Strain Recovery Research Articles

A novel Ni-alloyed Cu-Al-Ga-Fe shape memory alloy with excellent shape recovery performance

In this study, a novel Ni-alloyed Cu-Al-Ga-Fe single crystal alloys were reported for the first time. The microstructure, martensitic transformation, shape memory effect, and superelasticity of Cu-12.5Al-4Ga-3Fe-xNi (x = 2, 3, 5) and Cu-13.5Al-4Ga-3Fe-xNi (x = 3, 4, 5) were investigated. The results show that Cu-12.5Al-4Ga-3Fe-xNi (x = 2, 3, 5) single crystals consist of the parent phase, 18 R martensite, and β(FeAl) nano-precipitates. As the increase in Al content, Cu-13.5Al-4Ga-3Fe-xNi (x = 3, 4, 5) single crystals consist of the parent phase, γ1(Cu9Al4) phase, and β(FeAl) nano-precipitates. The martensitic transition finishing temperatures (Mf) of 12.5Al alloys are all higher than room temperature, while the reversible martensitic transition finishing temperatures (Af) of 13.5Al alloys are all lower than room temperature. Cu-12.5Al-4Ga-3Fe-3Ni single crystal shows the highest SME of 7.6 % with 100% strain recovery. Similarly, the Cu-13.5Al-4Ga-3Fe-3Ni single crystal demonstrated the highest superelasticity of 11 %. Additionally, the Cu-13.5Al-4Ga-3Fe-3Ni single crystal maintained 100 % strain recovery even after 4500 cycles of compression deformation at 6 %, although the critical stress of stress-induced martensite decreased with each cycle. These findings suggest that the Cu-Al-Ga-based shape memory single crystal alloys obtained in this study have promising applications in driving response devices.

Bionic lotus‐leaf‐structured fluorine‐free polyurethane by combining electrospinning and electro‐spraying for waterproof and breathable textiles

AbstractDeveloping environmentally friendly and high‐performing viable waterproof and breathable membranes (WBMs) is critical but significantly challenging. Herein, we combine electrospinning and electro‐spraying methods. First, elastic thermoplastic polyurethane (TPU) is used as the polymer, and poly(methylhydrosiloxane) (PMHS) with low surface energy is introduced to prepare TPU/PMHS fiber‐based membranes by electrospinning. Electro‐spraying is performed using a dilute solution of TPU/PMHS/SiO2, with fluorine‐free hydrophobic SiO2 nanoparticles introduced to facilitate the construction of a rough lotus‐leaf‐like surface structure, ultimately obtaining TPU/PMHS/SiO2 WBMs with bead‐on‐string fiber binary structures and a strong water repellency. The results indicate that the TPU/PMHS/SiO2 WBMs comprise a three‐dimensional porous structure formed by randomly arranged fibers and a lotus biomimetic structure consisting of bead‐like fibers, providing the membrane with excellent mechanical properties and waterproof breathability, including a tensile strength of 15.8 MPa, static water pressure of 33 kPa, water vapor transmission rate of 5169 g m−2 d−1, and contact angle of 142°. Additionally, the fiber membrane exhibits an excellent strain recovery after 1000 cycles of cyclic stretching under 100% strain, verifying the excellent elastic properties of the TPU/PMHS/SiO2 WBMs. This simple preparation process and unique composite structure can facilitate the design and preparation of environmentally friendly, waterproof, and breathable materials.

Harvesting optimization and Omega-3 recovery improvement from Schizochytrium DT3 using surfactant-aided dispersed air flotation: Response surface methodology

