Search Results (52)

Cu–Al–Ni shape memory alloys (SMAs) are attractive for structural and functional applications due to their cost-effectiveness and shape memory behavior. This study systematically investigated the effect of beryllium (Be) addition on the phase stability, microstructure, transformation temperatures, mechanical hardness, corrosion resistance, and wear behavior of Cu–Al–Ni alloys. Alloys with Be contents ranging from 0 to 1.5 wt.% were fabricated via arc melting and subjected to thermal treatment. Characterization techniques included dilatometry, X-ray diffraction (XRD), microhardness testing, potentiodynamic polarization, and pin-on-flat wear testing. The results showed that Be additions ≤ 0.4 wt.% stabilized the martensitic β′ phase, while higher concentrations favored the formation of austenitic β phase with a BCC structure. Hardness increased with Be content, especially in austenitic samples. Corrosion tests revealed that while the 0.2 wt.% Be alloy exhibited the most positive corrosion potential (Ecorr), it also had a higher corrosion rate. Overall, corrosion resistance declined with Be concentrations ≥ 0.6 wt.%. Wear tests demonstrated improved resistance in martensitic alloys, attributed to pseudoplastic deformation. These findings highlight the dual role of Be in modifying phase stability and functional properties, offering useful guidance for designing Cu-based SMAs with tailored performance. Full article

The superelasticity of CuZr shape memory alloys (SMAs) originates from stress-induced transformations between the B2 (austenite) and B19’ (martensite) phases. Grain size is a key parameter affecting the superelasticity of shape memory alloys. Previous studies on NiTi, Fe-based, and Cu-based SMAs confirm that altering grain size effectively regulates superelasticity. Current research on the influence of grain size on the superelasticity of CuZr shape memory alloys (SMAs) is relatively sparse. This study employs molecular dynamics simulations to evaluate the effect of grain size on the superelasticity of CuZr SMAs through uniaxial loading–unloading tests. Polycrystalline samples with grain sizes of 6.59 nm, 5 nm, and 4 nm were analyzed. The results indicate that reducing grain size can decrease the irrecoverable strain, thereby enhancing superelasticity. The improvement in superelasticity is attributed to a higher recovery rate of the martensite-to-austenite transformation, allowing more plastic deformation within the grain interior to recover during unloading, and thereby reducing the irrecoverable strain. The recovery rate of the martensite-to-austenite transformation is closely related to the elastic strain energy accumulated within the grain interior during loading. Full article

In the present study, the capacity of two commercial NiTi and NiTiCu shape memory alloy (SMA) wires to develop work-generating (WG) and constrained-recovery (CR) shape memory effects (SMEs), as well as the capacity of a commercial NiTiFe super-elastic wire to act as cold-shape restoring element, have been investigated. Using differential scanning calorimetry (DSC), the reversible martensitic transformation to austenite of the three NiTi-based wires under study was emphasized by means of an endothermic minimum of the heat flow variation with temperature. NiTi and NiTiCu wire fragments were further tested for both WG-SME and CR-SME developed during the heating, from room temperature (RT) to different maximum temperatures selected from the DSC thermograms. The former tests revealed the capacity to repetitively lift various loads during repetitive heating, while the latter tests disclosed the repetitive development of shrinkage stresses during the repetitive heating of elongated wires. The tensile behavior of the three NiTi-based SMA wires was analyzed by failure and loading–unloading tests. The study disclosed the actuation capacity of NiTi and NiTiCu shape memory wires, which were able to develop work while being heated, as well as the resetting capacity of NiTiFe super-elastic wires, which can restore the initial undeformed shape of shape memory wires which soften while being cooled down. These features enable the design of a robotic gripper based on the development of NiTi-based actuators with repetitive action. Full article

Heusler alloys, which were unintentionally discovered at the start of the 20th century, have become intriguing materials for many extraordinary functional applications in the 21st century, including smart devices, spintronics, magnetic refrigeration and the shape memory effect. With this review article, we would like to provide a comprehensive review on the recent progress in the development of Heusler alloys, especially Ni-Mn based ones, focusing on their structural crystallinity, order-disorder atoms, phase changes and magnetic ordering atoms. The characterization of the different structures of these types of materials is needed, where a detailed exploration of the crystal structure is presented, encompassing the influence of temperature and compositional variations on the exhibited phases. Hence, this class of materials, present at high temperatures, consist of an ordered austenite with a face-centered cubic (FCC) superlattice as an L2₁ structure, or body-centered cubic (BCC) unit cell as a B2 structure. However, a low-temperature martensite structure can be produced as an L10, 10M or 14M martensite structures. The crystal lattice structure is highly dependent on the specific elements comprising the alloy. Additionally, special emphasis is placed on phase transitions within Heusler alloys, including martensitic transformations ranging above, near or below room temperature and magnetic transitions. Therefore, divers’ crystallographic defects can be presented in such types of materials affecting their structural and magnetic properties. Moreover, an important property of Heusler compounds, which is the ability to regulate the valence electron concentration through element substitution, is discussed. The possible challenges and remaining issues are briefly discussed. Full article

