Equiatomic NiTi Alloy Research Articles

Thermodynamic Investigation of Propagating Behavior in SHS for Equiatomic and Non-Equiatomic NiTi Alloys Using Factsage Software

Thermodynamic Investigation of Propagating Behavior in SHS for Equiatomic and Non-Equiatomic NiTi Alloys Using Factsage Software

3D printing of shape memory Alloys for complex architectures of smart structures

Shape Memory Alloys (SMA) are materials used to design smart structures with intrinsic functional properties and improved efficiency. This is a key aspect of aerospace industry and makes SMA good candidates in this field. One of the most widespread SMA is the equiatomic NiTi alloy which, however, has the strong limitation of poor machinability, so only simple shapes can be obtained. Additive Manufacturing processes allow to overcome this limit and to design complex shapes. Compared to other metallic materials, the optimization of the process for NiTi alloy is complicated because, beside mechanical properties and presence of defects, considerable attention needs to be dedicated to the material functionality. The high temperatures involved in the additive process significantly affect the material properties due to possible evaporation of Ni and formation of precipitates that enable a shift of the phase transformation temperatures. This paper is focused on the optimization of the process parameters of the NiTi alloy printed through the Laser Powder Bed Fusion (L-PBF) to ensure optimal pseudo-elastic behaviour, which is essential for the design of structural dampers. This was accomplished starting from simple structures and then designing a damper that couples the pseudoelasticity of NiTi with load support capacity.The L-PBF is a powder-bed technique that selectively melts layers of micrometric metal powder. A pseudoelastic NiTi powder with 50.8 at. % of Ni content was selected and characterized through scanning electron microscope (SEM) and observations connected to an Energy Dispersive X-ray Spectroscopy (EDX) probe. After that, some cubic samples were manufactured, with the dimension of 3 × 3 × 15 mm3. A set of different laser powers and scanning speeds were used to find the set of process parameters that optimize the functional properties of the printed parts. Near fully dense specimens with density higher than 99.5 % were selected for further investigations. Differential scanning calorimetry (DSC) and mechanical tests were performed on as-built and heat-treated samples.Quasi-static mechanical tests were accomplished in compression mode, at different strains, up to 8 %. It was observed that the residual strain for cyclic loading at 4 % is lower than 1 %, so good recovery of the deformation was shown. Moreover, numerical analyses that mimic the pseudoelastic behaviour in compression tests were implemented.Finally, the best set of parameters was selected on the basis of the material's ability to recover deformations and its loss factor.

Characteristic variations in a pulsed‐anodized NiTi alloy surface by the lower voltage setting

Pulsed anodization of a nearly equiatomic NiTi alloy in HNO3 leads to the formation of a nearly Ni‐free oxide layer, resulting in the suppression of Ni‐ion release from the alloy surface. The core technology involves introduction of a lower‐voltage period, which promostes chemical reactions between the alloy and electrolyte, to obtain a surface layer with better corrosion protection. In this study, a higher voltage of 3.5 V was applied, and the lower voltage was varied within 0–3.5 V. As the lower voltage was increased, nanometer‐sized pores present on the anodized surface gradually expanded, while the layer thickness decreased. Although the corrosion protectivity of the layer did not change significantly during the electrochemical experiments, the amount of Ni‐ion released into the physiological solution significantly decreased when the voltage was below 1.8 V. X‐ray photoelectron spectroscopy analyses revealed the presence of Ni (OH)2 on the topmost surface, and its concentration decreased at a voltage <1.8 V. Lower voltages affected the concentration of Ni (OH)2 on the topmost surface and thus considerably influenced the Ni‐ion release behavior. These findings will contribute to the production of NiTi alloys with improved biocompatibility.

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.

