NiTi-based Shape Memory Alloys Research Articles

Microstructural features, martensitic transformation, and functional properties of multicomponent Ti–Ni based shape memory alloys

Microstructural features, martensitic transformation, and functional properties of multicomponent Ti–Ni based shape memory alloys

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.

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.

Hybrid self-centering braces with NiTi-SMA U-shaped and frequency-dependent viscoelastic dampers for structural and nonstructural damage control

This study proposes a novel hybrid self-centering brace (HSB) with structural and nonstructural damage-control capabilities for the seismic resilience of building structures. NiTi-based shape memory alloy (SMA) U-shaped dampers and frequency-dependent viscoelastic dampers are included in HSBs to provide self-centering capacity for avoiding structural damage and velocity-proportional damping for nonstructural damage control, respectively. This study includes two major parts, i.e. component-level mechanical behaviour and system-level dynamic response evaluation of HSBs. The working principle and desired nonlinear behaviour of the HSB were first presented. Then, the preliminary design procedures were developed. After validating the efficiency of the modelling of SMA U-shaped and viscoelastic dampers using prior test results, comprehensive 3D finite element model simulations were performed to study the hysteretic responses of HSBs under cyclic loadings, where the influences of loading frequency, loading amplitude and viscoelastic damper’s contribution were considered. Analysis results demonstrated that the designed HSB can attain the desired deformation mechanism and nonlinear hysteretic behaviour during dynamic cyclic loadings. Higher loading frequency and amplitude and more contributions of the viscoelastic damper can introduce higher loading and energy-dissipation capacity. Subsequent dynamic analyses of the single-degree-of-freedom systems were conducted to assess the dynamic performance and advantages of HSBs. Results showed that compared with traditional self-centering systems without velocity-proportional damping, the systems with HSBs can achieve zero residual deformations, lower base shear demands and smaller absolute acceleration responses, confirming the promising potential of HSBs in developing seismic resilient building structures by reducing structural and nonstructural damage.

Thermodynamic and Kinetic Simulations Used for the Study of the Influence of Precipitates on Thermophysical Properties in NiTiCu Alloys Obtained by Spark Plasma Sintering.

The thermodynamic and kinetic simulations based on the re-assessment of the thermodynamic and kinetic database of the Ni-Ti-Cu system were employed to predict the phenomena of mechanical alloying, spark plasma sintering and thermal properties of the intriguing Ni-Ti-Cu system. Thermodynamic calculations are presented for the stable and unstable phases of NiTiCu materials and support a correlation with the evolving microstructure during the technological process. Also, the thermal conductivity, the thermal diffusivity and the specific heat of spark plasma sintered and aged Cu-alloyed NiTi-based shape memory alloys (NiTiCu) with two compositions, Ni45Ti50Cu5 and Ni40Ti50Cu10, are evaluated and the influence of mechanical alloying and precipitates on thermal properties is discussed. Measurements of these thermal properties were carried out from 25 °C up to 175 °C using the laser flash method, as well as differential scanning calorimetry. The thermal hysteresis of the 20 mm diameter samples was between 8.8 and 24.5 °C. The observed T0 temperatures from DSC experimental transformation features are in reasonable accordance with the thermodynamic predictions. The determined k values are between 20.04 and 26.87 W/m K and in agreement with the literature results. Moreover, this paper can provide some suggestions for the preparation of NiTiCu shape memory alloys and their applications.

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 Review on Comparison between NiTi-Based and Cu-Based Shape Memory Alloys

One of the best types of smart materials is Shape Memory Alloys (SMAs), which have more applications in modern technology such as aerospace, automotive, biomedical, civil engineering, robotics, and so on. Among all parts of them NiTi-based, and Cu–based are more focused on because they have some interesting properties that have made them more widely used in modern technology. In this work, the classifications and applications of SMAs according to the different application fields like biomedical, automotive, and aerospace have been reviewed. Besides the characteristic of (NiTi-based, Cu – based) and the comparison between them in many areas such as: In terms of cost, creation, corrosion resistivity, density, and electrical and thermal conductivity were represented. Each of NiTi and Cu-based SMAs has many useful applications and strengths, but they have some limitations.

