TiNi-based Alloys Research Articles

Predictions of Lattice Parameters in NiTi High-Entropy Shape-Memory Alloys Using Different Machine Learning Models.

This work applied three machine learning (ML) models-linear regression (LR), random forest (RF), and support vector regression (SVR)-to predict the lattice parameters of the monoclinic B19' phase in two distinct training datasets: previously published ZrO2-based shape-memory ceramics (SMCs) and NiTi-based high-entropy shape-memory alloys (HESMAs). Our findings showed that LR provided the most accurate predictions for ac, am, bm, and cm in NiTi-based HESMAs, while RF excelled in computing βm for both datasets. SVR disclosed the largest deviation between the predicted and actual values of lattice parameters for both training datasets. A combination approach of RF and LR models enhanced the accuracy of predicting lattice parameters of martensitic phases in various shape-memory materials for stable high-temperature applications.

Aqua jet integrated fiber laser machining of quaternary NiTiCuZr alloy

ABSTRACT Low thermal diffusivity of NiTi-based smart alloy results in concentrated heat accumulation during laser beam machining which rises to adverse thermal effects like heat-affected zones, residual stresses, and alterations in microstructure of shape memory alloy (SMA) system. The addition of Cu and Zr can reduce the thermal hysteresis in the phase transformation of NiTi SMAs. Minimizing thermal hysteresis is desirable in SMA applications where precise control of the phase transformation temperatures is required, such as in actuators, sensors, and medical devices. Therefore, aqua jet integrated laser (AJ-FLBM) machining is a suitable choice for smart materials that need to be machined with high precision and low heat affected zone. In this current research, the NiTiCuZr SMA is machined using AJ-FLBM to investigate the effects of process variables on surface characteristics measured using RSM-BBD approach. The surface quality was enhanced by 68.93% with a gradual decrease in laser power (P), cutting speed (CS), and an increase in frequency (F). Aqua jet integrated FLBM technique may reduce heat affected zone due to the removal of heat in areas near the cutting location. This was confirmed by the DSC test which reveals that optimal machining inputs do not alter shape memory properties.

Next-generation multicomponent SMAs: leveraging HEA empirical parameters

Empirical rule calculation, commonly used for phase selection of high entropy alloys (HEAs), provides valuable guidelines for designing multicomponent solid solutions. It is unknown whether the technique can offer a similar design strategy for multicomponent intermetallics that undergo B2−B19′ transformation. Establishing such guidelines can significantly accelerate the pace of discovery for new shape memory alloys (SMAs) by narrowing down the search area within the vast compositional design space of multicomponent alloys. In this study, we compiled hundreds of data on medium and high-entropy NiTi-based alloys exhibiting B2−B19′ transformation. Our objective was to construct a more comprehensive Ω−δ map and locate multicomponent SMAs on it. Additionally, we conducted a thorough analysis to establish a correlation between transformation temperatures and compositional features in medium and high-entropy SMAs (ME and HE-SMAs). This correlation can be leveraged to design SMAs with targeted transformation temperatures.

Semi-quantitative analysis on sea buckthorn phenolic-rich extract coating bone-like open porous NiTi-based alloy

This study investigates the feasibility of coating Ni–Ti alloy with sea buckthorn extract via a hydrothermal method for targeted delivery of beneficial phenolic compounds to bone tissue. The qualitative analysis confirmed the presence of flavonoids and tannins in sea buckthorn extract, supporting its osteogenic potential. The microhardness of the NiTi alloy substrate was suitable for biomedical applications, and successful coating was achieved without compromising its properties. NiTi alloy samples were coated with 18.1, 20.1, and 12.4 mg of extract, respectively. Comprehensive evaluations confirmed the successful integration of the extract onto the alloy's surface. The coated system exhibited sustained release properties over five days, with the highest release occurring on the first day (on average 32.1 % for the first peak and 72.1 % for the second peak), as determined by HPLC analysis. The findings demonstrate the potential of this novel approach in developing dual-functionality implants for bone health promotion. Overall, this study underscores the promising potential of Ni–Ti alloy coated with sea buckthorn extract as a targeted drug delivery system for bone tissue.

