Ni Lattice Research Articles

Effects of milling and annealing on formation and structural characterization of nanocrystalline intermetallic compounds from Ni–Ti elemental powders

Nickel and Titanium elemental powders with a nominal composition Ni–50at.%Ti were mechanical alloyed in a planetary high-energy ball mill in different milling conditions (5, 10, 20, 40 and 60h). The investigation revealed that increasing milling time leads to a reduction in crystallite size, and after 60h of milling, the Ti dissolved in Ni lattice and NiTi (B2) phase was obtained. With milling time, morphology of pre-alloyed powders changed from lamella to globular. Annealing of as-milled powders at 1173K for 900s led to formation of nanocrystalline NiTi (B19′), grain growth and release of internal strain. The results indicated that this technique is a powerful and high productive process for preparing NiTi intermetallic compound with nanocrystalline structure and appropriate morphology.

Ni–SiO2 and Ni–Fe–SiO2 catalysts for methane decomposition to prepare hydrogen and carbon filaments

Active and stable Ni–Fe–SiO2 catalysts prepared by sol–gel method were employed for direct decomposition of undiluted methane to produce hydrogen and carbon filaments at 823K and 923K. The results indicated that the lifetime of Ni–Fe–SiO2 catalysts was much longer than Ni–SiO2 catalyst at a higher reaction temperature such as 923K, however, a reverse trend was shown when methane decomposition took place at a lower reaction temperature such as 823K. XRD studies suggested that iron atoms had entered into the Ni lattice and Ni–Fe alloy was formed in Ni–Fe–SiO2 catalysts. The structure of the carbon filaments generated over Ni–SiO2 and Ni–Fe–SiO2 was quite different. TEM studies showed that “multi-walled” carbon filaments were formed over 75%Ni–25%SiO2 catalyst, while “bamboo-shaped” carbon filaments generated over 35%Ni–40%Fe–25%SiO2 catalysts at 923K. Raman spectra of the generated carbons demonstrated that the graphitic order of the “multi-walled” carbon filaments was lower than that of the “bamboo-shaped” carbon filaments.

Ignition of an exothermal reaction by collision between Al and Ni crystals

Classical molecular dynamics methods have been used to investigate the atomic-scale dynamics of collisions between two Al and Ni crystals with rough surfaces. The crystals were approached along the direction perpendicular to the surfaces and simultaneously displaced along the direction parallel to them at relative velocities in the range between 1 and 10 nm ns−1. The mechanical stresses operating at collision determine a local deformation of Al and Ni lattices, accompanied by a significant temperature rise. As the Al melting point is reached, the Al crystal partially melts and Ni atoms start dissolving into the molten phase. The significant heat of mixing liberated further promotes the Al melting and the Ni dissolution processes. In the absence of neighboring Al-Ni interfaces, the heat dissipation processes and the limited rate of Ni dissolution gradually lead to the extinction of the reactive behavior. Conversely, the presence of Al-Ni interfaces in the vicinity of the Al-Ni one formed by collision permits the propagation of the high-temperature chemical reaction. It is shown that the ignition and propagation of the self-sustaining reaction is sensitive to the distance between Al-Ni interfaces and to their degree of chemical mixing.

Enhanced solid solution effects on the strength of nanocrystalline alloys

Solid solution strengthening in nanocrystalline alloys is studied using sputtered Ni–W as a model system. In the composition range of 0–20at.% W, the sputtered alloys have a nanocrystalline structure with a grain size that is independent of composition. Nanoindentation of these alloys shows that solute addition increases strength to very high levels, almost in proportion to the solute content. This behavior is not expected based on traditional solid solution strengthening mechanistic models of local dislocation pinning at solute atoms, but can be explained by further considering a global effect of solute on the average properties of the Ni lattice. The new strengthening term arises by considering grain boundaries as pinning points for dislocation motion in nanocrystalline materials and incorporating the effect of solutes on such a mechanism. Our discussion surrounding Ni–W also provides insights into other solid solution nanocrystalline systems, a variety of which we show can be accurately described using the same concept. These developments also explain the origin of solid solution softening in some nanocrystalline alloys.