Effects of Point Defects on the Monoclinic Angle of the B19″ Phase in NiTi-Based Shape Memory Alloys

This study investigated the effects of a variety of point defects on the monoclinic angle of the B19″ martensite in NiTi, including Ni and Ti vacancies, Ni–Ti anti-sites in the lattice, third element substitution, and Ni and Ti enrichments in the B2 stoichiometry. Their effects were studied by density functional theory calculations on the monoclinic B19″ phase as a proxy for the experimentally observed B19′ phase. It was found that vacancies and Ni–Ti anti-sites reduce the monoclinic angle of B19″, whereas third element substitution of Ni and Ti may affect the monoclinic angle in both directions depending on the third element and the host element it replaces. Enriching Ni or Ti content to deviate from the B2 stoichiometry decreases the monoclinic angle. A change in the monoclinic angle of the B19″ phase implies variation in the characteristic strains associated with the transformation. Thus, these findings may provide a new angle on possible interpretations of the variations of recoverable transformation strains experimentally observed of NiTi shape memory alloys.

Recoverable strain induced by martensitic transformation during holding of NiTi-based shape memory alloys under constant stress and temperature

The aim of the present paper was to study the strain variation in the NiTi-based shape memory alloys during isothermal holding under stress. The recoverable strain of Ni51Ti49 and Ti40.7Hf9.5Ni44.8Cu5 alloys was studied on holding under stress after cooling (Regime 1), after active loading (Regime 2), or during holding without stress in the sample that demonstrates the two-way shape memory effect (Regime 3). The strain variation was found during holding under a stress in all regimes. This strain was recovered on subsequent heating or unloading, thus, the strain variation on holding was caused by the isothermal martensitic transformation. This isothermal strain depended on the chemical composition of the alloy, the regime for the isothermal holding, the stress, and the holding temperature. The maximum isothermal strain was 3.4% in Ti40.7Hf9.5Ni44.8Cu5 (Regime 1) and 6.1% in Ni51Ti49 (Regime 2). Holding in Regime 3 was accompanied by a small strain (less than 0.3%). The influence of the stored elastic energy on isothermal martensitic transformation on holding was discussed, and it was shown that a large isothermal strain was found if the transformation was accompanied by a small stored elastic energy.

Role of Zr and thermomechanical treatment on phase transformation and functional properties of NiTi-based shape memory alloys

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.

An interatomic potential for ternary NiTiHf shape memory alloys based on modified embedded atom method

NiTi-based high temperature shape memory alloys (HTSMAs) can operate in high temperature applications, where binary NiTi cannot function. NiTiHf is among the most popular HTSMAs due to its lower preparation cost and good thermal stability. Various aspects of material properties of NiTiHf and the effect of different factors, such as composition, heat treatment and precipitates, on its behavior is yet to be understood. Molecular dynamics (MD) simulations can provide insights to acquire such an understanding. However, MD simulations of NiTiHf have not been possible due to the absence of a reliable interatomic potential, which is a necessary tool for such simulations. In this study, density functional theory (DFT) simulations were employed and a ternary Second Nearest Neighbor Modified Embedded Atom Method (2NN MEAM) potential was calibrated for NiTiHf. To perform this study, the related unary and binary potentials were calibrated by fitting the values of their reproduced physical properties to the DFT results. The final ternary MEAM potential was checked for reliability and transferability by performing MD simulations. The results showed that the developed potential can appropriately capture the temperature-induced and stress-induced martensitic phase transformations in NiTiHf.

