Search Results (222)

Loading and clamping schemes significantly influence the behavior of shape memory alloys, specifically, the course of their solid-state transformations. This paper presents experimental and numerical findings regarding the nonlinear response of samples of the above-mentioned type of smart materials observed during tensile tests. Hysteretic properties were studied to elucidate the superelastic behavior of the tested and modeled samples. The conducted tensile tests considered two configurations of grips, i.e., the standard one, where the jaws transversely clamp a specimen, and the customized bollard grip solution, which the authors developed to reduce local stress concentration in a specimen. The characteristic impact of the boundary conditions on the solid phase transformation in shape memory alloys, present due to the specific clamping scheme, was studied using a thermal camera and extensometer. Martensitic transformation and the plateau region in the nonlinear stress–strain characteristics were observed. The results of the numerical simulation converged to the experimental outcomes. This study explains the complex nature of the phase changes in shape memory alloys under specific boundary conditions induced by a given clamping scheme. In particular, variation in the martensitic transformation course is identified as resulting from the stress distribution observed in the specimen’s clamping area. Full article

The incorporation of advanced smart materials, such as shape memory alloys (SMAs), in civil engineering presents significant challenges, particularly in modeling their complex behavior. Traditional numerical SMA models often require material parameters that are difficult to estimate and validate. The objective of this paper is to introduce an equation-based approach to modeling the superelastic behavior of SMAs based on rheological models. The proposed phenomenological model accurately captures SMA superelasticity under isothermal conditions, with each material parameter directly correlated to data from standard mechanical experiments. Four modifications to the baseline rheological model are proposed, highlighting their impact on superelastic characteristics. The resulting constitutive relationships are expressed as non-linear ordinary differential equations, making them compatible with commercial finite element method (FEM) software through user-defined subroutines. The practical application of this modeling approach is demonstrated through the strengthening of a historical masonry wall subjected to seismic activity. Comparative analysis shows that ties incorporating SMA segments outperform traditional steel ties by reducing the potential damage and enhancing the structural performance. Additionally, the energy dissipation during the SMA phase transformation improves the damping of vibrations, further contributing to the stability of the structure. This study underscores the potential of SMA-based solutions in seismic retrofitting and highlights the advantages of equation-based modeling for practical engineering applications. Full article

In this study, the effect of aging heat treatment on the superelastic properties and microstructure of [001]-oriented Fe₄₁Ni₂₈Co₁₇Al_11.5Ti_1.25Nb_1.25 (at.%) single crystals was investigated using the cyclic superelastic strain test and a transmission electron microscope (TEM). The TEM results reveal that the average precipitate size is around 3–5 nm in the 600 °C/24 h samples, 6–8 nm in the 600 °C/48 h samples, and 10–12 nm in the 600 °C/72 h samples. The results indicate that precipitate size increases as aging time increases from 24 to 72 h. EDS analysis results show decreased Fe and increased Ni when the analyzed line crosses the precipitate region. The diffraction pattern results show that the precipitate has an L1₂ crystal structure. The thermo-magnetization curves of single crystals under the three aging conditions (600 °C/24 h, 600 °C/48 h, and 600 °C/72 h) show that the values of the transformation temperatures increased from 24 to 72 h. Magnetization was saturated at 140 emu/g under the magnetic field of 7 Tesla. When increasing the magnetic field from 0.05 to 7 Tesla, the transformation temperatures rose. The results indicate that magnetic fields can activate martensitic transformation. From the results of the superelasticity test at room temperature, [001]-oriented FeNiCoAlTiNb single crystals aged at 600 °C for 24, 48, and 72 h presented recoverable strains of 3%, 5.1%, and 2.6%, respectively. Digital image correlation (DIC) results of the aged samples show that two martensite variants were activated during the superelasticity test. The two variants form corresponding variant pairs (CVPs) and improve the recoverable strain of superelasticity. Although maximum recoverable strain was obtained for the 600 °C/48 h samples, the samples show poor cyclic stability at room temperature after applying the 6% strain. According to the DIC results, the retained martensite, which is pinned by dislocations, was observed after the test. The irrecoverable strain was attributed to the residual martensite. For the 600 °C/72 h samples, the large size of the precipitates poses an obstacle to dislocation transformation and formation. The dislocations increase the stress hysteresis width and stabilize the martensite, causing poor recoverability. Full article