Endothelium Cell Responses on Pulsed-Anodized NiTi Alloy with HNO<sub>3</sub>, NH<sub>4</sub>NO<sub>3</sub>, H<sub>2</sub>SO<sub>4</sub>, and (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> as Electrolytes

Pulsed anodization allows to form a Ni-free TiO2 layer on an almost equiatomic NiTi alloy. The surface is then hydrophilized and impedes the release of Ni ions from the alloy. This study aimed to assess the suitability of electrolytes used in the pulsed anodization process with the view of application as a biomaterial. Toward this end, the NiTi alloy was pulsed-anodized in four different aqueous electrolytes, HNO3, NH4NO3, H2SO4, and (NH4)2SO4, after which the endothelium cell behavior was compared. The use of H2SO4 as the electrolyte resulted in the formation of a TiO2 layer with a groove-like structure of several tens of nanometers wide, and this surface inhibited the activity of endothelium cells. The ability to prevent the release of Ni ions was diminished when using the HNO3 electrolyte, resulted in inferior cell proliferation. The other two electrolytes, NH4NO3 and (NH4)2SO4, formed a Ni-free TiO2 layer with a comparatively smooth surface, which more effectively suppressed the release of Ni ions; thus, the cell attachment as well as proliferation were superior to that of the other two electrolytes. The surface smoothness and Ni suppression were thus concluded to be the factors governing the selection of an electrolyte.

Synthesis and characterization of novel NiTi–Ni3Ti/SiC nanocomposites prepared by mechanical alloying and microwave-assisted sintering process

Equiatomic Ni–Ti alloy reinforced with 0, 2.5, 5, and 10 vol % SiC nanoparticles was synthesized through mechanical alloying and microwave sintering process. In this method, a mixture of Ti and Ni powders and SiC nanoparticles (0–10 vol%) were mechanically milled for 30 h, compressed under 1 GPa pressure, and then sintered at 900–1100 °C by microwave heating. Microstructural analysis of the synthesized samples was performed using X-ray diffraction and scanning/transmission electron microscopy, and their hardness was determined using a Vickers microhardness tester. Structural studies revealed the formation of a complex phase structure consisting of the austenite (B2–NiTi), the martensite (B19′-NiTi), and the nickel-rich (Ni3Ti) and titanium-rich (NiTi2) phases. HR-TEM investigations confirmed an increased Ni3Ti phase in the austenitic NiTi matrix nearby the martensitic NiTi phase. It was found that adding SiC nanoparticles to Ni–Ti increased the amount of hard Ni3Ti phase, promoted the stability of the martensite phase at room temperature, and retarded the matrix grain growth during the sintering process. Microhardness tests showed that when the content of SiC nanoparticles increased up to 5 vol %, the microhardness increased considerably to 9 GPa and then declined. The high microhardness of synthesized nanocomposites is due to the formation of Ni3Ti and NiTi martensitic phases and their nanocrystalline structure.

Depth distribution analysis of alloying and impurity elements in Ni-Ti alloy thin films using secondary ion mass spectrometer

Ni-rich Ni-Ti thin films of various compositions and thickness (69 nm - 141 nm) were deposited on Si (111) substrate by magnetron sputtering system using equiatomic NiTi alloy sputtering target. X-ray reflectivity analysis of the thin films deposited at room temperature showed the presence of a layered structure as: NiO/Ni-Ti/NiO/SiO2 on Si substrate. Secondary ion mass spectrometry (SIMS) was used to determine the qualitative (intensity) depth distribution of elements Ni, Ti, O and Si in the films using MCs+ approach. Relative sensitivity factor values were utilized to convert the qualitative depth profiles to concentration depth profiles. Surface and depth distribution of alloying elements in the deposited films were investigated. It was observed that the films deposited at high sputtering power and working pressure resulted in more non-uniform depth distribution of alloying elements compared to films deposited at low sputtering power and working pressure. SIMS depth profile data showed the presence of high oxygen contaminations at the surface and at the interface of the films. The Ti concentration in the deposited films depends upon the background oxygen contamination present in the deposition chamber during sputtering process. The concentration of Ni in the films deposited under similar working pressure increases with increase in the sputtering power. However, with increase in working pressure during deposition process results in decrease in the concentration of both Ni and Ti in the deposited films. In order to study the effect of insitu substrate heating on the distribution of alloying and impurity elements, one of the film was deposited on heated Si substrate. The film deposited on heated substrate represents the layer structure as NiTiO3 on Si substrate. The concentration of oxygen in the film deposited on heated substrate is comparatively higher than those deposited at room temperature.

