Pure Polycrystalline Nickel Research Articles

Toward single crystal nickel fabrication using WAAM – A first report

The present study is focused on investigating the feasibility of additive manufacturing of single crystal nickel components using MIG processed wire arc additive manufacturing (WAAM) technique. The feasibility was validated by depositing the material first on 99.999 % pure polycrystalline nickel substrate and then onto a 99.995 % pure single crystal nickel specimen oriented in <111> direction. Optical microscopy and EBSD results prove the possibility for regions of single crystal to be deposited. The process yielded distinct regions of single crystal. Columnar and equiaxed grains occurred in a region with the highest local thermal gradient and slowest solidification rate. EBSD analysis confirmed that the single crystal region revealed in optical microscopy was in one orientation and the etchant was effective in highlighting individual grain boundaries.

Structural, Optical and Magnetic Properties of Nickel–Silica Nanocomposite Prepared by a Sol–Gel Route

Nickel-silica nanocomposites have been synthesized by a sol–gel method using dextrose (C6H12O6) as the reducing agent. The dried gel is heat treated at 850 and 900 °C for 30 min in an inert atmosphere by N2 purging to obtain the composite material. The samples have been characterized by powder X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, transmission electron microscopy and selected area electron diffraction. Pure polycrystalline nickel granular particle has been found to form with face-centered cubic structure and is entrapped in amorphous silica matrix with particle sizes in between 10 and 30 nm and is almost spherical in shape. The strong ferromagnetic nature of Ni–SiO2 composite became evident from the M-H curve which is quite different from the bulk nickel. The band gap of the synthesized Ni–SiO2 nanocomposite is found to be 2.35 eV. The reported sol–gel technique is a convenient and effective method to prepare high purity nanopowders with uniform size distribution.

Repeated load relaxation testing of pure polycrystalline nickel at room temperature using nanoindentation

We present the results of repeated relaxation tests using nanoindentation to derive the activation volume of the dislocation velocity and the ratios of the dislocation density and dislocation velocity. An experimental technique, based on classical uniaxial relaxation experiments, was developed to establish a constant strain during repeated load relaxation transients and then to calculate the stiffness of unloading, and therefore the hardness, across the transients with acceptable results. We found that the activation volume of the dislocation velocity from our nanoindentation methodology was in good agreement when compared to the same reported for uniaxial experiments.

The ductile–brittle transition for nominally pure polycrystalline nickel

A ductile to brittle transition has been observed for a nominally high purity polycrystalline nickel of ∼200μm grain size, containing 50ppm sulphur. The nickel specimens were subjected to heat treatment at 1017°C for 0.5h and were then either furnace cooled or water quenched. Small notched bend geometry specimens were tested over the temperature range of −196°C to 200°C at a fixed strain rate. Both the work to initiate fracture and the proportion of intergranular fracture were determined. The elemental concentration of sulphur at the grain boundaries has been measured using scanning Auger spectroscopy and STEM-EDX microanalysis. These compositions are compared with predictions from a non-equilibrium segregation model. The segregation of sulphur to the grain boundaries has a strong effect in promoting brittle intergranular fracture and this is attributed to the change in cohesive strength of the grain boundaries as a result of the segregation.

Crystallographic orientation and indenter radius effects on the onset of plasticity during nanoindentation

Abstract

Quantification of the microstructures of high purity nickel subjected to dynamic plastic deformation

A quantitative microstructural analysis is presented for pure polycrystalline nickel processed by means of dynamic plastic deformation at high strain rates (102–103s−1) to strains from 0.3 to 2.9. This analysis covers a number of structural parameters, such as the spacing between and the misorientation angle across dislocation boundaries and high angle boundaries. These boundaries subdivide the structure on a finer and finer scale towards saturation at the highest strain. The structural evolution follows a hierarchical pattern from the formation of cells and cell blocks to a characteristic lamellar structure, which is similar to that observed in metals deformed at a low strain rate by conventional deformation processes. However, at a constant strain the increase in strain rate increases the dislocation density and reduces the distance between deformation-induced dislocation boundaries and high angle boundaries. Shear bands and twins have not been observed. In order to underpin the structural analysis, the mechanical properties as a function of strain have been determined by tensile and hardness tests. The flow stress is 850MPa, showing that high strain rate deformation has potential as a method to produce strong nanostructured metals by imposing only a moderate strain.

