Properties Of Shape Memory Alloys Research Articles

Mechanical behavior of shape memory alloys considering the effects of body fluids corrosion for biomedical applications

Abstract Shape memory alloys (SMAs) are widely used in biomedical engineering, including cardiovascular stents, artificial skeletons, and orthodontic implants. For the above applications, the body fluids corrosion processes will inevitably cause deterioration in the mechanical properties of the SMAs actuator during its service life, which will threaten the safety of human health. To analyze such problems, experimental measurements have been carried out to investigate the influence of body fluid corrosion on the mechanical properties of SMAs. Changes in the mechanical properties, such as Young’s modulus and phase transformation temperatures of SMAs under body fluids corrosion were tested firstly in the simulated body environment with the 0.9 wt. % NaCl solution at 37. With an increase of the immersion time, the results show that the Ti (titanium) percentage, austenitic (reverse) transformation start temperature, austenitic (reverse) transformation finish temperature, and maximum residual strain all increase, the Ni (nickel) percentage, martensitic transformation finish temperature, tensile strength, and yield strength decrease, and the martensitic transformation start temperature first decreases and then increases. The research in this work can provide an experimental basis for further study of the SMAs materials in biomedicine applications.

On the effect of the processing parameters in microstructure and thermomechanical properties of LPBF NiTi shape memory alloys

Recent studies have shown that variations in the parameters of the laser powder bed fusion (LPBF) process significantly impact the microstructure and thermomechanical properties of NiTi shape memory alloys (SMAs). Understanding how parameter selection influences the final NiTi SMA properties enables the strategic design of additively manufactured (AM) components with tailored mechanical and transformation behavior, suitable for specific applications in a plethora of scientific fields. However, the connection between processing and thermomechanical properties of NiTi SMAs is still not completely understood. This study investigates the effects of AM parameters laser power (72–185 W), scanning speed (668-1739 mm/s), and hatch spacing (28–85 μm) on a near equiatomic NiTi powder. Low porosity levels are found in the samples additively manufactured (AM) with the different LPBF parameter combinations. The findings also reveal that transformation temperatures and nickel evaporation trends correlate directly with volumetric energy density (VED). Additionally, the amount of Ti2Ni precipitates increases with higher VED. Thermal cycles under constant stress, training, and two-way shape memory effect tests have been performed; the results demonstrate a strain recovery ratio of 2–3.5% for most of the samples. The results arising from this study pave the way for the engineering of customized materials with tailored properties from the same alloy metal powder.

Effect of Strip Rolling and Wire Drawing Processes on NiTi Shape Memory Alloy Properties: A Comparative Study

Effect of Strip Rolling and Wire Drawing Processes on NiTi Shape Memory Alloy Properties: A Comparative Study

Enhancing CLT floor vibration mitigation with pre-strained shape memory alloy-tuned mass dampers

Cross-laminated timber (CLT), recognised for its environmental benefits, faces challenges in its excessive human-induced vibration due to its lightweight nature. This research investigates the application of a tuned mass damper (TMD) for vibration control in CLT floor slabs, focusing on enhancing the structural performance of using this sustainable building material. The study introduces a solution by integrating Shape memory alloy (SMA) into TMD systems (SMA-TMD), utilising the properties of SMAs for effective self-centring and consistent damping, and it aims to incorporate pre-strained SMAs into TMDs to improve the damping performance of the system. Experimental testing and finite element simulations confirm that pre-straining SMA bars not only contribute to the self-centring but also considerably increase the equivalent viscous damping ratio of the SMA-TMD. Findings from the simulations demonstrate that the optimised SMA-TMD system can substantially reduce CLT floor vibration and accelerates the energy dissipation effect under human footfall loadings, overcoming one of the primary limitations of CLT in construction applications. This advancement supports the broader adoption of CLT as a sustainable building material by providing a viable solution for vibration control.