Microalgae's potential to produce high value bioactives is contingent on the cost-effective harvesting of algal biomass. The use of CTAB (cetyl trimethylammonium bromide) assisted dispersed air flotation as a harvesting technique for Schizochytrium biomass, was optimized as a cost-effective means for recovery (R) and concentration factor (CF) using Response Surface Methodology. To the best of our knowledge, this is the first study to employ surfactant assisted dispersed air flotation (SDAF) in the recovery of a heterotrophic thraustochytrid strain. A Box-Behnken design of experiment investigating the operating parameters of CTAB amount, air flow rate and volume on Schizochytrium was employed. Initially, both responses were analyzed individually and then used to simultaneously maximize both variables. The optimized conditions of CTAB (500 mg/L), air flow of 2 L/min and volume of 600 mL resulted in a R of 91 % and CF of 19 times. Although a secondary step is necessary for further concentration, this technique utilizes 70 times less energy as compared to conventional centrifugation techniques which are used to recover Schizochytrium sp. This is significant as this technique can be easily adapted to existing bioreactors, as they are already equipped with gas spargers. In addition, the presence of surfactant carried through demonstrated an improvement in the recovery of long chain poly unsaturated fatty acid (PUFA) by 6 %, particularly in DHA and DPA which was not observed in washed biomass.

Data-Driven Design of Nickel-Free Superelastic Titanium Alloys.

In this paper, a CatBoost model for predicting superelastic strains of alloys was established by utilizing features construction and selection as well as model filtering and evaluation based on 125 existing data points of superelastic titanium alloys. The alloy compositions of a TiNbMoZrSnTa system were optimized and three nickel-free titanium alloys with potentially excellent superelastic properties were designed using the Bayesian optimization algorithm using a superelastic strain as the optimization target. The experimental results indicated that only Ti-12Nb-18Zr-2Sn and Ti-12Nb-16Zr-3Sn exhibited clear superelasticity due to the absence of relevant information about the alloys' β stability in the machine learning model. Through experimental optimization of the heat treatment regimens, Ti-12Nb-18Zr-2Sn and Ti-12Nb-16Zr-3Sn ultimately achieved recovery strains of 4.65% after being heat treated at 853 K for 10 min and 3.01% after being heat treated at 1073 K for 30 min, respectively. The CatBoost model in this paper possessed a certain ability to design nickel-free superelastic titanium alloys but it was still necessary to combine it with existing knowledge of material theory for effective utilization.

Understanding the influence of the arabinoxylan-rich psyllium (Plantago ovata) husk on dough elasticity and bread staling: Interplay between biopolymer and water dynamics

Bread staling is a complex phenomenon in which multiple interconnected mechanisms involving water operate, including the formation of amylopectin crystallites sequestering water, crumb-to-crust moisture migration, and the escalating immobilization of water reducing gluten plasticization. Considering the pivotal role of water-gluten-starch interactions, the integration of the highly-soluble, water-avid, arabinoxylan-rich psyllium husk (PSY) into a wheat dough/bread system was explored, to understand the impact of PSY on the water and biopolymer dynamics that could lead to lower staling. With the addition of PSY, optimal dough development required more water (from 68.0 to 119.5 % in control and 10 % wheat flour replacement with PSY, respectively). LF-NMR showed a parallel increase in the proton population attributed to free water entrapped by the continuous network of biopolymers surrounding granular starch (A23), and its relaxation time (T23). PSY resulted in freshly baked bread with similar specific volume, porosity and starch gelatinization than the control, but with a much higher proportion of intergranular free water (A23). Importantly, PSY did not affect amylopectin retrogradation nor shortened the relaxation time of water protons in exchange with gluten, suggesting that PSY did not withdraw plasticizing water from starch or gluten, respectively. During storage, PSY resulted in a dose-dependent delay of both crumb hardening (correlated to T23, r = −0.76, p < 0.05) and loss of cohesiveness (correlated to T23, r = 0.71, p < 0.05 and A23, r = 0.74, p < 0.05). PSY also resulted in greater crumb resilience, which was related to the self-assembly of PSY molecules and its impact on dough elasticity (with up to two-fold increase in recovery strain).

Effect of short heat treatment on mechanical properties and shape memory properties of Cu–Al–Ni shape memory alloy

Abstract The transformation of shape-memory alloys to high-temperature shape-memory alloys can be achieved through either the addition of alloying elements or heat treatment. However, heat treatment is more effective in improving the properties of the alloys. This research paper explores the impact of annealing on the mechanical and shape memory properties of Cu–Al–Ni shape memory alloys. The alloys were first cast, homogenized, and then machined before being aged at temperatures of 250, 300, and 350°C, and finally air-cooled. The results showed an increase in transformation temperature and recovery strain, as well as shape memory effect, and a decrease in hardness. Moreover, there was an increase in yield stress and strain. In conclusion, aging was found to improve the shape memory properties and mechanical properties better than thermomechanical treatment and some alloying elements.

