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Shape Memory Alloys (SMA) are materials used to design smart structures with intrinsic functional properties and improved efficiency. This is a key aspect of aerospace industry and makes SMA good candidates in this field. One of the most widespread SMA is the equiatomic NiTi alloy which, however, has the strong limitation of poor machinability, so only simple shapes can be obtained. Additive Manufacturing processes allow to overcome this limit and to design complex shapes. Compared to other metallic materials, the optimization of the process for NiTi alloy is complicated because, beside mechanical properties and presence of defects, considerable attention needs to be dedicated to the material functionality. The high temperatures involved in the additive process significantly affect the material properties due to possible evaporation of Ni and formation of precipitates that enable a shift of the phase transformation temperatures. This paper is focused on the optimization of the process parameters of the NiTi alloy printed through the Laser Powder Bed Fusion (L-PBF) to ensure optimal pseudo-elastic behaviour, which is essential for the design of structural dampers. This was accomplished starting from simple structures and then designing a damper that couples the pseudoelasticity of NiTi with load support capacity. The L-PBF is a powder-bed technique that selectively melts layers of micrometric metal powder. A pseudoelastic NiTi powder with 50.8 at. % of Ni content was selected and characterized through scanning electron microscope (SEM) and observations connected to an Energy Dispersive X-ray Spectroscopy (EDX) probe. After that, some cubic samples were manufactured, with the dimension of 3 × 3 × 15 mm3. A set of different laser powers and scanning speeds were used to find the set of process parameters that optimize the functional properties of the printed parts. Near fully dense specimens with density higher than 99.5 % were selected for further investigations. Differential scanning calorimetry (DSC) and mechanical tests were performed on as-built and heat-treated samples. Quasi-static mechanical tests were accomplished in compression mode, at different strains, up to 8 %. It was observed that the residual strain for cyclic loading at 4 % is lower than 1 %, so good recovery of the deformation was shown. Moreover, numerical analyses that mimic the pseudoelastic behaviour in compression tests were implemented. Finally, the best set of parameters was selected on the basis of the material's ability to recover deformations and its loss factor. • Optimization of process parameters for functional behaviour of SMA. • Design of damping structure using pseudoelastic effect of Nitinol. • Internal defects observation of manufactured specimens. • Compression tests on as built and thermal treated specimens with strain up to 8 %. • Numerical simulation of compression tests with experimental correlation. [ABSTRACT FROM AUTHOR]

Memory‐steel for smart steel structures – a review on recent developments and applications This study comprehensively examines recent advancements in the development and utilization of iron‐based shape memory alloy (Fe‐SMA), commonly known as memory‐steel, within the retrofitting of steel structures. The discussion is structured into several key areas. Initially, a thorough analysis of the material properties of Fe‐SMA is presented, focusing on aspects such as shape memory effect and super‐elasticity. Subsequently, the application of Fe‐SMA in prestressed strengthening of steel structures is elucidated, encompassing its role in reinforcing steel girders, connections, and addressing fatigue crack repairs. Various strengthening methodologies are explored, including the utilization of mechanically anchored or adhesively‐bonded Fe‐SMA. The study delves into an in‐depth examination of the behavior exhibited by Fe‐SMA‐to‐steel bonded joints. Notably, the research highlights the practical application of Fe‐SMA in strengthening a 113‐year‐old steel bridge. Furthermore, innovative applications of Fe‐SMA as pipe couplers are introduced, showcasing its versatility in diverse structural contexts. [ABSTRACT FROM AUTHOR]

Herein, the quality and accuracy to manufacture delicate parts from NiTi powder using electron beam powder bed fusion (EB-PBF) technology is investigated. Therefore, benchmarks with thin cylinders and thin walls are designed and fabricated using two distinct scan strategies of EB-PBF manufacturing (i.e., continuous melting and spotmelting) with different process parameter sets. After these optimizations, four different lattice structures (i.e., octahedron, cell gyroid, sheet gyroid, and channel) are manufactured and characterized. It is shown both continuous melting and spot melting modes are able to manufacture lattices with relative densities over 97%. And as-built lattice structures exhibit an excellent pseudoelasticity up to 8% depending on the design of the structure, e.g., the channel structure shows more deformation recoverability than the cell gyroid. This is attributed to the integrity of geometry as well as compressive mode of the mechanical loading. Of course, the compressive strength and ultimate compressive strength also increase with the increasing volume fraction. Moreover, the spotmelting can be used as an engineering tool to customize a delicate beamshaped structure with a superior pseudoelasticity. [ABSTRACT FROM AUTHOR]

A systematic investigation was conducted to evaluate the role of Zr addition on the martensitic phase transformation behavior and properties of NiTi-based shape memory alloys. Equiatomic binary NiTi alloy and ternary Ni50Ti45Zr5 and Ni50Ti40Zr10 alloys (at.%) were processed by a simple thermomechanical treatment including cold rolling followed by post-deformation annealing at 450 °C for 30 and 60 min. The microstructural investigation and thermal analyses reveal that martensitic phase transformation is suppressed by adding Zr. Solid solution strengthening due to Zr addition, strain hardening due to cold rolling and formation of (Ti,Zr)2Ni precipitates in a dual-phase microstructure during annealing leads to a significant increase in hardness in the thermomechanically treated alloys (e.g., up to 550 Hv for Ni50Ti45Zr5 alloy). The stress-strain curve of martensitic Ni50Ti50 alloy reveals the essential characteristics of shape memory effect, however, Ni50Ti45Zr5 alloy shows typical features of pseudoelasticity with significantly high stress plateau (∼600 MPa) and remarkable recovered strain of 3.5% during unloading.