Effect of carbon addition on the microstructure and mechanical behavior of near equiatomic NiTi alloy wire

According to the phase diagram, 0.03 wt% (C0.03) and 0.05 wt% (C0.05) carbon were added to Ni50.2Ti49.8 alloy, and the alloy wires with a diameter of 1.2 mm were prepared by vacuum induction melting and drawing. The results show that during melting and solidification, C combines with Ti in C0.03 and C0.05 to form eutectic TiC and primary TiC, respectively. Compared with the NiTi alloy wire without C, the tensile fatigue life (22866 cycles under 700 MPa tensile stress) and tensile strength (1536 MPa) of the alloy were increased by 39 % and 22 % respectively when 0.03 wt% C was added. With the further increase of C content to 0.05 wt%, the two properties began to deteriorate. This is mainly related to the changes of matrix phase composition, second phase morphology and internal microstructure caused by the addition of different amounts of C. The research results show that on the premise of following the requirements for C content (C ≤ 0.04 wt%) in the international standard ASTM F2063-18 for medical NiTi alloy composition, adding a certain amount of C (such as 0.03 wt%) to NiTi alloy is beneficial to improving the tensile strength and tensile fatigue life of wire.

Study on phase transformation behavior and mechanical properties of equiatomic ratio NiTi alloy formed by selective laser melting

PurposeThis paper aims to better control the mechanical properties and functional properties of NiTi alloy.Design/methodology/approachNiTi alloy samples with equal atomic ratio were formed by selective laser melting (SLM). X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy and tensile testing methods were used to study the effects of different laser power and scanning speed on the densification behavior, phase transformation characteristics and mechanical properties of NiTi alloy.FindingsCompared with the laser power, the variation of the keyhole effect caused by the change of scanning speed is more intense, which has a greater effect on the densification behavior of SLM NiTi alloy. The effect of the laser power on the phase transition temperature is small. The increase of scanning speed weakens the burning degree of Ni element, so phase transition temperature decreases. The results of DSC test and tensile test show that the scanning velocity can significantly change the phase transition temperature, martensite twins reorientation and stress–strain behavior of SLM NiTi alloy.Originality/valueThis study provides a potential method to regulate the mechanical properties and functional properties of NiTi shape memory alloy in the future and NiTi alloys formed by SLM with good elongation were obtained because the Supercellular crystal structure formed during the nonequilibrium solidification of SLM and the superfine precipitates dispersed in the alloy prevented the dislocation formation.

Influence of local thermal cycle on the lattice and microstructure evolution of twin-wire directed energy deposition-arc fabricated equiatomic NiTi alloy

In the present work, the authors wish to investigate the effect of repeated heating cycles on the phase and crystal structure few layers at a time. Equiatomic NiTi alloy is additively manufactured using twin-wire directed energy deposition-arc (TW-DED-arc) in an in-situ alloying process from pure Ni and Ti wire feedstocks. The central region is wide enough for the extraction of several metallographic and tensile specimens away from dilution zones. Microstructure characterization reveals the incorporation of silicon preventing the formation of NiTi2 and the existence of two types of B2 austenite in the matrix, which has a composition of 50.2 at.%Ni-49.8 at.%Ti. The lattice is observed to dilate near the top regions under tensile thermal stress with no contribution from Ni-rich precipitates. Mechanical test gives a maximum ultimate tensile stress of 782.5 MPa with a fracture surface indicative of quasi-cleavage failure. The results show TW-DED-arc is able to produce fully dense, chemically homogenous NiTi alloys without detrimental phases. In addition, the local thermal cycles experienced by the samples gradually promote the nucleation of Ti5Si3 precipitates and lead to a preferential growth of NiTi grains along the (0 0 1) direction.