A Nanoscale Study of Dislocation Nucleation at the Crack Tip in the Nickel-Hydrogen System

Strengthening and embrittlement are controlled by the interactions between dislocations and hydrogen (H)–induced defect structures that can inversely affect the deformation mechanisms in materials. Here we present a simulation framework to understand fundamental issues associated with H-assisted dislocation nucleation and mobility using Monte Carlo (MC) and molecular dynamics (MD). In order to study the interaction between H and dislocations and its effect on material failure, we extensively examined mode I loading of an edge crack using MD simulations. The MD calculations of the total structural energy in the nickel (Ni)–H system shows that H atoms prefer to occupy octahedral interstitial sites in the fcc Ni lattice. As H concentration is increased, the Young’s modulus and the energy required to create free surface decreased, resulting in H-enhanced localized plasticity. The MD simulations also indicate that H not only facilitates dislocation emission from the crack tip but also enhances dislocation mobility, leading to softening of the material ahead of the crack tip. While the decrease in surface energy suggests H embrittlement, the increase in local plasticity induces crack blunting and prohibits crack propagation. The mechanisms responsible for transitioning from a ductile to brittle crack behavior clearly depend on the H concentration and its proximity to the crack tip. Enhanced plasticity will occur within a general field of H atoms that results in lower stacking fault and surface energies, yet H interstitials on preferential slip planes can inhibit dislocation nucleation.

Effect of polymer coating on the magnetic properties of oxygen-stabilized nickel nanoparticles

A comparative study of the structural and magnetic properties of polymer coated (pc-) and uncoated (uc-)Ni nanoparticles prepared by chemical reduction method are reported. pc-Ni nanoparticles have been synthesized by reducing ${\text{Ni}}^{2+}$ cations to Ni particles by sodium borohydride in aqueous solution in the presence of polyvinyl alcohol (PVA) solution. Both samples have been identified to have a tetragonal crystal structure, different from its usual fcc structure, though some traces of fcc-Ni phase have been observed in x-ray diffraction and selected area electron diffractogram patterns for pc-Ni. This structural modification of fcc-Ni occurs due to the presence of interstitial oxygen atoms in the Ni lattice and results in appreciably modified magnetic properties in this new phase of Ni, as, for example, a nonhysteretic magnetization response with applied field at 300 K. A comparative magnetic study of the uc- and pc-Ni particles exemplifies the role played by the PVA matrix in modifying the magnetic properties of the uc-Ni sample. The $M\text{\ensuremath{-}}H$ and $M\text{\ensuremath{-}}T$ curves have been analyzed using superparamagnetic/ferromagnetic/paramagnetic component models and anomalies observed were explained using two-phase model.

Structural and magnetic properties of granular (Ni0.84Fe0.16)54(alumina)46 sputtered thin films

Abstract The microstructure and magnetic properties of sputtered (Ni 0.84 Fe 0.16 ) 54 (alumina) 46 (at%) thin films have been investigated by X-ray diffraction, electron microscopy, Mossbauer spectroscopy and magnetic measurements. The microstructure of the as-sputtered film is constituted of 4 nm large Ni(Fe) nanoprecipitates in an amorphous alumina matrix. This film exhibits a ferromagnetic character due to the existence of interactions between magnetic Ni(Fe) nanoprecipitates. Its Mossbauer spectrum consists of a magnetic broadened sextet indicating a large distribution of iron atom environment probably due to the chemical disorder on the Ni(Fe) lattice. One hour annealing heat treatments were applied to the films at temperatures ranging from 573 to 773 K. The annealing resulted in a simultaneous dewetting of the film, and coarsening of the nanoprecipitates. It was shown to be associated with an increase of the saturation magnetization of the films and a refinement of the Mossbauer spectrum.

Desorption of hydrogen from Ni—Cu—Cr catalyst on (θ + α-Al2O3 sorbent modified with cerium

We present the results of thermal desorption studies of H2 interactions with 8.9% Ni—Cu—Cr (1:3:0.1)/(θ+α)-Al2O3+2% Ce. It was demonstrated that three forms of hydrogen with different desorption temperatures and Tmax peak values are formed in the catalyst at 673 K: H2(ads), H(ads), and dissolved hydrogen. It was found that upon an increase in the adsorption temperature from 673 to 1173 K, H2(ads) and H(ads) remain on the surface (Edes = 14.4 kcal/mol, first order desorption reaction, H: Ni = 1.6—1.88) while hydrogen dissolution in the Ni lattice and its copper alloy increases (Edes= 32.9 kcal/mol, second order desorption reaction, H: Ni = 2.0).