Ultra‐Strong and Tough Bio‐Based Polyester Elastomer with Excellent Photothermal Shape Memory Effect and Degradation Performance

AbstractThe exploitation of bio‐based materials derived from renewable resources represents a pivotal strategic approach in addressing environmental pollution and alleviating the scarcity of fossil resources. 2,5‐Furandicarboxylic acid (FDCA) is the most potential substitute for terephthalic acid. Lignin is the most abundant aromatic biomass resource. However, the preparation of high‐performance lignin/FDCA‐based bio‐polyesters remains a formidable challenge. Herein, a multifunctional lignin‐modified polyester elastomer (LFPEe) is designed using FDCA‐based polyester oligomer (PPeF) and lignin (AOH) as building blocks. The LFPEe exhibits superior mechanical properties with the optimum tensile strength, fracture strain, and elastic recovery ratio up to 58.9 MPa, 610% and 88.9%, respectively, attributing to the formation of dual cross‐linking network with nanophase separation structure. Furthermore, leveraging the inherent characteristics of lignin, the LFPEe demonstrates excellent light‐controlled shape memory and excellent UV shielding performance. This innovative work not only breaks the performance dependence of FDCA‐based polyester on high molecular weight but also highlights a novel paradigm for value‐added utilization of lignin in sustainable bio‐polyesters.

Microstructures, transformation temperatures and superelastic properties of the rapidly solidified (TiZrHf)50Ni25Co10Cu15 HESMAs

In this study, the effects of the rapid solidification process on microstructures, transformation behaviors and superelastic properties of the multi-component (TiZrHf)50Ni25Co10Cu15 (at%) high-entropy shape memory alloy (HESMA) were investigated. The as-spun (TiZrHf)50Ni25Co10Cu15 fibers were prepared by a rapid solidification process. The solution-treated (TiZrHf)50Ni25Co10Cu15 alloy bulk specimen consisted of a (NiCoCu)-rich matrix, (TiZrHf)2(NiCoCu)-type phase and carbide, while the as-spun fiber specimen consisted of (TiZrHf)-rich matrix and carbide. The (TiZrHf)2(NiCoCu)-type phase is dissolved in the matrix of as-spun fibers due to the rapid solidification process. The martensitic transformation start temperature of the (TiZrHf)50Ni25Co10Cu15 alloy increased from 53.5 °C to 91.5 °C after the rapid solidification process. Both the (TiZrHf)50Ni25Co10Cu15 alloy bulk and fiber specimens showed clear superelasticity and the total superelastic recovery strain increased from 4.6 % to 5.7% after the rapid solidification process.

Biomimetic design and fabrication of thermally induced radial gradient shape memory scaffolds using fused deposition modeling (FDM) for bone tissue engineering

Bone defects pose a significant challenge, often exceeding natural healing capabilities. This study explores the potential of thermally induced radial gradient shape memory (RGSM) scaffolds for minimally invasive bone repair. Inspired by the natural porosity gradient of bone, these scaffolds feature a high-porosity inner zone that mimics cancellous bone and a low-porosity outer zone that resembles cortical bone. When the relationship between porosity and key properties was investigated, it was found that lower-porosity RGSM scaffolds exhibited higher compressive strength but experienced higher residual strain and lower shape recovery ratio compared to their higher-porosity counterparts. Despite this trade-off, the gradient design successfully mimicked the natural bone structure, potentially enhancing osseointegration and bone regeneration. These results demonstrate the feasibility of RGSM scaffolds for bone tissue engineering. This holds promise for advancing minimally invasive surgical techniques and improving the treatment of bone defects.

