Amorphous Ni Ti Alloys Research Articles

Effects of pore diameter and B2–NiTi crystal on plasticity of amorphous Ni–Ti alloy based on molecular dynamics simulation

The poor plasticity of metallic glasses (MGs) limits its applicability as engineering materials. Therefore, it is particularly important to find ways to improve the plasticity of metallic glasses. In this study, the effects of pore diameter and B2–NiTi crystal on the compressive deformation of Ni50Ti50 MGs are studied by molecular dynamics simulation. The results demonstrate that pores effectively inhibit rapidly stress reduction. Pore experiences pressure in the direction of the load under compression process, which induce the formation of high shear strain atoms around the pores. The presence of pore induces the formation of shear transition zones (STZs) and shear bands. Specifically, larger pore diameters result in lower yield strengths, flatter stress-strain curves, and increase plasticity of MGs. As the B2–NiTi crystal is highly resilient in MGs, it could not be easily destroyed. In the compression process, the B2–NiTi crystal reduce the deformation of the system and impede the propagation of shear bands. As a result, the shear bands only propagate in the cracks of crystals. The larger number of crystals lead to a larger inelastic deformation interval in the nanocomposites and improve the ability to block the propagation of shear bands. These findings elucidate the relationship between yield strength, shear band, plasticity, pore diameter, and B2-phase crystals, and provide theoretical guidance for the development of MGs with improved plasticity.

Preparation and Characterization of NiTi/PVA Nanofibers by Electrospinning

NiTi alloys are widely used in biomedical applications for their unique properties particularly the shape memory effect, superelasticity, and biocompatibility. In this research, NiTi/PVA composite nanofibers are fabricated by electrospinning technique, using a novel method of producing NiTi ultrafine particles by immersing amorphous NiTi alloy in dilute aqua regia solution. The NiTi particles are successfully embedded in the PVA matrix. The produced NiTi particles are analyzed by X-ray diffraction (XRD), Energy dispersive spectroscopy (EDS), and Particle size analyzer. The XRD pattern of ultrafine NiTi particles shows much better phases as compared to the XRD pattern of the amorphous NiTi alloy sample. The morphology of the produced NiTi/PVA composite nanofibers are characterized by Field emission scanning microscope (FESEM), and Energy dispersive spectrometry (EDS). The test results show regular continuous smooth bead-free nanofibers.

Mechanical response of mesoporous amorphous NiTi alloy to external deformations

The porous titanium nickelide is very popular in various industries due to a фunique combination of physical and mechanical properties such as shape memory effect, high corrosion resistance, and biocompatibility. The non-equilibrium molecular dynamics simulation was applied to study the influence of porosity degree on mechanical properties of porous amorphous titanium nickelide at uniaxial tension, uniaxial compression, and uniform shear. We have found that the porous amorphous alloy is characterized by a relatively large value of Young’s modulus in comparison to its crystalline analogue. It has been found that the system with a percolated network of pores exhibits improved elastic characteristics associated with resistance to tensile and shear. The system containing isolated spherical pores is more resistant to compression and less resistant to tensile and shear. These results can be applied to develop and improve the methods for making amorphous metal foams.

Achievement of nearly fully amorphous structure from NiTi alloys via differential speed rolling at 268 K and effect of annealing on superelasticity

High-ratio differential speed rolling (HRDSR) technique was applied to a Ni-rich NiTi alloy at a subzero temperature of 268 K and a nearly fully amorphous microstructure was obtained after a single HRDSR pass with a thickness reduction of 50%. This result was attributed to the severe plastic deformation under low roll pressure at low temperatures where high-density of dislocations and vacancies were rapidly generated while dynamic recovery was suppressed. Subsequent annealing at 673 K for 2 min resulted in the formation of homogeneous nanograins with sizes of 20–30 nm. With increasing annealing time, grain size continued to increase and the volume fraction of remaining amorphous phase continued to decrease. Compared to that for the sample with the ultrafine grains of 100–200 nm, the alloy with the nanograins of 20–30 nm exhibited the superelastic strain larger by a factor of 35 (3.6% vs. 0.1%), indicating that the nanograin refinement through amorphization and subsequent annealing is an effective method of improving superelasticity of NiTi alloys. The homogeneous refinement to nanosized grains contributes to enhancement of superelasticity by considerably increasing the resistance of austenite matrix to plastic deformation. The presence of some amount of amorphous phase that remained after heat treatment may also contribute to superelasticity through the load transfer effect or/and particle strengthening effect. The currently proposed technique provides a solution for the easy and continuous production of (nearly) amorphous NiTi alloys in sheet form.