First principles based predictions of the toughness of a metal/oxide interface

Abstract We describe a first-principles-based strategy to predict the macroscopic toughness of a γ-Ni(Al)/α-Al2O3 interface. Density functional theory calculations are used to ascertain energy changes upon displacing the two materials adjacent to the interface, with relaxation conducted over all atoms located within adjoining rows. Traction/displacement curves are obtained from derivatives of the energy. Calculations are performed in mode I (opening), mode II (shear) and at a phase angle of 45°. The shear calculations are conducted for displacements along <110> and <112> of the Ni lattice. A generalized interface potential function is used to characterize the results. Initial fitting to both the shear and normal stress results is required to calibrate the unknowns. Thereafter, consistency is established by using the potential to predict other traction quantities. The potential is incorporated as a traction/displacement function within a cohesive zone model and used to predict the steady-state toughness of the interface. For this purpose, the plasticity of the Ni alloy must be known, including the plasticity length scale. Measurements obtained for a γ-Ni superalloy are used and the toughness predicted over the full range of mode mixity. Additional results for a range of alloys are used to demonstrate the influences of yield strength and length scale.

Electrodeposited Ni–B alloy coatings: Structure, corrosion resistance and mechanical properties

Ni–B alloy coatings with different boron content ranging from 4 to approximately 28 at.% were prepared by electrodeposition in a nickel-plating bath containing sodium decahydroclovodecaborate as a boron source. The influence of the boron concentration in the coatings on their structure, morphology, electrochemical and corrosion behavior, physico-mechanical and electrical properties was investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and other methods. It was found that the electrodeposited Ni–B coatings with relatively low boron content (≤8 at.%) are nanocrystalline and comprise a solid solution of boron in f.c.c. Ni lattice having a mixed substituted-interstitial type. Further increase in the boron content (up to 10–15 at.%) leads to the appearance of heterogeneous amorphous-nanocrystalline structure, and the coatings with a high boron content (20 at.% and above) are X-ray amorphous. Polarization measurements in neutral NaCl solutions showed that the Ni–B coatings with relatively low boron content demonstrate a potential region of low anodic currents associated with the passive film formation at the alloy surface. The anodic current in this potential region increases significantly with increasing the boron content above 10 at.%, suggesting the non-protective nature of the anodic film formed on the amorphous Ni–B alloys. Immersion tests monitored by EIS measurements revealed a significantly better corrosion performance of the Ni–B coatings with low boron content (4 at.%) in comparison with that of the amorphous coatings. The microhardness and wear resistance of the Ni–B coatings essentially increases with increasing the boron content. Maximum microhardness and wear resistance were found for the coatings containing 8 at.% B.

First Principles Study of the Stability and the Formation Kinetics of Subsurface and Bulk Carbon on a Ni Catalyst

Ni-based catalysts are widely used in hydrocarbon conversion processes. First principles based analysis of the stability of subsurface and bulk carbon on Ni catalysts indicates that they might be formed under typical reaction conditions. It was shown that carbon diffusion to the octahedral sites of the first subsurface layer is thermodynamically favorable until about 75% of the subsurface sites are occupied. Activation barriers for diffusion to the subsurface octahedral sites were found to decrease with on-surface carbon coverage, and typical barriers of less than 110 kJ/mol were calculated for moderate to high surface coverages. Diffusion from the subsurface sites to the Ni bulk was found to be thermodynamically favorable if more than 50% of the subsurface octahedral sites are occupied. Such diffusion is, however, kinetically limited, with typical barriers above 150 kJ/mol. For carbon concentrations above 50% in the first subsurface layer, calculations indicate that the Ni lattice is likely to expand. This expansion decreases the diffusion barriers to below 100 kJ/mol and opens up a diffusion path. Hence, carbon diffusion into Ni catalysts is proposed to be a gradual process, first partially filling the subsurface octahedral sites before diffusing to the bulk.

