Superelasticity Effect Research Articles

Nickel–titanium alloy porous scaffolds based on a dominant cellular structure manufactured by laser powder bed fusion have satisfactory osteogenic efficacy

Nickel–titanium (NiTi) alloy is a widely utilized medical shape memory alloy (SMA) known for its excellent shape memory effect and superelasticity. Here, laser powder bed fusion (LPBF) technology was employed to fabricate a porous NiTi alloy scaffold featuring a topologically optimized dominant cellular structure that demonstrates favorable physical and superior biological properties. Utilizing a porous structure topology optimization method informed by the stress state of human bones, two types of cellular structures—compression and torsion—were designed, and porous scaffolds were produced via LPBF. The physical properties of the porous NiTi alloy scaffolds were evaluated to confirm their biocompatibility, while their osteogenic efficacy was investigated through both in vivo and in vitro experiments, with comparisons made against a traditional octahedral unit cell structure. NiTi alloy porous scaffolds can be nearly net-shaped via LPBF and exhibit favorable physical properties, including a low elastic modulus, high hydrophilicity, a specific linear expansion rate, as well as superelastic and shape memory effects. These scaffolds demonstrate excellent biocompatibility, support in vitro osteogenesis, and possess significant in vivo bone ingrowth capabilities. When compared to titanium alloys, NiTi alloys show comparable osteogenic properties in vitro but superior bone ingrowth properties in vivo. Additionally, among octahedral-type, torsion-type, and topologically optimized compression-type porous scaffolds, the latter demonstrates enhanced bone ingrowth properties. LPBF technology is effective for manufacturing porous NiTi alloy scaffolds with fine pore structures and excellent mechanical properties. The scaffolds based on topologically optimized dominant cellular structures facilitate satisfactory and efficient bone formation.

Friction and Wear Resistance of Nanostructured TiNi Shape Memory Alloy

TiNi shape memory alloys with a superelastic effect are widely used in tribological interfaces requiring high wear resistance. One of the common approaches to reducing the wear of various metals is the application of severe plastic deformation (SPD), resulting in structural refinement and corresponding hardening. This paper investigates the tribological behaviour of a nanostructured Ti49.3Ni50.7 shape memory alloy produced using SPD. The friction and wear characteristics of the alloy at room temperature are compared in the coarse-grained, nanostructured, and nanostructured aged states. Through hardness measurement and transmission electron microscopy, it is shown that the transformation of a coarse-grained state into a nanostructured state increases wear resistance and hardness, reduces the coefficient of friction, and changes the friction mechanism. Formed nanoparticles during ageing in a nanostructured state further increase wear resistance.

Black Body Cavity Apparatus for Measuring the Emissivity of Nickel-Titanium-Based Shape-Memory Alloys and Other Metals

Nickel-titanium (NiTi)-based shape-memory alloys (SMAs) have various applications in biomedicine, actuators, aerospace technologies, and elastocaloric devices, due to their shape-memory and super-elastic effects. These effects are induced in SMAs by a reversible martensitic phase transformation. This transformation can be achieved by applying stress or temperature differences. It is extremely difficult to measure the temperature of NiTi elements with contact thermometers because they disturb the phase transformation phenomenon. An alternative approach is contactless infrared thermography for which accurate emissivity data are mandatory. To determine the temperature distribution in NiTi components with an infrared camera, a newly constructed apparatus is presented to measure the emissivity of metals. For this purpose, a state-of-the-art infrared camera was employed to analyze the emissivity behavior with temperature, especially during the phase transformation. The emissivity of the NiTi samples was systematically studied by comparing them with a reference-quality black body cavity (BBC) having an emissivity better than 0.995. Calibration measurements revealed that the maximum deviation of the BBC temperature measured with the infrared camera was only 0.15% (0.45 K) from its temperature measured with the built-in contact thermometers. The apparatus was validated by measuring the emissivity of a polished aluminum sample and found to be in good agreement with literature data, particularly for temperatures above 340 K. Finally, the emissivity of two rough and one polished NiTi samples was measured covering the temperature range from 313 K to 423 K with an expanded uncertainty of about 0.02 (k=2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$k=2$$\\end{document}). The studied NiTi samples have an atomic percent composition of Ni42.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Ni}_{42.5}$$\\end{document}Ti49.9\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Ti}_{49.9}$$\\end{document}Cu7.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Cu}_{7.5}$$\\end{document}Cr0.1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Cr}_{0.1}$$\\end{document}. We observed that the emissivity of NiTi varies between 0.17 and 0.31 depending on temperature. As the temperature rises, the emissivity increases rapidly during the phase transformation, and it decreases gradually beyond the austenite finish temperature. In addition, the emissivity of NiTi depends on the microstructure of the martensite and austenite phases and the surface roughness of the samples.

