Nickel Specimens Research Articles

Towards Optimization of Grain-Boundary Structures in Annealed Nickel

Many properties of materials are influenced by intergranular structure. Increasing the number of grain boundaries with favourable properties, or ‘special’, boundaries, can enhance the performance of a material in service. However, the criteria that must be satisfied for a boundary to be classified as special are not widely understood. For example, it has been shown that Σ3 coincidence site lattice (CSL) boundaries alone present a resistance to corrosion greater than for other boundary types in nickel (fee). In contrast, Rolim Lopes et al. observed no statistical preference for Σ3s in the grain boundary network of FeAl (bcc), and Bouchet and Priester discovered that in nickel there was a weak propensity for segregation to several boundary types, including Σ3, Σ1 1 and Σl9a CSLs.Commercially pure and superpure nickel specimens have been subjected to the strain annealing treatments detailed in the Table. Fig. 1 shows the results from an electron backscatter diffraction study of the commercial purity specimens (CI to C4). It is clear that the only boundary types that increase in frequency with increased thermal processing are Σ3n (n=l,2,3) CSLs. An abundance of annealing twins was observed for sample C4, and is unique to this sample, and this accounts for the very high proportions of Σ3s. Samples CI to C3 were annealed under vacuum, whereas C4 was treated in air, and it is proposed that the dominant energy minimization mechanism has changed from grain/grain boundary rotation for samples CI to C3, to annealing twin formation for sample C4. If the structure of a grain boundary evolves towards the exact misorientation of what is considered a special CSL type, the special properties of that boundary will, in general, become more marked. Σ3s have been shown to possess special properties. Fig. 2 shows the average deviation from exact misorientation for the Σ3s in each sample, normalised by the Brandon criterion for maximum permitted deviation. This clearly indicates that the Σ3s created by annealing twin formation can be considered more special than those produced via grain/grain boundary rotations.

Microstructure of copper and nickel irradiated with fission neutrons near 230°C

The microstructures of pure copper and nickel specimens were examined by transmission electron microscopy (TEM) following irradiation at ∼ 230°C in a mixed spectrum fission reactor to damage levels between 0.01 and 0.25 displacements per atom (dpa). A high density of small stacking fault tetrahedra (SFT) and dislocation loops was observed in both materials, and a moderate density of small voids was observed in the irradiated copper specimens. From a comparison with published studies, the proportion of SFTs and dislocation loops in copper was observed to be nearly constant over a wide range of damage levels at temperatures between 200 and 250°C. This suggests that nucleation and growth of interstitial loops to visible sizes is rather difficult in copper in this temperature regime. On the other hand, the loop size and density in nickel increased steadily with increasing dose. The defect clusters in nickel formed a {001} planar wall pattern which became visible after damage levels of ∼ 0.1 dpa. Defect cluster patterning was not observed in the irradiated copper specimens.

The effect of grain size on the stored energy of cold work as a function of strain for polycrystalline nickel

It is well known that both the distribution and density of dislocations are affected by grain boundaries and that the stored energy of deformation is e function of the density of dislocations. Thus, the grain size affects the stored energy of deformation. In general, a fine grain-sized material has a higher dislocation density than a coarse grain-sized material for a given plastic strain. This paper presents measurements of the total stored energy as a function of strain for a series of polycrystalline nickel specimens of different grain size. The aim is to determine how the stored energy of deformation from the geometrically-necessary dislocations contributes to the total stored energy of deformation as a function of the strain.

Formation process of defect clusters in copper and nickel under irradiation with helium ions

The formation process of defect clusters in copper and nickel irradiated with He + ions has been studied to understand the effects of the small fraction of damage cascades and He atoms on it. Wedge-shaped specimens of copper and nickel were irradiated with 20–100 keV He + ions at 300–570 K in an ion accelerator or an HVEM on line with an ion accelerator. The results on microstructural evolution were analyzed in terms of the energy of He + ions, the irradiation temperature and the specimen thickness as well as the difference between copper and nickel. It is concluded that the small fraction of damage cascades induced by He + ions provides the direct nucleation of vacancy- and interstitial-clusters and that the threshold energy for forming vacancy clusters in nickel is slightly larger than in copper.

Microdamage accumulation in nickel

The results of an experimental and theoretical study on the failure of nickel specimens during impact have been presented. For the numerical analysis a model was used which assumes the formation of pores in the matrix which subsequently grow under the influence of stress. Using experimental data taken from the nickel damage, the parameters which were used in the model were determined. The numerical study was completed and a number of the processes by which damage accumulates were outlined.