A micromechanical view of enhanced reversibility in shape memory alloys

Transformation-induced plasticity (TIP) is the source of poor reversibility and functional fatigue in shape memory alloys (SMA)s undergoing martensitic phase transformation. The TIP is believed to originate from the defect generation at the highly-stressed austenite/martensite transition layer. It is suggested that under satisfaction of compatibility criteria (also known as cofactor conditions) the highly-stressed transition layer is removed and the phase transformation reversibility is highly enhanced. In this study, we employ the inclusion problem of micromechanics along with crystallographic theory of martensite to quantify the generated internal stresses associated with the nucleation of martensite in a wide range of SMAs. The micromechanical calculations show that the nucleation of martensite in SMAs with high degree of compatibility and enhanced reversibility generates much reduced internal stresses. For example, the maximum shear stress generated by nucleation of twinned martensite in Ni50.3Ti29.7Zr20 is ∼ 5 GPa, which is reduced to ∼ 400 MPa (about an order of magnitude reduction) in single-variant martensite of Ni39Ti50Pd11 with excellent reversibility and low functional fatigue. It is also shown that the internal stresses are much lower in a supercompatible twinned martensite microstructure (Cu25Au30Zn45) compared with a conventional twinned microstructure such as those formed in NiTi-based alloys. Analysis of the deformation state of the matrix shows that the underlying mechanism behind the reduced internal stresses is the reduction in the maximum shear strain ((λ3 - λ1)/2) imposed by the nucleation of martensite. The results present a quantified picture of internal (interfacial) stresses in different SMAs to better understand the origin of reversibility and functional fatigue.

High-temperature near-equiatomic TiNi strain glass

Strain glasses (STGs) exhibit many unique properties not available in their counterparts of martensites and have found applications in cutting-edge space technology. However, the strain glass transition temperature Tg is commonly too low to enrich applications, especially in prototypical TiNi-based shape-memory alloys (SMAs). Here we report a surprising finding of strain glass in Ti50.1Ni49.9 alloys with high Tg ∼290 K, which is the highest among existing TiNi STGs. It was achieved via a simple method of slight annealing at 573 K for 15 s on cold-rolled high Ms (martensitic transformation temperature) Ti50.1Ni49.9 SMAs. As a result, this near-equiatomic TiNi strain glass caused by cold-rolling-induced dislocations can inherit the high-Ms of Ti50.1Ni49.9 SMA and possess high Tg, manifested by a weak transition temperature decrease with cold-rolling reduction rate in the phase diagram.

Active operative heat storage application using thermoelastic martensitic transformation in TiNi-based alloy

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.

Electronic transport evolution across the successive structural transitions in Ni50–xFexTi50 shape memory alloys

Abstract TiNi-based shape memory alloys have been extensively investigated due to their significant applications, but a comprehensive understanding of the evolution of electronic structure and electrical transport in a system with martensitic transformations (MT) is still lacking. In this work, we focused on the electronic transport behavior of three phases in Ni50−x Fe x Ti50 across the MT. A phase diagram of Ni50−x Fe x Ti50 was established based on x-ray diffraction, calorimetric, magnetic, and electrical measurements. To reveal the driving force of MT, phonon softening was revealed using first-principles calculations. Notably, the transverse and longitudinal transport behavior changed significantly across the phase transition, which can be attributed to the reconstruction of electronic structures. This work promotes the understanding of phase transitions and demonstrates the sensitivity of electron transport to phase transition.

Selenium Inhibitor Coating on NiTi Shape Memory Alloys by Using Thermal Evaporation System

Abstract NiTi-based memory alloys are often used as intracorporeal implant materials because of their superelasticity and shape memory properties. However, over time, the release of nickel from the NiTi alloy into the body becomes unfavorable; therefore, studies should be conducted to reduce the negative impact of nickel leakage. In this study, the biocompatible selenium element was used to coat NiTi alloys with different thicknesses. The coating process was performed using the thermal evaporation technique. Scanning electron microscopy-Energy Dispersive X-ray Analysis measurements revealed that the surfaces of the samples were well covered. Also, the atomic force microscope device determined the surface roughness of the alloys. The roughness of the surface increased after the coating process. The Tafel test was accomplished to determine the biocompatibility of the coated and uncoated alloys in an artificial body fluid. The corrosion test reveals that Se-coated NiTi alloys have high corrosion resistance.

