Polycrystalline Nickel Research Articles

Modelling of Fatigue Crack Initiation in Hydrogen Charged Polycrystalline Nickel

Hydrogen Embrittlement (HE) leads to deterioration of the fracto-mechanical properties of metals. In spite of vast literature, it is still not clearly understood and demands significant research on this topic. For better understanding of the hydrogen effect on fatigue behaviour of metals, present work focuses on developing a computational framework for fatigue crack initiation studies in metals in the presence of hydrogen. The developed framework consists of a nonlocal crystal plasticity model coupled with hydrogen transport model to study the fatigue behaviour of hydrogen charged metals. The nonlocal crystal plasticity model accounts for the statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) in polycrytalline metal. Hydrogen transport model, on the other hand, accounts for diffusion and trapping behavior of hydrogen due to concentration gradient, pressure gradient, plastic strain-rate with dislocations as the only trapping sites along the slip systems. A polycrystalline representative volume element (RVE) with periodic boundary conditions is used in this study. Fatigue crack initiation criterion is proposed for the simulated RVE with controlled microstructure by considering a critical value of the fatigue indicator parameter (FIP). FIP is formulated based on the experimental observations of several crack initiation sites along the grain boundaries, their normal direction with respect to loading direction and the accumulated plastic strain in nickel polycrystalline samples. Developed simulation framework correctly accounts cyclic stress-strain behavior and multiple fatigue crack initiation sites observed experimentally in the presence of hydrogen.

Role of inert gas in the Cvd-graphene synthesis over polycrystalline nickel foils

Abstract The role of the inert gas during CVD-graphene growth over polycrystalline nickel foils is reported. Nitrogen, hydrogen and methane were used as inert, reduction and carbonaceous gases, respectively. Graphene samples were characterized by using Optical Microscopy, Raman Spectroscopy and SEM equipped with EDX. Four different colors were distinguished over the optical microscope images. These colors were analyzed by Raman Spectroscopy and correlated to each type of graphene deposited over the foil (monolayer, bilayer, few-layers and multilayer). An Excel-VBA application was developed, which was used for computing the percentage of each type of graphene, resulting in an estimation of the graphene quality value. It was verified that the incorporation of the inert gas in the reaction step during CVD-graphene synthesis greatly influenced the quality of the obtained graphene. Different temperatures and percentages of nitrogen were studied in order to define the optimal reaction conditions. A factorial design study was performed to find the best experimental conditions leading to the highest quality graphene value, which resulted to be the following ones: 947 °C and 13 vol % of nitrogen in the feed stream, 1 min of reaction time, a relation between CH4 and H2 of 0.07 v/v, and a total flow of gases of 80 Nml/min.

Extracting Grain Orientations from EBSD Patterns of Polycrystalline Materials Using Convolutional Neural Networks.

We present a deep learning approach to the indexing of electron backscatter diffraction (EBSD) patterns. We design and implement a deep convolutional neural network architecture to predict crystal orientation from the EBSD patterns. We design a differentiable approximation to the disorientation function between the predicted crystal orientation and the ground truth; the deep learning model optimizes for the mean disorientation error between the predicted crystal orientation and the ground truth using stochastic gradient descent. The deep learning model is trained using 374,852 EBSD patterns of polycrystalline nickel from simulation and evaluated using 1,000 experimental EBSD patterns of polycrystalline nickel. The deep learning model results in a mean disorientation error of 0.548° compared to 0.652° using dictionary based indexing.

Fast synthesis of thin graphite film with high-performance thermal and electrical properties grown by plasma CVD using polycrystalline nickel foil at low temperature

We have developed a high speed, low temperature synthesis method of thin graphite film by plasma-enhanced chemical vapor deposition (plasma CVD). The synthesis time was less than 60 min and the highest process temperature was 1300 °C. Raman spectroscopy, X-ray diffraction and cross-sectional transmission electron microscopy (TEM) show a high crystallinity of synthesized graphite film. The graphitization of the film was larger than 0.944, which suggests that the film was nearly completely graphitized. The thermal conductivity of the film was 1570 W/mK which is the highest among of graphite synthesized by CVD on transition metal substrates.

