Nanocrystalline Ni Samples Research Articles

Effect of grain size on the thermal stability of electrodeposited nanocrystalline nickel: X-ray diffraction studies

Nanocrystalline (NC) Ni coatings have widespread engineering applications, many of which are in high-temperature environments. Therefore, it is important to study the thermal stability of NC Ni. The present study compares the thermal stability of electrodeposited (ED) NC Ni of varying grain sizes: 20 nm, 50 nm, and 100 nm. In-situ high-temperature X-ray diffraction (XRD) investigations have been employed to follow the grain growth behavior of 〈111〉 and 〈200〉 grains under isochronal annealing up to 550 °C. Based on the calculated activation energies, the anisotropic growth mechanisms of 〈111〉 and 〈200〉 grains are evaluated. The studies revealed that the onset of grain growth occurred at progressively higher temperatures with increasing initial grain sizes. While near uniform grain size distribution is observed for the 20 nm grain size variant, bimodal distributions are observed for its counterparts. Electron Backscatter Diffraction studies of NC Ni samples in the final-processing temperature of XRD showed that <100>//ED texture is preserved during the annealing process. While high fractions of Σ3 coincidence site lattice (CSL) boundaries are observed in the 20 nm sample, reduced fractions with no particular trend is observed for the 50 nm and 100 nm grain-sized samples. It is shown here that the incomplete recrystallization arising from inhomogenous strain distribution is the reason for the observed bimodal distribution and deviation from an expected trend in the formation of Σ3 CSL as a function of initial grain size.

The spectrum of atomic excess free volume in grain boundaries

The grain boundary (GB) excess volume is an important structural factor that is strongly correlated with various thermodynamic and kinetic properties of GBs such as GB energy, GB mobility, GB diffusivity, and GB segregation energy, etc. However, the excess volume is usually reported as an average value of the entire GB. Such simplification does not consider the spectral nature of the excess volume in a GB, which cannot be used to describe the atomic mechanisms of some kinetic process, such as GB migration, that involves only a few atoms at a time. Here, we explore the spectrum of atomic excess volume in representative nanocrystalline Ni and Al samples as well as 388 Ni bicrystals based on the Olmsted dataset by using atomistic simulations. It is found that the nanocrystalline Ni and Al models show a skew-normal distribution in the spectrum of both the atomic excess volume and the atomic excess energy in the GBs, which show a weak inverse correlation between them. This is in stark contrast to the widely reported positive correlation between GB energy and excess volume based on the average value. We further show based on the statistical analysis that the correlation between the atomic excess volume and excess energy strongly depends on the GB type and a universal trend between them does not exist. While low ∑ Ni GBs generally shows a strong inverse linear correlation between these two properties, such correlation is weak for high ∑ Ni GBs. Moreover, we find that the spectrum of the excess volume shows characteristics distribution in some special Ni GBs. For example, twist GBs generally show a symmetrical unimodal distribution while most surveyed ∑3 Ni GBs with anti-thermal behavior show an apparent bimodal distribution. Nevertheless, a strong correlation is found between the atomic excess volume and the segregation energy based on the nanocrystalline Al model with Mg impurity, which implies a possible universal trend between the two properties. The current study thus shows that the excess volume provides useful insights in revealing the elemental structure–property correlations in GBs, which may be used as a structural variable in future thermodynamic modeling of GBs.

Comparative creep behaviour study between single crystal Nickel and ultra-fine grained nano crystalline Nickel in presence of porosity at 1120 K temperature

Molecular dynamics (MD) using embedded atom method (EAM) potential has been performed for the simulation of the creep behaviour of virtual single crystal (SC) Ni and nano-crystalline (NC) Ni having grain size ~ 4 nm with and without presence of porosity (porosity ~ 1 at.%). The dependence of the kinetics of creep deformation of SC Ni and NC Ni on atomic configuration is rationalized in this paper. NC Ni exhibits pronounced as well as extensive primary creep and tertiary creep regimes at operating temperature 1120 K. Grain boundary is observed to be broadened during creep process of NC Ni due to resultant diffusion fluxes across the grain boundaries. In addition overall randomness inside the NC Ni sample is found to increase as creep deformation process progress and the NC Ni sample becomes amorphous at tertiary part of creep curve. The MD study reveals that, the negative effect of porosity on creep behaviour of SC Ni is more significant compared to the effect of porosity on creep behaviour of NC Ni. Nano size SC Ni is found to be better creep resistant material in comparison with NC Ni.