This study evaluated the influence of heat treatment and thermomechanical training on the microstructural evolution and mechanical characteristics of conventional and additive-manufactured FeMnSi-based shape memory alloys. The conventional samples were produced by casting and rolling. The additive-manufactured samples were manufactured using the laser powder bed fusion (L-PBF) technique. Both specimens were subjected to the same heat treatment and thermomechanical training. The heat treatment involved solution annealing at 1050 °C for 2 h and aging at 750 °C for 6 h, and the thermomechanical training concluded with a 4% elongation at ambient temperature followed by annealing at 250 °C for 15 min. This training cycle was repeated four times for each sample after heat treatment. The heat treatment improved the pseudoelasticity and shape memory effect of the samples. Although training further enhanced the pseudoelasticity, it also reduced the shape memory effect. Thermomechanical training led to the formation of a large number of stacking faults, which facilitated the inverse phase transformation of martensite (ε) to austenite (γ) during unloading, resulting in improved pseudoelasticity. The heat-treated additive-manufactured samples showed the highest total recovery strain owing to the pseudoelasticity and shape memory effect. This characteristic could be due to the smaller grain size and higher volume fraction of precipitates. The precipitates and grain refinement improved the conditions for partial dislocation motion by increasing the back stresses on the martensite tip.

This study reviews the recent works on the development and application of iron‐based shape memory alloy (Fe‐SMA), the so‐called memory‐steel, for steel structures. First, the studies on the material properties of Fe‐SMA in terms of shape memory effect and superelasticity are discussed. Next, the use of Fe‐SMA in prestressed strengthening of steel structures is explained, including the applications in strengthening of steel girders, connections, and fatigue crack repairs. Various strengthening solutions such as using mechanically anchored or adhesively‐bonded Fe‐SMA, as well as the studies on the behavior of the Fe‐SMA‐to‐steel bonded joints, are discussed. The use and application of Fe‐SMA for strengthening of a 113‐years steel bridge has been explained. In addition, studies on the innovative application of the Fe‐SMA as pipe couplers are presented. At the end, innovative ongoing research on the additive manufacturing of architected Fe‐SMA (4D‐printing) are discussed. [ABSTRACT FROM AUTHOR]

The limited wear resistance of commercially pure titanium (CP-Ti) hinders its use in abrasive and erosive environments, despite its good strength–weight ratio and corrosion resistance. This paper reports the first study proposing a novel method for wear-resistant TiNi coating through Ni plating and electron beam (EB) irradiation in an in situ synthetic approach. Single-track melting experiments were conducted using the EB to investigate the feasibility of forming a TiNi phase by fusing the Ni plate with the CP-Ti substrate. Varying beam powers were employed at a fixed scanning speed to determine the optimal conditions for TiNi phase formation. The concentration of the melt region was found to be approximate as estimated from the ratio of the Ni-plate thickness to the depth of the melt region, and the region with Ni-48.7 at.% Ti was successfully formed by EB irradiation. The study suggests that the mixing of Ti atoms and Ni atoms was facilitated by fluid flow induced by Marangoni and thermal convections. It is proposed that a more uniform TiNi layer can be achieved through multi-track melting under appropriate conditions. This research demonstrates the feasibility of utilizing EB additive manufacturing as a coating method and the potential for developing TiNi coatings with shape memory effects and pseudoelasticity. [ABSTRACT FROM AUTHOR]

In order to develop novel high damping materials with excellent pseudoelasticity (PE) properties to meet the application requirements in aerospace, medical, military and other fields, porous Ni50.8Ti49.2 shape memory alloy (SMA) was prepared by the powder metallurgy method. Different contents of Nb element were added to regulate the microstructures. It was found that after adding the Nb element, the number of precipitates significantly decreased, and the Nb element was mainly distributed in the Ni-Ti matrix in the form of β-Nb blocks surrounded by Nb-rich layers. Property tests showed that with the increase in Nb content, the damping and PE increased first and then decreased. When the Nb content reached 9.0 at.%, the highest damping and the best PE could be achieved. Compared with the porous Ni-Ti SMA without Nb addition, the damping and PE increased by 60% and 35%, respectively. Correlated mechanisms were discussed. [ABSTRACT FROM AUTHOR]

<100>-, <112>- and <149>-oriented single crystals of Fe75.5Ga24.5 alloy were prepared by optical float zone melting method. The pseudoelasticity behavior and elastocaloric effect of the single crystals were investigated, as well as the associate microstructures. D03 phase structure was realized by solution treatment at 800 °C and annealed at 600 °C for 10 h. The compressive deformation behavior exhibits significant dependence on the crystalline directions. Excellent pseudoelasticity with recoverable strain up to 5% is obtained by compression along <149> direction. The pseudoelasticity disappears after five deformation cycles. Adiabatic temperature change is simultaneously detected during the pseudoelasticity, especially during loading process. [ABSTRACT FROM AUTHOR]

