Coarse-grained Nickel Research Articles

Effect of strain of cryorolling on structure and strength of nickel

The effect of isothermal rolling at liquid nitrogen temperature with reductions up to 90 % on the structural-mechanical behavior of coarse-grained nickel was evaluated by TEM, SEM-EBSD and X-ray analysis methods, and microhardness measurements. The highest hardening rate of the metal was found at the initial stage of rolling (up to about 50 %), after which it decreased slightly with further reductions. Detailed analyses of the evolution of dislocation density, grain size and grain boundary spectra have shown that the behavior found was attributed to an extremely low deformation temperature, resulted in a decreased rate of accumulation of crystalline defects, as well as the formation and rearrangement of dislocation structure. Due to the strong suppression of dynamic recovery and recrystallization, even when rolling to a reduction of 90 %, a relatively homogeneous coarse-fibered structure with a developed nanocellular substructure and less than 5 % fraction of nanoscale dynamically recrystallized grains was formed. It was concluded that the main structural strengthening factors in cryogenically rolled nickel was the formation of low-energy dislocation structures and their transformations into a well-developed substructure with nanosized crystallites separated predominantly by low-angle boundaries.

ПРИМЕНЕНИЕ СКАНИРУЮЩЕЙ ТУННЕЛЬНОЙ МИКРОСКОПИИ ДЛЯ ОЦЕНКИ СТЕПЕНИ НЕРАВНОВЕСНОСТИ ГРАНИЦ ЗЕРЕН В НИКЕЛЕ, ПОЛУЧЕННОМ КРУЧЕНИЕМ ПОД ВЫСОКИМ ДАВЛЕНИЕМ

The goal of the study is to estimate the relative free energy of grain boundaries in nickel deformed by high-pressure torsion (HPT) using scanning tunneling microscopy. In the process of work, the following studies have been carried out. The technique has been developed for sample preparation of Ni samples subjected to severe plastic deformation (SPD) by the HPT method for subsequent studies by scanning tunneling microscopy, which provides an average value of the root-mean-square surface roughness of approximately 2 nm. Using scanning tunneling microscopy, images of the nickel surface structure have been obtained, which have been processed using the Gwyddion application program. As a result, the grain boundary profiles have been calculated, based on which the values of the dihedral angles at the bottom of the etching grooves have been calculated and the relative energies of the grain boundaries have been calculated. It has been established that after HPT deformation, the average relative energy of grain boundaries significantly exceeds the relative energy of grain boundaries of coarse-grained nickel with grain boundaries of recrystallization origin. Thus, because of HPT, non-equilibrium grain boundaries are formed in the samples. It has been shown that with an increase in the degree of HPT deformation, an increase in the average relative energy of grain boundaries occurs, which indicates an increase in the degree of their non-equilibrium. It has been demonstrated that SPD by the HPT method makes it possible to achieve a higher level of relative energy of grain boundaries than deformation by the equal-channel angular pressing (ECAP) method.

Ultrasonic Welding of Nickel with Coarse and Ultrafine Grained Structures

Solid state joints of samples of coarse-grained (CG) and ultrafine-grained (UFG) nickel have been obtained for the first time using spot ultrasonic welding (USW). The UFG structure in disk-shaped samples was processed by means of high-pressure torsion (HPT). On the basis of lap shear tests, the optimal values of the clamping force resulting in the highest values of the joint strength are determined. The microstructures in the weld joints obtained at optimal parameters of USW are characterized by scanning electron microscopy. It is shown that during ultrasonic welding of coarse-grained nickel, a thin layer with an UFG microstructure is formed near the weld surfaces. The bulks of sheets retain the CG microstructure, but a significant dislocation activity is observed in these regions. During USW of samples having an UFG initial microstructure, significant grain growth occurs. Fine grains are observed only along the welding interface. An average lap shear strength of 97 MPa was obtained by welding the UFG samples, which was approximately 40% higher than the strength of samples processed by welding coarse-grained sheets (70 MPa). It is concluded that higher strength weld joints can be obtained by using sheets with the UFG structure as compared to the CG sheets.

Ultrastrong nanotwinned pure nickel with extremely fine twin thickness.