Coke Formation on Preoxidized Iron and Nickel in a CH4-H2Gas Atmosphere at 1,000°C

Coke formation on preoxidized iron (Fe) and nickel (Ni) specimen was studied in a 15% methane-hydrogen gas (CH4-H2) atmosphere at 1,00°C by thermogravimetry. Ground specimen of different surface finish were investigated also. An accelerated heterogeneous reaction resulted when the Fe specimen were preoxidized, resulting in a heavy coke deposition. This resulted from formation of an active “metallic” surface generated by the reduction of preoxidized scales. For Ni specimen, acceleration resulting from preoxidation was small. The different surface activities between the preoxidized Fe and Ni specimen were not attributed to the different surface areas. Based on the test results, a practical remedy to alleviate deposition of coke in high-temperature cracking environments was proposed.

The influence of isotopically controlled boron addition on void swelling of nickel irradiated in FFTF

Pure nickel specimens doped with several levels of boron-10 ( 10B) and natural boron ( nB) were irradiated in the Materials Open Test Assembly of the Fast Flux Test Facility (FFTF/MOTA) for the purpose of examining the applicability of boron addition to the study of helium production effects. It was shown that the boron addition enhances void nucleation as its transmulation effect and suppresses as its chemical effect. The former effect is dominant at higher temperatures.

Chemical machining of nickel in ferric chloride solution

Chemical machining is a method of removing an unmasked area from a metal surface by controlled selective dissolution with chemical reagents. This process is used in the production of machine and electronic parts and is also called chemical etching or system. milling. Sometimes current is applied at low current density as an assist. Many detailed studies have been 101 conducted recently to investigate the chemical etching of nickel and nickel alloys [1-3]. Nickel is highly resistant to a variety of corrosive media, due to the passive film that it forms on its surface. Moon and v Nobe [4] reported that the dissolution rates of nickel 1 0 0 increase as the concentration of chloride ions is E increased. Chevalet and Zutic [5] also proposed a reaction mechanism of nickel in chloride solution. The purpose of this study was to investigate the ~ol etching behaviour of nickel in ferric chloride solu._c tions over the temperature range 15-50°C. The ~ 10 -1 experiments were conducted by a gravimetric "' method. Low current densities were applied to enhance the etching rate of nickel in FeC13 solution in this study. Nickel specimens were prepared for etching tests from 99.99% purity nickel sheet (0.5 m m x 2.5 cm x2 .hc m) . The specimens were polished with 1000-grade silicon carbide paper, cleaned in acetone and dried. The prepared specimens were then used in immersion tests. For the measurement of etching rates, 1000ml t01 solutions were prepared in a constant-temperature water bath at a temperature (+ 0.5 °C) over the range 15-50 °C. Weighed specimens were totally 4" immersed for a period of 1 h at the end of which they were removed, rinsed in 10% sulphuric acid followed by distilled water and acetone, and then dried E and reweighed. For the purpose of accelerated etching for nickel specimens, a Nichia model G1001ED galvanostat o, was used to apply a constant anodic current on the .E enickel. Current densities of 0.01-0.1 Acm -2 were chosen to compare with the results from immersion tests. For the etching of nickel good and reproducible results were obtained. Results for the test solutions in the concentration range from 0.1 to 1.0 M at 26 °C are shown in Fig. 1. It is clear that the etching rate of nickel increases exponentially with FeC13 concentration. The etching rate of nickel in FeC13 containing

Activation parameters of high-temperature creep in polycrystalline nickel at ambient and high pressures

Polycrystalline specimens of pure nickel were deformed in uniaxial compression at temperatures of 1000–1550 K, strain rates of 1×10 −5-3×10 −3s −1 and pressures of 0.1–1500 MPa, in order to determine the activation parameters of high temperature creep. Experiments at 0.1 MPa were conducted in an MTS apparatus with the specimen immersed in a molten heat-treating salt to prevent oxidation. The data show a decreasing power-law stress exponent with decreasing normalized steady-state flow stress ( σ/ G), approaching the “natural law” value of n=3 at normalized stresses <10 −4. In contrast, the activation energy is constant over our range of temperatures ( T/ T m = 0.55−0.90), and is indistinguishable from the activation energy of self-diffusion (284 kJ/mol). High pressure experiments were conducted in a modified piston-cylinder apparatus using the same molten heat-treating salt for the confining medium. The small activation volume could not be resolved; however, the trend of the high pressure data parallels that of the 0.1 MPa data with a systematic offset, and is consistent with the measured activation volume of self diffusion. Specimens deformed at 0.1 MPa exhibited significant strain-enhanced grain growth; this effect is greatly reduced under hydrostatic pressure, whereas subgrain size was less affected.

Impedance Characteristics of Anodic Oxide Films Formed on NiAl in Molten Na2 CO 3

Impedance spectroscopy was employed to study anodic oxide films at the interface at 1000°C and 1 atm using a standard polarization cell with an alumina‐sheathed Pt, , reference electrode and a platinum counterelectrode. This system does not anodize easily; in most samples the anodic current was shunted through nodules that formed on the surface of the aluminided nickel specimens. Impedance behavior of successfully anodized samples in molten sodium carbonate at the rest potential was consistent with the continued growth of barrier‐type at short times (<25 h) and with the formation of a porous layer for longer times (>45 h).