Effect of substrate temperature on the surface and interface properties of NiTi atmospheric plasma sprayed coating

Atmospheric plasma spraying (APS) is one of the most preferred coating processes, which comes under thermal spraying technology. It has a wide range of applications due to its versatility in surface modifications. By the APS technique, different high-temperature application materials like cermets, and ceramics, can be coated on a substrate whose surface properties have to be modified in an extreme environment. Ni50Ti50 (at.%) alloy became more focus of interest for researchers and industrialists due to their two extraordinary smart properties such as shape memory effect (SME), and superelasticity (SE) which can protect any structural materials from catastrophic failure. Compared to various deposition routes, APS is an economical process to apply NiTi on mild steel. NiTi smart alloy also exhibits good wear resistance, corrosion resistance, damping behaviour, and high load-bearing capacity. In terms of mechanical properties, it has high strength and hardness at high temperatures. Owing to these properties, NiTi alloy can be used to protect structural materials such as mild steel from catastrophic failure. In this study, mild steel was coated at 100 °C, 200 °C, 300 °C and 400 °C preheating temperatures by APS techniques using an equiatomic NiTi alloy. The surface and interface characterization were performed using an optical microscope, SEM, XRD, Hardness test and 3D optical profilometer. It was found that along with the desired NiTi phase, some of the intermetallic and oxide phases are responsible for the mechanical strength. The surface and interface morphologies show that NiTi splat formation by APS results in better surface bonding and dense coating. With increase in the substrate temperature, the hardness of a coating interface increases with decrease in the number of pores at coating interface.

Study on laser beam butt-welding of NiTinol sheet and input-output modelling using neural networks trained by metaheuristic algorithms

Though NiTinol, the equiatomic Ni-Ti alloy, is revolutionizing different fields of engineering, manufacturing and medicine since its discovery as a functionally advanced material, poor formability and machinability, and the limitations of available fabrication techniques applied for this alloy are restricting its applications in the form of various desired products. Now-a-days, laser welding is the mostly reported joining technique applied to the alloy. Though the difficulties associated with fabrication method and the cost of the alloy are high enough, a computationally efficient model for quick and precise prediction of operating parameters in order to obtain the specified attributes of the laser welded NiTinol sheets is significantly missing in the literature. In the present work, basic metallographic study related to laser welding of NiTinol sheets in butt-joint configuration was investigated. At the same time, an efficient forward and reverse model had been developed with the help of artificial neural network (ANN). These ANNs were trained with the help of regression equation model using five different metaheuristic techniques separately for the development of both forward and reverse models. Laser power, scan speed, frequency, duty factor and focal positions were treated as the input process variables and three different weld attributes, namely bead-area, micro hardness of the bead and tensile strength of the bead were considered as output variables of the process. The developed model was capable enough of predicting the outputs, which were in good agreement with the experimental data, for both forward and reverse models. The novelty of this study deals with the development and testing of feed-forward neural network and recurrent neural network trained using five different metaheuristic techniques for both the forward and reverse modelling of fibre laser welding of NiTinol alloys.

The damping behavior of a Ni-50 at.%Ti shape memory alloy

Abstract In order to utilize the shape memory alloys (SMAs) for prospective damping applications, a near equiatomic NiTi alloy was investigated on a dynamic mechanical analyzer in the infrasonic frequency range. A high damping capacity stable to the variation of temperature can be obtained during a two-stage phase transformation and in the martensite state approaching to the martensite-parent phase transformation temperature in the NiTi SMA. The transient damping peaks associated with the phase transformation in the NiTi SMA shows a strong frequency dependence in both solutionized and annealed specimens. The frequency dependence parameter, n, was found to be in the range 0.3 – 0.6. For the applied strain amplitudes from 8.5 × 10–6 to 5.0 × 10 –5 a clear strain amplitude dependence was not observed in the damping peaks, except for the solutionized specimens. While the damping capacity of the martensite state, characterized at 300 K, showed obvious frequency dependences in both the solutionized and annealed specimens, only the annealed specimens manifested the strain amplitude dependence. The different damping behavior was attributed to the internal structure of martensite, including dislocations, the twinning types and variant interfaces, changeable with the prior thermomechanical treatments.