Joining wires made of NiTi alloys with shape memory effect and pseudoelasticity causes many technical and structural problems. They result from unwanted phase interactions that occur in high temperatures and negatively affect the characteristics of these materials. Such obstacles are challenging in terms of welding. Hence, an attempt was made to join NiTi wires via an economical and reliable basic laser welding technique which does not require complicated equipment and gas protection. The parameters such as spot diameter and pulse time were constant and only the laser power, calculated as a percentage of the total power, was optimized. The wires were parallelly connected with overlapping seam welds 10 mm long. The welds were examined regarding their microstructure, chemical and phase composition, reversible martensitic transformation, microhardness, and pseudoelasticity. The obtained results showed that the joint was completed at the 12–14% power. The weld revealed good quality with no voids or pores. As the laser power increased, the microhardness rose from 282 (for 4%) to 321 (for 14%). The joint withstood the stress-inducing reversible martensitic transformation. As the transformation was repeated cyclically, the stress value decreased from 587 MPa (initial wire) to 507 MPa (for the 14% power welded wire). [ABSTRACT FROM AUTHOR]

The present study investigates the corrosion behavior of a Fe–Mn–Al–Ni–Cr shape memory alloy in a 5.0 wt% NaCl solution in combination with the functional properties. In a single crystalline condition, the investigated alloy shows a superior superelastic response with a maximum recovery strain of 5.2%. Furthermore, the results of the potentiodynamic polarization and impedance spectroscopy show that the addition of chromium generally improves corrosion resistance by increasing the resistance to localized corrosion. The single crystalline condition, which is characterized by an austenitic matrix and scattered martensite plates, shows an enhanced, however, still unstable passive behavior due to the formation of corrosion pits along the martensite plates. [ABSTRACT FROM AUTHOR]

Today, two-way shape memory alloys have become increasingly popular in various industries. In this study, innovative nitinol-based bi-layers were fabricated using the additive approach. The purpose of the study was to examine the characteristics of a bi-layer sample made of Ni 50.8 Ti 49.2 that was deposited using Laser Powder Bed Fusion on a Ni 45 Ti 50 Cu 5 substrate made by vacuum arc remelting. Transmission electron microscopy with high resolution was used to study the bi-layer's microstructure. The as-printed bi-layer exhibits dislocation, stacking faults, precipitates such Ni 4 Ti 3 and Ti 2 Ni, compound twins, and Type Ⅱ twins, according to the transmission electron microscopy investigation. After going through annealing at a temperature of 1000 °C for a duration of 16 h, it was observed that Type Ⅰ and compound twins were the prevailing characteristics. Furthermore, it has been demonstrated that subjecting the material to heat treatment leads to enhanced elongation and reduced fracture stress. The sample aged at 500 °C for 30 min exhibited the most favorable properties, including a maximum elongation of 10 ± 0.1 % after 12 incremental cycles. In addition, the bi-layer samples exhibited both pseudoelasticity and shape memory effect, with approximately 80 % intrinsic reversible two-way shape memory behavior. The findings of this study can be applied in the production of various actuators. [ABSTRACT FROM AUTHOR]

Martensitic phase transformation and plasticity are two primary mechanisms of deformation in shape memory alloys (SMAs) and the interaction between them influences the behaviour of SMA during cyclic loading, specifically the pseudoelasticity behaviour and the shape memory effect. This interaction, which occurs in microscale, affects the reversibility and eventually the actuation capacity of SMAs. In order to capture this interaction in microscale, a discrete dislocation–transformation model was developed in Sakhaei et al. (Mech Mater 97:1–18, 2016) and was applied to simulate the single-crystalline NiTi samples under thermo-mechanical loads. In this study, the microscale coupling between phase transformation and plasticity as well as grain size and orientation effects is investigated in multi-crystalline shape memory alloys under thermal and mechanical loading by using the discrete dislocation–transformation framework through the representative numerical simulations. The results illustrated the dependency of dislocation slip and martensitic transformation to crystalline orientations as well as grain size and grain boundary densities in the multi-crystalline SMAs. [ABSTRACT FROM AUTHOR]

In this paper, superelastic (SE) NiTi wire is used to fabricate axially symmetric wire mesh dampers (WMDs) with the expectation of a higher damping capacity. However, the phase transformation damping of the NiTi WMD could be suppressed by the cold-work-induced dislocation. Therefore, the NiTi WMDs were heat-treated and then tested by a hydraulic universal testing machine. The NiTi WMD is found to achieve higher damping capacity when heat-treated at 200 °C. However, the WMD heat-treated at 250 °C suffers from a sharp decline in the loss factor in exchange for an improvement in the stiffness. The sine sweep test was then conducted to examine the dependency of the WMD's vibration isolation performance upon the heat treatment temperature and the excitation acceleration. The NiTi WMD outperforms the 304 stainless steel (SS 304) WMD in damping capacity only when the excitation acceleration magnitude is less than 1.5 g. The stiffness of NiTi WMD can be improved without significantly compromising its damping capacity by heat treatment at 200 °C for 30 min. The present work carries out comprehensive measurements of the NiTi WMD's response to dynamic mechanical test and sine sweep test and addresses how heat treatment influences the stiffness and damping capacity of the SE NiTi WMD. [ABSTRACT FROM AUTHOR]