Hyperfine magnetic field at Ta impurities in nickel: Perturbed angular correlation and first principle calculation study

The hyperfine magnetic field ( H h f ) in 0.2 at.% Hf–Ni alloy is measured at the 181Ta probe using the time-differential perturbed angular correlation (TDPAC) method, in the temperature range 78–675 K. The obtained value of 8.6 (3) T at room temperature is in good agreement with the previously reported measurements for similar Hf concentrations in Ni. X-ray powder diffraction (XRPD) experiments confirmed that small atomic concentrations of Hf atoms (<1 at.%) mainly substitute on Ni lattice sites in the fcc crystal lattice without forming any intermetallic phase. In addition, ab-initio calculation using all-electron augmented plane waves plus local orbitals (APW+lo) formalism is performed and the obtained result for the hyperfine magnetic field at Ta site is in accordance with the measurement.

Effect of heterogeneous solute distribution on the anomalous thermomechanical response of Cu–Ni alloy single crystals below 50 K

The anomalous temperature dependence of the critical resolved shear stress (CRSS) of Cu–Ni alloys observed below a certain temperature, T o = 50 K, has been accounted for by introducing a stress-concentration factor f(T) = [(T ′ + T o)/(T ′ + T)] in the monotonic CRSS–T formulation of the kink-pair nucleation model of solid-solution hardening. The empirical constant T ′ is found to depend not only on the solute concentration, c, but also on the nature of the solute distribution in the host lattice. It is found that the solute distribution is random for c ≤ 14 at.% Ni in the Cu lattice and for c ≤ 20 at.% Cu in the Ni lattice, whereas some sort of local ordering occurs for all other values of solute concentration.

The effect of silver coating on magnetic properties of oxygen-stabilized tetragonal Ninanoparticles prepared by chemical reduction

Ni–nickel oxide and Ni–Ag nanoparticles with core–shell morphology have beensynthesized by a simple two-step chemical route. It involves the reduction ofNi2+ cations to Ni particles with sodium borohydride(NaBH4) in aqueous solution, followed by coating part of the sample with a surface layer of silver.The coating was accomplished via a simple transmetallation reaction, , in aqueous AgNO3 solution at room temperature. Particles left uncoated have a thin spontaneous surfaceoxide layer of nickel oxide, presumably non-stoichiometric, as a passivating layer.From x-ray diffraction patterns, the Ni in as-prepared samples, both coated anduncoated, has been identified as being in a tetragonal crystal structure, differentfrom its usual fcc form. This structural modification is due to the presence ofinterstitial oxygen atoms in the Ni lattice, and results in appreciably modifiedmagnetic properties in this new phase of Ni, for example a linear non-hystereticmagnetization response with applied field at room temperature. A comparativemagnetic study of the uncoated and silver coated Ni particles exemplifies the roleplayed by the Ag shell in modifying the magnetic properties of the coated sample.This is evident from the smaller coercivity and remnant magnetization in thehysteresis loop at 5 K in the case of the Ag coated as-prepared sample, largermagnetic moment but smaller saturation magnetization and susceptibility of the Agcoated air annealed sample, and higher blocking temperature for this sample incomparison to the uncoated counterparts. These features have been coherentlyexplained on the basis of the structural modifications induced in these samples by Agcoating.

Cubic Laves ferromagnetTbNi2Mninvestigated through ambient-pressure magnetization and specific heat and high pressure ac magnetic susceptibility

$\mathrm{Tb}{\mathrm{Ni}}_{2}\mathrm{Mn}$ is a cubic Laves material, isostructural to $\mathrm{Tb}{\mathrm{Ni}}_{2}$, with the Mn sitting on both the Tb and the Ni lattice positions. It is ferromagnetic at ambient pressure at ${T}_{C}=151\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and has been investigated in detail at ambient pressure through magnetization and specific heat with various applied magnetic fields. It displays Curie-Weiss behavior above ${T}_{C}$, with an effective moment of $8.43{\ensuremath{\mu}}_{B}∕\mathrm{f.u.}$, and isothermal magnetization at various temperatures is consistent with narrow domain walls. The low-temperature specific heat results in a linear contribution of $65\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕\mathrm{mol}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{2}$ and a Debye temperature of ${\ensuremath{\theta}}_{D}=188\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The ac magnetic susceptibility has been investigated using designer diamond anvils up to $28.3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. A magnetic anomaly at ${T}^{*}$ is investigated as a function of both magnetic field and pressure, both of which have very little effect for fields less than the coercive force. The Curie temperature decreases at a rate of $d{T}_{C}∕dP=\ensuremath{-}1.96\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{GPa}$, and differences between hydrostatic and nonhydrostatic conditions are discussed.