In this paper, the crystal structure, microstructure, and deformation behavior in the Ti-33Nb alloy under furnace-cooling (FC) and water-quenching (WQ) conditions after holding at 950 °C for 0.5 h are reviewed. The stable and metastable phases obtained under FC and WQ heat treatments have significantly different influences on the mechanical properties of this alloy. The furnace-cooling specimens possess a β and α phase at room temperature, while water-quenched specimens are composed of a metastable β phase and martensite α″ phase. According to the results of the nanoindentation test, the hardness value of the FC specimens is 2.66 GPa, which is lower than that of the WQ specimens. It can be attributed to the presence of a large number of α phases. The indentation depth recovery ratio (η_h) and work recovery ratio (η_w) of the WQ specimens are 19.02% and 19.54%, respectively, indicating a better superelastic response than the FC specimens. In addition, the wear resistance (H/E_r) and yield pressure (${\mathrm{H}}^3/{\mathrm{E}}_{\mathrm{r}}^2$) of the WQ specimens are 0.0282 and 0.0030 GPa, respectively, suggesting a better wear resistance and resistance of plastic deformation. Full article

NiTi and NiMnGa stand out as prime thermal and magnetic shape memory alloys (SMAs), possessing a superior shape memory effect (SME) and superelasticity (SE). These alloys have crucial current and potential future applications across industries. Additive manufacturing (AM) offers a transformative approach to fabricating these materials into complex geometries; however, the quest to create integral additively manufactured structures with reliable thermal or magnetic shape memory properties remains a recent and fast-emerging research frontier. This article provides a comprehensive review on (i) the intricate principles giving rise to the thermal SME and SE in NiTi, and the magnetic SME in NiMnGa alloys, emphasising their specific relevance in the realm of AM, and (ii) the latest developments, recent findings, and ongoing challenges in the AM of NiTi- and NiMnGa-based SMAs, including their functional lattice structures. Based on this review, for the first time, novel, empirically derived AM process design maps tailored to maximise SME and SE in laser powder bed fusion- and directed-energy deposition-processed NiTi structures are proposed. Similarly, promising avenues to resolve the key challenges regarding the AM of NiMnGa magnetic SMAs are suggested. This article concludes by outlining the most promising future research directions shaping the trajectory of AM of these SMAs. Full article

The optimization of elastocaloric cooling systems based on Shape Memory Alloys (SMAs) faces significant challenges in practical implementation. Despite promising thermomechanical properties, the development of efficient and compact cooling devices is hindered by incomplete understanding of strain rate effects on transformation behavior and energy conversion efficiency. While previous research has broadly characterized general SMAs’ thermomechanical behavior, the specific relationship between strain rate and elastocaloric performance in Ni-Ti sheets requires systematic investigation to overcome these barriers. This study investigates the strain rate dependence of Ni-Ti sheets’ properties through systematic mechanical characterization across strain rates ranging from $2.56\times {10}^{-4}{\mathrm{s}}^{-1}$ to $6.15\times {10}^{-3}{\mathrm{s}}^{-1}$. Commercial Ni-Ti sheets underwent Shape Setting heat treatment and were characterized at eight different deformation levels using a universal testing machine equipped with a 50 kN load cell. Each deformation level was investigated through tests performed at four different crosshead speeds (1–24 mm/min), while monitoring stress-strain behavior and energy parameters. Results suggest distinct rate-dependent patterns in transformation stresses and energy dissipation characteristics across different strain rates. The analysis indicates that mechanical response and transformation behavior vary significantly between lower and higher strain rates, with implications for practical cooling applications. These findings aim to establish guidelines for optimizing elastocaloric performance by identifying suitable operating conditions for specific application requirements, considering factors such as energy conversion efficiency and cycling frequency. Full article

NiTi alloy has been widely used due to its excellent shape memory effect, superelasticity, and high damping performance. These excellent properties are mainly derived from its unique phase structure. In order to further explore the effect of different phase ratios on the performance of NiTi alloy, this study successfully prepared NiTi alloys with different atomic ratios by controlling the wire feeding speed to control the atomic ratio in the alloy. The results of TEM showed that the alloy with a lower Ni atomic ratio is enriched with Ti element, while the alloy with a higher Ni atomic ratio has a coexistence of NiTi phase and NiTi₂ phase. At the same time, the compression performance showed that the increase in Ni atomic ratio can improve the compression performance of the alloy. In addition, by constructing a molecular dynamics model of NiTi alloys with different phase ratios, the unloading recovery behavior and phase transformation characteristics of the alloy under external force were analyzed. The results showed that with the increase of the NiTi₂ phase ratio in the alloy, the irrecoverable strain also increases, exceeding the elastic strain limit of the NiTi₂ phase, resulting in the generation of disordered structure and plastic deformation in the late deformation stage. In addition, with the increase of the NiTi₂ phase ratio, the energy dissipation area of the hysteresis curve increases, reflecting a greater energy loss. Full article