Thermodynamic ripening induced multi-modal precipitation strengthened NiTi shape memory alloys by directed energy deposition

The functional properties of shape memory alloys made by additive manufacturing can be used in numerous applications. Adjusting the precipitation can be one way to tailor the functional properties. In this work, the NiTi shape memory alloys are fabricated through Directed Energy Deposition (DED) technology and facilitate Ni4Ti3 and NiTi2 precipitates by heat treatment. Experimental results show that increasing the heat treatment temperature can switch the precipitates distributed from the grain boundaries to the interior of the grains, accompanied by the increasing size and content of precipitates. The results pressent that the tailored microstructure affects the mechanical performance, manifested by the tensile recovery strain of ∼2 %, the tensile strength of ∼749 MPa, and the strain of ∼11 %. Our findings can provide information for designing enhanced shape memory properties and further microstructural studies of NiTi shape memory alloys.

Data-driven high elastocaloric NiMn-based shape memory alloy optimization with machine learning

Shape memory alloys (SMAs) have the potential to improve the efficiency of solid-state refrigeration technology through coupled excitation of multiple thermal effects. Aiming to achieve high elastocaloric NiMn-based SMAs, this paper utilized machine learning to predict the adiabatic temperature change and first-principle calculations to elucidate the mechanism. Based on the optimal XGB Regressor model, the Ni50Mn33Ti17 SMA through directional solidification is predicted to have the highest adiabatic temperature change of 10 K (test temperature = 298 K, applied stress = 300 MPa). In addition, the volume change ratio after martensitic transformation reaches 2.375 % with first-principles calculations, which is expected to provide sufficient entropy and thus obtain an excellent elastocaloric effect. This study provides an available pathway to design and optimize the elastocaloric property of NiMn-based SMAs.

Effect of Heat Treatment on Some Thermodynamics Analysis, Crystal and Microstructures of Cu-Al-X (X: Nb, Hf) Shape Memory Alloy

Heat treatment is an important technique that can improve the properties of shape memory alloys. In this study the effect of heat treatment at three different temperatures (973 K, 1073 K and 1173 K) on some thermodynamics parameters, crystal structure, and microstructure of Cu_86 Al_12 Nb_2 and Cu_86 Al_12 Hf_2 (mass %) Shape Memory Alloy has been investigated. After heat treatment, the change in the thermal properties of the samples was determined using Differential Scanning calorimetry (DSC). The effect of heat treatment on the crystal and microstructure of the alloy samples were determined at room temperature using X-ray diffraction (XRD), Scanning Electron microscopy (SEM) device. The DSC result showed that in both samples the temperature hysteresis was decreased after heat treatment. Also grain size was decreased by increasing the treatment temperature in both samples. And finally, optical images represented that Hafnium (Hf) is more dissolved in the alloy compared to Niobium (Nb).

An SMA-based compliant adjustable constant force gripper for micro-assembly

Ensuring precise and consistent force regulation in a confined workspace is critical for high-quality micro-assembly operations. To enhance structural efficiency and reduce system complexity, this study leverages the variable stiffness properties of the shape memory alloy (SMA) and presents a novel compliant adjustable constant force microgripper. In this work, a passive variable stiffness chained pseudo-rigid-body model is established to address the coupling analysis problem arising from the mechanical and SMA’s in-homogeneous phase transformation properties, thereby facilitating the integrated design of the SMA and the mechanism. Exploiting the variable stiffness property of SMA, an improved stiffness-tunable two-beam-binding constant force mechanism is proposed for the gripper jaws, which makes adjustable constant force operation via current control can be achieved straightforwardly. Further, inspired by differential actuation mode, a compact SMA-based actuator comprising two parallel guide mechanisms and SMA drivers is developed to meet the requirement for the large working stroke of the gripper. Numerical simulations and experimental studies validate the design model’s efficiency and the proposed structures’ capabilities, and also prove the gripper’s constant operation force value can be regulated in the range of 0.152 to 0.381 N. With dimensions of 35×56.5×7 mm3 and a maximum stroke of 2.077mm, the gripper demonstrates fine compactness and potential for continuous, cross-scale micro-assembly.