Effect of thermal cycling on the change in length and rupture of cylindrical nickel specimens

1. The elongation of nickel specimens after TC depends on their diameter and the maximum temperature of the cycle; the higher the values of these parameters, the greater the elongation. 2. The elongation of specimens upon TC between 830 and 20°C is connected with the formation, opening, and increase in number of cracks. 3. The initial structure of the specimens influences their length change. Specimens preliminarily annealed at 1020°C are more resistant to change in length than those annealed at 830°C.

Effects of preinjected helium in heavy-ion irradiated nickel and nickel-copper alloys

Pure nickel and two nickel-copper alloys (Ni-10 at. pct Cu and Ni-50 at. pct Cu) containing 50 appm preinjected helium have been irradiated with 14 MeV nickel ions at a constant homologous temperature of 0.45T m to 3 to 5 dpa at a depth of 1 μm from the surface. The radiationinduced lattice defects have been analyzed by transmission electron microscopy (TEM) with samples prepared in cross section. In the helium preinjected region of the pure nickel specimen, a substantial density of voids with an average diameter of 35 nm was observed. The nickelcopper alloys were found to contain only a high density of small helium bubbles (under 5 nm in diameter) and dislocation loops. The density of both dislocation loops and helium bubbles increases with increasing copper content, while the size decreases. The observed resistance of the nickel-copper alloys to void formation regardless of the presence of helium bubbles is considered to be the result of local clustering of like atoms which generates a high density of traps, and, thereby, disperses vacancies and gas atoms.

Metal surface roughness and optical reflectance

A description of metal surface roughness has been obtained by introducing dispersion of the roughness distribution as a new parameter. The optical reflectance of nickel, aluminium, copper and brass specimens have been investigated experimentally using HeNe and Ar + lasers. From the wavelength dependence of reflectance observed for nickel and aluminium specimens, it is feasible that an industrial on-line system could be used to monitor surface roughness.

The formation of helium bubbles near the surface and in the bulk in nickel during post-implantation annealing

A systematic TEM study on nickel specimens homogeneously implanted with about 1000 appm helium at room temperature and annealed for about 1 h between 873 and 1273 K is reported and interpreted. During annealing the observed bubble structure coarsens substantially. The coarsening rate is, however, much faster close to the surface than in the bulk. The apparent activation energies of the average bubble radius in these regions are 1.1 and 0.23 eV, respectively. Results from other investigations confirm and complete these findings. The enhanced coarsening near the surface is attributed to the relaxation of initially overpressurized bubbles by vacancies supplied by the nearby surface. The coarsening mechanism in this region is identified as Ostwald ripening. In the bulk, the latter is suppressed due to a reduction of the vacancy concentration by the overpressure in the bubbles. In this region bubble coarsening is attributed to coalescence via bubble migration controlled by surface diffusion.

The effect of ion beam surface modifications on fatigue crack initiation in polycrystalline nickel

Flexural fatigue tests at surface strain amplitudes of ±0.0012 were conducted on polycrystalline nickel to determine the effect of ion beam surface modifications of the Ni/Al system on fatigue crack initiation. An Ni-75at.%Al surface region 1000 Å thick was produced by evaporating alternate layers of nickel and aluminum followed by ion beam mixing with 3 MeV Ni 2+ ions to a fluence of 1 × 10 16 ions cm −2. A two-phase γ-γ′ surface structure was produced through implantation of 350 keV Al + ions to a fluence of 5 × 10 17 ions cm −2 into the nickel specimens at 723 K. Nickel specimens that were self-implanted with 350 keV Ni + ions to a fluence of 1 × 10 16 ions cm −2 were also analyzed. The addition of aluminum to the surface resulted in an increase in surface hardness and suppression of surface slip band features. The nickel self-implantation increased surface hardness but had very little effect on surface slip feature suppression. Transgranular crack initiation was seen in unmodified and self-implanted specimens, and intergranular crack initiation was prevalent in aluminum micro-alloyed specimens.