A Fe-30.5wt%Ni-0.155wt%C alloy was annealed at two different temperatures to produce two different austenite grain sizes. In the coarse-grained specimen, hierarchical configurations of variants are formed and carefully analyzed using EBSD. These typical patterns result from the alternate formation of two perpendicular plate groups of variants over several length scales, and two distinct types of mechanical couplings are shown to occur sequentially in the process of the transformation of an austenitic grain. In the fine-grained specimen, the martensite start temperature is depressed below liquid nitrogen temperature, and the martensitic transformation can only occur under stress assistance. Grain size reduction brings about a dramatic change in the morphology of martensite and its configurations. Martensite is fully twinned, and martensite variants arrange themselves into self-accommodating configurations involving all four variants of the same plate group. Those specific configurations share striking similarities with those usually encountered in conventional shape memory alloys. The reversion of such microstructures upon heating is believed to be at the origin of the observed shape memory effect. Full article

Bionic joints are crucial for robotic motion and are a hot topic in robotics research. Among various actuators for joints, shape memory alloys (SMAs) have attracted significant interest due to their similarity to natural muscles. SMA exhibits the shape memory effect (SME) based on martensite-to-austenite transformation and its inverse, which allows for force and displacement output through low-voltage heating. However, one of the main challenges with SMA is its limited axial stroke. In this article, a bionic joint based on SMA wires and a differential pulley set structure was proposed. The axial stroke of the SMA wires was converted into rotational motion by the stroke amplification of the differential pulley set, enabling the joint to rotate by a sufficient angle. We modeled the bionic joint and designed a proportional–integral (PI) controller. We demonstrated that the bionic joint exhibited good position control performance, achieving a rotation angle range of −30° to 30°. The proposed bionic joint, utilizing SMA wires and a differential pulley set, offers an innovative solution for enhancing the range of motion in SMA actuated bionic joints. Full article

Shape memory alloys (SMAs) demonstrate a shape memory effect and superelasticity that can provide recovery performance to structural members. In this study, a round SMA bar was designed to replace the conventional deformed steel bar, particularly within the plastic hinge section of structural members. To integrate the SMA bar and the existing steel bar, a mechanical coupler was proposed by utilizing the advantages of both one-touch and threaded couplers. Uniaxial tensile tests were conducted to analyze the performance of the proposed coupler and the mechanical properties of the SMA–steel connected bar. Stress and strain relationships were examined for steel bars mechanically connected with the SMA bar and for SMA bars before and after exhibiting the shape memory effect. To induce the shape memory effect, SMA should be heated above the finished austenite temperature. Due to the difficulty of accurately measuring strain on the heated bar using traditional contact methods, we employed digital image correlation technology for precise strain measurement of the heated SMA bar. The experimental results indicate the effective application of SMA bars within the plastic hinge region of structural members using the proposed mechanical coupler. Full article

This paper proposes a new topology optimization formulation for obtaining shape memory alloy actuators which are designed with prescribed two-way transforming shapes. The actuation behaviors of shape memory alloy structures are governed by austenite-martensite phase transformations effected by thermal-mechanical loading processes; therefore, to realize the precise geometric shape variations of shape memory alloy actuators, traditional methods involve iteration processes including heuristic structural design, numerical predictions and experimental validation. Although advanced structural optimization methods such as topology optimization have been used to design three-dimensional (3D) shape memory alloy actuators, the maximization/minimization of quantities such as structural compliance or inaccurate stroke distances has usually been selected as the optimization objective to obtain feasible solutions. To bridge the gap between precise shape-morphing requirements and efficient shape memory alloy actuator designs, this paper formulates optimization criteria with quantitatively desired geometric shapes, and investigates the automatic designs of two-way prescribed shape morphing shape memory alloy structures based on the proposed topology optimization method. The super element method and adjoint method are used to derive the analytical sensitivities of the objective functions with respect to the design variables. Numerical examples demonstrate that the proposed method can obtain 3D actuator designs that have the desired two-way transforming shapes. Full article