Methane Dissociation on Ni(111): The Role of Lattice Reconstruction

The effects of lattice motion and reconstruction on the dissociation of methane on Ni(111) are explored, using both electronic structure theory and quantum dynamical calculations. We show that during the reaction, the Ni lattice reconstructs, effectively lowering the barrier to reaction, in contrast with earlier models of this process.

Lattice Parameters and Local Lattice Distortions in fcc-Ni Solutions

The lattice parameters and the local lattice distortions around solute atoms in fcc-Ni solutions were studied using first-principles calculations. The solute atoms considered include Al, Co, Cr, Hf, Mo, Nb, Re, Ru, Ta, Ti, and W. The calculations were performed using supercells with 1 solute atom and 107 solvent atoms. It is found that the atomic size difference, the electronic interactions, and the magnetic spin relations between the solute and solvent atoms all contribute to the lattice distortions. Based on the results from first-principles calculations, the linear composition coefficients of fcc Ni lattice parameter for different solutes were determined, and the lattice parameters of multicomponent Ni-base superalloys as a function of solute composition were predicted. The results are compared with existing experimental measurements, and good agreements are obtained for both the compositional dependence of lattice parameters and the local lattice distortions.

Third-order elastic constants and low-temperature thermal expansion of the shape memoryalloy Cu–Al–Ni

Numerous theoretical and experimental investigations are being carried out onCu-based alloys due to their technologically important shape memory propertiesand pseudo-elasticity, which are intimately associated with the martensitictransformation. The austenite phase in Cu-based shape memory alloys is known forits high elastic anisotropy. The complete set of second-order elastic constants(SOECs) and third-order elastic constants (TOECs) of Cu–Al–Ni in theβ phase has been obtained. A combination of the deformation theory and finite strainelasticity theory has been employed to get the expressions for the strain energydensity of the lattice. The theory is based on the Keating’s approach by taking intoaccount the two-body as well as three-body interactions up to second nearestneighbors in the Cu–Al–Ni lattice. The elastic constants are compared with thereported experimental values. The relevant role of the comparatively large TOECs indescribing the instability of the bcc structure towards the martensitic transformationis also discussed. The aggregate elastic properties, such as the adiabatic bulkmodulus, shear modulus, Cauchy pressure and anisotropy factor, are calculated. Themode Gruneisen parameters of the acoustic waves are determined based on thequasi-harmonic approximation method. The low-temperature limit of the lattice thermalexpansion and the Anderson–Gruneisen parameter of Cu–Al–Ni are also obtained.

Epitaxy-Like Protective Layers for High-Performance Low-Cost Au/Pd/Ni Preplated Cu Alloy Leadframes

In this paper, we report the performance, the crystal microstructure, and defects of Au/Pd/Ni/Cu preplated lead frames (PPF). The wire pull test, the solderability test, and high-resolution transmission electron microscopy (HRTEM) are employed to characterize the PPFs in order to understand the relationship between performance and microstructure. The electroplated PPFs are composed of Au (3 nm)/Pd (5 to 15 nm)/Ni (0.7 mum)/Cu substrate. We optimize the electroplating profile and determine the minimum thickness of the Pd layer with the PPF performance satisfying the industry standards. Further increasing the Pd layer thickness beyond the critical thickness will not enhance the performance more, but increase the product cost. With the optimized electroplating profile, the electroplated Au layer is epitaxially deposited on the Pd layer, and so does the Pd layer on the Ni layer. The interface between the Au and Pd layers is in general coherent in spite of the about 4.8% mismatch strain between the Au and Pd lattices because the Au layer thickness is only 3 nm small. Misfit dislocations and nanotwins are present at the interface between the Pd and Ni layers, which are generated to release the about 10.4% misfit strain between the Pd and Ni lattices

A study of the behavior of helium atoms at Ni grain boundaries

AbstractUsing molecular dynamics simulations with an analytical embedded‐atom method (MAEAM) model, the diffusion behavior of helium atoms at two Σ = 5 symmetric grain boundaries of Ni was investigated in the present paper. The simulations demonstrate that the interstitial helium atoms are stable at the grain boundaries. Helium atoms in Ni lattices could diffuse easily to the grain boundary and precipitate into a cluster, which confirmed the fact that helium atoms could nucleate and grow up into bubbles at the grain boundaries. The diffusion activation energy of interstitial helium atom at grain boundaries was also determined. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)