Grain size gradient nickel alloys II: Fatigue properties

Grain size gradient nickel alloys were tested in a rotating-bending fatigue machine and found to display better fatigue behavior than non-gradient, homogeneous nickel specimens having the same average grain size. This demonstrates that the overall fatigue properties are improved through the concept of a grain size gradient. The following reasons are suggested for the improved fatigue behavior. For the case of a gradient alloy with fine grains at the exterior and coarse grains in the center, the combined effect of a fine-grained surface and the presence of Al 2O 3 particles (grain growth inhibitor) increases fatigue crack initiation resistance, while the coarse-grained central region increases fatigue crack propagation resistance. For an alloy having the reversed grain size gradient, the coarse-grained surface increases crack propagation resistance near the threshold region, and the central, Al 2O 3 - added region contributes to fatigue resistance by enhancing yield strength. In both cases, when the applied stress is small, the fatigue crack propagates intergranularly at the beginning of the fracture process, but changes to a transgranular mode with striations as the stress intensity factor ΔK increases. At higher stresses the fatigue crack growth occurred only in a transgranular mode with striations. Grain size was found to exert a stronger influence upon the overall fatigue behavior than do the dispersed Al 2O 3 particles, probably because of the two opposing effects of oxide particles on crack front deflection and cavity formation. The portion of intergranularly fractured surface area decreases with an increase in the applied stress, and fatigue life increases with an increase in the portion of intergranularly fractured surface area.

Effect of composition and structure of alloys on the change of shape and crack nucleation in heat cycling

1. It was established that the length of cylindrical specimens of copper and nickel, preliminarily annealed in the process of HC in the temperature range 953–293°K, increased after heating in a salt bath and cooling in water, and also after heating in air and cooling in water. 2. When specimens of copper M1 are subjected to HC, pores form inside the grains and on their boundaries, and twins also originate, and when they merge, they form discontinuities on the grain boundaries. An increase of the amount of impurities in copper M2 (from 0.1 to 0.5%) intensifies the process of formation of pores, their merger and formation of cavities on the grain boundaries. The nature of the increase in length of the specimens is nonlinear; this is possibly due to the grains coming closer together where there are discontinuities, and also to the turning of the grains at the joints. 3. When specimens of nickel are subjected to HC, cracks appear in the central part; they are situated on the grain boundaries perpendicularly to the axis of the specimen. Grain boundaries are apparently one of the weak spots where cracks (in nickel specimens) and discontinuities (in copper specimens) form. 4. A radical way of reducing the effect of change of length in HC is to add high melting elements to the alloys, thus increasing the strength of the alloy in the heated state; another device is to reinforce the grain boundaries by a carcass of intermetallic phases.

Strong traps for deuterium in krypton-implanted nickel

Experiments were carried out to investigate strong traps (binding enthalpies&amp;gt;0.50 eV) for deuterium that were found earlier when pure nickel was implanted with argon and krypton at temperatures ≤500 °C. In one experiment, three different areas of a nickel specimen were implanted with a fluence of 2×1016 cm−2 krypton at 600, 700, and 800 °C; a second experiment involved implanting krypton at three different fluences (2, 6, and 20×1015 cm−2) in nickel at 500 °C. Retained deuterium was monitored during subsequent isochronal anneals using nuclear reaction analysis. In a third experiment, depth profiles for both implanted krypton and trapped deuterium were determined simultaneously using secondary ion mass spectrometry. Traps for deuterium in krypton-implanted nickel were found with binding enthalpies ranging from 0.5 to 1.1 eV. At implantation temperatures &amp;lt;500 °C, the traps are in the same location as the krypton atoms. At implantation temperatures &amp;gt;700 °C, the traps are found where the krypton was originally implanted in spite of the fact that a significant fraction of the krypton apparently escapes through the surface during the implantation process. The observed fluence and temperature dependence indicate that the traps with binding enthalpies between 0.5 and 1.1 eV are associated with the surfaces of cavities formed during the implantation.

Simultaneous ion- and gas-driven permeation of hydrogen isotopes through first wall material

Abstract Deuterium was simultaneously ion- and gas-driven into specimens of nickel, 304 SS and vanadium, and its permeation rate was experimentally measured. The values of the permeation rate were used to evaluate the “recombination rate coefficient” k R . Results revealed that, under the conditions of the present study, the recombination process at surfaces was the rate-limiting step of the permeation for the metals investigated. Also, the values of k R were very sensitive to the surface impurity compositions. In such cases, treatment of the surfaces in respect to their impurity compositions was found to be effective in controlling the permeation rate.

Influence of chemical substituents on the superconductivity of the system Bi4Ca3Sr3Cu4Oy

The effect of the partial replacement of Bi by Eu, Gd, K and Pb, and Cu by Ni, on the superconducting transition temperature (Tc) of Bi4Ca3Sr3Cu4Oy (y=undetermined) is investigated by DC resistivity (R) and AC susceptibility ( chi ) measurements. Tc is not significantly altered by Eu and Gd substituents (Tc close to 65 K); however, K and Pb substituents tend to enhance the fraction of a phase superconducting around 110 K. Interestingly, for nickel specimens, Tc of a large fraction, as suggested by AC chi data, initially decreases to 40 K with increasing concentration of nickel, followed by an increase, whereas R data shows no appreciable variation of Tc.