Investigation of Copper Addition on Corrosion Behavior of a Ferrous Based Shape Memory Alloy for Biomedical Implant Applications

Permanent biomedical implants pose several issues in long term scenarios like infections, inflammation, implant fracture, tissue damage, cancerous tumors formation, and skin allergies. Biodegradable biomedical implants are a new interest that function by degrading internally after achieving the implant goal. Shape memory alloys like Nitinol and Iron based shape memory alloys have applications in biomaterials due to the excellent property of super elasticity and shape memory effect respectively with the ease of small surgery requirement. To achieve biodegradability, the alloy composition is to be set while not compromising other properties such as biocompatibility, mechanical properties, shape memory properties, and magnetic properties. Slow corrosion rates of Fe-Mn alloys are reported and alloying addition, surface modifications, and novel manufacturing techniques are suggested to overcome this problem. In this study, the effect of addition of copper addition effect on the degradation behavior of Fe-30Mn-5Si is investigated. Austenite is the major phase present in both samples and small amounts of martensite are also present. For 10% copper, an additional copper rich phase is formed along the grain boundaries as it was beyond the solubility limit of iron matrix. The electrochemical corrosion test shows that 10% Cu addition resulted in 1.72 times higher corrosion rate than that of 5% Cu addition. As 5% Cu addition is within the solubility limit of iron matrix, and it forms a solid solution with iron that creates a passive layer during corrosion testing results in slower degradation.

Achieving dual-phase structured Cu-Al-Mn-Si alloy with prominent shape memory properties via laser powder bed fusion

In this study, Cu-11.1Al-8.15Mn-0.37Si shape memory alloy was manufactured via laser powder bed fusion (LPBF) and compared with furnace-cooling cast samples. The LPBF-manufactured samples exhibited a dual-phase structure of L21 austenite and 2H martensite, along with uniformly distributed nanoscale Mn5Si3 precipitations observed via TEM. The 2H martensite presented a layer-like morphology and was confirmed to be induced by residual stress during the LPBF process. Such a dual-phase structure could be stabilized by Mn5Si3 precipitations, and yielded the concurrent presence of prominent superelastic (SE) and shape memory effect (SME) properties. Under 6 % pre-strain, LPBF-manufactured samples showed an 80 % SE rate and 83 % SME rate. The recovery SE and SME strain could be up to 4.2 % and 2.1 %. Compared to the cast sample, fracture compressive and tensile strain increased by 68.4 % and 33.3 %. This enhancement was due to regularly arranged columnar crystals and strong [001]//Z texture confirmed by EBSD. These findings suggest that LPBF has the potential to produce Cu-Al-Mn-Si alloys with prominent shape memory properties and provide references for an austenite-martensite dual-phase structure.

