NiTi Shape Memory Alloys

In the current investigation, the mechanical properties evaluation and optimization of process parameters of plasma-sprayed NITINOL coating on mild steel substrate has been performed. The relation between the mechanical properties and process parameters of the coating was established by proposing a nonlinear regression equation. The obtained coefficient of determination (R 2) and the mean relative error for microhardness (99.7 and 4.85%) and adhesion strength (99.2 and 6.88%) confirmed the statistical validity of the proposed nonlinear regression equation. The Genetic Algorithm optimization technique was implemented to obtain the optimized parametric setting for the plasma spray coating process. To check the quality of the coating fabricated by the optimized parametric combination (plasma arc current of 550 A and primary gas flow rate of 45 lpm), the characterizations (x-ray diffraction, scanning electron microscope, energy dispersive spectroscopy, and 3D surface roughness measurement) of the coating were performed. The SEM morphology of the surface and the interface of the coating revealed the better flattening behavior of the splat and the lamellar structure of the coating, respectively. The obtained phases (Ni, Ti, NiTi, Ti 2 Ni, Ni 3 Ti, Ni 4 Ti 3 , TiO, NiO) found in the XRD pattern were confirmed from the EDS analysis at various regions of the specimen surface, and the obtained coating surface roughness (roughness average (S a)) is 12.35 lm.

In the current investigation, an elemental blending of equiatomic Ni and Ti powder was considered to spray on mild steel by the atmospheric plasma spray process. The results revealed that the coating developed with 120 mm stand-off distance (SOD) has better mechanical properties such as microhardness and adhesion strength. Again, SOD predominantly influences the formation of intermetallics (NiTi, Ni 3 Ti, Ti 2 Ni, and TiO) that helps to enhance the microhardness (683.85 HV) as well as the mechanical interlocking and chemical bonding that is solely responsible for the high adhesion strength (43.17 MPa) of the coating. The failure analysis of the coating developed at too high and too low SOD revealed that rapid expansion of gas stream, reduction in enthalpy of particles, improper heat transfer, burning of splat, agglomeration of particles during flight, and oxidation are the key factors responsible for the reduction in mechanical properties of plasma-sprayed Ni-Ti alloy (NITINOL) coatings. In addition to the above, the solid particle erosion analysis revealed the increase in brittleness of the coating with increasing in SOD. The surface morphologies of the eroded surface depict various erosion mechanisms at both 45° and 90° impingement angles such as chip formation, lip formation, plastic deformation, scratches, etc.

A most used structural material, mild steel which is prone to catastrophic failure is studied to enhance the surface properties with the application of NiTi using atmospheric plasma spray
(APS) technique. Near equiatomic Ni-Ti composition is popular for its shape memory effect and superelastic behaviour. In this investigation, atmospheric plasma spray is a cost effective as well as the
user friendly process in which a homogenized elemental mixture of Ni and Ti powder has been taken as feeder materials. Physical and mechanical properties have been revealed for the coating materials
using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), x-ray diffraction (XRD) and Vickers hardness testing machine. Particle erosion wear test has been studied to find out
the principle of material removal and brittle/ductility behaviour on the coating surface. It is clear that the mechanical properties of the substrate (mild steel) enhanced fivefold with the application of NiTi
alloy. A detail microstructural behaviour has been explained before erosion and after erosion. There is an increase in hardness is due to the formation of intermetallic of NiTi and oxide phases on the
surface of the coating.

Shape memory alloys (SMAs) are used in many applications as actuators. The main drawbacks that limit the use of the SMAs in the field of mechanical actuation are the low mechanical bandwidth (up to a few Hertzs) and the unsatisfactory stroke (several millimeters). This paper contributes to enhancing the performances of SMA actuators by proposing a new SMA helical spring with a hollow section. The hollow spring is modeled, then it is constructed, and finally it is tested in compression to compare its performances with those of a spring with a solid cross section of equal stiffness and strength. Emptied of the inefficient material from its center, the hollow spring features a lower mass (37% less) and an extremely lower cooling time (four times less) than its solid counterpart. These results demonstrate that helical springs with a hollow construction can be successfully exploited to build SMA actuators for higher operating frequencies and improved strokes.

