Ti-based Shape Memory Alloys Research Articles

Evaluations of Mechanical Properties and Functionalities of the Al- and Zr-Tailored Ti–5.5Mo Shape Memory Alloys for Biomedical Applications

AbstractDue to the critical world population aging issue, biomaterials are important topics in the biomedical community. Among the biomedical materials, shape memory alloys (SMAs) are considered to be promising materials owing to their functionalities, such as shape memory effect (SME) and superelasticity (SE). Ti–Mo–Al-based alloys, which are biocompatible materials, are basic materials in the Ti-based SMAs due to the evaluation of the phase stability through the calculations of Mo and Al equivalents. This study, in addition, introduced Zr into the Ti–Mo–Al-based alloys for its highly biocompatibility and being a fine modifier to tune the phase stability. The mechanical (i.e., strength and elongation) and functional (i.e., SME and SE) properties have been investigated in this study to reveal the practicability of the biomedical applications for human usage. It was found that addition of Zr to the Ti–Mo alloy shows slight influence on the mechanical properties of the binary Ti–Mo alloy. With the addition of Al, that is, ternary Ti–Mo–Al alloys, two-stage yielding was found in the tensile examinations. Among all the alloys, the Ti–5.5Mo–4Al, the Ti–5.5Mo–4Al–6Zr, and the Ti–5.5Mo–8Al–6Zr (mol pct) alloys performed excellent shape recovery of about 100 pct in the bending and heating tests. Especially, the Ti–5.5Mo–4Al–6Zr and the Ti–5.5Mo–8Al–6Zr alloys further show high strength. Results in this study could be a guideline for the design of the fundamental Ti–Mo–Al-based SMAs.

Mechanical properties of various thermomechanical processed Ti-Mo-Al-Zr and pseudo-binary (Ti-Zr)-Mo shape memory alloys for biomedical applications

Shape memory alloys (SMAs) have attracted much attention due to the great demand for biomedical materials. Among the SMAs, the Ti-based SMAs are promising materials owing to their high biocompatibility, non-toxicity, excellent functionalities, and proper mechanical properties. Concerning the Ti-based SMAs, adding other elements is an often-used strategy, and the combinations of the addition materials are unlimited. Among the various Ti-based alloy systems, the Ti-Mo-Al-Zr alloy is one fundamental alloy system, since Mo and Al are served as the specific metals for the calculations of the Mo equivalent and Al equivalent for the evaluation of the alloy stability. While Zr is a neutral element for fine-tuning the properties of the alloys. This fundamental quaternary alloy was thus chosen in this study. In this research series, it has been found that the Ti-5.5Mo-8Al-6Zr alloy performed proper mechanical properties. Therefore, this specific alloy was further subjected to various thermomechanical treatments to further improve its properties. In addition to the Ti-5.5Mo-8Al-6Zr alloy, pseudo-binary alloys of (Ti-Zr)-Mo were also investigated due to their intrinsic low phase transformation temperature compared to the pure Ti and Zr elements. Strategies for the development of these SMAs by conducting thermomechanical treatments have been proposed in this study.

Investigations on the interaction of laser parameters for efficient microdrilling of NiTiV smart alloy system

Ni–Ti shape memory alloys (SMAs) are popular in current research due to their usefulness and mechanical properties. At different temperatures, Ni–Ti alloys transition from austenite to martensite. To restore high-temperature memory in nickel-titanium SMAs, vanadium (V) is added as an alloying element. For Ni–Ti-based SMAs, the fiber laser is one of the best machining procedures for bio-implants, actuators, and aircraft engine parts. Using a Box–Behnken design to experiment with laser power, nozzle distance, cutting speed, and frequency, this study examines fiber laser micro-drilled Ni50Ti48V2 SM alloy material removal and hole taper angle. By increasing power (P), frequency (F), and cutting speed (C S ), Ni50Ti48V2 alloy material removal rate (MRR) increased by 75.79%. The hole taper angle (HTA) dropped 75.33% when cutting speed, laser power and frequency decreased. Lowering cutting speed and laser power increases micro-hole circularity and reduces HTA. Upon surface topographical inspection, debris and molten materials were found on the drilled surface. The flow of nitrogen gas caused materials to diffuse on the Ni50Ti48V2 alloy’s entry and exit surfaces, changing surface roughness. High parameters influence surface roughness, HTA, and circularity due to nitrogen gas flow. The material’s DSC and XRD tests confirmed its suitability for biomedical microhole production.