Influence of wire-electric discharge machining process parameters on surface integrity of Ni-rich Ni-Ti-Hf alloys

Ni-Ti-Hf Shape memory alloys (SMAs) have shown promising results in high-temperature applications in aviation, space and energy exploration, actuators, etc. In the past decade, extensive work has been carried out to understand the behavior of High-Temperature SMAs (HTSMAs). NiTi-based SMAs are grouped as hard-to-machine materials, and machining these materials through traditional methods leads to high tool wear, dimensional inaccuracy, degradation of Shape Memory properties, etc. Therefore, Non-Conventional machining processes are a better choice for machining these alloys. It is evident from previous studies that Wire Electric Discharge Machining (WEDM) yields better results compared to other processes. The current study investigates the effect of WEDM input variables such as servo gap voltage (SV), spark-ON duration (TON), wire electrode feed rate (WF), and spark-OFF duration (TOFF) on the machining of Ni-Ti-Hf HTMSAs. The surface integrity of the machined samples was analyzed by investigating characteristics like machined surface morphology, machined surface quality, subsurface microhardness, and recast layer thickness. TON emerged as the most critical parameter for surface roughness and Material Removal Rate. Various defects like micro-cracks, micro-pores, craters, and globules were found on the machined surfaces, and approximately 30% harder surface was found near the machined region. The average thickness of the recast layer observed for Hf-15 and Hf-20 samples was around 12 μm and 50 μm for samples with lower and higher discharge energies, respectively.

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.

Microstructural evolution and superelastic properties of ultrafine-grained NiTi-based shape memory alloy via sintering of amorphous ribbon precursor

NiTi-based shape memory alloys (SMAs) are considered as cutting-edge intelligent functional materials. However, it remains a great challenge to obtain ultrafine-grained (UFGed) bulk materials with mm-scale size as well as outstanding superelastic properties. Here, UFGed bulk Ti35Zr15Ni35Cu15 NiTi-based SMA is successfully prepared via spark plasma sintering of amorphous ribbon precursor at different sintering temperatures, and microstructural evolution and superelastic properties are symmetrically investigated. It is found that its grain size ranges from UFG to micro-grain with increased sintering temperature regardless of the predominant B2 matrix in all bulk samples. Interestingly, the orientation relationships between B2 matrix and nano-scale fcc (Ti,Zr)2Ni precipitate evolve from coherent to incoherent. Consequently, the UFGed samples exhibit perfect superelasticity with the high recoverable strain of ∼5.8%, the stable recovery rate above 99%, and the great critical stress inducing martensitic transformation higher than 1 GPa, far superior to the corresponding ones of suction-cast micro-grained TiZrNiCu SMAs. Fundamentally, the perfect superelasticity is attributed to the good resistance to dislocation slip or grain boundary slip by residual nano-scale amorphous phase or secondary phase of coherent and semi-coherent fcc (Ti,Zr)2Ni precipitate. In addition, the gentle superelastic plateau is associated to the favorable transfer stress and the strong ability to accommodate dislocation movement, which is generated by the coherent interface between nano-scale fcc (Ti,Zr)2Ni and UFGed B2 matrix. These results suggest that spark plasma sintering of amorphous alloy precursor is a feasible route to obtaining excellent superelasticity in NiTi-based SMAs.

Investigation of structure and properties of melt-spun NiTi based shape memory alloys

In this work, we investigated the structure, mechanical properties and corrosion resistance of Ti50Ni50 and Ti16.667Zr16.667A16.667Ni25Cu25 (A = Hf, Nb, Co, Cr and Ga) shape memory alloys (SMAs) fabricated by using melt-spinning method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses reveal that the alloy ribbons are partially crystallized with B19' martensitic structure in the added alloys. The crystalline phase formation and the atomic size difference (δ = 4.33 - 10.25%) significantly affect the hardness, tensile strength, tensile strain, elastic modulus and corrosion resistance of the alloys. The hardness of the alloy gradually increases from 583 HV to 873 HV when adding elements in the order of Hf, Nb, Co, Cr and Ga. Tensile strength, tensile strain and elastic modulus simultaneously reach their maximum of 669.2 MPa, 0.899% and 28.82 GPa, respectively, when Hf is added to the alloy. Ga enhances the corrosion resistance of the alloy ribbons more than other additional elements.