The effect of cobalt alloying on the phase transformation kinetics of Ni-Ti alloys

This study investigates the influence of cobalt (Co) alloying addition and heat treatment temperature on the phase transformation behaviour controlling the superelasticity and shape memory effect (SME) of Nickel-Titanium (Ni-Ti) alloys, commonly known as nitinol. The microstructural evolution upon heat treatment conducted at a temperature ranging from 440 to 560 °C was thoroughly analyzed via Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), and Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS). Increase in heat treatment temperatures from 470 °C up to 530 °C led to the dissolution of particles present in as-received (cold-worked) condition. It was determined that Co addition into the Ni-Ti alloy system resulted in a change in the nucleation and growth kinetics of Ti-rich precipitates, leading to the formation of larger and fewer particles during processing. Both binary and ternary alloys showed a decrease in austenite finish temperature (Af) with increasing heat treatment temperatures, however, the rate of decrease was found to be higher for Co containing ternary alloys. This is linked with faster structural relaxation when Co is present and evidenced by lattice size variation during heat treatment. It is highlighted that heat treatment methodology needs to be tailored to the specific alloy composition for controlling superelasticity and SME via alloy design.

Ternary Mg-Sc-based TRIP alloys: Design strategy based on Sc-equivalent

This study addresses the transformation-induced plasticity (TRIP) effect observed in bcc single-phase Mg–Sc alloys, aiming to circumvent the strength-ductility tradeoff inherent in traditional Mg-based alloys. Introducing a third element (X = Zn, Y, Nd, Gd) significantly enhanced the strength of the binary Mg–Sc TRIP alloys, simultaneously preserving ductility due to the TRIP effect. Consequently, a ternary Mg–Sc–X TRIP alloy with a bcc single-phase, for instance, achieved an ultimate tensile strength of approximately 380 MPa and a fracture elongation of around 41%, surpassing the conventional tradeoff. Furthermore, we introduce the design strategy of Sc-equivalent (Sceq) to articulate the combined influence of Sc and X elements on the stability of the bcc phase. This novel approach demonstrates the room-temperature deformation behavior of the Mg–Sc–X ternary alloy can be predicted and modulated, showcasing superelasticity (17.5 < Sceq < 19.5) and TRIP effect (19.0 < Sceq < 21.0).

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

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

Corrosion and tribocorrosion resistance of superhydrophobic LPBF-NiTi alloys surface fabricated by nanosecond laser and ultrasonic fluorination

NiTi alloys have been extensively studied for their excellent super-elasticity (SE) and shape memory effects (SME). With the development of laser powder bed fusion (LPBF), it is possible to process the complex structure of NiTi alloy, which results in the mechanical properties of LPBF-NiTi alloys receiving great attention. However, the processing method is different from the traditional one, which not only has an impact on the mechanical properties but also seriously affects the anti-corrosion resistance and causes a substantial deterioration. In the research work of this paper, nanosecond laser processing was used to fabricate the bionic mastoid-shaped micro-nano structure on the LPBF-NiTi alloys. An efficient and convenient ultrasonic fluorination treatment was used to achieve the full surface coverage of FAS-17 in 10 min, and the superhydrophobic LPBF-NiTi alloy surface (CA ~ 157°) was successfully achieved. The constructed superhydrophobic LPBF-NiTi alloy surface exhibits excellent anti-corrosion resistance compared with the original surface, and the icorr from (4.42 ± 0.5) × 10−7 A·cm−2 reduced to (3.21 ± 0.5) × 10−9 A·cm−2. Meanwhile, the superhydrophobic surface showed outstanding stability in the 15-day immersion test n 3.5 wt% NaCl solution. And it is also stable after the tribocorrosion tests in 3.5 wt% NaCl solution at 1 N and 5 N pressures. The remarkable anti-corrosion resistance and stability of the superhydrophobic surface can be attributed to the micro-nano structure rich in the TiO2 oxide layer, the air layer formed by the superhydrophobic surface insulates the corrosive liquid and the stable hydrophobic agent film.