3D imaging of backside metallization of SiC-SBD influenced by annealing

In this study, the influence of annealing at 250 °C for 500 h on Ohmic contact of backside electrode of Silicon Carbide Schottky-Barrier-Diode (SiC-SBD) was investigated. Considering the prevalence status of the SiC-SBD as power device applications, the experiment was implemented on a commercial based SiC-SBD device. The annealing has brought remarkable transition in the structure and the impurity profile at the surface and the Ag/Ni interface of the device as a result of oxidation. For these evaluations, dual-beam Time-of-flight Secondary Ion Mass Spectrometry is regarded as an available analytical technique on the high sensitivity and imaging ability. In the deeper region of the backside electrode, an amorphous Ni–Ti alloy layer was found between the Ni/Ti interface of the annealed sample, which is also considered the result of diffusion due to the annealing. On the other hands, some nuggets of glassy-carbon and Ti cubic structures were observed in the Ni(Si)–C layer of both the non-annealed and the annealed samples. These structures are deduced to be formed during the fabrication process of the backside electrode and to be stable against the annealing. For these evaluations, Transmission Electron Microscopy (TEM) and Scanning TEM (STEM)—Energy Dispersion X-ray analysis were optimized with the high lateral resolution.

Crystallization of amorphous NiTi shape memory alloy fabricated by severe plastic deformation

Based on the local canning compression, severe plastic deformation (SPD) is able to lead to the almost complete amorphous nickel-titanium shape memory alloy (NiTi SMA), in which a small amount of retained nanocrystalline phase is embedded in the amorphous matrix. Crystallization of amorphous NiTi alloy annealed at 573, 723 and 873 K was investigated, respectively. The crystallization kinetics of the amorphous NiTi alloy can be mathematically described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. NiTi SMA with a complete nanocrystalline phase is obtained in the case of annealing at 573 K and 723 K, where martensite phase transformation is suppressed due to the constraint of the grain boundaries. Crystallization of amorphous NiTi alloy at 873 K leads to the coarse-grained NiTi sample, where (001) martensite compound twin is observed at room temperature. It can be found that the martensitic twins preferentially nucleate at the grain boundary and they grow up towards the two different grains. SPD based on the local canning compression and subsequent annealing provides a new approach to obtain the nanocrystalline NiTi SMA.

Comparison between Einstein and Debye models for an amorphous Ni46Ti54 alloy produced by mechanical alloying investigated using extended x-ray absorption fine structure and cumulant expansion

We investigated an amorphous Ni(46)Ti(54) alloy produced by mechanical alloying using extended x-ray absorption fine structure (EXAFS) technique and cumulant expansion considering Einstein and Debye models for the temperature dependence of the cumulants. Results obtained from both models were compared and very similar values were obtained. From them, we found information about the structure of the alloy besides thermal and structural disorder, anharmonicity, thermal expansion, and asymmetry of the partial distribution functions g(ij)(r). The cumulants C(1)(*), C(2)(*), and C(3)(*) also allowed us to reconstruct the g(ij)(r, T) functions from EXAFS.

Adhesion performance of TiN coating with amorphous NiTi alloy interlayer onto 316L stainless biosteel deposited by sputtering process

In the present study, TiN coatings were deposited with and without NiTi interlayer onto 316L stainless steel substrates to observe the effect of amorphous NiTi layer on the adhesion properties of TiN coatings. The deposition technique was carried out at a temperature (100–150°C) without any substrate heating to form the amorphous NiTi coating, using the closed field unbalanced magnetron sputtering system. The qualitative and quantitative adhesion properties of TiN coatings with and without amorphous NiTi interlayer were measured using the Rockwell C test, the scratch test and the pull-off test. This research demonstrates how the amorphous NiTi interlayer improved the adhesion of TiN coating when a thickness of the NiTi interlayer applied was equal to 0·5 μm. A thin layer of amorphous NiTi coating was shown to exert a significant influence over the adhesion performance of the TiN coatings and demonstrating its suitability for the tribological applications.