Thermodynamic rationale for transformation-induced dislocations in shape memory alloys

Shape Memory Alloys (SMAs) undergo stress-induced martensitic phase-transformation affording a “superelastic” behavior with functional applications. Such behavior is undermined by the formation and accumulation of irreversible residual strains in each cycle. These residual strains arise from transformation-induced dislocation-emission and current understanding has postulated the microstructural role of emitted dislocations to accommodate lattice-mismatch while also observing a preference to occur during reverse-transformation. This study develops a thermodynamic framework to offer a causal explanation for dislocation-emission from Gibbs’ free energy considerations. Superelastic stress-strain curves for a reversible pathway without emitted-dislocations and for an irreversible pathway with emitted-dislocations are derived. The role of emitted-strain in relaxing the transformation-strain of the martensitic-inclusion and in accruing residual strain is proposed. It is shown that both pathways obey the first law of thermodynamics but it is the second law of thermodynamics that dictates the path preference. It is shown that the irreversible path achieves the critical condition for spontaneous reverse-transformation at a higher stress-level than the reversible path. Thus, the irreversible pathway initiates earlier during unloading and is thermodynamically selected during reverse-transformation. The driving-forces associated with the irreversible pathway are analyzed to establish the cause of emitted-dislocations and why it is thermodynamically preferred despite offering a higher lattice-friction barrier. Consequently, a new approach to target fatigue-resistant SMA properties is offered, focusing on the interplay of the individual driving-forces coming from the elastic strain-energy, work-interaction, and lattice-friction, as revealed by the thermodynamic framework.

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

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

Effect of Heat Treatments on Microstructure and Mechanical Properties of Fe-Mn-Ni-Al-Gd Shape Memory Alloy

There are significant scientific and industrial efforts to develop and optimize Iron-based shape memory alloys (SMA) such as FeMnNiAl for cost-sensitive applications. This alloy system shows shape memory and superelastic properties across a large temperature range. However, many studies have pointed out the need for rather complex thermo-mechanical treatments for the optimization of the SMA properties. In addition, works considering the effects of alloying on the development of microstructures that are more conducive to pseudo-elasticity in this system remain limited. Hence, systematic studies aiming at the investigation of the microstructural evolution of the FeMnNiAl(Gd) system are of great interest. In this study, solution heat treatment is done to tune the microstructure for optimum mechanical properties. The effect of phase distribution on mechanical properties is investigated at different heat treatments. Whereas cyclic heat treatment induced abnormal grain growth (AGG) in all samples, so large grains were obtained. The phase variation and elemental composition are analyzed by X-ray diffraction and Energy Dispersive Spectroscopy, respectively. The microstructure and phase distribution are observed using Scanning Electron Microscope and then related to the microhardness results. The microstructure has a good correlation with mechanical properties where the fine distribution of phases results in a higher hardness number.

Development of an Automated Experimental System for Thermomechanical and Electrical Characterization of NiTi Shape Memory Alloys

BackgroundComprehensive datasets quantifying the coupled thermo-mechanical and electrical properties of shape memory alloys (SMAs) are lacking, as are standardized techniques for robust characterization. This hampers accurate modeling and design of SMA-based components. Objective: This work develops an automated experimental system to enable simultaneous measurement of stress-strain-temperature behavior and electrical resistivity evolution in NiTi SMA wires under controlled stress conditions. Methods: Customized test frames apply precise mechanical stresses while allowing for in situ electrical measurements and infrared imaging during complete thermal cycling protocols. Specialized instrumentation including a Keithley 2002 multimeter, Agilent E3631A programmable power supply, and FLIR A615 thermal camera are integrated with LabVIEW-based software routines for complete automation of the characterization process. Rigorous metrology principles are implemented throughout the measurement procedure to improve accuracy, repeatability, and consistency compared to prior manual techniques. Results: Extensive datasets are generated which reveal pronounced stress-dependencies in key SMA material parameters including transformation temperatures, recoverable strain, and electrical resistivity. A 3D regression model describes the comprehensive relationship between resistivity, temperature, and applied stress across the entire characterization domain. Conclusions: The automated measurement framework and methodology establishes a foundation for high-fidelity, reliable acquisition of coupled SMA property data. This will enable more accurate modeling and design of components and systems incorporating SMA actuation or sensing functions.