Meso-scopically homogeneous superelastic transformation and related elastocaloric effect in a textured Ti-based shape memory alloy

Homogeneous superelastic behavior in shape memory alloys (SMAs) has been confirmed to be crucial for functional and structural fatigue properties, and more importantly, for stable elastocaloric effect (eCE). Here, the Ti–22Nb–4Zr–2Ta plate was prepared by successive cold rolling at 98.5%, followed by annealing at 1023 K for 30 min, forming a strong recrystallized texture, denoted as {112}β < 110>β. Such favorable texture contributed to a completely recoverable superelastic strain of ∼2.4% with narrow hysteresis of 35/45 MPa under isothermal/quasi-adiabatic conditions, which resulted in homogeneous martensitic transformation (MT) and adiabatic temperature changes (ΔTadi) of 4.8 K upon loading and 4.4 K upon unloading. The observation on the evolution of space-resolved strain and temperature throughout the sample revealed that the intrinsic feature of diffuse transformation along with favorable texture contributed to meso-scopically homogeneous transformation and related eCE. A standard deviation of strain was determined to be only 0.05%, corresponding to a small temperature deviation of 0.2–0.3 K, which quantitatively demonstrated the homogeneity, potentially high solid-state cooling performance efficiency, and long structural and functional fatigue life in Ti-based SMAs.

Parametric optimization of electric discharge machining of Ni 55.65Ti based shape memory alloy using NSGA II with TOPSIS

Nickel–Titanium based shape memory alloys (SMAs) are a unique category of smart materials that possess remarkable properties like super-elasticity, shape memory effect, high wear and corrosion resistance, thereby rendering them appropriate for uses such as composite structures, aerospace, and biomedical uses etc. These distinctive features make these alloys ‘difficult in machining’ using the traditional approach of machining process. In the present investigation, machinability aspects of Ni55.65Ti-SMAs using non-traditional electric discharge machining process has been studied. The effect of input process parameters i.e., pulse time ON, duty factor, peak current on the surface roughness and material removal rate has been investigated. The output responses viz. Minimum surface roughness along with maximum material removal rate having values of 6.828 μm and 4.552 mm3/sec respectively were achieved. Furthermore, empirical modelling and ANOVA study have been executed based on the full factorial design analysis. The non-dominated sorting genetic algorithm IIalgorithm IIalg2 commonly known as NSGA II, used crowding distance approach for multi-objective process optimization. The results obtained with NSGA-II were again ranked using Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) approach to give a more thorough understanding of the solution space and opt for the best possible solution.

Deformation mechanism of the (β + α”) dual-phase Ti-42Zr-13Nb alloy exhibiting an abnormal work-hardening behavior

In-situ high-energy X-ray diffraction combined with post-mortem transmission electron microscopy was used to reveal the deformation mechanism of the (β + α”) dual-phase Ti-42Zr-13Nb (wt.%) alloy exhibiting a two-stage yielding behavior under tension. It is found that the first yielding stage is dominated by two processes, i.e., the reorientation of pre-existing α” phase via variant selection and the preferential induction of α” variants during martensitic transformation. These two processes make the {020}α” plane perpendicular to the loading direction (LD) and lead to an abnormal increase in the work-hardening rate at 0.036–0.082 strain. When the strain exceeds 0.082, the stress-induced martensitic transformation ceases and the reorientation of pre-existing α” phase gradually weakens, resulting in a continuous decrease in the work-hardening rate. Moreover, the second yielding stage is dominated by the reorientation of stress-induced α” martensite, which tends to reorient the plane perpendicular to LD from {020}α” to {110}α”. Our investigation provides new insights into the reorientation scenarios of both pre-existing and stress-induced α” phases, as well as the underlying physical mechanism responsible for the abnormal work-hardening behavior in Ti-based shape memory alloys.