Universal strain–temperature dependence of dislocation structure evolution in face-centered-cubic metals

The combined effect of strain and temperature on the microstructural evolution of plastically deformed face-centered-cubic (fcc) metals is explored systematically. In particular, the detailed nanoscale, internal structure of dislocation boundaries is determined in pure polycrystalline aluminum, nickel and gold and compared to earlier results in copper. In all the metals studied, dislocations within the boundaries tend to rearrange themselves with increasing strain in the same sequence from tangles into dislocation cells with tangled boundaries, followed by dislocation boundaries consisting of wavy, parallel dislocations and finally into arrays of parallel dislocations. The strain at which rearrangement occurs decreases with increasing temperature. The results are represented by microstructural maps on the strain–temperature plane. The topology of the microstructural maps is found to be similar for all metals studied, suggesting a universal strain–temperature dependence in deformed fcc metals.

Passivation and Electrocatalytic Behavior of an Amorphous Nickel–Carbon Film in Sulfuric Acid

The electrochemical behavior of a sputtered film of carbon and nickel, of approximate stoichiometry , is presented. This film is quite passive in hot sulfuric acid solution. The material also displays considerable electrocatalytic activity toward the hydrogen reaction, both anodically and cathodically. The electrocatalyst is passivated by the carbon component after dissolution of some of the surface nickel. The remaining surface nickel, entrapped by carbon, provides electrocatalytic activity comparable with that of pure polycrystalline nickel.

Effects of grain size on dislocation organization and internal stresses developed under tensile loading in fcc metals

The relationship between deformation and dislocation properties has been studied for pure polycrystalline nickel and austenitic stainless steel AISI 316L in stage III. Special care was taken to study statistically the effects of the grain size and grain orientation on dislocation densities and distribution. It is shown that the nature of dislocation cells depends on grain size and crystallographic orientation. The dimensional parameters, which depend on grain size, i.e. the inter-boundary spacing (λ) and boundary thickness (e), define three domains of crystallographic orientation and depend on the grain size. Scaling hypotheses reveal two physical mechanisms which, at this level of plastic strain, are correlated to a specific value of the noise, associated with distribution functions. Similarities between structural parameters and dislocation densities in each phase (walls and inter-walls spacing) are identified and discussed in terms of kinetic equations describing dislocation density evolution and fluctuations of certain physical parameters. This similarity provides physical signification of the scaling distribution obtained on λ and e in terms of a stochastic approach to dislocation distribution. The origin of Hall–Petch behaviour observed at large strain is interpreted in terms of an interaction between inter- and intra-granular long-range internal stresses, which depends on grain size. We conclude that, at high strain, the Hall–Petch phenomenological relationship is a consequence of plastic strain history and strain gradient in grains. From this last point, a length scale arises naturally, which depends on stacking fault energy.

Microstructure Evolution in Deformed Copper and Nickel

ABSTRACTThe combined effect of strain and temperature on the microstructure and detailed internal structure of dislocation boundaries was systematically studied in compressed pure polycrystalline copper and nickel and compared to the microstructure of compressed polycrystalline aluminum. Below 0.5Tm the microstructure of Cu and Ni consists of dislocation cells, however, only in Cu second generation microbands are formed. In Cu and Ni, the dislocations inside the boundaries rearrange themselves from tangles to ordered arrays of parallel dislocations following interplay between strain (requirement for cross slip) and temperature (dislocation mobility and ease of cross slip). The ordered detailed structure is similar to that observed in Al deformed at room temperature and lower strain levels. The amount of strain and temperature applied to Cu and Ni in order to achieve the same detailed structure formed in Al depends on the stacking fault energy (SFE) of the metal- higher strain and temperature as the SFE is lower.