NiTi shape memory alloy (SMA) thin films were fabricated using biased target ion beam deposition (BTIBD), which is a new technique for fabricating submicrometer-thick SMA thin films, and the capacity to exhibit shape memory behavior was investigated. The thermally induced shape memory effect (SME) was studied using the wafer curvature method to report the stress-temperature response. The films exhibited the SME in a temperature range above room temperature and a narrow thermal hysteresis with respect to previous reports. To confirm the underlying phase transformation, in situ x-ray diffraction was carried out in the corresponding phase transformation temperature range. The B2 to R-phase martensitic transformation occurs, and the R-phase transformation is stable with respect to the expected conversion to the B19′ martensite phase. The narrow hysteresis and stable R-phase are rationalized in terms of the unique properties of the BTIBD technique.

Nanoskiving is a novel nanofabrication technique to produce shape memory alloy nanowires. Our previous work was the first to successfully fabricate NiTi alloy nanowires using the top-down approach, which leverages thin film technology and ultramicrotomy for ultra-thin sectioning. For this work, we utilized biased target ion beam deposition technology to fabricate nanos-cale (i.e., sub-micrometer) NiTi alloy thin films. In contrast to our previous work, rapid thermal annealing was employed for heat treatment, and the B2 austenite to R-phase martensitic transformation was confirmed using stress-temperature and diffraction measurements. The ultramicrotome was programmable and facilitated sectioning the films to produce nanowires with thickness-to-width ratios ranging from 4:1 to 16:1. Energy dispersive X-ray spectroscopy analysis confirmed the elemental Ni and Ti make-up of the wires. The findings exposed the nanowires exhibited a natural ribbon-like curvature, which depended on the thickness-to-width ratio. The results demonstrate nanoskiving is a potential nanofabrication technique for producing NiTi alloy nanowires that are continuous with an unprecedented length on the order of hundreds of micrometers.

Titanium oxide film with a graded interface to NiTi matrix was synthesized in situ on NiTi shape memory alloy(SMA) by oxidation in H2O2 solution. In vitro studies including contact angle measurement, hemolysis, MTT cytotoxicity and cell morphology tests were employed to investigate the biocompatibility of the H2O2-oxidized NiTi SMAs with this titanium oxide film. The results reveal that wettability, blood compatibility and fibroblasts compatibility of NiTi SMA are improved by the coating of titanium oxide film through H2O2 oxidation treatment.

Single‐bi‐layer of Ni–Ti thin film was deposited using DC and RF magnetron sputtering technique by layer‐wise deposition of Ni and Ti on Si(100) substrate in the order of Ni as the bottom layer and Ti as the top layer. The deposition of these amorphous as‐deposited thin films was followed by annealing at 300 °C, 400 °C, 500 °C, and 600 °C temperature with 1‐h annealing time for each to achieve crystalline thin films. This paper describes the fabrication processes and the novel characterization techniques of the as‐deposited as well as the annealed thin films. Microstructures were analysed using FESEM and HRTEM. Nano‐indentation and AFM were carried out to characterize the mechanical properties and surface profiles of the films. It was found that, for the annealing temperatures of 300 °C to 600 °C, the increase in annealing temperature resulted in gradual increase in atomic‐cluster coarsening with improved ad‐atom mobility. Phase analyses, performed by GIXRD, showed the development of silicide phases and intermetallic compounds. Cross‐sectional micrographs exhibited the inter‐diffusion between the two‐layer constituents, especially at higher temperatures, which resulted either in amorphization or in crystallization after annealing at temperatures above 400 °C. Copyright © 2015 John Wiley & Sons, Ltd.

