Nickel Specimens Research Articles

Effective electronic properties and coupling for two-temperature model-molecular dynamics simulation of ultrafast laser ablation of nickel

ABSTRACT The accuracy of hybrid two temperature model-molecular dynamics (TTM-MD) simulations in predicting the ultrafast laser ablation phenomena in metals is influenced by the functional definition of subsystem properties and electronic-lattice coupling. In this work, laser ablation of nickel is simulated using hybrid TTM-MD with empirical and density function theory (DFT) derived electronic and coupling parameter sets. The investigation revealed that the empirically derived temperature-dependent parameterisation of electronic specific heat, thermal conductivity, and coupling factor enhanced the fidelity of electron–phonon nonequilibrium dynamics when used in conjunction with simple optical absorption models, aligning closely with prior experimental observations for nickel specimens. The TTM-MD setup is then coupled utilising two widely used coupling techniques – the temperature difference scaled and Langevin thermostat. It is found that the former exhibits increased tensile stress due to artificial enhancement of the collision cascade owing to deterministic nature of the coupling force. While the probabilistic nature of the Langevin thermostat offers more accurate predictions of the ablation threshold/yield. Under these conditions, the TTM-MD scheme is tested for a range of absorbed laser fluence, with ablation threshold, and the onset of phase explosion is determined as 115 and 270 mJ/cm2, respectively for a 1 picosecond (ps-) laser pulse.

Current rate flash sintering of nickel at ambient temperature in <1 min

AbstractWe show that powder pressed specimens of nickel can be sintered to 99.96% density by injecting electrical current, without the use of a furnace. Full sintering could be accomplished in 10 to 52 s by changing the current rate from 5 to 1 A/s. In all instances, the samples sintered abruptly at a current density of ∼20 A mm−2. The grain size of the sintered samples was somewhat larger than the nickel powder particle size (∼60 μm vs. 40 μm). Tensile testing yielded a yield strength of 98 MPa, ultimate tensile stress of 323 MPa, and ductility of ∼17%. Four in‐operando measurements are reported: (i) sintering, (ii) the change in resistance with current density, (iii) the temperature, and (iv) electroluminescence. The change in resistance during flash sintering exhibited a high peak followed by a steep decline in resistance; the transient is attributed to the breakdown of particle–particle interface resistance. The same cycle repeated with the flash‐sintered, dense sample, did not show the spike, and gave reproducible results. The resistance data for these latter cycles, when viewed as a function of temperature exhibited sigmoidal behavior: initially lower, and then higher than the literature values. This unusual behavior reflects the influence of defects generated during flash. We have also measured the endothermic enthalpy, expressed by the difference between the in situ input electrical energy and the radiation, convection, and specific heat losses. Dividing by the formation energy of Frenkel pairs yields the concentration of defects, estimated to be 0.3–0.4 mol %. These concentrations are far above thermal equilibrium; it is concluded that flash of metals is a far from equilibrium phenomenon.

POOL BOILING HEAT TRANSFER CHARACTERISTICS OF POROUS NICKEL MICROSTRUCTURE SURFACES

Pool boiling is an effective heat dissipation approach in electronic cooling, battery thermal management, etc. This study used the electrochemical deposition method to fabricate one smooth nickel specimen (named Ni-smooth) and three specimens with a porous nickel-stacked structure. The three porous specimens were created with deposition current densities of 0.5 A·cm<sup>-2</sup> (named Ni-0.5), 2.0 A·cm<sup>-2</sup> (names Ni-2.0), and 5.0 A·cm<sup>-2</sup> (named Ni-5.0), respectively. The four samples underwent microstructural characterization via scanning electron microscopy. The increasing current density led to the porous nickel surface exhibiting a more distinct pore structure, and the nickel sphere grains became more refined, developing a loose "mound-like" structure. A marked increase in the nickel film thickness was also observed. Through visual experiments, we evaluated their wettability, and through pool-boiling experiments, we tested their boiling heat-transfer properties. Our findings suggest that samples incorporating a porous nickel structure consistently outperform unmodified samples regarding heat-transfer efficiency. Specifically, sample Ni-0.5A demonstrated the most optimal boiling heat-transfer performance, evidenced by a 32.2% reduction in temperature at the onset of boiling, a 19.9% increase in critical heat flux density, and a 78.6% larger maximum heat-transfer coefficient compared to the smooth nickel sample. These marked improvements are intrinsically linked to the specific characteristics of the porous nickel structure. The higher performance of samples Ni-0.5 can be attributed to the presence of additional nucleation sites within the porous structure and the formation of smaller micro-crystalline dendritic constructs due to the specific current density applied during electrodeposition. Understanding this relationship between surface characteristics and electrodeposition is essential in maximizing heat-transfer efficiency.