A dynamic trap well model of hydrothermal shape-memory effect in amorphous polymers undergoing tailorable shape recovery behaviour

A dynamic trap well model is developed to describe the complex relaxations of functional segments, and explore the working principles behind the hydrothermal coupling effect in shape memory polymers (SMPs). A constitutive relationship among shape fixity strain, shape recovery strain and relaxation time has been formulated to characterize the hydrothermal coupling effect using geometrical parameters (i.e. width and height) of trap wells. Moreover, effects of temperature and solvent absorption on dynamic relaxation behaviours of SMPs have been formulated based on the Flory-Huggins theory and Fokker-Plank probability equation. The trap well model effectively analyzes the shape fixity ratio and shape recovery ratio within ranges of 50–100% and 0–100%, respectively. Finally, an extended Maxwell model is proposed to formulate the dynamic mechanical behaviours of SMPs with hydrothermal shape-memory effect (SME), and the analytical results have been verified using the experimental results reported in literature. A good agreement between the analytical results obtained from the proposed model and the experimental data is present, where the correlation coefficient ( R 2 ) is 95%. The present study firstly introduces the dynamic trap well model for shape memory behaviours and intricate relaxations, and then accurately predicts the dynamic shape recovery of SMP in response to hydrothermal stimulus.

Headed coupling behavior of large diameter Cu-Al-Mn shape memory alloy bars: Mechanical testing and microstructural analyses

Cu-Al-Mn (CAM) superelastic alloy bars (SEAs) have received increasing attention in bridge applications. To take full advantage of their strain recovery and energy dissipation capacity, CAM SEAs are usually only applied in the plastic hinge regions of columns as part of the longitudinal reinforcement and coupled with steel rebar through mechanical splices. In this study, the feasibility of using headed coupling to connect large diameter CAM SEAs with steel rebar was investigated. Five large diameter CAM SEAs with headed ends were prepared: one 30 mm diameter and four 20 mm diameter samples. First, mechanical tests were performed on the five headed samples where each sample was coupled with one steel rebar at each end. Monotonic, incremental and constant strain cyclic loading was applied to simulate earthquake loading. The key mechanical properties were extracted and discussed. Second, microstructural analyses including electron backscatter diffraction (EBSD), metallographic imaging, Vickers hardness testing and fractographic evaluation were performed. The crystal orientation, phase composition and fracture surfaces were investigated to understand the effect of heading process on the stress induced martensitic transformation (SIMT), phase composition and failure of the headed CAM SEAs. It was found that the heading process only affects the near end portion of the specimen and it has no influence in the middle portion of the bars. Therefore, the strain recovery capacity of the CAM SEAs after heading was not reduced. Heading process led to high density of bainite phase precipitation at the end portions, which strengthened the sample. In order to uniformly strengthen the headed end of large diameter CAM SEAs, the key is to ensure a consistent cooling rate in the central and peripheral regions of the headed end after the heading process.

Reactive melt‐grafting of maleic anhydride onto olefin block copolymer toward high‐performance polar polyolefin thermoplastic elastomer

AbstractThe block structure of olefin block copolymer (OBC) elastomer gives it good elasticity and heat resistance relative to other polyolefin elastomers. However, the non‐polar nature of OBC chains limits its comprehensive properties like dyeing and toughening for polar engineering plastics. In this work, maleic anhydride (MA) and dicumyl peroxide (DCP) were selected as a typical polar monomer and a radical initiator to modify the polarity of OBC elastomer. During the melt‐grafting process, the DCP content was tuned to optimize the grafting efficiency while the MA‐to‐OBC ratio was fixed at 5 wt%. It was found that all the melt‐grafting reactions show high efficiency and can be finished in 3 min at 170 °C. With the increase of DCP content, the estimated grafting degree of OBC‐g‐MA elastomer increases from 0.40% to 1.16% along with increased gel content from 4.27% to 22.44%. A suitable grafting process was obtained with a DCP content of 3.0 wt% by balancing grafting efficiency and final mechanical properties. The optimized OBC‐g‐MA elastomer with a grafting degree of 0.92% has a mechanical strength of 17.15 MPa, a Young's modulus of 26.35 MPa, elongation of 1564% and strain recovery of 59.73%. Additionally, the contact angle test shows that polar grafting can increase surface hydrophilicity while excessive crosslinking promotes hydrophobicity. Therefore, our work has demonstrated the feasibility of the reactive melt‐grafting method to prepare OBC‐g‐MA elastomer and can also provide a processing reference for other chemical functionalization of polyolefin thermoplastic elastomers. © 2024 Society of Industrial Chemistry.