Microstructure and Phase Transformation Behavior of NiTiCu Shape Memory Alloys Produced Using Twin-Wire Arc Additive Manufacturing

NiTiCu thin walls were produced by twin-wire arc additive manufacturing (T-WAAM) using commercial NiTi and Cu wires as the feedstock materials. This approach aims to solve the problems typically associated with large phase transformation hysteresis in NiTi shape memory alloys. The microstructure, mechanical properties, and phase transformation behavior of the as-deposited NiTiCu alloy were comprehensively examined. The results revealed that the as-deposited NiTiCu alloy was well-formed, with its microstructure showed columnar, equiaxed, and needle-like grains, depending on the location within the deposited walls. The microhardness gradually increased from the first to the third layer. The Cu content was 20.80 at%, and Cu-based precipitates were formed in the as-deposited NiTiCu. The volume fractions and lattice parameters of the matrix and precipitates in the as-deposited NiTiCu material were analyzed using high-energy synchrotron X-ray diffraction. The martensitic phase was identified as a B19 crystal structure, and the as-deposited NiTiCu underwent a one-step B2-B19 phase transformation. The tensile strength and fracture strain were approximately 232 MPa and 3.72%, respectively. In particular, the addition of Cu narrowed the phase transformation hysteresis of the as-deposited NiTiCu alloy from 24.4 to 7.1 ℃ compared with conventional binary NiTi alloys. This study expands the potential of T-WAAM in modifying the phase transformation behavior of NiTi-based ternary alloys.

Effect of aging treatment on microstructure and wear performance of Ni-rich NiTiFe alloy

NiTi-based alloys with high nickel content have been utilized as a novel bearing material because of their exceptional hardness, superior wear resistance and corrosion resistance etc. In this work, microstructure, hardness and wear behavior of (Ni56Ti44)95Fe5 alloy were investigated as a function of aging duration. It is found that with increasing aging duration, Ni4Ti3 phase formed continuously and transformed into Ni3Ti phase in a short time due to the addition of Fe. In this case, the highest hardness (729 HV) and the lowest coefficient of friction (0.611) were obtained in the sample aged at 400 °C for 5 h. The dependence of hardness and wear behavior of (Ni56Ti44)95Fe5 alloy was attributed to the combined effect of Orowan strengthening by nano-sized Ni4Ti3 phase and the soften effect of Ni3Ti phase. The wear behavior of (Ni56Ti44)95Fe5 alloy was governed by the combined mechanism of adhesive wear, abrasive wear and oxidative wear.

Continuous operating elastocaloric air-cooling device

<p>Elastocaloric cooling has shown significant potential as an alternative to traditional vapor-compression technology because it relies on nonflammable, non-toxic, and zero-global warming potential solid refrigerants (usually NiTi and TiNi-based shape memory alloys). However, the uniaxial loading mode commonly used in existing elastocaloric cooling devices is often accompanied by a lower operating frequency. Additionally, using water as the heat exchange medium presents challenges in further cooling the air with cold water. These issues will hinder the application of this type of refrigerators in space cooling scenarios. Here, we report a continuous operating tension-based elastocaloric air-cooling device that utilizes the bundles of NiTi wires. The driving mode based on the cam structure realizes the alternating loading-unloading cycle of multiple NiTi bundles, facilitating continuous the cooling process. Our air-cooling device achieves a temperature decrease of 4.4 �� in continuous air flow and provides a cooling power of 21 W. These results demonstrate the possibility of applying elastocaloric technology in space cooling and provide a technical reference for designing household air conditioners using this technology.</p>

A Thermal, Mechanical, and Materials Framework for a Shape Memory Alloy Heat Engine for Thermal Management.

Shape memory alloy (SMA) heat engines possess an inherent property of sensing a change in temperature, performing work, and rejecting heat through the shape memory effect resulting from a temperature-induced phase transformation. This work presents a framework for the design and implementation of an SMA-based Stirling heat engine for maximum torque or speed incorporating and combining mechanical, thermal, and material aspects. There is a growing need for such engines for reliable thermal management and energy recovery in both ground and space applications. Mechanical aspects were addressed from force balances in the SMA element and focused on the resulting stress distribution. Thermal aspects considered heat transfer between the SMA element and both the heat source and the heat sink. Materials aspects considered the chemical, elastic, and frictional contributions to the enthalpy of the transformation. The roles of nano- and microstructure through composition, precipitates, variant interfaces, training, cycling, texture, defects, nucleation sites (bulk vs. surface), and multi-step transformations (e.g., a trigonal R-phase transformation) in NiTi based-alloys are also emphasized. The aforementioned aspects were combined to present a figure of merit to aid in the design and implementation of a Nitinol Stirling heat engine operating to maximize torque or maximize speed.