Effect of elastic-plastic deformation by biaxial tension on the magnetic characteristics of the nickel

The studies were carried out in a test facility based on the original biaxial testing machine created at the IES UB RAS, designed to determine the physical properties of materials during elastic-plastic deformation independently along two axes. Finite element modeling of the stress-strain state of a cruciform nickel specimen has shown that, under symmetric biaxial tension, equivalent von Mises stresses are uniformly distributed in the central part. The results of microstructural studies indicate that dynamic recrystallization takes place in the material during biaxial deformation at room temperature. The results of studying the behavior of the hysteretic magnetic characteristics, magnetostriction, Barkhausen magnetic noise parameters, and electromagnetic parameters of nickel under biaxial symmetrical tension have been presented. A unique relationship between the coercive force and equivalent stresses in the region of plastic deformation has been revealed. It has been shown that such magnetic parameters as maximum magnetic permeability and the maximum in the field dependence of differential magnetic permeability, as well as the amplitude and frequency of the eddy current signal determined at a frequency of 10 kHz, vary monotonically with increasing equivalent stresses under both elastic and plastic deformations.

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.

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.

Impact of corrosion on mechanical properties of additively manufactured Invar 36

AbstractInvar alloys are used for high‐precision applications due to their invariance of the coefficient of thermal expansion over a temperature range up to about 250 °C. The present study determines the mechanical and corrosive properties of Invar 36 (64 % Iron – 36 % Nickel) specimens produced by laser powder bed fusion. Tensile specimens were fabricated to investigate the mechanical properties. The influence of a horizontal and vertical building orientation is considered. An apparent mechanical anisotropy depending on the respective building is observed. Based on this, further tensile specimens were stored in a hydrochloric acid pickling solution, a sulphate buffer solution and a sodium chloride solution to analyse the corrosion behaviour. The findings after eight weeks in the individual media show that outsourcing the Invar 36 specimens in a sulphate buffer solution results in significant changes in the mechanical properties, especially fracture elongation.

In situ transient Laue x-ray diffraction during high strain-rate tension.

In situ transient synchrotron Laue x-ray diffraction based on high-energy and broadband x rays under high strain-rate tensile loading was developed at a superconducting wiggler beamline at the Beijing Synchrotron Radiation Facility. A split-Hopkinson tensile bar is utilized to realize this dynamic loading condition, while the transient Laue x-ray diffraction captures the transient internal structure of monocrystalline materials. Plastic deformation of a monocrystalline nickel specimen was investigated to prove the ability of this instrumentation in the characterization of a dynamic response of monocrystalline materials during a high strain-rate impact process with 5 µs time resolution.

The deformation behaviour of electrodeposited nanocrystalline Ni in an atomic force microscope with a newly developed in situ bending machine

Abstract Four-point-bending and strain-controlled fatigue experiments were performed on electrodeposited nanocrystalline nickel specimens at room temperature. For the first time, the deformation behaviour of bulk nanonickel specimens was studied in an atomic force microscope with a newly developed in situ bending machine. Results show that grain boundary sliding plays an important role during deformation, and the fatigue crack propagates along grain boundaries.