Angular, temperature, and impurity effects on secondary electron emission from Ni(110)

The secondary electron emission from a temperature-controlled Ni(110) sample was examined for 50–1500 eV electrons impacting at 0°–35°, 50°, and 78°. Measurements showed a non-cosine dependence on an electron incidence angle: the yield has a maximum at 0°, minima at ±12°, and increases at larger angles up to 35°. This trend in angular dependence is characteristic of single crystal materials and is due to increased secondary electron generation when primary electrons are directed along a close-packed direction. For example, compared to polycrystalline nickel, the yield for Ni(110) from primary electrons at 0° (i.e., along the [110] direction) is up to 36% larger. Additionally, secondary electron yields are highly sensitive to incident electron energy (most notably between 0 and 500 eV) and to the presence of adsorbed carbon monoxide [with an up to 25% decrease compared to clean Ni(110)]. However, yields are independent of sample temperature between 300 and 600 K and of exposure to deuterium ions leading to formation of subsurface hydrogen. These results reaffirm the unique secondary electron emission properties of single crystals materials and highlight the importance of crystal orientation. Results are important for plasma-enhanced chemistry applications that utilize Ni(110) catalysts, since larger secondary electron emission may facilitate reactions of adsorbed species.

Detailed analysis reveals differences between single-crystal and polycrystalline nickel features

Characterizing the secondary electron emission behavior of Ni(110), a single-crystal form of nickel, demonstrates potential implications for plasma-enhanced chemistry.

Elucidating the contribution of mobile hydrogen-deformation interactions to hydrogen-induced intergranular cracking in polycrystalline nickel

Uniaxial mechanical testing conducted at room temperature (RT) and 77 K on hydrogen (H)-exposed nickel was coupled with targeted microscopy to evaluate the influence of deformation temperature, and therefore mobile H-deformation interactions, on intergranular cracking in nickel. Results from interrupted tensile tests conducted at cryogenic temperatures (77 K), where mobile H-deformation interactions are effectively precluded, and RT, where mobile H-deformation interactions are active, indicate that mobile H-deformation interactions are not an intrinsic requirement for H-induced intergranular fracture. Moreover, an evaluation of the true strain for intergranular microcrack initiation for testing conducted at RT and 77 K suggests that H which is segregated to grain boundaries prior to the onset of straining dominates the H-induced fracture process for the prescribed H concentration of 4000 appm. Finally, recent experiments suggesting that H-induced fracture is predominately driven by mobile H-deformation interactions, as well as the increased susceptibility of coherent twin boundaries to H-induced crack initiation, are re-examined in light of these new results.

Lithium phosphorous oxynitride (LiPON) coated NiFe2O4 anode material with enhanced electrochemical performance for lithium ion batteries

Polycrystalline nickel ferrite (NiFe2O4) thin films were fabricated by pulsed laser deposition (PLD) and coated with nanometer layer of lithium phosphorus oxynitride (LiPON) by radio frequency magnetron sputtering (RF sputtering). Physical characterizations demonstrated that NiFe2O4 thin films were not destroyed by LiPON coating with 50 nm thickness. Discharge capacity of LiPON coated NiFe2O4 thin film after 50 cycles at 5 μA cm−2 was 849 mAh g−1, much higher than the counterpart value of pristine NiFe2O4 thin film (449 mAh g−1), while rate capability was also greatly enhanced. Ex situ TEM investigation confirmed the two-steps reaction mechanism between NiFe2O4 and Li, which includes an irreversible decomposition reaction of NiFe2O4 in the first discharge process and reversible reactions between NiO, Fe2O3 and their nanosized metal particles afterward. The significant improvement on the electrochemical performance was ascribed to the amorphous LiPON layer on the surface, which suppressed the volume expansion of electrode, the formation of delaminated particles and generation of cracks on electrode surface, as proved by ex situ SEM and AFM results. The LiPON coating is an effective approach to enhance the electrochemical performance of conversion materials for lithium ion batteries.