Characterization of Rolled and Recrystallized High-Purity Nickel Using EBSP

99.995% purity polycrystalline nickel, purchased received from Alfa Aesar, was rolled to either 90 %, 95 % or 98 % thickness reduction at either 293 K or 77 K. The rolled specimens were isothermally annealed in air at their primary recrystallization temperatures and higher temperatures up to 1273 K for 1 h. For SEM/EBSP examination, specimens were mechanically polished, followed by electro-polishing in 10% sulfuric acid in methanol, at a voltage of ~14V, a current density of ~65mA/cm and a temperature < 273 K.

Formation of intermetallic layers at high intensity ion implantation

The experimental results are presented on a study of intermetallic phase formation in the surface zone of metal target at high intensity ion implantation. High intensity ion implantation allow to obtain the surface-alloyed layers of a much greater thickness in comparison with ‘ordinary’ ion implantation. Pure polycrystalline nickel was chosen as the target. The nickel samples were irradiated with the aluminum ions using the vacuum-arc ion beam and plasma flow source ‘Raduga-5’. The RBS and TEM were used for the investigations presented. It was established that the fine dispersed intermetallic precipitates are formed in the surface alloyed nickel layer. The alloyed layer thickness is equal to 150 nm and more, while the ion projected range that is equal to 70 nm. Compositions of these intermetallic precipitates are close to Ni 3Al and NiAl phases. The solid solution of aluminum in nickel is also formed. The depth dependence of the formation of intermetallic phases can be deduced from the Ni–Al phase diagram.

Microstructure and strength of nickel at large strains

A quantitative microstructural analysis is presented for pure polycrystalline nickel (99.99%) cold rolled to reductions from 70 to 98% ( ε vM 1.4–4.5). Applying transmission electron microscopy (TEM) techniques, key structural parameters, such as spacing between dislocation boundaries and high angle boundaries, as well as the misorientations across their boundaries, have been measured and analyzed. Application of scaling and similitude hypotheses to these microstructural parameters and their distributions revealed that the structures maintain a similar character with increasing strain. This similarity indicates that the measured parameters capture the important features of the structure. Scaling and similitude provide governing principles for structure formation. Based on this structural information and a detailed description of the morphology, structural parameters are identified, strength determining parameters chosen, and strength–structural relationships discussed. The suggestion is then made that two strengthening contributions should be considered: (i) dislocation strengthening due to the presence of low angle boundaries and (ii) grain boundary strengthening due to medium to high angle boundaries. The calculated individual strength contributions evolve differently with the strain and their addition leads to flow stress values and hardening rates in good agreement with those observed experimentally. No saturation of the calculated or experimental flow stress was observed.

Interaction of Propene with Pure and Hydrogen-Precovered Nickel Films. Studied by Means of Isothermal Reaction Mass Spectrometry and Thermal Desorption Spectroscopy

A special experimental setup was used to study the equilibrium gas composition after interaction of propene with pure thin polycrystalline nickel films and nickel films precovered with hydrogen under ultrahigh vacuum conditions. Isothermal reaction mass spectrometry allows the analysis of the gas phase composition under equilibrium conditions. The analyzed gas phase consists of desorbed reaction products formed in the adsorbate phase after adsorption of propene on nickel films at 273 K. This technique was combined with thermal desorption spectroscopy and measurements of adsorption-induced changes in the electrical resistance of the polycrystalline films. After interaction with pure nickel films, in addition to propene, hydrogen, methane, propane, and butane could be detected as reaction products. The interaction of propene with nickel films precovered with hydrogen leads to a higher yield of butane and additionally to the formation of pentane. Thermal desorption experiments carried out after adsorption of...

Surface behaviour of first-wall materials due to the synergistic effect of helium and hydrogen isotopes

Scanning electron microscopy has been used to investigate changes in surface morphology due to helium implantation and hydrogen charging. Pure polycrystalline nickel, OFHC copper and Cu1.8Be0.2Co (CAD 172) alloy have been studied. The influence of helium implantation parameters on blister formation and growth was investigated. Hydrogen charging (cathodic or thermal-gas) was found to lower the helium content needed for blistering and surface exfoliation. The effect of heating, carried out after hydrogen charging, was also studied. For the copper samples, hydrogen damage was produced by oxide reduction at the oxide-metal interface. This damage was found to be lower when the sputtering due to helium implantation increased. The CuBe alloy showed a greater hydrogen resistance due to the stability of the surface BeO.