Ni–Ti shape memory alloy (SMA) foams, capable of bringing revolutionary changes to crucial fields such as aerospace, energy engineering, and biomedical applications, are at the forefront of materials science research. With the aim of designing Ni–Ti SMA foams with complex structures, near-equiatomic Ni–Ti SMA foams featuring a rhombic dodecahedron (RD) structure were fabricated using selective laser melting (SLM) technology. Damping, superelasticity, and quasi-static compressive mechanical tests were carried out on the resultant foams. The findings indicated that the smaller the unit structure of the RD or the larger the rod diameter, the higher the damping and compressive strength of the foams would be. Foams with a cell structure of 2 mm × 2 mm × 2 mm and a rod diameter of 0.6 mm exhibited the highest damping, reaching up to 0.049, along with the highest compressive strength, reaching up to 145 MPa. Furthermore, if the specimen underwent solution and aging heat treatments, its strength could be further enhanced. Meanwhile, the specimens also exhibited excellent superelasticity; even when the pre-strain was 6%, the elastic recovery could still reach 97%. Based on microstructure characterization and finite element simulation, the property mechanisms and deformation rules of the foams were revealed. 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

Nickel–titanium (NiTi) shape memory alloys are promising materials for orthopedic implants due to their unique mechanical properties, including superelasticity and shape memory effect. However, the high nickel content in NiTi alloys raises concerns about biocompatibility and potential cytotoxic effects. This review focuses on the recent advancements in surface modification techniques aimed at enhancing the properties of NiTi alloys for biomedical applications, with particular emphasis on low-temperature glow discharge plasma oxidation methods. The review explores various surface engineering strategies, including oxidation, nitriding, ion implantation, laser treatments, and the deposition of protective coatings. Among these, low-temperature plasma oxidation stands out for its ability to produce uniform, nanocrystalline layers of titanium dioxide (TiO₂), titanium nitride (TiN), and nitrogen-doped TiO₂ layers, significantly enhancing corrosion resistance, reducing nickel ion release, and promoting osseointegration. Plasma-assisted oxynitriding processes enable the creation of multifunctional coatings with improved mechanical and biological properties. The applications of modified NiTi alloys in orthopedic implants, including spinal fixation devices, joint prostheses, and fracture fixation systems, are also discussed. Despite these promising advancements, challenges remain in achieving large-scale reproducibility, controlling process parameters, and reducing production costs. Future research directions include integrating bioactive and antibacterial coatings, enhancing surface structuring for controlled biological responses, and expanding clinical validation. Addressing these challenges can unlock the full potential of surface-modified NiTi alloys in advanced orthopedic applications for safer, longer-lasting, and more effective medical implants. Full article

In the space environment, the elastic vibrations of satellite solar panels are caused by various factors that disturb satellite missions. Therefore, we propose a multi-layered high-damping yoke structure based on a passive control method. To optimize the proposed yoke structure, we performed a free vibration test on various multi-layered blade specimens and designed a yoke structure with the maximum damping performance based on the test results. This high-damping yoke structure was mounted on a dummy solar panel with flexible mode (0.79 Hz) and basic characteristic tests were performed to validate the effectiveness of the solar panel vibration suppression. The test results demonstrated that the proposed multi-layered high-damping yoke is effective in suppressing the vibrations of the first and second modes. In addition, a thermal vacuum test was performed to investigate the delamination between multi-layered structures, and the test results proved the applicability of the proposed yoke structure in an actual space environment. Full article

Shape memory alloys (SMAs) are functional materials with a wide range of applications, from the aerospace sector to the biomedical field. Nowadays, there is a worldwide interest in developing SMAs through powder metallurgy like additive manufacturing (AM), which allows innovative building processes. However, producing SMAs using AM techniques is particularly challenging because of the microstructure required to obtain optimal functional properties. This aspect is critical in the case of Cu–Al–based SMAs, due to their high elastic anisotropy, making them brittle in polycrystalline form. In this work, we approached the processing of a Cu–Al–Ni SMA following a specific powder metallurgy route: gas atomization of a pre-alloyed melt; compaction of the atomized powders through hot isostatic pressing; and a final hot rolling plus thermal treatments. Then, the microstructure of the material was characterized by electron microscopy showing a specific [001] texture in the rolling direction that improved the functional behavior. The successive processing steps produce an increase of about 40 °C in the martensitic transformation temperatures, which can be well controlled and reproduced through the developed methodology. The thermomechanical functional properties of superelasticity and shape memory were evaluated on the final SMA. Outstanding, fully recoverable superelastic behavior of 4.5% in tension, as well as a ±5% full shape memory recovery in bending, were reported for many cycles. These experiments demonstrate the enhanced mechanical and functional properties obtained in polycrystalline Cu–Al–Ni SMAs by powder metallurgy. The present results pave the road for producing this kind of SMA with the new AM technologies, which always produce polycrystalline components and can improve their processes taking the powder metallurgy SMA, here produced, as reference material. Full article