Effect of heat treatment on the microstructure and superelasticity of NiTi alloy via selective laser melting

In this study, a Ni50.4Ti49.6 shape memory alloy (SMA) was fabricated by selective laser melting (SLM), and it is found that heat treatments were effective to change the phase transformation behavior, microstructure, and superelasticity under compression for SLM Ni50.4Ti49.6. Detailed results based on EBSD demonstrated that, for SLM Ni50.4Ti49.6 during solid solution and aging (450 °C) treatment, the ultrafine cellular and columnar grains (∼16.5 μm) in as-fabricated Ni50.4Ti49.6 became equiaxed grains (∼70.2 μm), and the frequency of low-angle grain boundaries (26.7%→13.5%) and kernel average misorientation (0.37 → 0.21) decreased. The SEM and TEM analyses show that Ti-rich Ti2Ni/Ti4Ni2Ox and Ni-rich Ni4Ti3 precipitate phases were formed in the matrix simultaneously. After solid solution treatment, the size and distribution of Ti2Ni/Ti4Ni2Ox almost unchanged with aging temperature increasing from 350 °C to 550 °C, however, the size of Ni4Ti3 in aging-treated NiTi samples increased with aging temperature obviously, and it is interesting to find that the distribution of Ni4Ti3 became uneven and dislocation networks were formed in the matrix when the aging treatment reached up to 550 °C, which originated from the internal stress generated by supersaturated Ni atoms. Furthermore, the results of superelasticity for aging-treated NiTi demonstrated that the characteristics of Ni4Ti3 precipitate phase were powerful to influence the process of superelasticity decay/fatigue instead of determining the ultimately recovery strain after 10 compressive cycles. The aging-treated NiTi with evenly-distributed, high-density, and relatively-small Ni4Ti3 precipitate phase shows minimum recovery strain at 1st cycle compression and best superelasticity stability among three aging-treated NiTi, which was attributed to strong obstruction effect of these Ni4Ti3 precipitate for the occurrence of stress-induced martensitic transformation process and formation of dislocations. In contrast, the aging-treated NiTi with unevenly-distributed and low-density Ni4Ti3 precipitate shows maximum recovery strain at 1st cycle compression and worst superelasticity stability. These results in this work can bring some meaningful insights into tailoring the microstructure and superelasticity of SLM NiTi SMAs.

Microstructure and Super-Elasticity of Fe-33Mn-17Al-8.5Ni (at. %) Alloy for Structural Applications

The control of the residually stressed γ’-FCC phase in the grain boundaries that affects super-elasticity in the promising Fe-Mn-Al-Ni shape memory alloy (SMA) and grain size enhancement was an epitome for research in the current study. New composition Fe-33Mn-17Al-8.5Ni (at. %) was designed with the help of thermocalc software TCFE 11 database, produced in an electric arc furnace under an argon atmosphere and systematically investigated in the as-cast and heat-treated conditions. Characterization was performed using optical microscopy, X-ray diffraction measurements (XRD), and compression tests. Controlling the cooling conditions after heat treatment (HT) with high flowrate air cooling helped to reduce on the formation of the detrimental phase, γ’ at the grain boundaries as well as observed some grain growth in the microstructure without necessarily causing cracking as reported previously with quenching in cold water. The yield strength depicting the stress-induced martensitic transformation was 925 MPa for as cast and 909 MPa upon heat treatment. From cyclic compression loading/deloading training, a recovery strain of 2.1% and 2.3% was attained at 800 MPa maximum stress in the as-cast and heat treated-conditions, respectively.

Ultrasonic scattering behavior in austenitic NiTi with varying grain size and precipitate characteristics