Nonlinear dynamics analysis of the attachment system and design of variable stiffness connecting bracket based on the complete aero-engine system

During the operation of the aero-engine, the attachment and brackets are subjected to various complex internal and external loads, which can pose risks to the structural integrity of the engine. To address this issue, this paper introduces a variable stiffness bracket designed for vibration reduction in the attachment system. Firstly, a finite element model (FEM) of the aero-engine was established using the Lagrangian method, incorporating the dual rotor-bearing system and bracket-attachment system. Subsequently, the vibration response of the system was solved using the Runge-Kutta method. The sources and propagation paths of vibration within the system were identified. Lastly, leveraging the material properties of Shape Memory Alloy (SMA), a stiffness-adjustable bracket for the aircraft engine was proposed to achieve active vibration reduction in the system. The results demonstrate that the variable stiffness bracket effectively reduces the resonance amplitude and frequency in the attachment system, exhibiting excellent vibration reduction and isolation capabilities.

Cryogenic rolling induces quasi-linear superelasticity with high strength over a wide temperature range in TiNi shape memory alloys

Materials with high functional and strength performance are finding wide range of applications. In this study, we propose an approach to enhance the synergistic relationship between recoverable strain and strength in TiNi shape memory alloys (SMAs) by utilizing cryogenic rolling. Comparative analysis of the rolled samples, which experienced a thickness reduction of up to 13% at a nitrogen temperature, revealed good cycling stability, greater recoverable strain, reduced modulus, and increased strength at room temperature when compared to their undeformed counterparts. Notably, these rolled samples exhibited a significant recoverable strain (>4%) and impressive strength (0.8∼1.6 GPa) over a wide temperature range (∼400 K). Our findings suggest that these enhancements can be attributed to the emergence of the B19′ strain glass transition. This research not only illuminates the potential advantages of cryogenic rolling in shaping the functional and structural properties of SMAs but also offers a promising avenue for effectively regulating properties.

Shape memory alloy-reinforced UHPC tube confined bridge piers for enhancing the seismic resistance of highway bridges

Multi-span continuous highway bridges supported with reinforced concrete (RC) bridge piers may experience large permanent deformation after strong earthquakes. To overcome the limitations, an innovative UHPC tube-confined SMA reinforced bridge pier, i.e., shape memory alloy (SMA) replacing the steel rebars at the plastic hinge region and a prefabricated UHPC tube substituting the cover concrete, is proposed. To fully utilize the tensile strength of UHPC, the UHPC tube is embedded into the foundation. A performance-based seismic design procedure considering target residual drift as engineering demand parameter (EDP) is proposed to design this novel UHPC-SMA pier. The seismic responses of novel bridge systems supported by UHPC piers (Bridge II) and UHPC-SMA piers (Bridge III) are investigated and compared with reference bridge (Bridge I) having RC piers. The results indicate that both UHPC piers and UHPC-SMA piers are efficient in improving the seismic resistance of highway bridges. Compared with Bridge I, the prefabricated UHPC tube significantly decreases the residual deformation of Bridge II piers by 22.9 % as a result of the bridging effect of steel fibers. Due to the excellent self-centering property of SMA, the residual deformation of Bridge III piers decreases further by 57.8 % in comparison to reference bridge. The UHPC tube increases the low stiffness of bridge piers caused by SMA rebars at the plastic hinge region. The combination of UHPC tube and SMA can effectively prevent core concrete from severe damage with less dissipated energy of bridge piers.

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.

Hybrid Technique for Vibration Control using Synchronized Switch Damping on Inductor Modal Approach and Shape Memory Alloy Integration

Vibration control is a critical aspect of various engineering applications, including automotive and aerospace systems. This research paper presents a novel approach to enhance the performance of the Synchronized Switch Damping on Inductor (SSDI) technique by incorporating Shape Memory Alloys (SMAs). SSDI has proven effective in managing energy dissipation during electronic switching transitions, mitigating voltage spikes and reducing electromagnetic interference. However, the method consumes a substantial amount of energy. In this study, we propose a hybrid approach that combines the benefits of SSDI with the unique properties of SMAs to achieve improved vibration control. The integration of SMAs introduces a semi-passive element to the system, exploiting the inherent properties of these alloys to undergo reversible shape changes in response to external stimuli. By strategically placing SMAs within the vibration-prone components of the system, we aim to harness the mechanical forces generated during shape recovery to counteract and dampen vibrations. This innovative approach aims to capitalize on the efficient energy conversion capabilities of SMAs, thus minimizing the additional energy consumption associated with traditional semi-passive methods. The research involves theoretical modeling and simulation studies to assess the effectiveness of the proposed technique. Preliminary results demonstrate a significant reduction in vibration amplitudes and enhanced damping capabilities, validating the potential of the integrated SSDI-SMA system. The findings of this study not only contribute to advancements in vibration control methodologies but also provide insights into the broader applications of SMA-enhanced semi-passive techniques in improving the overall performance and efficiency of systems.