This work utilizes short time heat treatments of submicrometer-thickness NiTi alloy films fabricated using biased target ion beam deposition and investigates crystallization. Films were fabricated on Si substrates, and thicknesses were about 150 nm, which were much less than conventional thicknesses on the order of micrometers. To understand the composition dependence, Ni concentrations were varied such that alloys ranged from Ti-rich to near-equiatomic. Rapid thermal annealing was used for the heat treatment and temperatures ranged from 465 up to 540 C for 10 min. X-ray diffraction measurements for each of the NiTi alloy compositions revealed that the crystallization temperature was equivalent ($490 C) and the B2 austenitic atomic crystal structure existed. Evolutions of surface morphologies, measured using atomic force microscopy, as a function of heat treatment temperature confirmed the composition independence of the crystallization temperature. To investigate the structure using transmission electron microscopy, 150 nm-thickness films were also deposited on ultrathin SiN substrates and heat treated, which confirmed equiaxed grains existed. Crystallization and annealing heat treatments for nanoscale films can be carried out for time on the order of minutes, which should curtail detrimental diffusion effects known to compromise shape memory behavior.

This paper presents a review of the research on the fatigue performance of superelastic polycrystalline nanocrystalline NiTi shape memory alloys (nc NiTi SMAs). A brief introduction to some focal definitions and basic concepts of fatigue measurement and response in NiTi SMAs were given. Results found in the literature on fatigue in nc NiTi SMAs are discussed. Mechanical behavior and energy dissipation capacity of nc NiTi SMAs are explored and collectively compared with coarse-grained NiTi SMAs, along with an assessment of the influence of grain size refinement and thermomechanical treatments. Several conclusions and suggestions are made, including that nc NiTi SMAs exhibit larger functional fatigue resistance, lower crack tolerance, higher superelasticity, and smaller hysteresis loss.

An ultrafine grain layer consisting of nanocrystallites as well as submicrometer grains is produced on NiTi shape memory alloy by surface mechanical attrition treatment (SMAT) and the effects of the ultrafine grain layer on the tribological properties are investigated under dry sliding conditions. Compared to the coarse grain (CG) NiTi, the SMAT NiTi has smaller friction coefficients and improved wear resistance at applied loads from 5 to 15 N due to the grain refinement effect. Examination of the worn surfaces indicates that materials delamination and particles co-exist on both the CG and SMAT NiTi samples. Our results indicate that delamination is the main wear mechanism on CG NiTi whereas abrasive particles dominate the wear process on SMAT NiTi.

The phase constituents and transformation behavior of the martensite B19′ NiTi shape memory alloy after undergoing surface mechanical attrition treatment (SMAT) are investigated. SMAT is found to induce the formation of a parent B2 phase from the martensite B19′ in the top surface layer. By removing the surface layer-by-layer, X-ray diffraction reveals that the amount of the B2 phase decreases with depth. Differential scanning calorimetry (DSC) further indicates that the deformed martensite in the sub-surface layer up to 300 μm deep exhibits the martensite stabilization effect. The graded phase structure and transformation behavior in the SMATed NiTi specimen can be attributed to the gradient change in strain with depth.

Despite many investigations on the corrosion behavior of NiTi shape memory alloys (SMAs) in various simulated physiological solutions by electrochemical measurements, few have reported detailed information on the corrosion products. In the present study, the structure and composition of the corrosion products on NiTi SMAs immersed in a 0.9% NaCl physiological solution are systematically investigated by scanning electron microscopy (SEM), x-ray energy dispersion spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS). It is found that attack by Cl− results in nickel being released into the solution and decrease in the local nickel concentration at the pitting sites. The remaining Ti reacts with dissolved oxygen from the solution to form titanium oxides. After long-term immersion, the corrosion product layer expands over the entire surface and XPS reveals that the layer is composed of TiO2, Ti2O3, and TiO with relatively depleted Ni. The growth rate of the corrosion product layer decreases with immersion time, and the corrosion product layer is believed to impede further corrosion and improve the biocompatibility of NiTi alloy in a physiological environment. It is found that the release rate of nickel is related to the surface structure of the corrosion product layer and immersion time. A corrosion mechanism is proposed to explain the observed results.