Evaluation of the Magnet Breakaway Force Measurement Accuracy of the NT-800 Sensors for Early Detection of Defects of Their Manufacturing

Сontrol of mechanical stresses formed with the deposition of nickel coatings plays an important role in the diagnosis of coatings’ technical condition. Large internal stresses can lead to cracking or flaking of coatings which is completely unacceptable for critical parts and assembly units used, for example, in space technology for which reliability is of paramount importance. An important aspect of internal stresses monitoring is the measurement error of the instruments used. The purpose of this work was to determine the characteristics of the device sensors, which make the assessment of their manufacturing possible at the preliminary stage of the measuring equipment assembling in order to maintain the required accuracy of subsequent measurements.In most cases the measurement error assessment is possible only after the equipment manufacture and calibration. In this paper it is proposed to evaluate the accuracy characteristics of device sensors based on the precision (repeatability and reproducibility) of the primary informative parameter recording. In the case of the NT-800 device that was developed at the Institute of Applied Physics of the National Academy of Sciences of Belarus the effect of precision characteristics deterioration on the eventual measurement error is demonstrated. Determining the precision parameters before establishing correlation dependences between the primary informative parameter and the measured characteristic is proposed in order to reject poorly manufactured sensors and reduce labor costs.In particular, measurements of the magnitude proportional to the magnetic breakaway force were carried out using the NT-800 device with nickel specimens simulating coatings with a thickness of 200 to 700 μm and a rolling value from 0 to 40 %. It was established that in the case of well-made sensors the variation coefficient calculated from the dispersion of repeatability is in the range 0.2–0.6 %, and the variation coefficient calculated from the dispersion of reproducibility does not exceed 0.9 %. In the case of a sensor with the sensitive element parameters worsened, the variation coefficient of repeatability and reproducibility were up by one and a half times. Deterioration of the precision characteristics resulted in significant changes in the readings of the calibrated instrument. Thus the absolute measurement error for a sensor with a poorly made sensitive element turned out to be approximately 3 times higher in the range of 200– 300 MPa than that for a sensor with good precision parameters.

Hydrogen assisted crack initiation in metals under monotonic loading: A new experimental approach

Hydrogen is foreseen as a promising energy carrier that can control global warming by reducing CO2 emissions. However, hydrogen is associated with an embrittlement phenomenon that imparts substantial damage to the infrastructure by reducing the ductility, fracture strength, strength bearing capacity, etc., of metallic components. Therefore, understanding hydrogen-induced crack initiation mechanisms in metals are of prime importance. Greater insights into this critical phenomenon are expected if the hydrogen-induced crack initiation can be correlated with the local microstructure and corresponding stress-strain state towards their propensity for hydrogen accumulation. With this motivation, in this work, crack initiation is studied for the uncharged and hydrogen charged nickel specimens during in-situ tensile loading under the scanning electron microscope. By assuming the material to be elastic at low strains, a novel approach is implemented for generating the microstructural stress maps through strain and stiffness tensor extracted at each point in the region of interest on the specimen surface using high-resolution digital image correlation (HR-DIC) and Euler angles (given by electron backscattered diffraction data), respectively. Based on this analysis at low strain, the crack initiation sites for uncharged and hydrogen charged nickel specimens are correlated with microstructural maps of maximum Schmid factor, elastic modulus in the loading direction, hydrostatic stress, von Mises stress, and triaxiality factor. The analysis highlighted two independent factors responsible for hydrogen enhanced decohesion (HEDE) based intergranular failure observed only at the random grain boundaries, (i) strain localization due to hydrogen enhanced localized plasticity (HELP) mechanism of hydrogen embrittlement, and (ii) hydrostatic stress-based hydrogen diffusion to the crack initiation sites. These critical insights thus can help to design hydrogen embrittlement resistant metals. In addition, the novel experimental approach can be used to calibrate advance micromechanical models while providing quantitative estimate of the hydrogen distribution in realistic metallic microstructure responsible for hydrogen-assisted crack initiation with deformation.

Informative properties of the envelope of the magnetoelastic acoustic emission signal

Parameters of the envelope of the magnetoelastic acoustic emission signal are proposed to be used as informative ones. To study the properties of the envelope of the signal, experimental studies on steel St.3 and nickel specimens are investigated. The signal envelopes for a number of values of the amplitude of the remagnetizing field induction are estimated by several imple¬mentations. It is shown that the shape and duration of the envelope of the magnetoelastic acoustic emission signal depends on the amplitude of the remagnetization field induction, which requires ensuring their stability when using the parameters of the envelope as informative by diagnosing ferromagnetic objects. To test the new informative parameter, uniaxial tensile forces were applied to nickel and steel specimens of the same size and shape (for nickel the stresses were changed from 0 to 110 MPa, for steel to 280 MPa), the specimens were remagnetized with the outside field and the magnetoelastic acoustic emission signals were registered. Estimates signals of the envelope for different values of the applied load are found. The dependences of duration of the magnetoelastic acoustic emission signals on the magnitude of the applied stresses, which can be used as calibration curves for diagnosing residual stresses in ferromagnetic objects of long-term operation, are constructed.