Shape memory alloys like NiTi exhibit high strain recovery due to a reversible martensitic transformation when cooled or stressed, enabling actuation and biomedical applications. The characteristics of the stress-induced martensitic transformation in Ni-rich NiTi depend on the heat treatment. For example, aging induces the precipitation of Ni;4Ti;3 and lowers the critical stress needed for martensitic transformation. However, comprehensive characterization of these nanoscale precipitates is challenging as only small areas can be inspected efficiently. Ultrasonic immersion testing may assist in developing linkages between structure and transformation behavior as ultrasonic scattering is sensitive to the microstructure and easily mapped on the bulk scale. To this end, the ability of scattering behavior to discern between different precipitation characteristics must be probed to establish the sensitivity of ultrasonic parameters to changes in precipitation. This work explores the dependence of wave speed, attenuation, and backscattered energy on varying annealing and aging heat treatments of Ni-rich NiTi. The annealing treatments result in a gradient of grain sizes, while the aging treatments result in nucleation and growth of precipitate phases with increasing hold time. Variations in the ultrasonic parameters elucidate the sensitivity to changes in the microstructure, which are more quantitatively explored through existing analytical scattering models.

Influence of applied stress on shape memory characteristics of Ni50Ti45Cu5 (at.%) alloy subjected to thermomechanical cycling

Shape memory alloys have made rapid progress in many domains, primarily biomedical (endovascular stents, orthodontic archwires), and engineering (smart actuators, robotics, hydraulic couplings). The selection of a shape memory alloy for the indented application is based on its characteristic phase transformation temperatures. These characteristic temperatures are influenced by myriad parameters, such as composition, microstructure of the alloy, defect density, etc. When an shape memory alloy under an external load is subjected to cyclic operations to perform useful work, for example, actuators, these characteristic temperatures are modified. This study, therefore, aims to understand the influence of external loading on the shape memory characteristics of a Ni50Ti45Cu5 (at.%) alloy. A wire of 1.43 mm diameter and length of 100 mm was subjected to heating and cooling between its phase transformation temperatures in a cyclic manner under constant stress (of up to 60 MPa). The maximum recovery strain, actuation/retraction rate, and the stress influence coefficient were determined and compared with those of the other Ni-Ti and Cu-based shape memory alloys. The results show that raising the load level causes an increase in the transition temperatures, especially the Ms (martensite start temperature) rather than the other phase transformation temperatures (martensite finish (Mf), austenite start (As), austenite finish (Af)). It also significantly affects the recovery strain and the rate of retraction during forward transformation and the symmetry of operation.

Unveiling the mutual effects of hydroxyl and dibenzhydryl groups in α‐diimino‐nickel precatalysts for direct synthesis of polyethylene elastomers

The properties and practical applications of polyethylene are closely associated with the polymer molecular weight, polydispersity, degree of branching, and the type of branches and its arrangement within microstructure. In this study, a set of structurally diverse unsymmetrical 1‐(2,6‐dibenzhydryl‐4‐hydroxylphenylimino)‐2‐(2,6‐(R)‐4‐(R1)phenylimino)acenaphthene‐nickel dibromide precatalysts (where Ni2Me [R = Me, R1 = H], Ni2Et [R = Et, R1 = H], Ni2iPr [R = iPr, R1 = H], Ni3Me [R, R1 = Me], Ni2Et,Me [R = Et, R1 = Me]) has been prepared and studied for ethylene polymerization. Molecular structure analysis of Ni2Me and Ni2Et complexes revealed distorted tetrahedral geometries at nickel. The polymerization activities are extremely high for all precatalysts upon activation with either EASC or MMAO aluminum reagents, a trend notably pronounced in the case of the EASC‐activated systems, entering activity in the range of 10 million g mol−1 h−1 at 30°C. The Ni2Me demonstrated exceptional performance at higher temperatures, achieving an activity of 2.4 × 106 g mol−1 h−1 at 100°C and generated high molecular weight polyethylene (Mw = 105 g mol−1) with narrow polymer dispersity across all reaction temperatures (Ð ≈ 1.5). High temperature 13C NMR spectra identified 97–142/1000C branches in resulting polyethylene, a feature significantly dependent on the reaction temperature. In terms of mechanical properties, stress–strain analysis indicated that polyethylene synthesized at lower temperatures displayed superior tensile strength compared with that produced at higher reaction temperatures, while a reverse trend was observed in strain recovery analysis. The high strain recovery (up to 72%) of these polyethylene highlights the characteristic properties of thermoplastic elastomers.