Effect of Zr addition on creep properties of ultra-fine grained nanocrystalline Ni studied by molecular dynamics simulations

Molecular dynamics (MD) simulation has been carried out using embedded atom method potential to assay the variation of creep behaviour of nanocrystalline (NC) Ni and Ni–Zr alloy (Zr addition of 3at.%, 6at.% and 12at.%) having grain size ∼6nm due to the Zr addition. In this study, the creep process is evaluated at constant temperature of 1209K and constant applied load of 1.0GPa with Zr additions both randomly distributed in the NC Ni sample and segregated at the grain boundaries. Centro-symmetry parameter analysis, radial distribution function evolution, Wigner-Seitz defect analysis and Voronoi cluster analysis have been performed to provide the insight of underlying mechanism for accounting the deviation in creep properties owing to Zr addition in NC Ni. In this work the underlying mechanism based on structure property correlation is evaluated for accounting the improved creep properties of NC Ni-Zr alloy having segregated Zr atoms at grain boundaries compared to that of NC Ni and Ni–Zr alloy containing randomly distributed Zr atoms.

Core and surface/interface magnetic anisotropies in nanocrystalline nickel

By making use of the micromagnetic theory for the ‘approach-to-saturation’ in magnetization, the effective magnetic anisotropy (MA) constant (MA energy density), Keff, as a function of temperature in the range 2 K ≤ T ≤ 300 K, is deduced from the magnetization versus magnetic field isotherms taken on nanocrystalline Ni samples with average crystallite size, d, ranging from 10 to 40 nm. For a fixed d and at any given temperature, the contributions to Keff(T) arising from the volume (core) magnetocrystalline anisotropy, K1c(T), the surface/interface anisotropy, Ks(T), and shape anisotropy, Kshape(T), are assumed to be additive. The result that a linear relationship exists between Keff × d and d not only validates this assumption in the present case but also permits an accurate determination of K1c(T), Ks(T) and Kshape(T). Thermal activation over an effective anisotropy energy barrier (which separates two local energy minima and depends on both temperature and magnetic field), as the mechanism for magnetization reversal, is shown to account for the observed temperature dependence of the coercive field. Another important finding is that the magnetic moment per Ni atom, μNi, deduced from the saturation magnetization at 2 K, increases with d in accordance with the relation μNi(d) = μNi(d = ∞) [1 − b × d−3/2], with the magnetic moment per Ni atom in the bulk at T = 2 K, μNi(d = ∞) = 0.616 μB, and b = 0.50(5).

Compression Deformation Mechanisms of Nanocrystalline Ni Prepared by Plasma Evaporation Method

To investigate mechanical behaviors and deformation mechanisms of nanocrystalline materials, bulk nanocrystalline Ni samples were prepared by a plasma evaporation method combined with hot pressure sintering. The compressive mechanical properties of the bulk samples were tested under different quasi-static strain rates at room temperatures, and the evolution of the microstructure of the bulk sample before and after compression was studied. Results indicate that the bulk samples show a good combination of strength and ductility during the compressive testing. Meanwhile, the grain size of the compressed sample decreases compared with that of the sintered sample, while the microstrain of the grain increases after compression. Based on experimental results, it is can be inferred that both grain boundary dislocation glide and grain boundary sliding are the main deformation mechanisms for the bulk NC Ni samples.