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.

Mechanical Properties of NiTi Based Shape Memory Alloy used in Manufacturing of Mars Rover Wheels: Short Review

Shape memory alloys (SMA) are innovatively designed for engineering applications including, but not limited to, medical devices, actuators, micromechanical systems, deployable structures, and elastocaloric devices. Prior to the manufacture of any component for the above mentioned applications, an extensive simulation study is recommended as the basic properties are required to be filled in the software. The unavailability of to-the-point literature makes this task difficult for researches. In this paper, an overview of binary NiTi based shape memory alloy is given, where its characteristic physical properties i.e. shape memory effect and super elastic effect, and mechanical properties like fatigue failure, stress-strain characteristics and yield and ultimate strength is stated for several different specimens prepared using thermomechanical methods.

On the crystallographic texture and torsional behavior of NiTi shape memory alloy processed by laser powder bed fusion: Effect of build orientation

There is a growing industrial and academic interest in additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) both for producing parts with complex shapes and for tailoring the microstructure and properties of the resulting parts. This interest when it comes to NiTi-based shape memory alloys (SMA) has a third reason due to the poor machinability and need for several steps of additional treatments to create functional parts from these alloys. Generating the printability map and achieving fully dense parts is one of the priorities in AM communities, despite the aforementioned benefits of AM that open new doors for controlling the microstructure and thereby enhancing thermomechanical properties of additively manufactured components. Motivated by crystallographic texture-dependence of NiTi thermomechanical properties, a build orientation approach is utilized for controlling the crystallographic texture of LPBF-processed NiTi specimens. The microstructural and phase analyses of parts fabricated at varying angles show the significant effect of building orientation on the crystallographic texture of the as-printed samples. The mechanism of the effect of the proposed approach for controlling texture is discussed. SMA rotary actuator is a potential application of this methodology. We assessed the thermomechanical properties of the as-printed rod specimens under simple torsion using 3D digital image correlation (DIC) to capture the shear strain field. The recovery shear strains of the samples are then correlated to the crystallographic textures. It is shown that the recovery strain of the textured NiTi samples is in good agreement with the theoretical transformation strains obtained from lattice deformation theory.

Деформационная способность сплава с памятью формы TiNiHf при прокатке с импульсным током

Introduction. The deformation capacity of materials is one of the main mechanical characteristics that determine the possibility of its production using various technological processes for metal forming. Among intermetallic compounds, a special role belongs to alloys with a high-temperature shape memory effect (SME) based on TiNi with the addition hafnium. Most of these alloys are not only difficult to deform, but also quite brittle. Therefore, the development of any technological schemes to increase the deformation capacity of these alloys is relevant. The purpose of the work: to study the deformation capacity and the possibility of using electric pulsed current during cold rolling of the TiNiHf alloy. This processing method has not previously been applied to these alloys. In this work, the deformation capacity during cold rolling of a strip 2 mm thick made of a hard-to-deform high-temperature TiNi-based shape memory alloy with the addition of hafnium is studied. To increase the deformability, an external action in the form of a high-density pulsed current of more than 200 A/mm2 is investigated. The research methods are: X-ray analysis to assess the initial phase state; analysis of the evolution of true and engineering deformation to failure (appearance of visible macrocracks in the deformation zone); optical microscopy with magnification from 50 to 100 and measurement of Vickers hardness at room temperature. Results and discussion. An increase in the deformability under the influence of a pulsed current compared to rolling without current and the achievement of a maximum strain of 1.7 (true) and 85% (engineering) are established. The initial coarse-grained equiaxed martensitic microstructure (50 μm) is transformed into a microstructure elongated along the rolling direction, while the hardness increases by 50%. The absence of noticeable structural changes and the observed hardening may indicate a nonthermal effect of the current in increasing the deformability. Thus, the results of the conducted studies indicate the prospects of the method of rolling with a current of a hard-to-deform TiNiHf shape memory alloy as a method of metal forming.