Effects of Direct Aging Heat Treatments on the Superelasticity of Nitinol Produced via Laser Powder Bed Fusion

This work investigates the effects of short-time direct aging heat treatments on the mechanical properties and microstructure of additively manufactured Nitinol (NiTi) alloy. Cylindrical samples were produced through laser powder bed fusion (L-PBF), directly aged at different temperatures and compared to the solution annealed and aged conditions. Compression tests were carried out at room temperature both in cyclic mode at constant strain and incremental cyclic mode, to provide a comprehensive analysis on the superelastic features of NiTi after direct aging heat treatments. Furthermore, cyclic compression tests were performed at 37 °C to evaluate the superelastic effect at the body temperature and, therefore, the possibility to use these treatments for biomedical components. The effects of direct aging on the microstructure were characterized using transmission electron microscopy (TEM). High cyclic stability and superelastic recovery up to 10 pct of deformation emerged for the direct aged alloys. The comparable results obtained with and without the solution treatment points out that this step was not necessary in reaching superelasticity, proving the effectiveness of direct aging.

Numerical modeling of systems based on SMA wires and pulleys: A finite element formulation within the Arbitrary Lagrangian–Eulerian framework

Thanks to their properties of superelasticity and shape memory effects, SMA wires are often considered for the design of damping devices or actuators coupled to pulleys. In order to dimension such devices for commercialization, it is necessary to develop reliable, robust and fast numerical tools. To this end, simplifying the management of the wire/pulley contact using an Arbitrary Lagrangian–Eulerian (ALE) formulation is an original and effective solution. In this work, the wires are modeled by three-dimensional truss thermo-mechanical finite elements associated with a superelastic law. For each node, the curvilinear abscissa is used as an additional degree of freedom in order to deal with material flow from one element to another. After presenting the ALE formalism and the necessary precautions (advection of variables), as well as the material behavior and thermo-mechanical contact laws at the pulleys, elementary validation tests are studied. Finally, two devices taken from the literature are modeled using the ALE formalism. The results obtained demonstrate the relevance and effectiveness of the approach adopted, as well as the importance of not neglecting friction at the pulleys and thermal effects on the material mechanical response.

Microstructural characterization and analysis of nitinol-based self-healing metals matrix composite of A356 alloy produced by high-pressure die casting

A self-healing non-ferrous metal-matrix composite is prepared by the high-pressure die-casting process. It includes casting set-up, sample preparation of metal matrix composite (MMC), microstructural characterization, and analysis of its ability to close the crack. Aluminum alloy (A356) is deployed as a matrix material in the MMC. Nitinol is a smart alloy produced by a combination of Nickel and Titanium in equal mass proportion. Apart from excellent mechanical properties it also exhibits super-elasticity and shape-memory effect. The wire of the Nitinol is integrated as reinforcement within the matrix of A356 alloy through a high-pressure die-casting process. The recovery percentage of the metal matrix composite and microstructural evaluation are reported. The deployment of shape memory wire provides the ability to recover the matrix material even from plastic strain by just heating the sample slightly above the activation temperature of the Nitinol wire. Microstructural evaluation indicates fair integration of the reinforcement within the matrix material. Gaining the ability for 30.27% angular restoration and 19.37% crack closer is a very positive sign for designing self-healing metallic materials.

Effects of Zr and Hf on superelasticity, shape memory effect and microstructure of β-type (Ti–Zr–Hf)–Nb–Sn multi-principal element alloys with low magnetic susceptibility

(Ti–Zr–Hf)(97−x)–xNb–3Sn (Ti:(Zr + Hf) = 1:1) alloys were designed for biomedical superelastic alloys with magnetic resonance imaging (MRI) compatibility. The effects of Zr and Hf on constituent phases, superelasticity, shape memory effect and magnetic susceptibility were clarified. The specific Nb content for superelasticity and shape memory effect was also explored. It was found that the (Ti–Zr)–6.5Nb–3Sn alloy exhibited a superelastic recovery strain of 5.2 % with lower magnetic susceptibility compared to conventional Ti–Ni and Ti–Nb alloys. The magnetic susceptibility decreased with increasing Hf content. The (Ti–Hf)–9Nb–3Sn alloy showed a superelastic recovery strain of 0.9 % and very low magnetic susceptibility less than half of that of the conventional alloys. Based on the lattice parameters of these alloys, it was found that these alloys exhibited larger transformation lattice strain between a β phase and an α” phase than those of the Ti–Nb alloys, suggesting high potential to exhibit a large superelastic recovery strain. According to orientation analyses of constituting grains, the larger superelastic recovery strain in the (Ti–Zr)–6.5Nb–3Sn alloy than that of the (Ti–Hf)–9Nb–3Sn alloy is considered to be caused by a strong recrystallization texture of {001}β <110>β. By optimization of heat-treatment temperature, superelastic recovery strain of 3.6 % was achieved in the (Ti–0.5Zr–0.5Hf)–7.5Nb–3Sn alloy.