Strain-rate sensitivity of scratch hardness and deformation mechanism in nanocrystalline Ni under micro-scratch testing

To investigate the strain-rate sensitivity in nanocrystalline (NC) materials using a single experimental technique over a wide range of strain rate, the micro-scratch testing technique was selected to obtain consistent and systematic data of the strain-rate sensitivity in a fully dense, high purity, and well-characterized electrodeposited NC Ni sample. The scratch characterizations and mechanical properties of the sample under the different scratch velocities were discussed in details. First, some critical parameters under the micro-scratch testing, such as strain rates, scratch ditch widths, were confirmed. Then, the scratch hardness under the different scratch velocity was investigated. From the results, the sample exhibits strain-rate sensitive mechanical properties. Further, the average values of strain-rate sensitivity of the sample were calculated. The values increase with increasing scratch velocity, and exhibit higher values that are of the order of 0.03–0.1. The greater strain-rate sensitivity exponents indicate that the NC Ni samples have different fundamental physical deformation mechanisms. Finally, the fundamental physical deformation mechanism under scratch testing was inspected using SEM and TEM technique. From the SEM and TEM morphologies, the grain boundary dislocation pile-ups should be a carrier of plastic flow under scratch testing in the NC Ni sample.

Thermal demagnetization due to spin waves in nanocrystalline Ni

An elaborate analysis of the decline of magnetization with increasing temperature, observed in nanocrystalline (nc-) Ni samples with average crystallite size, d, varying from d=10nm to 40nm, permits us to (i) completely rule out a possible Stoner single-particle contribution to the thermal demagnetization, M(T), at external magnetic fields 10kOe≤H≤70kOe and temperatures T≤350K, (ii) demonstrate that spin-wave (SW) excitations alone account for the observed MH(T) and (iii) the thermal renormalization of spin-wave stiffness is primarily due to the magnon–magnon interactions. For fields H≥20kOe, the spin-wave stiffness at 0K,D(T=0,H), is found to decrease with field as D(T=0,H)∼H1/2 for all the nanocrystalline samples. Extrapolation of the D(T=0,H)−H1/2 straight line to H=0 yields D0≡D(T=0,H=0) for a sample of given d. D0 varies with d as d4/3. This power law behavior of D0 asserts that the crystallite size is the relevant length scale for spin waves. Strong departures from the spin-wave behavior are observed at low temperatures T≤T†(H) in nc-Ni with d=10nm. Such departures, marked by the T4/3 power law behavior of [MH(T)]2, are a manifestation of damped spin waves, which act as non-propagating spin fluctuations and get suppressed by magnetic field in accordance with the H1/2 power law.

Model of the magnetization of nanocrystalline materials at low temperatures

A theoretical model incorporating the material texture has been developed to simulate the magnetic properties of nanocrystalline materials at low temperatures where the effect of thermal energy on magnetization is neglected. The method is based on Landau-Lifshitz-Gilbert (LLG) theory and it describes the magnetization dynamics of individual grains in the effective field. The modified LLG equation incorporates the intrinsic fields from the intragrain magnetocrystalline and grain boundary anisotropies and the interacting fields from intergrain dipolar and exchange couplings between the neighbouring grains. The model is applied to study magnetic properties of textured nanocrystalline Ni samples at 2K and is capable to reproduce closely the hysteresis loop behaviour at different orientations of applied magnetic field. Nanocrystalline Ni shows the grain boundary anisotropy constant K1s=−6.0×104J/m3 and the intergrain exchange coupling denoted by the effective exchange constant Ap = 2.16 × 10–11 J/m. Analytical expressions to estimate the intergrain exchange energy density and the effective exchange constant have been formulated.

Disorder-induced non-fermi liquid behavior of electrical resistivity in nanocrystalline Ni

Irrespective of the strength of the applied magnetic field (0≤H≤90kOe), strong departures from the Fermi liquid behavior of resistivity, ρ, (ρH~T2) have been observed at temperatures below 15K in the nanocrystalline Ni samples with average crystallite sizes 10nm and 20nm. A combination of T2 and T4 power laws adequately describes ρH(T) in the temperature interval 16K≤T≤65K whereas the variation with temperature of ρH slows down to ~T3/2 in the range 130K≤T≤300K. These observations assert that the electron–electron scattering dominates over the coherent electron–magnon scattering in the temperature range 16K–65K and the incoherent electron–magnon scattering takes over at the temperatures T>130K.