An experimental investigation on the cyclic stability of solutionized Ni44.8Cu5Ti45.2Hf5 and Zr‐substituted Ni44.8Cu5Ti40.2Hf5Zr5 (atomic percent, at%) medium‐entropy shape memory alloys (MESMAs) was carried out. The shift of the peak temperature for the forward B2→B19′ transformation in Ni44.8Cu5Ti45.2Hf5 alloy after 50 differential scanning calorimetry (DSC) cycles was measured to be 23 °C, substituting 5 at% Zr for Ti increased it to 41 °C. No residual strain accumulation was observed in Ni44.8Cu5Ti45.2Hf5 alloy after 10 thermal cycles under a tensile stress of 200 MPa, and meanwhile the shape memory strain increased from 5% to 6.5%. A pseudoelastic strain (εPE) of ≈2.7% was obtained in Zr‐substituted Ni44.8Cu5Ti40.2Hf5Zr5 alloy at 55 °C (Af + 10 °C) for a maximum deformation (εmax.) of 4.4%. In addition, the stability of pseudoelasticity in Ni44.8Cu5Ti40.2Hf5Zr5 alloy during 100 loading–unloading cycles was examined in the solution annealed (1000 °C for 1 h) and aged (400 °C for 1 h) conditions. The results showed that aging treatment could effectively retard the degradation of pseudoelasticity in the alloy. These preliminary findings suggest that the idea of increasing chemical complexity in NiTi‐based SMAs holds promising potential in developing novel SMAs with enhanced cyclic stability. [ABSTRACT FROM AUTHOR]

Nickel–Titanium (Ni–Ti) alloys have exemplary properties such as good thermo-mechanical response and biocompatibility. Hence, they are one of the prime materials of interest in today's context for the fabrication of advanced biomedical devices. Such properties can be exemplified with certain techniques such as heat-treatment techniques. However, peer-reviewed literature for the same appears to be inadequate to efficiently quantify the effect of such techniques on the overall performance of such materials. Therefore, the present study focuses on analysing the effect of low-temperature heat treatment on the superelastic behaviour and corrosion resistance of titanium-rich Ni–Ti alloys. The superelastic behaviour has been evaluated for different strains. Meanwhile, the Tafel extrapolation method is incorporated to assess the electrochemical corrosion of these materials. It is noticed that the low-temperature heat treatment has marginal effects on the superelastic behaviour of the alloy. The Ni–Ti alloy annealed at 450°C at 6% predetermined strain rate showed the best enhancement in the superelastic behaviour and the corrosion resistance increased upto treatment temperatures of 350°C. Therefore, the most optimum temperature of annealing is found to be 350°C, resulting in about 35.72% improvement in the corrosion resistance of the alloy when compared to their untreated counterparts. [ABSTRACT FROM AUTHOR]

Nitinol is a well-known shape memory alloy (SMA) which is widely used due to its unique properties such as shape memory effect and pseudoelasticity. However, challenges fabricating Nitinol parts have limited the use of this alloy. Nowadays, additive manufacturing methods, specifically selective laser melting (SLM), are being used as an alternative to conventional methods for fabricating Nitinol specimens. Achieving a dense structure and controlling the transformation temperatures in such products have been among the most important challenges for several research groups. In the present study, fabrication of dense Nitinol parts by SLM together with control of their transformation temperatures is investigated with the main purpose of achieving pseudoelastic products at room temperature. For this purpose, the effect of process parameters on density, transformation temperatures, microstructure, hardness, and shape memory response are studied. The influence of process parameters on transformation temperatures varies depending on the amount of power so that the effect of scan tracks spacing for high powers is more pronounced than that for low powers. The hardness and compressive strength of the parts are also affected by the process parameters. Accordingly, optimal parameters are found to fabricate dense pseudoelastic parts with the ability of strain recovery at ambient temperature. [ABSTRACT FROM AUTHOR]

The use of shape memory alloys (SMAs) in civil structures attracted the attention of researchers worldwide. This interest is due the unique properties of SMAs, the shape memory effect (SME) and pseudoelasticity. Other desirable attributes for SMA include biocompatibility, high specific strength, wear resistance, anti-fatigue characteristics, and high yield strength. These beneficial qualities allow for a wide range of applications ranging from aerospace field to the medical and civil engineering fields. To intelligently utilize shape memory alloys and widen their application potential, certain characteristics including the heat activation and heat transfer mechanisms must be thoroughly understood. Such understanding would allow the development and optimization of systems utilizing SMA. Hence, this paper presents a state-of-the-art review about shape memory alloys with specific focus on the heat activation mechanisms and the heat transfer modes.