A unified physically-based model describing complex unloading behavior in cold and hot deformation process of aluminum alloys

During the cold and hot deformation of metals, the nonlinearity in unloading is mainly caused by dislocation-associated behaviors and creates challenges for the prediction of springback law and residual stress distribution after springback. In this paper, a unified physically-based model for describing the dislocation density evolution in unloading after cold and hot deformation was proposed firstly by considering the combined effects of reverse slip of reversible mobile dislocations (under back stress and the static recovery) and recrystallization (after deformation) on the dislocation density evolution in unloading. Furthermore, an anelastic strain model associated with dislocation behaviors was established for predicting the microplastic strain in unloading after cold and hot deformation by considering the effects of strain-induced dislocation multiplication and the dislocation annihilation/consumption caused by dynamic recovery and dynamic recrystallization on the reversible mobile dislocation density evolution during deformation. The established dislocation density evolution model and anelastic strain model were evaluated using the experimental results obtained from the hot compression of homogenous aluminum alloys and the cold compression of extruded aluminum alloys. The results indicated that the established models are able to accurately predict nonlinear unloading behavior and nonlinearly varying microplastic strain. Meanwhile, it was also found that, for cold deformation, the reverse slip of reversible mobile dislocations driven by back stress is the leading reason for the nonlinearity of unloading, and the strain hardening and recovery during deformation are main factors affecting the anelasticity of unloading; for hot deformation, the reverse dislocation slip, static recovery and recrystallization collectively affect the nonlinearity in unloading, and the anelasticity is affected not only by strain hardening and dynamic recovery but also by dynamic recrystallization during loading process.

The effect of Hf on the microstructure, transformation temperature, and the shape memory properties of Zr–Cu-based high-entropy shape memory alloys

The microstructures, phase transformation temperatures, and shape memory properties of Hf-modified Zr50−xHfxCu25Ni7.5Co17.5 and Zr50−xHfxCu25Ni10Co15 high-entropy shape memory alloys (SMAs) were investigated. Hf additions can alter the valence electron concentration, strengthen the alloy matrix, and introduce more secondary phase particles, which significantly influence the properties of the alloys. The results reveal that high austenitic transformation temperature (over 600 K) with very small thermal hysteresis (70–80 K) can be achieved by tailoring the Hf contents, and the best shape memory recovery strain (up to 2.44%) compared with other Zr–Cu-based SMAs could be obtained in Zr42Hf8Cu25Ni10Co15 alloy. This study demonstrated that high-performance Zr–Cu-based high-entropy SMAs can be realized by incorporating Hf additions.

Study of nanoindentation behavior of NiCrCoAl medium entropy alloys under indentation process using molecular dynamics

Abstract The mechanical properties and deformation behavior of CoCrNiAl medium entropy alloy (MEA) subjected to indentation by an indenter tooltip on the substrate are explored using molecular dynamics (MD) simulation. The study investigates the effects of alloy compositions, temperature variations, and ultra vibration (UV) on parameters, such as total force, shear strain, shear stress, hardness, reduced modulus, substrate temperature, phase transformation, dislocation length, and elastic recovery. The findings indicate that higher alloy compositions result in increased total force, hardness, and reduced modulus, with Ni-rich compositions demonstrating superior mechanical strength. Conversely, increasing alloy compositions lead to reduced von Mises stress (VMS), phase transformation, dislocation distribution, and dislocation length due to the larger atomic size of Ni compared to other primary elements. At elevated substrate temperatures, atoms exhibit larger vibration amplitudes and interatomic separations, leading to weaker atomic bonding and decreased contact force, rendering the substrate softer at higher temperatures. Additionally, higher initial substrate temperatures enhance atom kinetic energy and thermal vibrations, leading to reduced material hardness and increased VMS levels. Increasing vibration frequency enlarges the indentation area on the substrate’s surface, concentrating shear strain and VMS with vibration frequency. Higher vibration amplitude and frequency amplify force, shear strain, VMS, substrate temperature, and dislocation distribution. Conversely, lower vibration amplitude and frequency result in a smaller average elastic recovery ratio. Moreover, increased amplitude and frequency values yield an amorphous-dominated indentation region and increased proportions of hexagonal close-packed and body-centered cubic structures. Furthermore, this study also takes into account the evaluation of a material’s ability to recover elastically during the indentation process, which is a fundamental material property.