Transformation temperature and corrosion behavior of porous NiTi and NiTiCu shape memory alloy

The present research work deals with the fabrication of porous NiTi and NiTiCu shape memory alloy (SMA) by powder metallurgy process. These SMAs have number of applications in medical and space industries. The transformation temperature of both SMAs is measured by differential scanning calorimetry (DSC). Corrosion behavior is measured by potentiodynamic polarization curve in the presence of artificial saliva (pH value 7.4). Result shows that transformation temperature decreases by 8 °C to 10 °C after copper addition. Also, the corrosion current density decreases for NiTiCu, which exhibit more corrosion resistant than equi-atomic NiTi alloy. The corrosion current density of NiTiCu is less than NiTi by a factor of 31.

Twin-based martensite stabilizing and improving the shape memory response of near equiatomic NiTi alloy through multi-axial forging

In the present study, a high cumulative true strain (∼4.5) was applied on a near equiatomic NiTi shape memory alloy through multi-axial forging in the course of “isothermal” and “continuous cooling” regimes. It was found that the processed sample under decreasing temperature condition (from 900 °C to 660 °C) represented a full shape memory response without any residual strain. This behavior was discussed considering the effects of the compound twins on stabilizing the pre-existed martensite structure and favoring the detwinning stage compared with type I and type II deformation twins.

Influence of Batch Mass on Formation of NiTi Shape Memory Alloy Produced by High-Energy Ball Milling

A high-energy ball milling technique was used for production of the equiatomic NiTi alloy. The grinding batch was prepared in two quantities of 10 and 20 g. The alloy was produced using various grinding times. Scanning electron microscopy, X-ray diffraction, hardness measurement and differential scanning calorimetry were used for materials characterization at various milling stages. The produced alloy was studied by means of microstructure, chemical and phase composition, average grain and crystallite size, crystal lattice parameters and microstrains. Increasing the batch mass to 20 g and extending the grinding time to 140 h caused the increase in the average size of the agglomerates to 700 µm while the average crystallites size was reduced to a few nanometers. Microstrains were also reduced following elongation of milling time. Moreover, when the grinding time is extended, the amount of the monoclinic phase increases at the expense of the body-centered cubic one—precursors of crystalline, the B2 parent phase and the B19′ martensite. Crystallization takes place as a multistage process, however, at temperatures below 600 °C. After crystallization, the reversible martensitic transformation occurred with the highest enthalpy value—4 or 5 J/g after 120 and 140 h milling, respectively.

Molecular dynamics study of phase transformations in NiTi shape memory alloy embedded with precipitates

The present study utilizes molecular dynamics simulations to study the athermal and stress-induced martensitic transformation of B2 to B19′ phase in a Ni-Ti alloy and the effect of precipitates on the phase transformation. The simulations demonstrate the existence of an intermediate B19 phase between martensite and austenite phases. The Nickel-Titanium shape memory alloy with precipitates is studied by introducing Ni3Ti, NiTi2 and Ni4Ti3 precipitates individually using atomistic simulations. The results show that the phase transition temperature decreases in the presence of a large volume fraction of precipitates. A blended precipitates model with Ni3Ti and NiTi2 is simulated to study the phase transformation in equiatomic NiTi alloy. The results indicate that the precipitates would initiate the emergence of the B19 phase and reduce the transition temperature. In addition, the variation of Nickel content by embedded precipitates would lead to a change in the microstructural phenomena.