A Co-Ni-Ga high-temperature shape memory alloy has been additively manufactured by directed energy deposition. Due to the highly anisotropic microstructure, i.e. columnar grains featuring a strong near-⟨001⟩ texture in build direction, the as-built material is characterized by a very low degree of constraints and, thus, shows excellent superelasticity without conducting a post-process heat treatment. As characterized by in situ deformation testing and post-mortem microstructural analysis, additive manufacturing employing directed energy deposition seems to be highly promising for processing of shape memory alloys, which often suffer difficult workability.

Herein, a new torsional device based on shape memory alloy elements and operating as torque limiter is presented. It consists of a set of pseudoelastic NiTi tapes working in series configuration. The tapes are assembled radially with respect to the rotational direction and work sequentially in beam‐like bending mode. The trend of the resulting torque may span from pulsating to continuous by changing the number of tapes. Furthermore, the use of NiTi material enables a temperature‐dependent response. Due to the particular mounting, the flag‐shaped pseudoelastic mechanical behavior typical of NiTi is partially hidden; however, the use of pseudoelastic elements provides a stable cyclic response and enables the possibility to tune the output torque. [ABSTRACT FROM AUTHOR]

This paper examines the influence of 0-0.2 wt.%B-doping on the microstructure, mechanical properties, and pseudoelastic behavior of Cu-Al11.5-Be0.57 shape memory alloys (SMAs). This microstructure study reveals that the addition of boron leads to significant grain refinement in β 1 -type polycrystalline Cu-Al-Be SMAs. A maximum refinement size of 50 µm was achieved with the addition of 0.15 wt.%B. The fine-grained (Cu-Al11.5-Be0.57)-B0.15 SMA with serrated grain boundaries exhibited the maximum enhancement of ultimate tensile strength, 744.65 ± 29.34 MPa, and ductility of 21.93 ± 0.56%. The fracture morphology revealed the transformation of intergranular to transgranular fracture in the SMAs with boron-doping. Maximum pseudoelasticity of 4% was achieved in the SMA with 0.15 wt.%B and suits as a damper in seismic applications. [ABSTRACT FROM AUTHOR]

We propose a new concept of heat-storage material that utilizes thermoelastic martensitic transformation in a TiNi-based alloy. In the material, the stored thermal energy is maintained over a wide temperature range between the thermal absorption and desorption temperatures, and the stored energy is released on demand by external stress on the ground of a latent heat of the stress-induced martensitic transformation. The heat-storage temperature range was effectively increased through a thermomechanical treatment without changing the amount of latent heat. The tensile force applied to the treated sample released to the environment more heat than the unbiased desorption temperature. The amount of heat released was in agreement with the values expected from the Clausius–Clapeyron equation and differential scanning calorimetry profiles. The amount of heat released was confirmed to remain unchanged up to the 9th cycle when the release process was repeated at 286 K by applying tension after heat absorption. [Display omitted] [ABSTRACT FROM AUTHOR]

In this paper, superelastic (SE) NiTi wire is used to fabricate axially symmetric wire mesh dampers (WMDs) with the expectation of a higher damping capacity. However, the phase transformation damping of the NiTi WMD could be suppressed by the cold-work-induced dislocation. Therefore, the NiTi WMDs were heat-treated and then tested by a hydraulic universal testing machine. The NiTi WMD is found to achieve higher damping capacity when heat-treated at 200 °C. However, the WMD heat-treated at 250 °C suffers from a sharp decline in the loss factor in exchange for an improvement in the stiffness. The sine sweep test was then conducted to examine the dependency of the WMD’s vibration isolation performance upon the heat treatment temperature and the excitation acceleration. The NiTi WMD outperforms the 304 stainless steel (SS 304) WMD in damping capacity only when the excitation acceleration magnitude is less than 1.5 g. The stiffness of NiTi WMD can be improved without significantly compromising its damping capacity by heat treatment at 200 °C for 30 min. The present work carries out comprehensive measurements of the NiTi WMD’s response to dynamic mechanical test and sine sweep test and addresses how heat treatment influences the stiffness and damping capacity of the SE NiTi WMD.

Pseudoelastic NiTi structural components have been shown to exhibit tension/compression asymmetry. Under tension, phase transformation induces localized strains of nearly 7% that spread under nearly constant stress, while under compression the transformation leads to hardening at significantly higher stress, the strain is smaller and grows homogeneously. Bending of tubes ovalizes their cross-section, a nonlinearity that reduces the bending rigidity and precipitates instabilities that can lead to collapse. This paper uses experiment and analysis to examine the interaction of these geometric nonlinearities with the complex material behavior of pseudoelastic NiTi tubes of moderate diameter-to-thickness ratios under bending. An experiment on a tube with a diameter-to-thickness ratio of 18.7 shows that transformation causes the nucleation of several dispersed high strain banded patterns of martensite on the tensioned side. The zone where the bands coalesce into intersecting diamonds undergoes excessive local ovalization causing buckling and collapse at a relatively low overall tube curvature. The bending experiment is simulated with a finite element model coupled with a custom constitutive model of pseudoelastic NiTi that allows for plastic deformation of the martensitic phase. The analysis reproduces the moment-end rotation response, and the nucleation and evolution of the high strain diamond shaped patterns. Zones that develop such patterns exhibit accelerated growth of ovalization that degrades the local bending rigidity. In the presence of small geometric imperfections this degradation evolves into a buckle in the form of diffuse local ovalization that precipitates collapse that can negate the recoverable property of NiTi. Parametric studies show that as the tube D/t increases, the interaction between the material and geometric nonlinearities becomes stronger making buckling more imperfection sensitive. [ABSTRACT FROM AUTHOR]