Fracture mechanisms induced by microporosity and precipitates in isothermal fatigue of polycrystalline nickel based superalloy

Isothermal fatigue (IF) of polycrystalline nickel based superalloy Inconel 718 was studied to reveal the effects of microporosity and TCP phase on the damage mechanisms and fatigue life. Two groups of as cast and heat isostatic pressed (HIP) superalloys were subjected to the isothermal fatigue at three temperatures (500 °C, 575 °C, and 650 °C), followed by microstructure characterization and fracture mode analysis via scanning electron microscope (SEM) and transmission electron microscope (TEM). Resulting data suggested that microporosity significantly shorten the fatigue life of the superalloy even temperature and applied stress load were controlled at low level. Fatigue cyclic life of the superalloy decreased with the temperature and applied stress load increasing. While as cast superalloy with low porosity exhibited a long fatigue life at 500 °C and 650 °C. Microporosity accelerated the crack growth rate in crack initiation zone and assisted the rapid propagation of secondary crack. Laves, as a topologically close packed (TCP) phase, cannot hinder enough dislocations before brittle rupture, while precipitation of strengthener γ′′ promoted formation of dislocation slip bands leading to high fatigue resistance. Abnormal strain localization at scripted carbide preferentially changed the shape of fatigue striation and introduced twin. As crack source, surface oxide scale can generate complicated crack networks.

Dislocation-type evolution in quasi-statically compressed polycrystalline nickel

The nature of dislocation generation as a function of applied plastic strain in quasi-statically compressed polycrystalline pure nickel has been studied experimentally at ambient temperature. First, to ensure representative datasets of the geometrically-necessary dislocation densities (ρGND) associated with non-uniform plastic deformation, measurements over large (several millimeter square) areas were made using Hough-based EBSD methods. In addition, the total dislocation density (ρT) responsible for the overall work hardening is estimated from the measured flow stress based on Taylor's hardening model. Next, the statistically stored dislocation (SSD) density (ρSSD) is calculated by subtracting the GND density from the total dislocation density. The results demonstrate that in quasi-statically deformed nickel: i) the measured GND density varies linearly as a function of plastic strain in the range between 0.05 and 0.46; although Ashby's model predicts linearity for GND density evolution over entire range of strains, this study does not cover strains below 0.05; ii) the SSD density increases at a rate much faster than GND density; and iii) the SSD density exceeds the GND density at above 0.09 plastic strain. Both i) and ii) are in agreement with Ashby's prediction, while the magnitudes of GND density (iii) differ from Ashby's model prediction, particularly at large applied strains. Overall, this study enables the interplay of GNDs and SSDs in the hardening of nickel to be gleaned in a quantitative sense. This study illustrates that GNDs are the more important for the strength of polycrystalline metals in the early stages of work hardening, whereas SSDs contribute more to the strength at larger strains. Over the range of strain in this study, work hardening is predominantly through rapid multiplication of SSDs; whereas the GND density is initially higher than SSD density at 0.05 probably due to non-linear evolution of GNDs at low strains (<0.05), which will be the subject of future investigation.

Initiation of Carbon Nanofiber Growth on Polycrystalline Nickel Foam under Low Ethylene Pressure

AbstractThe initiation of carbon nanofiber (CNF) growth on polycrystalline Ni foam was investigated by using a combination of ex situ and in situ methods, which include SEM, XRD, and Raman spectroscopy. Experiments were performed at a low hydrocarbon partial pressure to slow down the initiation process. Very little to no CNFs were observed on reduced samples, which is caused by diffusion of C to the bulk of the Ni foam. This prevents the formation of Ni3C as a precursor of Ni nanoparticles at a low hydrocarbon partial pressure from acting as active particles for CNF formation. CNF growth was significant on oxidized samples and the initiation was slowed down by using an extremely low ethylene pressure. Ni nanoparticles are able to catalyze CNF growth, provided these are isolated from the Ni bulk by unreduced NiO, which results from the incomplete reduction of the NiO layer.

Effects of Strain Rate, Temperature and Grain Size on the Mechanical Properties and Microstructure Evolutions of Polycrystalline Nickel Nanowires: A Molecular Dynamics Simulation

Through molecular dynamics (MD) simulation, the dependencies of temperature, grain size and strain rate on the mechanical properties were studied. The simulation results demonstrated that the strain rate from 0.05 to 2 ns–1 affected the Young’s modulus of nickel nanowires slightly, whereas the yield stress increased. The Young’s modulus decreased approximately linearly; however, the yield stress firstly increased and subsequently dropped as the temperature increased. The Young’s modulus and yield stress increased as the mean grain size increased from 2.66 to 6.72 nm. Moreover, certain efforts have been made in the microstructure evolution with mechanical properties association under uniaxial tension. Certain phenomena such as the formation of twin structures, which were found in nanowires with larger grain size at higher strain rate and lower temperature, as well as the movement of grain boundaries and dislocation, were detected and discussed in detail. The results demonstrated that the plastic deformation was mainly accommodated by the motion of grain boundaries for smaller grain size. However, for larger grain size, the formations of stacking faults and twins were the main mechanisms of plastic deformation in the polycrystalline nickel nanowire.