In vitro corrosion and cytotoxicity on microcrystalline, nanocrystalline and amorphous NiTi alloy fabricated by high pressure torsion

Bulk nanocrystalline and amorphous Ni50.2Ti49.8 alloy samples were successfully prepared from commercial microcrystalline Ni50.2Ti49.8 alloy discs by high pressure torsion (HPT) technique. Then their corrosion resistance, surface wettability and cytotoxicity were further studied from the viewpoint of biomaterials. In both Hank's solution and artificial saliva, bulk nanocrystalline and amorphous Ni50.2Ti49.8 alloys showed significantly higher pitting corrosion potentials than that of microcrystalline Ni50.2Ti49.8 alloy. Meanwhile, the amount of Ni ion release after immersion in Hank's solution was minor, far below the threatening threshold of daily diet. Murine fibroblast and osteoblast cell lines were indirectly co-cultured with experimental sample extracts, indicating no cytotoxicity. Amongst all samples, the nanocrystalline Ni50.2Ti49.8 shows promising as best biomaterial candidate for its good combination of mechanical property, corrosion resistance and cytocompatibility.

Consolidation of MA amorphous NiTi powders by spark plasma sintering

Spark plasma sintering, as a recent innovation in rapid powder consolidation was used to sinter amorphous NiTi alloy obtained from mechanical alloying. Densified samples with Ni 3Ti, NiTi and NiTi 2 phases were produced. The grain sizes were retained to within 500 nm at the sintering temperature of 900°C and 1 μm for the sintering temperature of 1100°C. Based on the results of SEM and TEM, it is found that the formation of crystalline structure is controlled by solid state reaction when the sample was sintered at 900°C, but in the sample compacted at 1100°C, liquid was introduced for the low melting point phase NiTi 2 melt. The products after sintering associated with the advantage of spark plasma sintering and the property of amorphous NiTi alloys after mechanical alloying is discussed.

Structural relaxation in amorphous Ni-Ti alloys prepared by mechanical alloying

Structural changes in amorphous states during prolonged milling after the completion of amorphization have been investigated through the crystallization behaviors with Ni-Ti alloys of compositions corresponding to the intermetallic compounds. In 33Ni-67Ti, the crystallization to NiTi 2 takes place in two steps, a higher crystallization temperature dominating after prolonged milling. In 50Ni-50Ti, structural relaxation leading to the crystallization to NiTi 2 and Ni 3Ti takes place. In 30Ni-70Ti, which is close to the border of the amorphization range, the initially formed amorphous phase turns into a supersaturated solid solution of Ti in Ni.

Muon spin relaxation in hydrogen-loaded amorphous Ni-Ti alloys

Muon spin relaxation in the amorphous alloy Ni33Ti65 containing interstitial hydrogen has been measured in longitudinal geometry at zero applied magnetic field, at temperatures in the range 18-340 K. The relaxation profiles at all temperatures were consistent with time-dependent magnetic fields at the muon site and were analysed by means of a dynamic form of the Kubo-Toyabe relaxation theory. The correlation time of the local field at the muon below 80 K was found to be about 6 mu s and to vary only slowly with temperature. Above approximately 250 K, where the diffusion hopping rate of the hydrogen atoms is about 107-108 s-1, as measured by nuclear magnetic relaxation methods, the activation energy of the correlation time is similar to the activation energy of the hydrogen diffusion. The relaxation of the muon spin at these temperatures is ascribed to the highly correlated motion of the muon and the hydrogen atoms. Some Monte Carlo calculations of the fluctuation rates of the local fields under such conditions are reported. According to the outcome of these calculations the experimental data are consistent with the muon having an intrinsically lower diffusion rate than the hydrogen atoms.

The corrosion behaviour of sputter-deposited amorphous NiTi alloys in 1 M HCl

Amorphous NiTi alloys were prepared by d.c. magnetron sputtering. The corrosion behaviour of amorphous NiTi alloys in 1 M HCl was investigated by electrochemical measurements. Particular attention was paid to the compositions of the surface film and underlying alloy surface just below the surface film. The amorphous NiTi alloys were spontaneously passive. An X-ray photoelectron spectroscopy analysis revealed that the thickness of the surface film was increased linearly with an increase in the polarization potential. The growth rates for amorphous Ni32Ti and Ni62Ti alloys were the same as each other and slightly lower than the growth rate of sputter-deposited titanium. The passive film formed on the sputter-deposited amorphous NiTi alloys was enriched in titanium cation. In the underlying alloy surface just below the surface film after polarization for 1 h, the titanium content increased linearly with the increase in the potential. At relatively low potentials, nickel was concentrated in the underlying alloy surface, while at relatively high potentials the content of titanium was higher than that in the bulk alloy.