Novel applications of shape memory alloys (SMAs) in various fields of medicine and engineering necessitate theoretical models that can capture non-inherent material behaviors such as the two-way shape memory effect (TWSME). Most often, this requires the use of completely different sets of models or parameters to distinguish the "virgin" material exhibiting the fundamental behaviors like superelasticity (also known as pseudoelasticity), pseudoplasticity, and one-way shape memory effect, from the "trained" alloy showing the TWSME. Although using a single model (or set of parameters) can provide much insight on the changes within the material during various loading paths and histories from the initial virgin state to the trained state, it is quite challenging. In this study, an attempt was made to use a developed general multi-mechanism material model and a single set of parameters to predict both the inherent pseudoelastic behavior and non-intrinsic two-way shape memory effect characteristics of a NiTi shape memory alloy. To this end, several simulations were carried out to investigate the effect of pseudoelastic training conditions on the corresponding actuation strain, cyclic stability, and transformation temperatures of the NiTi under zero-load TWSME. Whilst previous studies considered external indicators such as the magnitude of plastic or irrecoverable strain as a prerequisite for TWSME, the present model focused primarily on internal stresses. Based on the magnitude of the zero-load actuation strain (measured from the trained material under no external load), the model results indicated that the magnitude of the pseudoelastic training strain (in comparison to the training temperature, and the number of training cycles) has a pronounced effect on the cyclic TWSME actuation characteristics. Moreover, observations from the internal state (hidden) variables of the model, representing the intrinsic residual stress state in the trained material (under zero observable/external stress) showed the possibility for an SMA to generate TWSME even if there is no significant accumulation of permanent strains. [ABSTRACT FROM AUTHOR]

Scarcity of stiff, yet compliant, materials is a major obstacle toward biological‐like mechanical systems that perform precise manipulations while being resilient under excessive load. A macroscopic cellular structure comprising two pre‐stressed elastic "phases" is introduced, which displays a load‐sensitive stiffness that drops by 30 times upon a "pseudoductile transformation" and accommodates a fully recoverable compression of over 60%. This provides an exceptional 20 times more deformability beyond the linear‐elastic regime, doubling the capability of previously reported super‐elastic materials. In virtue of the pre‐stressing process based on thermal‐shrinkage, it simultaneously enables a heat‐activated self‐formation that transforms a flat laminate into the metamaterial with 50 times volumetric growth. The metamaterial is thereby inherently lightweight with a bulk density in the order of 0.01 g cm−3, which is one order of magnitude lower than existing super‐elastic materials. Besides the highly programmable geometrical and mechanical characteristics, this paper is the first to present a method that generates single‐crystal or poly‐crystal‐like 3D lattices with anisotropic or isotropic super‐elasticity. This pre‐stress‐induced adaptive stiffness with high deformability could be a step toward in situ deployed ultra‐lightweight mechanical systems with a diverse range of applications that benefit from being stiff and compliant. [ABSTRACT FROM AUTHOR]

To understand heavy truck crash severity, a total of 1,818 sample crashes occurring over a six-year period (2011-2016) were recorded on four typical expressway segments in mountainous regions of Jiangxi and Shaanxi, China. The crashes were investigated using an ordered logit approach to determine the significant variables contributing to truck crash severity. Elasticity analysis was then used to measure the pseudoelasticity of each independent variable. A total of 12 explanatory variables, including less than three years driving experience of truck driver, type of commercial transport task of the vehicle, brake failure, speeding behavior, overloading condition, steep downhill grade of the road and slippery surface condition, hours between midnight and dawn, winter season, adverse weather, and head-on and rear-end collision types, were found to be statistically significantly correlated with the severity of truck crashes occurring on Jiangxi and Shaanxi mountainous expressways. Only truck drivers older than 55 years of age, presence of tunnel and runoff collision type were identified to have a significant and positive effect on the severity of truck crashes on Shaanxi expressways. Taken together, these findings provide useful guidance to develop safer design standards, stricter regulations and more efficient technical countermeasures in order to reduce or prevent the occurrence of serious heavy-truck crashes on mountainous expressways in China. [ABSTRACT FROM AUTHOR]

The study is focused to understand the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress induced martensitic transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is investigated. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The use of shape memory alloys (SMAs) in civil structures attracted the attention of researchers worldwide. This interest is due the unique properties of SMAs, the shape memory effect (SME) and pseudoelasticity. Other desirable attributes for SMA include biocompatibility, high specific strength, wear resistance, anti-fatigue characteristics, and high yield strength. These beneficial qualities allow for a wide range of applications ranging from aerospace field to the medical and civil engineering fields. To intelligently utilize shape memory alloys and widen their application potential, certain characteristics including the heat activation and heat transfer mechanisms must be thoroughly understood. Such understanding would allow the development and optimization of systems utilizing SMA. Hence, this paper presents a state-of-the-art review about shape memory alloys with specific focus on the heat activation mechanisms and the heat transfer modes. [ABSTRACT FROM AUTHOR]