Tribocorrosion behaviour of NiTiNOL60 alloy in an alkaline environment

NiTi-based alloys subjected to aggressive tribocorrosion working conditions can deteriorate by the combined action of mechanical and chemical wear. The synergistic interaction under sliding contact and corrosive environment formed the basis of the current study. This study employed a linear reciprocating tribometer coupled with an electrochemical cell of a three-electrode configuration to investigate the behaviour of NiTiNOL60 alloy sliding against an alumina ball (Al2O3) in a NaOH environment. The resulting wear track surfaces were examined using a scanning electron microscope, optical microscope, energy dispersive spectroscopy (EDS), and stylus profilometry. While the synergistic interactions reveal that abrasive and oxidative wear mechanisms exist concurrently, an increase in applied load reduces the corrosion potential, thereby leading to a higher wear volume and increased corrosion rate. The experimental results showed that a higher corrosion rate and wear volume occurred at a higher load. The wear track analysis illuminated the various wear mechanisms at play, including abrasion, debris adhesion, pitting, delamination, ploughing, and cracking. These mechanisms were influenced by both mechanical and chemical wear, the latter notably due to corrosive attacks. EDS elemental analysis showed an increase in oxygen content at higher loads, which suggests increased corrosion due to the delamination of the oxide layer during the reciprocating sliding.

Effect of Ternary Element Addition on Properties of TiNi-Based Shape Memory Alloys for Engineering and Medical Applications

Shape memory alloys (SMAs) are a type of smart material and have excellent engineering and medical applications. TiNi binary alloys possess remarkable shape recovery, mechanical properties, corrosion resistance, and excellent biocompatibility. By ternary elements addition just like Au, Pt, Pd, Hf, and Zr, increases transformation temperatures, leading to high-temperature shape memory alloys (more than 100°C) but other elements (Fe, Cu, Co, and Mo) form low-temperature shape memory alloys, (lower than 100°C). In the present work, it is reported that the effect of ternary element addition on microstructural properties, shape memory properties, mechanical properties, corrosion resistance, and biocompatibility of ternary shape memory alloys. Ag, Au, and Cu-based TiNi ternary alloys have excellent biocompatibility. The addition of ternary elements such as Ag and Nb increases corrosion resistance, Fe rises the hysteresis loop, Hf enhances thermal stability, and Mo raises workability.

Improving the functional stability of TiNi-based shape memory alloy by multi-principal element design

This study investigated the evolutions of the superelasticity and elastocaloric effect of a multi-principal element Ti40Zr10Ni40Co5Cu5 strain glass alloy during 3001 cyclic compressions. The results were compared with a conventional solution-treated Ti49Ni51 shape memory alloy. The Ti40Zr10Ni40Co5Cu5 strain glass alloy exhibited a higher hardness, lower residual strain, and lower dislocation density due to multi-principal element solid solution strengthening and strong lattice distortion. Furthermore, it showed stable critical stress and elastocaloric cooling temperature drop during the cyclic test, and the values were maintained at 90.9 and 83.3 % after 3001 cycles, respectively. Digital image correlation analyses showed that the Ti40Zr10Ni40Co5Cu5 strain glass alloy exhibited a more homogeneous martensitic transformation behavior instead of the localized Lüders-type one. This feature originated from the high density of atomic defects, which in turn improved the temperature uniformity of the material during elastocaloric cooling. Experimental results demonstrated that the multi-principal element design method effectively suppressed dislocation slip and thus improved the functional stability and transformation homogeneity of the shape memory alloy, so such alloys are potential candidates for solid-state refrigerants.