The strength of nanocrystalline and nanotwinned metals stops increasing or even starts decreasing when their grain size or twin thickness is below a critical size-a phenomenon known as Hall-Petch breakdown-which hinders the attainment of ultrahigh strength. Here, we report continuous strengthening in nanotwinned pure Ni with twin thicknesses ranging from 81.0 to 2.9 nm. An unprecedented strength of 4.0 GPa was achieved at extremely fine twin thickness of 2.9 nm, which is about 12 times stronger than that of conventional coarse-grained nickel. This ultrahigh strength arises from the excellent stability of twin boundaries and their strong impedance to dislocation motion. In particular, we find that secondary nanotwins are activated to sustain plastic deformation, which also contribute to the high strength. These results not only advance the understanding of the strengthening mechanisms in nanotwinned metals but also offer an alternative pathway to develop engineering materials with ultrahigh strength.

Effect of ultrasonic treatment on the structure of coarse-grained nickel

Commercially pure coarse-grained nickel was studied. On the electrically polished surface of four samples an area 1 × 1 mm2 in size was marked by a special indenter and studied by EBSD analysis. All the samples were subjected to ultrasonic treatment (UST) with stress amplitudes of 20–80 MPa. The marked area was studied after that. In all the samples, X-ray diffraction analysis, TEM study, microhardness measurements and surface roughness analysis were carried out before and after the UST. It was shown that the UST results in an increase in dislocation density, an increase in the fraction of low-angle boundaries, an increase in microhardness and root-mean-square microstrain. The values of these parameters depend on the UST amplitude.

Ion sputtering rate of nanostructured FCC, BCC and HCP metals processed by severe plastic deformation

The effect of nanostructuring on the ion sputtering rate of metals and the glow discharge current is studied. Nanostructured samples were processed by severe plastic deformation using high pressure torsion. For comparative measurements, coarse-grained samples were obtained by annealing of nanostructured samples. The sputtering rates of nanostructured and coarse-grained nickel, iron and zirconium were measured after ion sputtering in glow gas-discharge. It has been found that nanostructuring of metals leads to an increase in the ion sputtering rate and the glow discharge current. Possible causes of changes in the ion sputtering rate due to nanostructuring are analyzed.

Structure evolution in coarse-grained nickel under ultrasonic treatment

Changes in the structure of coarse-grained nickel due to ultrasonic treatment (UST) are studied. For this purpose, commercially pure nickel was annealed at a temperature of 1300 °C for 1 h to obtain a coarse-grained structure with a low dislocation density. On the electrolytically polished surface of the sample, a 2 × 2 mm2 sized area was marked and investigated by EBSD analysis. Further, the sample was subjected to UST with an oscillating stress amplitude of 20 MPa, and the marked area was polished and scanned again. X-ray diffraction analysis and microhardness measurements were carried out before and after UST as well. It is shown that UST leads to dislocation generation, an increase in the fraction of low-angle boundaries, changes in the shape of twins and grain boundaries. The dependence of structural parameters and microhardness on the UST amplitude is also described.

Reduced hardening of nanocrystalline nickel under multiaxial indentation loading

Abstract

Effect of Plastic Deformation of the Initial Components and Particle Size Reduction on the Structure and Properties of the PN85YU15-Ni Composite Material Produced by Spark Plasma Sintering

Structure and mechanical properties of the PN85YU15 - Ni composite materials obtained by spark plasma sintering were investigated. Two types of powder mixtures, namely, nickel mixed with coarse-grained nickel aluminide and nickel mixed with fine-grained nickel aluminide were used to obtain the composites. Nickel aluminide and nickel powders were taken in the ratio 7:3 respectively. The effect of the initial nickel aluminide particle sizes and plastic deformation due to the ball milling on the structure and mechanical properties of materials sintered at 1100 °C and pressure of 40 MPa was determined. Plastic deformation and refining the initial intermetallic powder particle sizes leads to increasing the sintered material relative density to 95%. The tensile strength of the PN85YU15-Ni composite material obtained by sintering of the milled PN85YU15 powder and nickel in the ratio 7:3 was 1060 MPa. This value is almost twice as high as the tensile strength of the composite containing a no significant plastic deformed coarse-grained intermetallic compound powder (590 MPa), and three times higher than the tensile strength of the sintered nickel aluminide powder (380 MPa).

The formation of a multipeak relief on the surface on nanostructured nickel and field electron emission from it

Studies of the relief formed on the surface of nanostructured nickel with nonequilibrium grain boundaries formed upon ion-beam sputtering are presented. It has been shown that the relief on the nano-structured nickel surface differs substantially from the relief formed on the surface of the coarse-grained nickel with equilibrium grain boundaries. In particular, a multipeak relief with submicron dimensionality of the peaks forms on the surface of nanostructured nickel under certain conditions. This relief is advantageous for the field electron emission. For the sample with multipeak relief, a CVC of field electron emission is measured.

A methodology for evaluating detection performance of ultrasonic array imaging algorithms for coarse-grained materials.