This work proposes a phenomenological level-set model for ferromagnetic shape-memory alloys (FSMAs), developed under consistent thermodynamic analysis. A set of adequate internal and external variables is chosen so as to describe the rate-independent magneto-mechanical response of a FSMA single crystal. Almost every phenomenological model in bibliography adopts martensitic volume fractions as internal variables. The novelty of this work is the introduction of a continuous level-set function as an internal variable of state, which accounts implicitly for the dissipative twin boundary motion in the evolution of inelastic reorientation of martensitic variants. Following the usual internal variable formalism within the framework of standard magnetomechanics, the evolution equations for the level-set function are derived for the forward and reverse variant reorientation processes. The model is implemented in a two-dimensional special case, where the reduced equations are numerically solved capturing the important effects of stress-induced and magnetic field-induced martensitic variant reorientation, such as magnetization hysteresis, pseudoelastic/partial pseudoelastic behavior and magnetic shape-memory effect. The proposed constitutive model is capable of explaining the constitutive response caused by the reorientation of martensitic variants in FSMAs and may be further improved so as to be implemented in even more complicated situations, such as dynamic and rate-dependent analysis. [ABSTRACT FROM AUTHOR]

The paper elucidates the superelastic characteristics of high entropy shape memory alloy (HESMA) (TiZrHf) 50 Ni 25 Co 10 Cu 15 over a wide range of deformation temperatures. Axial compression experiments are presented for different heat treatments and local recoverable strains up to 5% are noted. Superelasticity is observed over a large temperature range up to 175°C. Depending on heat treatment, the critical stress exhibits a Clausius-Clapeyron (CC) slope between 1.4 and 6 MPa/°C up to 80 °C deformation temperature. Obvious deviations from linearity however and significantly higher transformation stresses reaching 1200 MPa are reported at higher temperatures. The temperature change during transformation is reported at 9 °C. Image, graphical abstract [ABSTRACT FROM AUTHOR]

A Co-Ni-Ga high-temperature shape memory alloy has been additively manufactured by directed energy deposition. Due to the highly anisotropic microstructure, i.e. columnar grains featuring a strong near-⟨001⟩ texture in build direction, the as-built material is characterized by a very low degree of constraints and, thus, shows excellent superelasticity without conducting a post-process heat treatment. As characterized by in situ deformation testing and post-mortem microstructural analysis, additive manufacturing employing directed energy deposition seems to be highly promising for processing of shape memory alloys, which often suffer difficult workability. The present work establishes a new pathway towards realization of high performance shape memory alloys by additive manufacturing and, thus, will stimulate further research in this field directed towards application. [ABSTRACT FROM AUTHOR]

Summary: We present a novel method for computational design of adaptive shape‐memory alloy (SMA) structures via topology optimization. By optimally distributing a SMA within the prescribed design domain, the proposed algorithm seeks to tailor the two‐way shape‐memory effect (TWSME) and pseudoelasticity response of the SMA materials. Using a phenomenological material model, the thermomechanical response of the SMA structure is solved through inelastic finite element analysis, while assuming a transient but spatially uniform temperature distribution. The material distribution is parameterized via a SIMP formulation, with gradient‐based optimization used to perform the optimization search. We derive a transient, bilevel adjoint formulation for analytically computing the design sensitivities. We demonstrate the proposed design framework using a series of two‐dimensional thermomechanical benchmark problems. These examples include design for optimal displacement due to the TWSME, and design for maximum mechanical advantage while accounting for pseudoelasticity. [ABSTRACT FROM AUTHOR]

An assembly of hemispherical Ag nanoparticles is prepared by solid‐state dewetting of thin Ag film deposited on the sapphire substrate. The in situ nanomechanical compression testing of the particles with a flat diamond punch inside the scanning electron microscope demonstrates the deformation behavior typical for the nucleation‐controlled plasticity: high elastic deformation followed by an abrupt particles collapse. The latter is associated with the dislocations nucleation in otherwise pristine particle. The average contact pressure in the contact zone at the onset of dislocation‐controlled plasticity is about 8 GPa, and does not depend on particle size. This observation supports the hypothesis that the pseudoelasticity of much smaller Ag nanoparticles is intrinsically related to their ultrahigh strength. A stress‐induced diffusion along the particle–substrate and particle–punch interfaces is identified as a factor controlling the pseudoelastic deformation. The corresponding diffusion model allows estimating the room‐temperature self‐diffusion coefficient of Ag along the Ag–W and Ag–zirconia interfaces, which is quite close to the estimated value of the grain boundary self‐diffusion coefficient in Ag. Based on this finding, the map of pseudoelastic deformation of crystalline materials is proposed. [ABSTRACT FROM AUTHOR]