Compression Behavior and Mechanical Modeling of Nanocrystalline Ni under High Strain Rate Loading

To investigate mechanical properties and understand deformation mechanisms of nanocrystalline materials under high strain rate, the dynamic compression tests for nanocrystalline Ni bulk prepared by high-energy ball milling and hot-pressure sintering were carried out at different high strain rate on Hopkinson bar and a mechanical modeling based on deformation mechanism under high strain rate loading was developed. The experimental results indicate that the nanocrystalline Ni sample has higher strength and good ductility and the strength of the sample increased with the increasing strain rate. Meanwhile, The predictions by the mechanical modeling based on mechanism of dislocation gliding and grain boundary sliding at high strain rates show good agreements with the experimental data.

Friction and Wear Behavior of Nanocrystalline Nickel in Air and Vacuum

A bulk and dense nanocrystalline (NC) Ni with a mean grain size of 26 nm and a microhardness of 370–382 HV were fabricated by an electro-deposition technique. The microstructure and microhardness of the NC Ni samples were examined. The friction and wear behavior of the NC Ni samples under dry condition both in air and in vacuum were investigated in comparison with a conventional coarse-grained (CG) Ni with a mean grain size of 2.2 μm and a microhardness of 80–90 HV using a pin-on-disk type tribometer. The results show that the NC Ni sample possesses lower friction and higher wear resistance both in air and in vacuum, compared with the CG Ni sample. An important and interesting result is that for the both samples tested, the friction coefficients in vacuum are higher than those in air, while the wear loss occurs reverse case. In the air atmosphere, the NC Ni exhibits a mild wear, and avoids the severe wear that happens on the CG Ni, which is mainly attributed to which is mainly attributed to formation of a mechanical mixed layer on the worn surface of the harder NC Ni. In the vacuum atmosphere, compared with CG Ni, NC Ni shows a significant decrease in the plastic deformation and adhesive wear.

Evolution of shear banding in fully dense nanocrystalline Ni sheet

Compared with the coarse-grained counterpart, nanocrystalline (NC) metals have higher strength simultaneously with a decrease in ductility, strain localization is a main factor contributed to the early failure of NC metals during plastic deformation. This work deals with the study of shear banding in fully dense electrodeposited NC Ni sheet with sample dimensions at tens of millimeters under quasi-static uniaxial tensile load through the use of a strain gage calculated by digital image correlation technique. Shear band nucleation, broadening process and failure point were recognized. It is identified that maximum shear strain happens in the middle of the shear band where crack initiates first in this experiment. This indicates that the shear banding induces the failure of the NC Ni sample. Meanwhile, physical characteristics of the shear band, such as inclination and width of single full-developed shear band, were determined quantitatively. The results show that the inclination of shear band is about 63°, as well as the width of shear band is in sub-micrometer range. To investigate the micro-mechanisms during the shear banding process in the NC Ni sample, in situ tensile testing in a transmission electron microscope was conducted, the results suggest that grain boundary migration and grain coalescence are the main carriers during the propagation of shear band.

Investigations on the Fatigue Behavior of Nanocrystalline Metals

In this paper data on fatigue lifetime of nanocrystalline (nc) Ni samples is presented. The nc-Ni samples show high lifetimes and can resist large stress amplitudes. Furthermore a new method to determine the fatigue limit based on a Fourier analysis [7] is presented. First results of a load increase test suggest strong parallels to thermal and resistive measurements which can be used to determine the fatigue limit. Microstructural investigations on as received and failed samples show a strong increase in grain size induced by the cyclic loading. The processes behind this are not understood so far and will be investigated in the future.

High strength and high ductility in as-deposited nanocrystalline Ni

In the present study, an electrodeposited nanocrystalline (nc) Ni sample with high strength and superior ductility relative to many other electrodeposited nc-Ni was prepared. The superior ductility in the present nc-Ni sample free of defects was ascribed to mixed grains, the size of which spanned nano- and sub-micro scales at its as-deposited state with a grain size distribution ranged from 5 to 120nm. Obvious dislocation motion happening in coarse-grained polycrystalline was observed in large grains of nc-Ni matrix resulting in a remarkable enhanced ductility without a decrease in the strength. The present nc-Ni with an average grain size of 27.2nm prepared by direct current electrodeposition shows the average ultimate tensile strength of 1200MPa and the average elongation to failure of 10.4%.