Theoretical analysis and design of Ti-based shape memory alloys correlating composition and electronic properties to transformation temperatures for high temperature applications

The influence of electronic parameters, such as the atomic number of the alloy (Z), valence electron concentration (cv), number of valence electrons per atom (ev/a ratio), and pseudopotential radius (P), on the phase transformation temperatures (Ms, As) of binary, ternary and quaternary shape memory alloys (SMAs) based on Ti is investigated. These temperatures (Ms, As) are correlated with the composition and ev/a of the alloys using multiple linear regression. The dependence of thermal hysteresis on the atomic radius of the alloys is also studied. The alloys investigated comprise of both low (ev/a < 5) and medium (5 < ev/a < 7.5) valence electrons groups. The phase transition temperatures are raised with an increase in ev/a ratio, Z and P for medium ev/a group alloys, while for low ev/a group alloys, the trend is quite the opposite. As cv increases, the transformation temperatures (TTs) decrease for the medium ev/a group, while they increase with cv for the low ev/a group. The two different trends observed for low and medium ev/a groups are explained in the light of elastic modulus of the alloys. The effect of Z, cv, electron orbital occupancy and alloying elements on the elastic modulus, and in turn on the temperatures of transition of the alloys, is also discussed.

Superelastic metastable Ti-Mo-Sn alloys with high elastic admissible strain for potential bio-implant applications

The demand for titanium alloys simultaneously having high elastic admissible strain and large recovery strain for bio-implant applications is increasing. Ni-free Ti-based shape memory alloys are promising candidates for obtaining the required multifunctional properties. In this study, a wide content range of (0–15)wt% of low-cost, toxicity-free, and high-biocompatible Sn element was added to the Ti-8Mo (wt%) alloy to study its effect on the superelastic recovery and mechanical properties of biomedical Ti-Mo-Sn alloys. By tailoring Sn content, desired multifunctional properties of high elastic admissible strain and room temperature superelasticity were achieved in the studied Ti-Mo-Sn alloys. It was found that the increase in Sn content stabilized the β phase and a single β phase was obtained at room temperature in Ti-8Mo-(13, 15)Sn alloys. The addition of Sn modified the lattice parameters of the α″ martensite and β phase and affected the lattice deformation stain of β → α″. The lattice deformation strain along the [011]β direction was found to be decreased by –0.26%/wt% Sn. The room temperature superelasticity with a recovery strain of 3.1% and an elastic admissible strain of 1% was obtained in the Ti-8Mo-13Sn alloy. As Sn content increased to 15 wt%, a high elastic admissible strain of 1.56% and a recovery strain of 2.0% were obtained. These Ti-Mo-Sn alloys with excellent multifunctional properties are promising candidates for bio-implant applications.

Effect of annealing temperature on microstructure and mechanical response of sputter deposited Ti-Zr-Mo high temperature shape memory alloy thin films

The shape memory thin films are most suitable candidates as actuators in MEMS applications than other smart materials due to its ability to produce larger recovery strains at low operation voltages. Recently, Ti based shape memory alloys are gaining attention due to its ability to produce stable shape memory properties at high temperatures. In current study, Ti-Zr-Mo high-temperature shape memory thin films were fabricated using multi-target DC/RF magnetron sputtering system on Si (100) substrate. Deposition time and target power were varied to obtain the desired composition. Post-annealing was carried out at 500 °C, 600 °C and 700 °C for 15 min each to crystallize the thin films that were amorphous in the homogenized condition. The annealing temperature was varied to observe the structural and morphological changes. The total thickness of the film was found to be around 300 nm measured from the cross-sectional micrographs. The results suggest that flatness of surface topography is attributed to higher ad-atom mobility with increasing the annealing temperature. The martensite phase (α’) was observed after annealing at 500 °C and the formation of silicides was observed with annealing at higher temperatures. The transformation temperature of 220 °C was obtained from electrical resistivity measurement. The enhanced mechanical properties with maintaining structural stability was observed in Ti-Zr-Mo thin film annealed at 500 °C for 15 min.