In order to enhance the surface wear resistance and nitrogen diffusion during plasma treatment, orthopedic NiTi alloy is subjected to surface mechanical attrition treatment (SMAT) and a nanocrystalline and partial amorphous structure is fabricated in the surface layer. It is found that hardness in the surface layer is notably improved. The corrosion behavior is systematically studied in a 0.9% NaCl physiological solution by electrochemical methods. Potentiodynamic polarization measurements indicate that the corrosion resistance of SMAT NiTi with the surface nanocrystalline and partial amorphous structure is significantly enhanced compared to the bare NiTi with coarse grains. Both corrosion potential (Ecorr) measurements and electrochemical impedance spectroscopy (EIS) reveal that a passive oxide layer is readily formed on the SMAT NiTi during early immersion in the 0.9% NaCl solution. When the passive oxide layer has stabilized after long exposure in the 0.9% NaCl solution, corrosion induced by Cl− begins to degrade the passive oxide film. The observed corrosion behavior of SMAT NiTi is considered to be associated with the surface nanocrystalline and amorphous structure.

NITINOL is the most popular and economic shape memory alloy (SMA) used in various industries. For temperature associated applications, the shape memory effect (SME) is the major phenomenon for the shape and strain recovery of materials after deformation. The machinability and cold working ability of NiTi alloys are poor compared to conventional alloys. So to reduce the post-processing of the finished product with good homogenization of elements, the powder technology route is suitable for making these SMAs. Due to the simplicity and cost-effectiveness, the uniaxial press and sinter process was used for pellet making. In this paper, before sintering 450, 475, 500, 525, 550, 575 and 600 MPa compaction pressures were analysed by their green densities and 600 MPa compacted pellet yielded the better density. Sintering was done at 950, 1000, 1050, 1100 and 1150 °C with the variation of sintering time 0.5, 1, 1.5 and 2 h. The XRD and SEM studies showed that samples sintered at 950 °C have oxide phases with elemental Ni and Ni 3 Ti phases. For samples sintered at 1000, 1050 and 1100 °C, NiTi and Ti 2 Ni formed as major phases with minor phases of βTi and Ni 4 Ti 3 precipitates formed inside the NiTi matrix. It was difficult to detect the minor phases for the sample sintered at 1150 °C. Needle shaped martensitic NiTi forms inside the austenitic NiTi matrix and was detected at a very high magnification of 5000× in SEM. From the Differential Scanning Calorimetry study, it was found that an increase in sintering time and sintering temperature results in a faster shape memory response.

In the present work the machinability of nickel-titanium (Nitinol) shape memory alloy has been discussed. Nitinol is known as a difficult-to-machine alloy due to its high hardness, which requires a large amount of cutting force, resulting in high rate of tool wearing. Therefore, researchers have made an effort to ameliorate the machinability of this material to achieve a finer surface quality. The previous studies found that the cutting speed will remarkably influence the surface properties of machined nickel-titanium alloy in turning process. Tool wear and cutting force are at minimum values in a particular range of cutting speeds so that it leads to diminishing machining barriers such as burr formation, and chip-breaking. Lower cutting force and consequently lower temperature and stresses in the machining process improve the mechanical properties as well as reducing hardness, distortion, and residual stress. The machining process was optimized by applying a numerical approach thro...

Nickel-Titanium (NiTi) alloys are difficult to machine materials because of their superelastic and shape memory properties. The present work focuses on study of effects of different input parameters of wire electric discharge machining on output responses like surface roughness and material removal rate during machining of NiTi alloys. The various concepts of design of experiments have been used for planning the experiments, developing the models and for investigating the influence of parameters on performance measures. The results indicate that the pulse on time and spark gap set voltage are predominant factors in maximizing material removal rate and minimizing surface roughness.