Crystal plasticity based constitutive model for deformation in metastable β titanium alloys

Due to attractive mechanical properties, metastable β titanium alloys have become very popular in many industries including aerospace, marine, biomedical, and many more. It is often the complex interplay among the different deformation mechanisms that produces many of the sought-after properties, such as enhanced ductility, super-elasticity, and shape memory effects. Stress induced martensitic transformation is an important deformation mechanism for these alloys. Understanding of it and the influence it has on the microstructural evolution of materials is of great importance. To this end we have developed a crystal plasticity based constitutive model which accounts for both martensitic phase transformation and slip based plasticity simultaneously in metastable β titanium alloys. We present a new formulation for the evolution of martensite transformation, based on physical principles and crystal plasticity theory. To understand and demonstrate this feature of the model, a parametric assessment of the newly developed constitutive model is conducted. This is followed by first of its kind analyses of stress induced martensitic transformation in metastable β titanium alloys. We firstly present validations against uniaxial loading experiments for different metastable β titanium alloys exhibiting stress induced martensite transformation. As part of this, single crystal simulations in metastable β titanium alloys are used for the first time to investigate the interaction of individual transformation systems during unconstrained transformation. This study shows good agreement between the experimental and simulated responses during all stages of deformation in which elastic, transformation and finally the slip stage are exhibited. Relatively ‘strong’ and ‘weak’ orientations for transformation are observed, consistent with experimental studies. The work done here demonstrates the ability of this crystal plasticity finite element method to capture physical mechanisms while bringing new insight about the interaction of different deformation mechanisms in metastable β titanium alloys.

Microstructure of Stress‐Induced Martensite in Micron‐Sized Zirconia Single Crystal Spheres

Martensitic phase transformation of zirconia (ZrO2), when realized from the superelastic effect, has immense potential in energy applications, such as micro‐actuation and high‐energy dissipation/damping. In this study, martensitic transformation in ZrO2 is studied using a newly developed microscale phase field model, which is scale‐independent and contains an averaged gradient term. The model is implemented using the FEniCS package in Python. With a thorough explanation of the model construction, the mechanics of phase transformation are detailed, and the material behaviors under different stress conditions are discussed. Mesh sensitivity, multivariant evolution, and the effect of particle size are analyzed to understand the martensite evolution in superelastic ZrO2. This work provides new insight into the martensitic phase transformation behaviors of micron‐sized spherical ZrO2 particles.

Characterization of the temperature-dependent superelastic and elastocaloric effects of a NiTi tube under compression at 293–330 K

Comprehensive characterizations of the superelastic and elastocaloric effects of NiTi and NiTi-based shape memory alloys (SMA) in the operation temperature region are highly desirable for using them in elastocaloric coolers with a large temperature lift. In this article, we report the superelastic and elastocaloric effects of a commercially available superelastic polycrystalline NiTi SMA tube with an outer diameter of 5 mm and a wall thickness of 1 mm between 293 and 330 K. The NiTi tube sample was subjected to a training of 250 cycles to stabilize its superelastic and elastocaloric effects. We observed that temperature dependencies existed for both superelastic and elastocaloric effects of the NiTi tube, and stress–strain curves differed much between isothermal and adiabatic loading conditions. The largest temperature rise and temperature drop measured at 293 K under an applied strain of 3.66% and a strain rate of 0.1 s−1 during loading and unloading were 21 and 11 K, respectively. The loading conditions (loading function and holding time) also impacted the superelastic effect of the NiTi tube. We identified two major reasons for the irreversibility of the adiabatic temperature change: the hysteresis heat dissipation and the temporary residual strain after unloading, and they affected the cooling performance of the elastocaloric cooler in different ways. We investigated the dependencies of the superelastic and elastocaloric effects on the maximum applied strain and the temperature distribution on the NiTi tube during loading and unloading. The results are beneficial to the modeling of elastocaloric coolers with large temperature lifts.