Effect of thermo-mechanical treatment on microstructural evolution and mechanical properties of a superelastic Ti–Zr-based shape memory alloy

Achieving a combination of good superelastic performance and high strength in the bio-functional nickel-free Ti-based shape memory alloys is always required for the superelastic biomedical devices. For achieving this combination, in this paper, the thermo-mechanical treatment's influence on the microstructural development, mechanical performance and superelastic behavior of a Ti–45Zr–8Nb–2Sn (at.%) shape memory alloy was studied by using X-ray diffraction (XRD), transmission electron microscope (TEM), and tensile tests. 773 K and 823 K annealed specimens after cold rolling showed (β+α) two-phase microstructure. The grain size increased from 0.11 μm in 773 K annealed specimen to 125 μm in 1173 K solution-treated specimen. A maximum recovery strain of 5.4% was achieved at room temperature in the 1073 K annealed specimen due to some extent of the favorable recrystallization texture together with reasonable grain size but the tensile strength was 830 MPa. A combination of a large recovery strain of 5.0%, a high tensile strength of 1030 MPa and good ductility with fracture strain of 13.2% was achieved at room temperature in the 823 K annealed specimen due to the fine β grains of 0.74 μm, which can be beneficial for biomedical applications.

Combinatorial Synthesis and High-Throughput Characterization of Microstructure and Phase Transformation in Ni–Ti–Cu–V Quaternary Thin-Film Library

Abstract Ni–Ti–based shape memory alloys (SMAs) have found widespread use in the last 70 years, but improving their functional stability remains a key quest for more robust and advanced applications. Named for their ability to retain their processed shape as a result of a reversible martensitic transformation, SMAs are highly sensitive to compositional variations. Alloying with ternary and quaternary elements to fine-tune the lattice parameters and the thermal hysteresis of an SMA, therefore, becomes a challenge in materials exploration. Combinatorial materials science allows streamlining of the synthesis process and data management from multiple characterization techniques. In this study, a composition spread of Ni–Ti–Cu–V thin-film library was synthesized by magnetron co-sputtering on a thermally oxidized Si wafer. Composition-dependent phase transformation temperature and microstructure were investigated and determined using high-throughput wavelength dispersive spectroscopy, synchrotron X-ray diffraction, and temperature-dependent resistance measurements. Of the 177 compositions in the materials library, 32 were observed to have shape memory effect, of which five had zero or near-zero thermal hysteresis. These compositions provide flexibility in the operating temperature regimes that they can be used in. A phase map for the quaternary system and correlations of functional properties are discussed with respect to the local microstructure and composition of the thin-film library.

Microstructure, mechanical and superelastic behaviors in Ni-free Ti-Zr-Nb-Sn shape memory alloy fibers prepared by rapid solidification processing

Ni-free Ti-based shape memory alloy with superior superelasticity is promising and attractive for biomedical applications. In this study, Ti-18Zr-12.5Nb-2Sn (at.%) alloy fibers were prepared by rapid solidification method and then the effect of various heat treatments on phase constitutions, microstructures, mechanical properties, and superelasticity was investigated by means of X-ray diffraction (XRD), optical microscope (OM), transmission electron microscope (TEM) and tensile test. α phase was observed in the 673 K and 773 K annealed alloy fibers, leading to the degraded ductility. The total recovery strain at room temperature decreased, grain size and transformation temperature increased with increasing annealing temperature from 873 K to 1173 K. Athermal ω was observed in the 1173 K annealed alloy fibers. A good combination of clear superelasticity and high yield stress was achieved by annealing at 873 K followed by aging at 573 K due to the combined effect of the age-hardening caused by isothermal ω phase and small grain size, which indicates these alloy fibers have a good potential in biomedical applications.