Thermal cycling at constant stress for thin films over a wide temperature range: Design and fabrication of a specific device for the study of microactuators

The characterization of the mechanical behavior of shape memory thin film alloys is essential for their application as microactuators. Depending on the particular application, different elements have been considered for the fabrication of ternary NiTi-based alloys in order to shift the martensitic transformation toward a lower or a higher temperature range. This work presents the design and implementation of a specific device to perform thermal cycles at constant stress on film-like samples over a wide temperature range, between −130 and 300 °C. The uniaxial stress application, the elongations and contractions measurements associated with the martensitic transformations, and the cooling and heating methods are detailed in the present work. The fabrication and operation features of this relatively low cost device facilitate and promote the study of shape memory thin films.

Influence of high configuration entropy on damping behaviors of Ti-Zr-Hf-Ni-Co-Cu high entropy alloys

TiNi-based shape memory alloys (SMAs) have been used as damping materials to eliminate noise and mechanical vibration. However, their application is limited by low working temperatures and damping capacity. In this work, two novel Ti-Zr-Hf-Ni-Co-Cu high entropy shape memory alloys (HESMAs) with different transformation temperatures and damping properties were investigated. The results show that Ti25Zr8Hf17Ni30Co5Cu15 has superior damping performance arising from martensitic transformation, shape memory effect (thermal cycle at constant load) as well as superelasticity. Compared to traditional TiNi-based SMAs, the as-cast HESMAs exhibit a much higher ultrahigh yield strength (∼2 GPa) and storage modulus (∼50 GPa). The high configuration entropy of the HESMAs with high uneven internal stress and severe lattice distortion is revealed as the underlying mechanisms governing distinctive damping performance. The effects of high configuration entropy and microheterogeneity on the martensitic transformation behavior and damping performance of HESMAs are clarified in this work, which provides a basis for designing alloys with superior damping properties.

Influence of Heat Treatment and Deformation on the Structure, Phase Transformation, and Mechanical Behavior of Bulk TiNi-Based Alloys

We present a brief overview of the structural and phase transformations and mechanical properties of bulk binary TiNi shape memory alloys, which demonstrate attractive commercial potential. The main goal of this work was to create a favorable microstructure of bulk alloys using both traditional and new alternative methods of thermal and thermomechanical processing. It was found that the implementation of an ultrafine-grained structure by different methods determined an unusual combination of strength, ductility, reversible deformation, reactive resistance of these alloys to subsequent tensile or torsion tests at room temperature, and, as a consequence, the highly reversible effects of the shape memory and superelasticity. It is shown that the alloys Ti49.8Ni50.2 and Ti49.4Ni50.6 are incapable of aging, and, after being subjected to ECAP, were characterized by their high strength (σu up to 1200 MPa) and ductility (δ up to 60–70%). A combined treatment of multi-pass rolling and HT of the Ti49.5Ni50.5 and Ti49Ni51 alloys prone to aging have provided even greater strength (σu up to 1400–1500 MPa) with slightly lower ductility (25–30%). The microstructure, phase composition, and martensitic transformations in Ti-Ni alloys with varying Ni concentrations ranging from 50 to 51 wt.% were investigated by TEM, SEM, and X-ray methods. The mechanical behavior of the alloys was studied during tensile and torsion tests.

Influence of heat treatment on the phase transformation, thermodynamical parameters, crystal microstructure, and of Cu-Al-X (X: Cr, Ti) shape memory alloys

Shape memory alloys are known for their ability to return to their original shape under the influence of external factors (temperature, magnetic field and mechanical stress). Although NiTi-based alloys come to mind first when the shape memory effect is mentioned, CuAl-based alloys are very popular alternatives. The low cost of copper-based alloys is the biggest reason to study. In this study, the heat treatment effects of Cu-Al-X (X: Cr, Ti) (% weight) on some thermodynamic parameters, crystal structure and microstructure of shape memory alloy were investigated at three different temperatures (973 K, 1073 K and 1173 K). The changes in the thermal transformation of the samples were determined by DSC (Differential Scanning Calorimetry) and the changes in the crystal structure were determined by X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and optical microscope device. According to DSC measurement, the temperature hysteresis of the samples decreased after the heat treatment. Besides, as the entropy change decreased, the thermal stability of the samples increased. It can be seen that the particle size of the CuAlCr alloy decreased with increasing temperature. The particle size of the CuAlTi alloy increased with increasing temperature. SEM and optical images showed that chromium (Cr) was more dissolved in the alloy compared to titanium (Ti) into CuAl alloy.