Experimental study on the diffusion of hydrogen along individual grain boundaries in nickel

Whether hydrogen diffusion can be accelerated by grain boundaries (GBs) still remains controversial. In this study, the impact of different types of GBs within the directionally solidified polycrystalline nickel specimens on hydrogen diffusion was directly observed at room temperature by combining Electron Back Scattered Diffraction (EBSD) and scanning Kelvin probe force microscopy (SKPFM). The results suggest that the coherent Σ3 twin GB provides a fast path for hydrogen diffusion, while the random GB cannot significantly accelerate the transport of hydrogen atoms. In addition, this study confirms that SKPFM is a reliable technique to allow the detection of the local diffusion and distribution of hydrogen in metals with high lateral resolution down to the GBs.

Helium ion irradiation-induced microstructure evolution on the surfaces of thin nickel foils

Abstract Polycrystalline nickel foil (sample I) with thickness of ∼950 nm was irradiated with 0.5–1.2 MeV helium ions at room temperature. Another piece (sample II) with same thickness was mounted behind to receive the irradiation of transmitted helium ions (∼0.026–0.537 MeV). Morphology evolutions on irradiated surfaces were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results show that the open cracks, which were located mainly at grain boundaries, occurred on the surfaces of both irradiated samples. Interestingly, ripple patterns were observed to be regularly arranged on the front surface of sample I. The compressive stress resulting in the sliding on close-packed (1 1 1) planes was regarded as the origin of the ripple formation. Moreover, protrusion islands and its surrounding microstructures were observed on the front surface of sample II. The mass transport driven by the lateral stress generated in the helium ion irradiation were discussed as possible reasons.

Effects of Stress Ratio and Microstructure on Fatigue Failure Behavior of Polycrystalline Nickel Superalloy

The effects of microstructure and stress ratio on high cycle fatigue of nickel superalloy Nimonic 80A were investigated. The stress ratios of 0.1, 0.5 and 0.8 were chosen to perform fatigue tests in a frequency of 110 Hz. Cleavage failure was observed, and three competing failure crack initiation modes were discovered by a scanning electron microscope, which were classified as surface without facets, surface with facets and subsurface with facets. With increasing the stress ratio from 0.1 to 0.8, the occurrence probability of surface and subsurface with facets also increased and reached the maximum value at R = 0.5, meanwhile the probability of surface initiation without facets decreased. The effect of microstructure on the fatigue fracture behavior at different stress ratios was also observed and discussed. Based on the Goodman diagram, it was concluded that the fatigue strength of 50% probability of failure at R = 0.1, 0.5 and 0.8 is lower than the modified Goodman line.

Fatigue deformation in a polycrystalline nickel base superalloy at intermediate and high temperature: Competing failure modes

The microstructural configurations that favor early strain localization and fatigue crack initiation at intermediate and high temperature (400 °C–650 °C) have been investigated using novel experimental techniques, including high resolution digital image correlation and transmission scanning electron microscopy. Cyclic fatigue experiments in the high and low cycle fatigue regimes have been performed on a René 88DT polycrystalline nickel-base superalloy at temperatures up to 650 °C and compared to previous fatigue results obtained from tests in the very high cycle fatigue regime. Competing failure modes are observed along with an inversion in the temperature fatigue life dependence of fatigue strength from the low to high cycle fatigue regime. Oxidation-assisted processes are dominant at high applied stresses while cyclic plastic localization and accumulation govern fracture at low applied stresses. In addition, a second competing mode exists in the high and very high cycle fatigue regime from non-metallic inclusions as compared to internal intrinsic initiation sites. The grain-scale features that exhibit strain localization and crack initiation were investigated in detail. Transmission electron microscopy (TEM), transmission scanning electron microscopy (TSEM) and electron channeling contrast imaging have been conducted on samples removed from targeted regions with microstructural configurations that favor crack initiation to characterize the associated dislocation sub-structure and its evolution with temperature. Plasticity is observed to be less localized during cyclic loading at high temperature compared to room temperature. The microstructural features that drive initiation across the temperature range investigated are: twin-parent grains pairs that are at the upper end of the size distribution, are oriented for near maximum elastic modulus mismatch, and have high stresses along planes parallel to the twin boundaries.

