenization method is employed and further extended to describe the interaction between two phases and calculate the macroscopic overall stress-strain response of Mg–NiTi composites. Finally, comparisons between the simulated results and the corresponding experimental ones show that the temperature-dependent deformation of the composites with different volume fractions of NiTi SMA phase can be well captured by the proposed model. Predicted results demonstrate that the unusual temperature-dependent deformation of Mg–NiTi composites originates from the change in the inelastic deformation mechanism of NiTi SMA with the variation of temperature.

First-principles calculations of thermal properties of the mechanically unstable phases of the PtTi and NiTi shape memory alloys

The design of shape memory alloys (SMA) demands detailed knowledge of their thermodynamic properties and of the underlying atomic-level mechanisms governing their shape-memory behavior. This knowledge is traditionally obtained via atomistic simulations, but these systems have so far resisted such efforts due to the presence of phonon instabilities and associated spontaneous symmetry breaking. In this study, we investigate the thermodynamics of PtTi, a novel and promising SMA (and of the related better-known NiTi SMA, for comparison purposes) using recently developed first-principles computational method specifically designed to tackle this issue. The method efficiently explores the system's potential energy surface by discrete sampling of local minima, combined with a continuous sampling of the vicinity of these local minima via a constrained harmonic lattice dynamic approach. Our calculations provide a complete and atomic-level-based model for these compounds' free energy and shed some light on the ongoing search for the precise structure of dynamically stabilized high temperature phases in SMAs.

Deflection control of an aeroelastic system utilizing an antagonistic shape memory alloy actuator

This paper presents position control of the control surface of an aeroelastic typical section using shape memory alloy (SMA) actuators. The actuator was designed using an antagonistic pair of SMA wires, thus allowing its use to move the control surface to up and down. The mathematical model adopted here describes the relations among aeroelastic section, models of SMA wire heat convection, constitutive law and phase transformations of the SMA. Three types of controllers are presented based on variable structure control approach and the deflection of the control surface via two pairs of antagonistic wires are made in a closed loop system. The effectiveness of the actuation system in positioning the control surface is evaluated numerically.