Fabrication of nickel-graphene composites with superior hardness

Abstract Bulk nickel-graphene (Ni-Gr) composites are in the high demand for most of the engineering applications. Despite many efforts made, the main problem remains the homogeneous dispersion of graphene into the Ni matrix without destroying the structural integrity of Gr. The present paper addresses the fabrication, structural features and mechanical characteristics of bulk Ni-Gr composites, manufactured from nanosized nickel powder and exfoliated graphite platelets (GP) via modified powder metallurgy procedure including micromechanical GP splitting. The combination of XRD, SEM, Raman spectroscopy and EBSD analysis revealed homogeneous distribution of Gr in Ni matrix and the significant grain refinement observed for Ni-0.1 wt% Gr composite. Bimodal grain size distributions were obtained for all samples studied, and the mean grain size depends on Gr content. Raman spectroscopy proved the complete GP-to-graphene conversion for composite with 0.1 wt% Gr. Bulk nickel specimen and composites exhibit ∼3 times higher hardness and moderate tensile strength values compared to Ni, manufactured from micron-sized powders.

On the transition of fracture toughness in metallic materials with thickness: An atomistic viewpoint

From thick to very thin specimens, fracture toughness of metallic materials peaks out before hitting much lower value compared to the constant plane strain fracture toughness value obtained using standard fracture mechanics testing of thick specimens. Understanding this behavior is essential for improving reliability of small-scale metallic devices currently being used for various critical applications. To understand this behavior, it is essential to study the process of dislocations emission and interaction at the crack front and its variation with specimen thickness. To this end, atomistic fracture simulations of pre-cracked single crystal FCC metal (Nickel) specimens representing both thick and thin specimens are performed. At first, stress-state dependent single crystal yield function based on the generalized Schmid-law is associated with the dislocation emission process at the crack front for both thin and thick specimens. Fracture simulations are then performed on single crystal specimens representing different thickness cases. Due to low stress triaxiality prevailing throughout the thickness of thin specimens, dislocation interaction with each other inside the specimen and then with the specimen surface leading to wedge-shaped groove formation on opposite surfaces at crack front is found to be the responsible mechanism of crack propagation in thin specimens. This mechanism provides enhanced fracture toughness to thin specimen compared to thick specimen in which crack propagation is based on high stress triaxiality at the core of the crack front making formation of microvoids and their coalescence as a dominant mechanism of crack propagation. The dislocation configurations generated at the crack front for thick and thin specimens are also studied and the mechanisms for dislocation multiplication in thin specimens compared to thick specimens are highlighted.

Predicting crack initiation site in polycrystalline nickel through surface topography changes

Fatigue loading generates plastic deformation and strain localization on the surface of polycrystalline metals, which are known to be the precursor for crack initiation. The plastic deformations manifest themselves in the form of surface roughening that can be characterized using surface profiling techniques. In this study, surface topology changes on the surface of a nickel specimen under cyclic loading are monitored and analyzed to investigate the correlation between surface topography changes and crack initiation. Defining the surface roughness parameters as the damage index, an algorithm was formulated to predict the crack initiation location well before the formation of a visible crack. In addition, the crack initiation time can also be determined based on the saturation of the surface roughness damage index as well as the rebound of the surface height near the crack initiation location.