Fast synthesis of high-quality large-area graphene by laser CVD

Few layer graphene (FLG) has attracted tremendous interest in recent years because of its unique properties. However, how to synthesize graphene quickly and control the layer number is an important issue. Herein, high-quality and layer-controlled graphene was obtained on polycrystalline nickel foil by laser chemical vapor deposition (LCVD) within a few minute, which is a cold-wall CVD type based on irradiation of a continuous wave laser with Super-Gaussian distribution spot. The number of graphene layers and quality was controlled by controlling the growth time, laser power and cooling rate. The growth mechanism of the FLG has also been discussed.

Microstructure evolution and mechanical performance of nickel based superalloy C1023 at elevated temperatures

Microstructure evolution of polycrystalline nickel based superalloy C1023 casting was investigated by comparing as cast superalloy and heat treated superalloy in the paper. Influences of strengtheners and carbides on the tensile behaviors of heat treated superalloy at different temperatures (20 °C, 650 °C, and 850 °C) were studied. Resulting data from deep corrosion suggested that growth mode of scripted carbide included nucleation on the vertex of blocky carbonitride, expansion of dendrite arm in C-rich area, and enlargement of dendrite cap. Decomposition of MC carbide generated numerous octahedral M23C6 carbide as well as nanoscale Cr-Mo-rich nuclei. Diffusion of Mo from γ to precipitates such as γ', MC, and M23C6, occurred during the heat treatment while irregular γ' (4 nm–500 nm) transformed into cuboidal particles. At 650 °C, no significant deterioration of tensile strength was found while plastic deformation ability of γ base was improved. Oxidation of grain boundaries (GBs) dominated the crack behavior of the superalloy at 850 °C.

Unexpected pressure induced ductileness tuning in sulfur doped polycrystalline nickel metal

The sulfur induced embrittlement of polycrystalline nickel (Ni) metal has been a long-standing mystery. It is suggested that sulfur impurity makes ductile Ni metal brittle in many industry applications due to various mechanisms, such as impurity segregation and disorder-induced melting etc. Here we report an observation that the most ductile measurement occurs at a critical sulfur doping concentration, 14 at.% at pressure from 14 GPa up to 29 GPa through texture evolution analysis. The synchrotron-based high pressure texturing measurements using radial diamond anvil cell (rDAC) X-ray diffraction (XRD) techniques reveal that the activities of slip systems in the polycrystalline nickel metal are affected by sulfur impurities and external pressures, giving rise to the changes in the plastic deformation of the nickel metal. Dislocation dynamics (DD) simulation on dislocation density and velocity further confirms the pressure induced ductilization changes in S doped Ni metal. This observation and simulation suggests that the ductilization of the doped polycrystalline nickel metal can be optimized by engineering the sulfur concentration under pressure, shedding a light on tuning the mechanical properties of this material for better high pressure applications.

Creep deformation mechanisms and CPFE modelling of a nickel-base superalloy

Different cooling paths from a supersolvus temperature have been applied to FGH96, a polycrystalline nickel base superalloy for turbine disc applications, in order to simulate the different microstructures that exist through the thickness of a disc following an industrial heat treatment. The microstructures have been evaluated in terms of γ’ size distribution, morphology and volume fraction for the different heat treatments using SEM and post digital image software. The results illustrate that γ’ size decreases with cooling rate and interrupted cooling leads to a higher tertiary γ’ but lower secondary γ’ volume fraction. The heat treated samples were creep tested under 690 MPa at 700 °C. It was found that the creep response it inversely proportional to secondary γ’ size. Tertiary γ’ volume fraction was also found having a great impact on the material's creep properties. The TEM micrographs show that matrix dislocations could be the precursor of microtwin formation and creep deformation modes are closely related to tertiary γ’ precipitates. Tertiary γ’ size determines whether a/2<110> dislocations shear or dissociate before entering tertiary γ’ and the tertiary γ’ volume fraction determines how matrix dislocation dissociates. A CPFE model has been developed based on the experimental results. The simulation results for both the single element and the poly-crystalline model indicated that the modelled secondary stage creep rate is in very good agreement with the experimental results. The modelling enables the consequences of this to be investigated for representative different spatial microstructural disc conditions.