Both nanocrystalline and amorphous phases are observed from the near surface of nickel titanium shape memory alloy (NiTi SMA) with the B2 austenite phase after surface mechanical attrition treatment (SMAT). The microstructure and phase changes are systematically studied by cross-sectional and plane-view transmission electron microscopy. The strain induces grain refinement and it is accompanied by increased strain in the surface layer triggering the onset of highly dense dislocations and dislocation tangles (DTs), formation of the martensite plate via stress-induced martensite (SIM) transformation (B2 to B19′), and dislocation lines (DLs) as well as dense dislocation walls (DDWs) inside the martensite plate leading to the subdivision of the martensite plate. In addition, reverse martensite transformation (B19′ to B2) and amorphization take place concurrently in the surface region, and successive subdivision and amorphization finally result in the formation of well separated nanocrystalline and amorphous phases in the near surface. The average grain size of the nanocrystallites is about 20 nm. Owing to the almost complete reverse martensite transformation as well as thermal stability, the strain-induced nanocrystalline structure has the B2 austenite phase in the surface layer and no transformation occurs.

In this study, the surface integrity of nickel-titanium (NiTi) shape memory alloys (SMAs) was investigated after face milling processes with cryogenically treated/untreated cemented carbide cutting tools at the conditions of dry cutting and minimum quantity lubrication (MQL) of cutting fluids depending on the changing cutting parameters. The integrity of surface layer of the workpiece material was evaluated according to the mean surface roughness, microstructure and hardness, as well as according to the resultant cutting force and flank wear of inserts. Cutting tests were carried out at three different cutting speeds (20, 35 and 50 m/min), feed rates (0.03, 0.07 and 0.14 mm/tooth) and a constant axial cutting depth (0.7 mm). The influence of these parameters on the surface integrity was extensively investigated. The face milling tests of NiTi SMA at optimal cutting parameters show that the surface integrity enhanced at a cutting speed of 50 m/min and feed rate of 0.03 mm/tooth using boron-added cutting fluid (EG + %5BX) with deep cryogenic heat treated (2 196°C) CVD coated S40T grade cutting tool. Under MQL conditions, the minimum mean surface roughness (0.278 lm), resultant cutting force (268.2 N) and flank wear (0.18 mm) were obtained due to the high thermal conductivity and lubrication property of EG + %5BX cutting fluid. The highest hardness values (343 HV) were measured at the zone subjected to the highest deformation, while the lowest one (316 HV) was measured at the zone at the least deformation.

We report on free-standing NiTi alloy nanowires (120 nm × 75 nm) fabricated using a technique referred to as “nanoskiving”, which complements conventional thin film sputter deposition with ultramicrotomy for thin sectioning. To date, the technique has been limited to pure metals without exploring metallic alloys. Leveraging the technique for the fabrication of shape memory alloy (SMA) nanostructures meets two critical requirements: compositional control (via film deposition) and controlled dimensions (via film deposition and programmable sectioning). Microstructure and composition analysis confirm continuity of the produced nanowires and Ni and Ti elemental uniformity. Free-standing NiTi nanowires are robust and remain intact throughout physical manipulation. The fabrication of NiTi alloy nanowires by nanoskiving will advance fundamental characterization of small scale SMA behavior.

Shape memory alloys (SMAs) are increasingly used in the fields of aviation, automotive and biomedicine due to their unique properties. Nickel-Titanium (NiTi) alloy materials, which are one of the shape memory alloys, are among the most frequently used alloy materials. The shape memory and super elastic effects of NiTi alloys, high ductility and deformation hardening make it difficult to shape burr. An additional problem is the formation of a white layer during machining. In this study, surface milling operations were performed in dry cutting conditions with uncoated cutting tools with different nose radii. The processing parameters were determined based on the experience gained as a result of the preliminary tests. Tungsten carbide cutting tools with different nose radii (0.4mm and 0.8mm) were used for the milling operations. Milling was carried out at three different cutting speeds (20, 35, 50 m/min), feed rates (0.03, 0.07, 0.14 mm/tooth), and a constant axial cutting depth (0.7 mm). As a result of our experimental studies, the best tool life was found to be in 0.8 mm nose radius cutting tools at 20 m/min cutting speed and 0.03 mm/tooth feed rate (0.264 mm). The minimum average surface roughness was found after milling with 0.8 mm nose radius cutting tool at 20 m/min cutting speed and 0.03 mm/tooth feed rate (0.346 μm). It has been determined that increasing the cutting tool nose radius reduces both the flank wear over the cutting tool and the average surface roughness.