In-situ Study of the Effect of Hydrogen on Fatigue Crack Initiation in Polycrystalline Nickel

Correlating hydrogen embrittlement phenomenon with the metallic microstructural features holds the key for developing metals resistant to hydrogen-based failures. In case of fatigue failure of hydrogen charged metals, in addition to the hydrogen-based failure mechanisms associated with monotonic loading such as HELP, HEDE etc., microstructural features such as grain size, type of grain boundary (special/random), fraction of special grain boundaries; their network and triple junctions can play a complex role. The probable sites for fatigue crack initiation in such metals can be identified as the sites of highest hydrogen concentration or accumulated plastic strain. To this end, we have developed an experimental framework based on in-situ fatigue crack initiation and propagation studies under scanning electron microscope (SEM) to identify the weakest link in the metallic microstructure leading to failure. In-situ fatigue experiments are performed on carefully designed polycrystalline nickel (99.95% pure) specimens (miniaturised, shallow-notched & electro-polished) using a 10 kN fatigue stage inside the SEM. Electron Back Scattering Diffraction (EBSD) map of the notched region surface helps identify the distribution of special/random grain boundaries, triple junctions and grain orientation. The specimen surface in the shallow notched region for both the hydrogen charged and un-charged specimens are then carefully studied to correlate the microstructural feature associated with fatigue crack initiation sites. Such correlation of the fatigue crack initiation site and microstructural feature is further corroborated with the knowledge of hydrogen trapping and grain’s elastic anisotropicity to be either the site of high hydrogen concentration, accumulated plastic slip or both.

Specific Features of Deformation of the Nitinol Alloy After Electrolytic Hydrogenation

Specific features of the effect of hydrogenation on the susceptibility of a Ni–Ti alloy with shape memory to deformation are determined with the use of metallographic, electrochemical, and mechanical studies. Three sections are detected in the tensile curves of the specimens of nickel–titanium alloy in the initial state. The first section is linear due to the elastic deformation of the alloy with initial austenitic structure. The second section is nonlinear and associated with pseudoelastic structural transformations of the original austenitic structure into a martensitic structure. The third section is also linear and caused by the elastic deformation of martensite formed in the course of deformation of austenite. After hydrogenation of the Ni–Ti alloy, the pseudoelastic structural transformation starts at a somewhat lower level of stresses than without hydrogenation. In this case, the specimens are destroyed after the termination of this transformation for a much lower level of plasticity than in the nonhydrogenated alloy. It is assumed that the electrolytic hydrogenation of the alloy promotes the formation of a very brittle hydride phase on the surface of Ti-type inclusions revealed in the structure of alloy in the initial state. Its thickness is determined by the duration of the process of hydrogenation rather than by the current used for hydrogenation.

Fatigue life assessment using stress gradient factor

In general, a fatigue life is assessed on SN curves obtained from small-sized smooth specimens. However, it is known that a fatigue life prediction at a stress concentration region based on a peak stress is always too conservative due to the stress gradient. To eliminate the conservatism and rationalize a fatigue design, the authors applied the stress-gradient approach proposed by Siebel to a Nickel based superalloy. Fatigue tests of notched and smooth round bar specimens of Nickel based superalloy were performed at 550℃. The relation between the stress gradient and the fatigue reduction rate was determined from the round bar fatigue tests. In addition, plate specimens with a fillet were subjected to fatigue tests to verify the stress-gradient approach. The predicted life well agreed with the experimental fatigue life.

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.

In Ukrainian

A method of synthesis of single-domain nanoparticles of (Fe 1-x Ni x )Fe 2 O 4 solid solutions of oxide ferrimagnetics (where х = 0÷1) has been developed. To suppress coagulation of particles during their synthesis, we used sodium chloride as an indifferent dispersant. After washing, the precipitate was recrystallized at 373 K for the time required for forming particles of a certain size. Then the precipitate was washed in acetone once more and kept in ethyl alcohol. The synthesized products are characterized by X-ray diffraction. Magnetic moment hysteresis loops were measured with use of a laboratory vibrating magnetometer at the room temperature. Demagnetized nanoparticles of (Fe 1-x Ni x )Fe 2 O 4 solid solutions were used as specimens. To suppress interparticle interactions, the nanoparticles were distributed in paraffin matrices. Materials with the known value of saturation magnetization (M s ) were used as the reference specimen: the tested specimen of nickel and the Fe 3 O 4 (98 %) nanoparticles fabricated by Nanostructured & Amorphous Materials Inc., USA. The dependences of saturation magnetization and coercive force on nickel content have been studied. The synthesized material may be applied in micro- and optoelectronics, electrical engineering, biomedicine and other fields.