Structural determination of amorphous NiTi thin films using electron diffraction analysis

Structural analysis of amorphous Ni x Ti 1− x alloys ( x = 0.3–.07), obtained by vapor deposition, ion implantation and electron irradiation, has been made by electron diffraction techniques. Radial distribution functions (RDF) were derived from microdensitometer traces of diffraction patterns. From a simple fit routine, used to account for the decreasing width and position of the first RDF peak as the Ni concentration increases, it has been deduced that this first peak is the result of an overlap of two atomic coordination shells containing approximately 12 and 6 atoms, respectively; these coordination numbers are typical of a close packed structure. This may suggest that such atomic arrangements exist in the form of short range order in the amorphous structure. No major differences, in terms of peak positions and coordination numbers, were found between the structures of the amorphous NiTi alloys obtained by vapor deposition ion implatation and electron irradiation.

Electronic transport properties of amorphous NiTi alloys

Measurements of electrical resistivity, magnetoresistance and Hall effect on amorphous Ni100−xTix alloys withx=39, 43, 60, 65, 70, and 76 are presented. The resistivity at 4.2 K is in the range 210 μΔcm to 325 μΔcm. The Hall coefficient turns out to be negative for low Ti contents and positive for high Ti contents with values around −6×10−11 m3/As and +7×10−11 m3/As, respectively. At low temperatures, the temperature and magnetic field dependences of the quantities measured are analysed and discussed in terms of weak localisation and electron-electron interaction effects.

Hydrogen in amorphous NiZr and NiTi alloys

Various results on properties (solubility, diffusivity and local modes) of hydrogen in amorphous Ni1 − x Zrx (x = 0.35, 0.5, 0.65) and Ni0.35Ti0.65 will be presented as a function of hydrogen concentration. The results are interpreted in the framework of the site energy distribution model. Within experimental error the measured distribution of site energies can be described by superimposing gaussian distributions for the five possible tetrahedral sites (Zr4, Zr3Ni, Zr2Ni2 etc.) and assuming a random distribution of the metal atoms with a maximum number of interstices varying between 1.2 and 1.7 per metal atom. The local modes of hydrogen vibration are not affected very much by filling different tetrahedral sites with hydrogen, whereas hydrogen diffusivity changes over two or three orders of magnitude. The concentration dependence of the diffusion coefficient is in semiquantitative agreement with the model of constant saddle point energies and discrepancies may arise from correlations between site and saddle point energies or from diffusion via sublattices formed by the different types of tetrahedra.

Passivity of amorphous and crystalline NiTi alloys

In dilute sulphuric solution, amorphous and crystalline NiTi alloys with titanium content ⩾40 at.% had almost the same anodic behavior and photoelectrochemical properties in so far as they had the same composition, i.e. they were spontaneously passivated and showed a positive photocurrent. Band-gap energies of the surface film on amorphous and crystalline NiTi alloys were all about 3.3 eV although they were dependent on the applied potential. The thickness of the surface film formed on amorphous and crystalline Ni40Ti alloys was, in the range of several nanometers, within the potential region examined. It increased with potential. In the surface film Ti 4+ ions were concentrated relative to the bulk alloy composition.

Hydrogen in amorphous Ni-Zr and Ni-Ti alloys

Hydrogen in amorphous Ni-Zr and Ni-Ti alloys

Processing and properties of nanophase amorphous metallic alloys: Ni-Ti

Amorphous Ni-Ti alloys were synthesized by producing nanoscale amorphous Ni-Ti particles and compacting these powders, in situ, in a high vacuum environment. X-ray diffraction measurements revealed that crystallization occurs in these alloys in the temperature range 450–550 °C, although measurements of electrical resistivity and heat release showed that other relaxation processes occur at lower temperatures. The Vicker’s hardness of these alloys is ≊350 kg/mm2. The results are discussed in terms of interparticle or grain boundaries in the amorphous alloy.

A photoelectrochemical and ESCA study of passivity of amorphous nickel-valve metal alloys

Photoelectrochemical and ESCA methods were applied to the passive films formed on a variety of amorphous NiTi alloys and pure titanium in a deaerated sulfuric acid at 303 K polarized at various anodic potentials. Photocurrents were measured as a function of wavelength, potential and alloy composition. All the specimens showed positive photocurrent. From the photocurrent spectra, a band-gap energy of 3.2 eV was determined for the passive films on pure titanium and amorphous NiTi alloys. The photocurrent increased linearly with the Ti content in the alloys. The passive films on Ni40Ti alloy were enriched with Ti in all the potential regions examined in comparison with bulk alloy composition. Dilution of photoactive Ti ions in the passive film by the presence of Ni ions, therefore, results in a negative effect more than that expected from the passive film composition.