In this review, we systematically reviewed the recent advances in the development of ultrafine shape memory alloys with unique shape memory effects and superelastic behavior using amorphous metallic materials. Its scientific contribution involves defining and expanding the range of fabrication methods for single-phase ultrafine/nanocrystalline alloys with multicomponent systems. In multicomponent amorphous alloys, the crystallization mechanism depends on the alloy composition and is a selectable factor in the alloy designing method, considering the thermodynamic and physical parameters of constituent elements. The crystallization kinetics can be controlled by modulating the annealing condition in a supercooled liquid state with consideration of the crystalline temperature of the amorphous alloys. The phase stability of austenite and martensite phases in ultrafine shape memory alloys developed from amorphous precursors is determined according to alloy composition and grain size, which strongly influence the shape memory effect and superelastic behavior. A methodological framework is subsequently suggested to develop the ultrafine shape memory alloys based on the systematic alloy designing method, which can be considered an important strategy for developing novel ultrafine/nanocrystalline shape memory alloys with excellent shape memory and superelastic effects. Full article

Shape-memory Mn-rich austenitic stainless steels have a low high-temperature oxidation resistance because Mn tends to inhibit the formation of protective oxides. Mn depletion from oxidation also creates a ferritic Mn-depleted layer. A Mn-depleted layer formed via vacuum annealing has been associated with increased oxidation resistance. Thus, in the present study, a Mn-depleted layer was created with a pre-oxidation treatment conducted at 1000 °C for 30 min. Then, pre-treated and untreated samples were oxidized at 800 °C for up to 200 h. The resulting oxide layers were analyzed, as well as the metal/oxide interface roughness and the ferritic layer thickness. After pre-treatment, a 9 μm thick ferritic layer as well as an oxide layer richer in Cr-containing oxides than those usually observed in FeMnSiCrNi alloys oxidized at 800 °C were detected. After 200 h at 800 °C, the metal/oxide interface roughness of pre-treated samples was considerably lower. The oxidation rate of pre-treated samples was one order of magnitude lower for the first 50 h, but the effect significantly decreased afterward. The pre-existing ferritic layer was unable to stop Mn-rich oxides from being incorporated into the oxide layer, making its effect short-lived. Full article

Cu–Sn shape-memory microw ires were fabricated by a glass-coated melt spinning method. Effects of Sn content on the microstructure and mechanical properties of microwires were investigated. The phase transforms from martensite to austenite with an increase in Sn from 14.0 atomic percent (at.%) to 16.5 at.%. When the Sn content exceeds 16.5 at.%, a highly ordered intermetallic phase, δ, formed. The fracture stress (σ_f) and the critical stress for martensitic transformation (σ_Ms) increases with an increase in Sn content. The mechanical properties as well as the superelasticity were greatly improved by a high cooling rate in the glass-coated melt spinning method. A bamboo-grained structure was formed in the Cu–Sn microwire with a Sn content of 16 at.% by annealing at 750 °C for 5 h before quenching in water. The results indicate that two opposite strategies of refining the grain size to the micrometer level, or increasing the grain size to a one dimensional size of specimen, e.g., the diameter of the wire, are both effective in improving the superelasticity of the Cu–Sn alloy. Full article

In this study, the effect of heat treatment parameters on the optimized performance of Ni-rich nickel–titanium wires (NiTi/Nitinol) were investigated that were intended for application as actuators across various industries. In this instance, the maximum recovery strain and actuation angle achievable by a nitinol wire were employed as indicators of optimal performance. Nitinol wires were heat treated at different temperatures, 400–500 °C, and times, 30–120 min, to study the effects of these heat treatment parameters on the actuation performance and properties of the nitinol wires. Assessment covered changes in density, hardness, phase transition temperatures, microstructure, and alloy composition resulting from these heat treatments. DSC analysis revealed a decrease in the austenite transformation temperature, which transitioned from 42.8 °C to 24.39 °C with an increase in heat treatment temperature from 400 °C to 500 °C and was attributed to the formation of Ni₄Ti₃ precipitates. Increasing the heat treatment time led to an increase in the austenite transformation temperature. A negative correlation between the hardness of the heat-treated samples and the heat treatment temperature was found. This trend can be attributed to the formation and growth of Ni₄Ti₃ precipitates, which in turn affect the matrix properties. A novel approach involving image analysis was utilized as a simple yet robust analysis method for measurement of recovery strain for the wires as they underwent actuation. It was found that increasing heat treatment temperature from 400 °C to 500 °C above 30 min raised recovery strain from 0.001 to 0.01, thereby maximizing the shape memory effect. Full article