Problems of modeling elements with shape memory alloys of military equipment

Shape memory alloys (SMAs) are used in various fields, including military equipment, to create means with autonomous or improved properties. It can be especially useful in structural elements that are capable of self-healing after deformations or damage as a result of explosions, impacts or other loads that occur during military operations.
 SMAs are materials that have the ability to remember and restore the accumulated deformation under certain conditions. This makes them particularly useful in robotics, where they can be used to create moving parts, sensors, actuators and other components.
 One of the main applications of SMA in robotics is the creation of moving elements. For example, they can be used to create bendable segments in jobs that require flexibility and mobility. SMAs can be used to create manipulators that allow a robot to easily move and perform tasks in different environments. One of the most important properties of SMAs is their ability to retain their shape during deformation. This makes them useful for building sensors that can detect differences in the shape of an object. SMAs can be used to create pressure sensors that respond to a change in shape under the influence of external pressure. These sensors can be used to develop robots that can recognize objects and distinguish them by shape. SMA can also be used to create actuators. Actuators are components that convert electrical, mechanical, or other types of energy into motion. SMA are materials that have the ability to remember and restore the accumulated deformation under certain conditions. This is achieved due to the peculiarities of their microstructure and thermoelastic effect. When SMA is subjected to thermal influence in the temperature zone where it can transition between two states: austenitic (higher temperature) and martensitic (lower temperature), under an existing external load, the phase strain increases, and with the degree of load, the temperature of the beginning and end of the emerging of straine changes. Each of these states has its own crystal structure and characteristics determined by its chemical composition. The article considers the issue of taking into account the shift of characteristic temperatures under the influence of an external load of SMA in terms of the synthetic theory of irreversible deformation and proposes a non-linear formula for finding the characteristic temperatures of metals with memory during loading. The obtained ratio was used to find the proportion of the new phase and the rate of formation of the new phase as a function of the magnitude of the external load, the rate of loading, the absolute temperature, the rate of temperature change and the characteristic temperatures in terms of the effective temperature.

Solving the puzzle of hierarchical martensitic microstructures in NiTi by (111)-oriented epitaxial films

The martensitic microstructure decides on the functional properties of shape memory alloys. However, for the most commonly used alloy, NiTi, it is still unclear how its microstructure is built up because the analysis is hampered by grain boundaries of polycrystalline samples. Here, we eliminate grain boundaries by using epitaxially grown films in (111)B2 orientation. By combining scale-bridging microscopy with integral inverse pole figures, we solve the puzzle of the hierarchical martensitic microstructure. We identify two martensite clusters as building blocks and three kinds of twin boundaries. Nesting them at different length scales explains why habit plane variants with ⟨011⟩B19' twin boundaries and {942} habit planes are dominant; but also some incompatible interfaces occur. Though the observed hierarchical microstructure agrees with the phenomenological theory of martensite, the transformation path decides which microstructure forms. The combination of local and global measurements with theory allows solving the scale bridging 3D puzzle of the martensitic microstructure in NiTi exemplarily for epitaxial films.

Advances in Shape Memory Alloy-Based Reinforcement in Steel Structures: A Review

The utilization of shape memory alloys (SMAs) to reinforce steel structures has been proven to be an efficient and reliable method, the structural strengthening needs can be met without the need for tensioning equipment by activating the SMAs to generate restoring stresses. This paper firstly introduces the properties of SMA, and then presents the latest research progress, opportunities and challenges of SMA in the field of steel structural reinforcement, both in terms of basic components and applications. In terms of components, the construction forms and working mechanisms of Fe-SMA strips, SMA/CFRP composite patches and SMA dampers are introduced. On this basis, the application of SMA in steel structures reinforcement is introduced, and its effect is analyzed from three aspects: crack restoration, seismic retrofitting and structural strengthening. Finally, the results of the current research are summarized and the shortcomings are analyzed, hoping to provide a reference for the research of SMA in the field of steel structures reinforcement.