Effect of Ni Addition on the Corrosion Resistance of NiTi Alloy Coatings on AISI 316L Substrate Prepared by Laser Cladding

In this paper, an equiatomic NiTi (55NiTi) alloy powder was mixed with pure Ni powder to prepare laser cladding coatings on a 316L stainless steel substrate to study the effect of Ni addition on the microstructure and corrosion resistance of the coatings. The microstructure and phase composition of the coatings were analyzed using a scanning electron microscope (SEM) with configured energy-dispersive spectrometer (EDS) and X-ray diffractometer (XRD). OCP (open-circuit potential), PD (potentiodynamic polarization) and EIS (electrochemical impedance spectroscopy) experiments were conducted by a Gamry electrochemical workstation, and corresponding eroded morphologies were observed to evaluate the coating’s anti-corrosion performance. The addition of Ni led to fine and uniform dendrites and dense microstructure under the metallurgical microscope, which were beneficial for the formation of the passive film mainly consisting of titanium dioxide (TiO2). The results show that the pitting potential of the 55NiTi + 5Ni coating was 0.11 V nobler than that of the 55NiTi coating, and the corrosion current density was less than half that of the 55NiTi coating. The corrosion initiated preferentially at the interfaces of dendrites and inter-dendritic areas, then spread first to dendrites rather than in the inter-dendritic areas.

Effect of Zirconium Oxide Reinforcement on Microstructural, Electrochemical, and Mechanical Properties of TiNi Alloy Produced via Powder Metallurgy Route

Abstract Equiatomic TiNi alloy composites, reinforced with 0, 5, 10, and 15 vol% ZrO2, were synthesized using conventional sintering approach. Equiatomic TiNi pre-alloyed powder and ZrO2 powder were mixed in planetary ball mill for 6 h followed by cold compaction and pressure-less sintering, respectively. The sintered density was found to vary inversely with the addition of ZrO2 content. The X-ray diffraction (XRD) spectra have shown the formation of multiple-phases which were resulted from the decomposition of the B19′ and B2 phases of the equiatomic TiNi alloy due to the addition of ZrO2 and higher diffusion rate of Ni than that of Ti in the alloy composite. An increase in hardness was noted due to the addition of ZrO2, measured by micro and nanoindentation techniques. Potentiodynamic polarization scan revealed a 10% decrease in the corrosion rate of the composite containing 10 vol% ZrO2. Electrochemical impedance spectroscopy (EIS) results indicated an increase in passive layer resistance (Rcoat) due to the increase in charge transfer resistance (Rct) caused by the reduced leaching of ions from the surface.

Effect of low-temperature annealing on the superelastic response and electrochemical corrosion behaviour of equi-atomic Ni-Ti alloy

ABSTRACT Nickel–Titanium (Ni–Ti) alloys have exemplary properties such as good thermo-mechanical response and biocompatibility. Hence, they are one of the prime materials of interest in today’s context for the fabrication of advanced biomedical devices. Such properties can be exemplified with certain techniques such as heat-treatment techniques. However, peer-reviewed literature for the same appears to be inadequate to efficiently quantify the effect of such techniques on the overall performance of such materials. Therefore, the present study focuses on analysing the effect of low-temperature heat treatment on the superelastic behaviour and corrosion resistance of titanium-rich Ni–Ti alloys. The superelastic behaviour has been evaluated for different strains. Meanwhile, the Tafel extrapolation method is incorporated to assess the electrochemical corrosion of these materials. It is noticed that the low-temperature heat treatment has marginal effects on the superelastic behaviour of the alloy. The Ni–Ti alloy annealed at 450°C at 6% predetermined strain rate showed the best enhancement in the superelastic behaviour and the corrosion resistance increased upto treatment temperatures of 350°C. Therefore, the most optimum temperature of annealing is found to be 350°C, resulting in about 35.72% improvement in the corrosion resistance of the alloy when compared to their untreated counterparts.