Superelastic metastable β-Ti-Nb alloys are attractive low-modulus materials for use in biomedical implants. The antibacterial properties of silver and its ability to lower the modulus of Ti-Nb-based transforming alloys make it an appealing ternary addition, but the Ti-Nb-Ag system is poorly characterised at present. This study elucidates the microstructure, equilibrium phases, and mechanical behaviour of a systematic series of Ti–24Nb–XAg (X = 0, 2, 6) (at.%) alloys. The mutual solubility of Nb and Ag in Ti overcame the immiscibility of Nb and Ag and produced an alloy with a single-phase β microstructure for low Ag concentrations. However, at silver concentrations above approximately 5 at.%, the solubility limit was reached and precipitates began to form. These precipitates were found to form quickly during recrystallisation, refining the grain size by Zener pinning, and persisted even after a 500 h heat treatment at 1100 °C. All three alloys showed non-linear-elastic behaviour typical of transforming alloys. The addition of up to 2 at.% Ag to Ti–24Nb was found to decrease the elastic modulus, suppress formation of the ω phase, and cause the critical transformation stress to decrease, though the transformation stress increased above that of Ti–24Nb when 6 at.% Ag is added. These results indicate that Ti-Nb-Ag alloys are a promising candidate for developing new low-modulus implants. Full article

Background: Flatfoot deformity is a common condition in children and teenagers that may increase the risk of knee, hip, and back pain. Most of the insoles suggested to treat flatfoot symptoms are not designed to adapt to foot temperature during walking, and they are either too soft to provide support or hard enough to be uncomfortable. Purpose: This study aims to develop an advanced solution to diagnose and treat flexible flatfoot (FFT) using infrared thermography measurements and a hybrid insole reinforced by nitinol (NiTiCu) smart-memory-alloy wires (SMAWs), this super-elastic alloy can return back to its pre-deformed shape when heated, which helps to reduce the local high-temperature points caused by the uneven pressure of FFT. This approach achieves a more uniform thermal distribution across the foot, which makes the hybrid insole more comfortable. Methods: The study involved 16 subjects, divided into two groups of eight flat-footed and eight normal. The procedure includes two parts, namely, designing a prototype insole with SMAW properties based on thermography measurement by using SolidWorks, and evaluating this design using Ansys. Second, a hybrid insole reinforced with SMAWs is customized for flatfoot subjects. The thermography measurement differences between the medial and lateral sides of the metatarsophalangeal line are compared for the normal and flatfoot groups before and after wearing the suggested design. Results: The results show that our approach safely diagnosed FFT and significantly improved the thermal distribution in FFT subjects by more than 80% after wearing the suggested design. A paired t-test reported significant (p-value > 0.001) thermal decreases in the high-temperature points after using the SMAW insole, which was closely approximated to the normal subjects. Conclusions: the SMAW-reinforced insole is comfortable and suitable for treating FFT deformity, and infrared thermography is an effective tool to evaluate FFT deformity. Full article

The work described in this article is aimed at investigating the properties of a group of Ti-Zr-Nb alloys. In modern orthopedics and traumatology, the use of materials for bone implants with a minimum modulus of elasticity is becoming increasingly important. This is due to a number of advantages that allow for better integration of the implants with the bone tissue, including the reduction in the detrimental effect of the load-shielding effect, a better load distribution, and stress distribution, which allows for increasing the life of the implant. It is known that the lowest modulus of elasticity in titanium alloys at normal temperature is achieved by the phase composition consisting of metastable β-phase. It is possible to achieve the desired structure by a combination of alloy composition selection and heat treatment. Quenching of titanium alloys allows for the high-temperature β-phase to be fixed. This paper provides justification of the choice of compositions of the studied alloys by calculation methods. The structure of alloys after melting in a vacuum electric arc furnace in an argon environment was studied. The ingots obtained had a dendritic structure. Homogenizing annealing in a vacuum furnace at 1000 °C for 4 h was used to equalize the composition. The structure of the alloyed sheets after hot rolling and hot rolling and quenching was investigated. The microstructure of the plates in both variants had uniform grains of polyhedral shape. X-ray phase analysis of the plates showed that the content of metastable β-phase was 100% before and after quenching. Microhardness testing of the plates showed no significant effect of quenching. The result of the mechanical properties study showed an increase in the plasticity of the material after quenching, with the tensile plots of the samples after quenching reflecting the area where the reverse phase transition of β’<-> α’’ occurs. Mechanical studies by cyclic loading showed the presence of a superelasticity effect. The Young’s modulus study gave a result of 51 GPa for one of the compositions studied. The combination of properties of the materials under investigation has the potential for promising use as a basis for bone implants. Full article