Experimental and computational study of the NiTi thin wires mechanical behavior

NiTi wires used for biological implants demonstrated ductile fracture. NiTi 40, 60, and 90 µm thick wires were tested in uniaxial tension to fracture and loading-unloading. Uniaxial stress-strain curves demonstrate superelastic behavior. The inelastic martensite transformation strain is completely recovered upon unloading, forming thermo-mechanical hysteresis. A mathematical model was developed to describe superelasticity effects in NiTi wires. Modeling results are qualitatively and quantitatively similar to experimental data, and capture elastic deformation of austenite, forward martensite phase transformation stress plateau before the onset of martensite elastic deformation and the entire unloading range. The elastic limit and strength increase with the wire thickness.

Research on Superelasticity of Shape Memory Alloys Based on Reproducing Kernel Particle Method

Shape memory alloys (SMAs), known for their unique superelastic effect, exhibit superior mechanical properties compared to traditional materials. In this study, the reproducing kernel particle method (RKPM) is applied to investigate the superelastic effect of the SMAs. A segmented linearization RKPM model is developed to establish the stress–strain relationship for the superelastic effect, and relevant formulas of superelasticity for the SMAs are derived. Finally, the applicability and accuracy of the RKPM in analyzing the superelastic effect of the SMAs are demonstrated through numerical examples.

Enhanced superelasticity and reversible elastocaloric effect in nano-grained NiTi alloys with low stress hysteresis

Solid-state cooling technologies have been considered as potential alternatives for vapor compression cooling systems. The search for refrigeration materials displaying a unique combination of pronounced caloric effect, low hysteresis, and high reversibility on phase transformation was very active in recent years. Here, we achieved increase in the elastocaloric reversibility and decrease in the friction dissipation of martensite transformations in the superelastic nano-grained NiTi alloys obtained by cold rolling and annealing treatment, with very low stress hysteresis (6.3 MPa) under a large applied strain (5%). Large adiabatic temperature changes (ΔT max = 16.3 K at ε = 5%) and moderate COPmater values (maximum COPmater = 11.8 at ε = 2%) were achieved. The present nano-grained NiTi alloys exhibited great potential for applications as a highly efficient elastocaloric material.

Shape memory and self-healing in a molecular crystal with inverse temperature symmetry breaking.

Mechanically responsive molecular crystals have attracted increasing attention for their potential as actuators, sensors, and switches. However, their inherent structural rigidity usually makes them vulnerable to external stimuli, limiting their usage in many applications. Here, we present the mechanically compliant single crystals of penciclovir, a first-line antiviral drug, achieved through an unconventional ferroelastic transformation with inverse temperature symmetry breaking. These crystals display a diverse set of self-restorative behaviors well above room temperature (385 K), including ferroelasticity, superelasticity, and shape memory effects, suggesting their promising applications in high-temperature settings. Crystallographic analysis reveals that cooperative molecular displacement within the layered crystal structure is responsible for these unique properties. Most importantly, these ferroelastic crystals manifest a polymer-like self-healing behavior even after severe cracking induced by thermal or mechanical stresses. These findings suggest the potential for similar memory and restorative effects in other molecular crystals featuring layered structures and provide valuable insights for leveraging organic molecules in the development of high-performance, ultra-flexible molecular crystalline materials with promising applications.