Microstructure and corrosion behaviour of chromium nitride grown by dynamical ion beam mixing

A chromium nitride coating on pure polycrystalline nickel was obtained by dynamical ion beam mixing (DIBM). The chemical composition and microstructure of the surface layer were examined by Auger electron spectroscopy and transmission electron microscopy, and their effects on corrosion behaviour were studied. It was found that the DIBM CrN specimen surface region consisted of amorphous Cr-N-C, Cr 2(CN), Cr, CrN, Cr 2N, Ni-Cr amorphous phases and a dislocation cell structure. Three-sweep potentiokinetic measurement of the specimens in de-aerated CH 3COOH-CH 3COONa (pH 6.7, 25°C) showed that the critical current density and minimum anodic passivating current density are lowered by two orders of magnitude for the DIBM CrN specimens. The reasons for the improvement in corrosion resistance are discussed with respect to morphology and the microstructure of DIBM CrN.

The modification in fatigue life of pure polycrystalline nickel by carbon ion implantation: Xu, M.H., Patu, S. and Wang, Z.G. Vacuum 1989 39, (2–4), 297–299

The modification in fatigue life of pure polycrystalline nickel by carbon ion implantation: Xu, M.H., Patu, S. and Wang, Z.G. Vacuum 1989 39, (2–4), 297–299

Structural characterization and fatigue behaviour of a carbon-implanted pure polycrystalline nickel

Pure polycrystalline nickel was implanted with 150 keV carbon ions to fluences of 2 × 10 17 ions cm −2 at room temperature. Surface layer structural characterization for carbon-implanted and post-implantation heat-treated specimens, and its effect on fatigue behaviour, were investigated. The microstructural changes by the direct implantation of carbon into nickel were examined by transmission electron microscopy. It was found that the implantation results in the formation of amorphous phase, Ni 3C compound and some other metastable phases corresponding to different depths below the surface. The crystallization temperature of amorphous NiC is about 500°. The specimens were tested under load-unload tensile fatigue. A 22% increase in endurance limit was found after implantation. Fatigue behaviour of post-implantation heat-treated specimens is the same as before implantation. The fatigue surface was examined by scanning electron microscopy. Grain boundary cracking is another crack nucleation mode of the implanted specimens besides slip band cracking which is the only crack initiation mode of pure nickel. Fatigue cracks of post-heat-treated specimens nucleate at large Ni 3C particles. Transmission electron micrographs show that the cyclic loading leads to recrystallization of carbon-ion-induced amorphous NiC.

The modification in fatigue life of pure polycrystalline nickel by carbon ion implantation

Plates of nickel with and without carbon ion implantation have been tested in tension-zero fatigue. A 22% increase in endurance limit was found with implantation. The fatigued surfaces have been examined by SEM. Surface deformation favorable to crack initiation is greatly decreased by carbon ion implantation. Grain boundary cracking is also one of crack nucleation modes of modified materials besides slip band cracking which is the only crack initiation mode of unmodified specimens.

THE PROGRESSION OF BENDING FATIGUE IN NICKEL

Abstract— The progression of fully reversed bending fatigue, in a 99.6% pure polycrystalline nickel (NO2200) at room temperature, was monitored utilizing Nomarski interference contrast microscopy in conjunction with microhardness measurements. It was found that a gradual hardening, without early saturation, occurred from about 95 to 160 HV. Similar data were obtained with indenter weights of 15–500 g, from which it was concluded that a different behaving surface layer did not develop during fatigue. Cracks initiated at coarse slip bands within the first 1% of the life, when the hardness reached 140 HV. The cracking of these bands initiated away from grain boundaries. The slow growth of these slip band cracks dominated the life subsequently. Growth of the cracks both across and beyond the initial grain was difficult. The significant hardening prior to crack initiation and the confinement of the cracking mainly to coarse slip bands, contributed to a good inherent fatigue resistence of nickel. with a fatigue limit above the yield strength. Non‐propagating cracks were not observed below the fatigue limit.