Strengthening reinforced concrete elements with externally bonded prestressed fiber reinforced polymer (FRP) sheets has become a popular reinforcement technology in recent years. However, in practical engineering applications, due to the limitations of construction operation space and the need for specialized design of tensioning and anchoring devices, it is very cumbersome to apply prestressing force to FRP sheets. Therefore, using the recovery effect of shape memory alloys (SMA) to introduce prestressing into FRP sheets can innovate a new approach by combining FRP sheets and SMA wires. In order to study the basic mechanical properties of FRP/SMA composites, carbon fiber reinforced polymer and shape memory alloys were used to make the composite specimens, and uniaxial tensile tests were carried out on them. The mechanical properties such as the stress-strain curve, failure mode, ultimate tensile strength and fracture strain were obtained. The test results show that CFRP sheet exhibits obvious linear elastic behavior in tensile tests. The stress-strain curve of SMA wire can be divided into four stages: the linear elastic stage, yield stage, strengthening stage and failure stage. The fracture strain at failure can reach 7%, which indicates excellent deformation properties. The loading and unloading cycles have little effect on the mechanical properties of SMA wire. With the increase in the loading rate, the ‘stress plateau’ section of the phase transformation section of the SMA wire hysteresis curve gradually transits to an oblique upward curve. Increasing the pre-strain value within a certain range can improve the resilience of SMA wires. SMA wires with a pre-strain value of 8% can provide a maximum resilience of 514 MPa after heating to the austenitic state. A prediction model for the number of temperature cycles and maximum recovery force of SMA was proposed and validated. According to this model, the SMA wires can still provide stable resilience after 30 cycles. Increasing the amount of wire (volume ratio) can improve the maximum fracture strain and ultimate tensile strength of CFRP/SMA composite specimens, and the more wire is added, the greater the residual strength after fracture. The diameter of the fiber can significantly reduce the maximum fracture strain and ultimate tensile strength of the FRP/SMA composite specimen. Full article

An attempt to improve the functional characteristics of a degradable Fe-Mn-5Si shape memory alloy by means of structure refinement by equal-channel angular pressing (ECAP) was made. In the course of ECAP, an austenitic ultrafine-grained structure was obtained. In shear bands with a thickness of 301 ± 31 nm, twins 11 ± 1 nm in size were formed. Due to the resulting structure, the tensile strength was doubled up to 1419 MPa, and the yield strength was increased up to 1352 MPa, four times higher compared with the annealed state. Dynamic indentation tests revealed a decrease in Young’s modulus by more than 2.5 times after ECAP compared to values measured in the annealed state. The results of the study of hemolytic and cytotoxic activity in vitro, as well as the local and systemic reactivity of the body of laboratory animals after implantation of the test samples indicate the biocompatibility of the alloy after ECAP. Biocompatibility, high specific strength and low modulus of elasticity open prospects for Fe-Mn-5Si alloy after ECAP to be used for the production of degradable implants that can effectively provide the fastening function in osteoreconstruction. Full article

In recent years, there has been an increase in the development of medical robots to enhance interventional MRI-guided therapies and operations. Magnetic resonance imaging (MRI) surgical robots are particularly attractive due to their ability to provide excellent soft-tissue contrast during these procedures. This paper describes a novel design for a tendon-driven gripper that utilizes four shape memory alloy (SMA) spring actuators and variable stiffness joints controlled by SMA coils for use in MRI surgical robot applications. The contact force of the gripper link is determined by the mechanical properties of the SMA spring actuators (SSA) and the angle of each linkage, and the joint stiffness can be adjusted by varying the electrical current applied to the SMA coil. To enhance the efficiency of the SSAs, a new cooling system using water has been proposed and implemented. To validate the effectiveness of our proposed gripper, we conducted three types of experiments, namely, a single SSA experiment, a single SMA coil experiment, and a whole gripper experiment. The experimental results demonstrate that the proposed water-cooling system can effectively solve temperature issues of SMA, and the joint stiffness in the austenite state is higher than that in the martensite state. Moreover, our experiments show that the presented gripper is capable of grasping and holding objects of various shapes and weights. Full article