The force transmissibility characteristics of a passive vibration isolator in the form of shape memory alloy bar are investigated. The shape memory alloy bar, together with a rigid mass, constitutes a single-degree-of-freedom system. The force isolation ability of the oscillator is evaluated for both isothermal and convective environmental conditions. The transmissibility curve of an isothermal pseudoelastic oscillator displays single and double jumps depending upon the forcing amplitude. The shape memory alloy oscillator with coupled thermomechanical behaviour depends on the cooling rate near resonant frequencies. Increased cooling rate reduces both peak amplitude and the resonant frequency of the transmissibility curve. The force isolation provided by shape memory alloy oscillator is independent of the operating conditions. [ABSTRACT FROM AUTHOR]

This paper reports a study on non-linear temperature dependences of the critical pseudoelastic stress and stress hysteresis for stress-induced martensitic transformation deformation of a nanocrystallined Ni 47 Ti 50 Fe 3 alloy. Such non-linear temperature dependences were found over a wide low temperature range from -196 °C to 20 °C and they defy the predictions of the Clausius-Clapeyron law for the transformation. The pseudoelastic behaviour exhibited two unusual characteristics, including a progressively decreasing d σ / d T slope and a rapidly widening stress hysteresis with decreasing the temperature. In-situ X-ray diffraction analysis indicated that the starting microstructure of the alloy for pseudoelastic deformation was a mixture of the B2 and a crystallographically suppressed R phase (denoted R*) within -8 °C to -196 °C. The volume fraction, the rhombohedral lattice distortion and the internal atomic shuffle of the suppressed R phase increased upon cooling. These continued evolutions of the crystal structure of the R* phase and the microstructure of the alloy upon cooling are ascribed to be responsible for the abnormal non-linear temperature dependences of the pseudoelastic stresses and the widening of the stress hysteresis. [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMAs) undergo large recoverable deformation due to their inherent diffusionless martensitic phase transformation. Two factors need to be taken into account to incisively emulate the behaviour of SMA based structures: (i) consideration of large displacement and rotation (LDR) effect, and (ii) capturing the consequence of transformation induced material level coupling. In this study, both these effects are apprehended by extending the infinitesimal strain-based constitutive model of SMA as proposed by Lagoudas et al. (2012), assuming small elastic strain but with finite inelastic strain. To preclude any spurious stress generation out of large rotation, the Jaumann stress rate is used, and incremental objectivity for finite rotation between two successive time instants is conserved by rotating the strain increment and spin tensor to the rotation-independent mid-point configuration following Hughes–Winget (Hughes and Winget, 1980) algorithm. In addition, the contribution of thermoelastic and latent heat evolved during transformation is taken into consideration in the thermal equilibrium equation. Moreover, the effect of partial phase transformation yielding minor hysteresis loop has also been captured. The equilibrium equation is expressed in the current configuration following the Updated Lagrangian formulation. The mechanical and thermal equilibrium equations are solved concurrently in block matrix form using the Newton–Raphson (NR) iterative technique, considering the coupling terms resulting from the phase transformation. The efficacy and robustness of the developed finite element model are corroborated through various practical applications of SMA-based members, e.g., SMA ring, SMA-actuated beam, morphing of corrugated airfoil, orthodontic palatal expander, compliant gripper, undergoing LDR while subjected to different thermomechanical loading conditions. The combined effects of LDR along with thermomechanical coupling yield a stiffening behaviour during loading and sluggish response at the time of thermal recovery. • Coupled thermomechanical model of SMA considering large displacement and rotation (LDR) and partial phase transformation. • The mechanical and thermal equilibrium equations are solved concurrently following incremental-iterative finite element scheme. • The effect of LDR results in a stiffening nature in displacement response, especially in the case of thin members. • Coupling terms, i.e., latent heat and thermoelastic heat evolved during phase transition, leads to a sluggardly response. [ABSTRACT FROM AUTHOR]

The influence of temperature on the fatigue properties of pseudoelastic NiTi under low-cycle fatigue are investigated. Tests were performed under the uniaxial tensile deformation (pull-pull) at 0°С and 20°С which is above the austenite finish temperature. Experimental results indicate that the fatigue life of NiTi alloy increases with the decrease of test temperature from 20°С to 0°С in the case of presenting the results depending on the strain range and Odqvist’s parameter. Regardless the test temperature, with the increase of number of cycles to failure, the stress and strain ranges, as well as the dissipation energy decrease, and the total dissipation energy and Odqvist’s parameter increase. The slope of the fatigue curves of NiTi alloy is greater at the temperature of 0°С in comparison with the 20°С in the case of employing the stress range, strain range, Odqvist’s parameter, and total dissipation energy as the failure criteria, and is less while employing the dissipation energy as the failure criterion.

A microstructural informed thermodynamic model is utilized to tailor the pseudoelastic performance of a series of Fe-Mn-Al-Ni shape-memory alloys. Following this approach, the influence of the stability and the amount of the B2-ordered precipitates on the stability of the austenitic state and the pseudoelastic response is revealed. This is assessed by a combination of complementary nanoindentation measurements and incremental-strain tests under compressive loading. Based on these investigations, the applicability of the proposed models for the prediction of shape-memory capabilities of Fe-Mn-Al-Ni alloys is confirmed. Eventually, these thermodynamic considerations enable the guided enhancement of functional properties in this alloy system through the direct design of alloy compositions. The procedure proposed renders a significant advancement in the field of shape-memory alloys., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)