SEM/EDS analyses on shape memory alloy subjected to electrochemical corrosion

The aim of the present paper was to investigate the modifications occurred in the Ti-based shape memory alloy subject to electrochemical corrosion in artificial saliva. By 2D and 3D microscopy and by qualitative determinations of the luminous variation we could notice the effects of electrochemical corrosion tests on the surface of the metallic material, and by EDS determinations (Line and Mapping modes) of the surface chemical composition we could determine the chemical modifications produced following the corrosion tests.

Buckling and free vibration analyses of composite sandwich plates reinforced by shape-memory alloy wires

Nowadays, shape-memory alloys (SMAs) are used in many applications to improve the mechanical behaviors of the structures. In this paper, the natural vibration and buckling behaviors of the composite sandwich plates reinforced by SMA wires are studied. Three-dimensional (3D) finite element method is employed to model and analyze the sandwich plates. The face sheets of the sandwich plates are considered to be layered orthotropic composite plate, while a soft isotropic material is used for the core. The face sheets are armed by the Ni–Ti-based SMA wires. The Active Property Tuning method is used for modeling the SMA-embedded sandwich plates. The modal and eigenvalue buckling analyses are performed to calculate the natural frequencies and buckling loads of the plates. The effects of plate’s thickness, face sheet’s thickness and plate aspect ratio and also boundary conditions on the natural frequencies and buckling loads of the plates are inspected. Comparison between obtained numerical results and those published in the literature confirms that the present modeling and vibration and buckling analyses of the SMA-reinforced composite sandwich plates are reliable and applicable.

Microstructural Characterization of a Laser Surface Remelted Cu-Based Shape Memory Alloy

Cu-based shape memory alloys (SMAs) present some advantages as higher transformation temperatures, lower costs and are easier to process than traditional Ti-based SMAs but they also show some disadvantages as low ductility and higher tendency for intergranular cracking. Several studies have sought for a way to improve the mechanical properties of these alloys and microstructural refinement has been frequently used. It can be obtained by laser remelting treatments. The aim of the present work was to investigate the influence of the laser surface remelting on the microstructure of a Cu-11.85Al-3.2Ni-3Mn (wt%) SMA. Plates were remelted using three different laser scanning speeds, i.e. 100, 300 and 500 mm/s. The remelted regions showed a T-shape morphology with a mean thickness of 52, 29 and 23 µm and an average grain size of 30, 29 and 23µm for plates remelted using scanning speed of 100, 300 and 500 mm/s, respectively. In the plates remelted with 100 and 300 mm/s some pores were found at the root of the keyhole due to the keyhole instability. We find that the instability of keyholes becomes more pronounced for lower scanning speeds. It was not observed any preferential orientation introduced by the laser treatment.

My Experience with Ti–Ni-Based and Ti-Based Shape Memory Alloys

The present author has been studying shape memory alloys including Cu–Al–Ni, Ti–Ni-based, and Ni-free Ti-based alloys since 1979. This paper reviews the present author’s research results for the latter two materials since 1981. The topics on the Ti–Ni-based alloys include the achievement of superelasticity in Ti–Ni alloys through understanding of the role of microstructures consisting of dislocations and precipitates, followed by the contribution to the development of application market of shape memory effect and superelasticity, characterization of the R-phase and monoclinic martensitic transformations, clarification of the basic characteristics of fatigue properties, development of sputter-deposited shape memory thin films and fabrication of prototypes of microactuators utilizing thin films, development of high temperature shape memory alloys, and so on. The topics of Ni-free Ti-based shape memory alloys include the characterization of the orthorhombic phase martensitic transformation and related shape memory effect and superelasticity, the effects of texture, omega phase and adding elements on the martensitic transformation and shape memory properties, clarification of the unique effects of oxygen addition to induce non-linear large elasticity, Invar effect and heating-induced martensitic transformation, and so on.