Strain localization of fully dense nanocrystalline Ni sheet

The fully dense electrodeposited nanocrystalline Ni sample with dimensions at tens of millimeters was characterized under quasi-static uniaxial tensile load. During testing, the sample exhibited high strength simultaneously with a decrease in tensile strain. To inspect this early failure of the nanocrystalline Ni sample in this article, the strain fields of the sample were quantified with digital image correlation algorithm, and strain localization phenomenon in the form of shear band was captured successfully. Meanwhile, the failure mode and fracture surface of the Ni sample were examined in detail. The results suggest that the shear banding is a main deformation mode in nanocrystalline Ni sample, and it is an important factor to induce the early failure of the sample during plastic deformation.

Investigation of shear-banding mechanism in fully dense nanocrystalline Ni sheet

Evolution of shear banding in fully dense electrodeposited nanocrystalline Ni was successfully monitored by using a digital image correlation technique under a quasi-static uniaxial tensile load. To investigate the microscopic physical mechanism of the shear banding, in-situ tensile testing for the nanocrystalline Ni sample was conducted in a transmission electron microscope and fracture surface of the sample was examined by field emission scanning electron microscope. The results suggest that grain boundary migration based on atomic diffusion is a main carrier of the shear banding.

A polycrystalline mechanical model for bulk nanocrystalline materials using the energy approach

To systematically understand the grain size, strain rate and defect development dependent mechanical behavior of bulk nanocrystalline materials, a new constitutive model is proposed to describe the deformation mechanism, microstructure evolution and mechanical response of bulk nanocrystalline materials using the energy approach. In this model, the grain interior and grain boundary were not taken as two independent phases with different volume fractions, but as an integral object sustained dislocation and accommodated grain boundary sliding mechanisms. Meanwhile, defect creation and evolution and their effects on the overall stress–strain relation as well as the failure process of bulk nanocrystalline materials were considered in the model. For experimental verification, we have prepared nanocrystalline Ni powder by the DC arc plasma evaporation method. Bulk nanocrystalline Ni samples were then made by compaction and hot sintering. Experimental measurements on the mechanical response of bulk nanocrystalline Ni were performed under different strain rates and grain sizes. Comparison between experimental data and model predictions show that the method developed appears to be capable of describing the mechanical response of bulk nanocrystalline materials. The model applications to nanocrystalline Mg and Cu have shown that it can reflect the asymmetric defect development between tension and compression under quasi-static conditions; this results in its good capacity to describe the dynamic strain rate sensitivity and strain hardening behavior over a relatively large strain range under both compression and tension conditions.

DEFORMATION MECHANISM OF BULK NANOCRYSTALLINE NI PREPARED BY PLASMA EVAPORATION METHOD

Bulk nanocrystalline Ni samples were prepared by plasma evaporation method combined with hot pressure sintering. The compressive mechanical properties of the bulk samples were tested under quasi-static strain rates at room temperature and the evolution of microstructure of bulk sample before and after compression was studied. The experimental results indicated that the bulk samples show a good combination of strength and ductility and have obvious strain rate sensitivity. Both dislocation glide mediated and grain boundary mediated deformation mechanism were found to be main operating deformation mechanisms for the bulk samples based on the mechanical behaviors and grain refinement after compression.

Anomalous electrical transport behavior in nanocrystalline nickel

We have prepared nanocrystalline Ni (n-Ni) samples of grain sizes 40–100 nmusing a polyol method and investigated the electrical transport on theircompacted pellets in the temperature range 3–300 K. The resistivity,ρ, decreases nearly linearly with increase in compaction pressure but without a change in its slope,dρ/dT.ρ isanomalously large, and is strongly temperature and grain-size dependent. The resistivity at room temperature,ρ300 K, is in the range∼40–759 µΩ cm but with a positivecoefficient of resistivity α (metallic). This is associated with the significantly enhanceddρ/dT with increase inresidual resistivity ρ0. These characteristics are attributed to the disorder in the grain boundaries thatrepresents effectively a series resistor network.