Microstructural Analysis of Ti-Based Shape Memory Alloys Following the Electrochemical Corrosion in Artificial Saliva

The investigations carried out aimed to highlight the structural modifications occurred in the Ti-based shape memory alloys subject to electrocorrosion in Afnor artificial saliva. The behavior to corrosion was highlighted by fast electrochemical tests, mainly by dynamic potentiometry. From the microstructural analysis we noticed that the specimens of the two Ti-based shape memory alloys show traces of “pitting” corrosion on their surface of diverse sizes, a fact that will raise issues in terms of cytotoxicity due to the corrosion products released.

Effect of laser solution-treatment on a Ti-based shape memory alloy

Ti-based shape memory alloys are usually heat-treated to produce consistent properties for engineering applications. The objective of this study is to investigate the effects of laser heat treatment on the mechanical properties and superelasticity of Ti72Nb15Zr10Al3 alloy. Samples heat-treated with a single-mode fiber laser (1070nm) were compared with a sample conventionally heat-treated in a furnace at 800°C for 1800s. The samples that were laser heat-treated at 50W for 10s and 100W for 1s had similar mechanical properties. The deformation-induced phase transformation behavior was measured by in situ X-ray diffraction. Recrystallization and grain coarsening during laser heating were observed by high-energy X-ray diffraction. Laser heat treatment achieved the same effects as conventional heat treatment in 1/1000 of the time. Thus, laser heat treatment may be a useful method, especially for fine and precision parts.

Optimizing Ni–Ti-based shape memory alloys for ferroic cooling

Due to their large latent heats, pseudoelastic Ni–Ti-based shape memory alloys (SMAs) are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. The present work aims for a chemical and microstructural optimization of Ni–Ti for ferroic cooling. A large number of Ni–Ti-based alloy compositions were evaluated in terms of phase transformation temperatures, latent heats, mechanical hysteresis widths and functional stability. The aim was to identify material states with superior properties for ferroic cooling. Different material states were prepared by arc melting, various heat treatments and thermo-mechanical processing. The cooling performance of selected materials was assessed by differential scanning calorimetry, uniaxial tensile loading/unloading, and by using a specially designed ferroic cooling demonstrator setup. A Ni[Formula: see text]Ti[Formula: see text]Cu5V[Formula: see text] SMA was identified as a potential candidate material for ferroic cooling. This material combines extremely stable pseudoelasticity at room temperature and a very low hysteresis width. The ferroic cooling efficiency of this material is four times higher than in the case of binary Ni–Ti.

Effect of deformation on the microstructure, transformation temperature and superelasticity of Ti–23 at% Nb shape-memory alloys

Extensive research on the effect of spark-plasma sintering and free forging on the microstructure, transformation temperature, and superelasticity of Ti-23at% Nb shape-memory alloys (SMAs) was undertaken. Based on the obtained results, it was found that the specimen showed significant results after deformation than the homogenization step, from which it can be concluded that the deformation has considerable effects on the mechanical properties and shape-memory characteristics of Ti-based SMAs. The microstructure of homogenized and deformed Ti-23at% Nb SMAs consists of β, α, α′ and ω phases, while the sintered sample shows only β and α phases. Endothermic and exothermic peaks of the austenite↔martensite transformation are seen in the homogenized sample, then after being deformed, the exothermic peak disappeared. The highest austenite start and finish temperatures were observed with the deformed sample due to disappearance of TiNb4 intermetallic compounds and a high percentage of β phase. The ductility of the deformed sample shows the highest value compared with the homogenized sample due the elimination of coarse α precipitates and brittle intermetallic compounds of TiNb4, along with the suppression of the brittle ω phase and retaining β matrix.