Improving the ultrasound inspection capability for coarse-grained metals remains of longstanding interest and is expected to become increasingly important for next-generation electricity power plants. Conventional ultrasonic A-, B-, and C-scans have been found to suffer from strong background noise caused by grain scattering, which can severely limit the detection of defects. However, in recent years, array probes and full matrix capture (FMC) imaging algorithms have unlocked exciting possibilities for improvements. To improve and compare these algorithms, we must rely on robust methodologies to quantify their performance. This article proposes such a methodology to evaluate the detection performance of imaging algorithms. For illustration, the methodology is applied to some example data using three FMC imaging algorithms; total focusing method (TFM), phase-coherent imaging (PCI), and decomposition of the time-reversal operator with multiple scattering filter (DORT MSF). However, it is important to note that this is solely to illustrate the methodology; this article does not attempt the broader investigation of different cases that would be needed to compare the performance of these algorithms in general. The methodology considers the statistics of detection, presenting the detection performance as probability of detection (POD) and probability of false alarm (PFA). A test sample of coarse-grained nickel super alloy, manufactured to represent materials used for future power plant components and containing some simple artificial defects, is used to illustrate the method on the candidate algorithms. The data are captured in pulse-echo mode using 64-element array probes at center frequencies of 1 and 5 MHz. In this particular case, it turns out that all three algorithms are shown to perform very similarly when comparing their flaw detection capabilities.

Влияние условий электроосаждения никеля на физико-химические и коррозионные свойства покрытий

Galvanic coating of steel products is one of the main methods of corrosion protection, and nickel is one of the most common metals used in the process. Nickel-based electrochemical coatings are widely used as corrosion-resistant and protective coatings. This study focuses on the investigation of formation of nickel-based galvanic coatings under various conditions: at different temperatures, current densities, and using various mixing methods. The coatings were produced by the process of galvanostatic deposition using Watts nickel electrolyte. The effects of the conditions of electrodeposition process on the characteristics and the corrosion resistivity of the resulting coatings were studied. By using optical microscopy, a relation between the electrolysis conditions and the surface morphology or the size of the growing nickel crystals was discovered. It was established that increase of current density increases the microhardness of the coatings, due to the formation of coarse-grained nickel deposits. Highest corrosion resistivity is achieved by creating finely-grained nickel coatings, which are formed under low current density, low temperature and with mechanical mixing. The optimal conditions for producing high quality nickel coatings were established as follows: current density of 25 mA/cm 2 , temperature 20 o C, and using mechanical stirring. The nickel coatings, deposited under these conditions, have a high quality and the process of electrodeposition of nickel characterized by a high current efficiency.

In situ observations of crack propagation and role of grain boundary microstructure in nickel embrittled by sulfur

In situ observations of crack propagation in sulfur-doped coarse-grained nickel were performed for the specimens with grain boundary microstructure pre-determined by SEM/EBSD analysis. The role of grain boundary microstructure was studied in the crack propagation in nickel embrittled by grain boundary segregation of sulfur. It was found that the main crack tends to predominantly propagate along random boundaries, and the crack propagation rate can be locally accelerated at the grain boundary network with a high connectivity of random boundaries. On the other hand, the cracks can propagated along fracture-resistant low-Σ coincidence site lattice (CSL) boundary only when the trace of the grain boundary is arranged being almost parallel to slip bands in the adjacent grains. The local crack propagation rate was found to become lower when a crack propagated along low-Σ CSL boundaries. Moreover, when the crack propagation is inhibited by low-Σ CSL boundaries, the branching of propagating crack occurs at partially cracked triple junctions. The crack propagation can locally slow down due to the occurrence of crack branching. The optimum grain boundary microstructure for the control of sulfur segregation-induced brittle fracture is discussed on the basis of new findings obtained from the in situ observations on crack propagation and fracture processes in polycrystalline nickel.

Tribological Behaviors of Nanocrystalline Nickel Coatings under Lubricated Conditions

In this paper, nickel coatings (on phosphor bronze substrates) were prepared using pulse electrodeposition method. The friction and wear behaviors of nanocrystalline nickel coating compared with coarse-grained nickel coating against AISI 52100 steel were investigated using l-butyl-3-octylimidazolium tetrafluoroborate ionic liquid and perfluoropolyether as a reference lubricant on reciprocating ball-on-flat UMT-2MT sliding tester. The morphologies of the worn surfaces were observed using a scanning electron microscope. The chemical states of several typical elements on the worn surfaces were examined by means of X-ray photoelectron spectroscopy. The results indicated that NC nickel coatings have better tribological properties than that of coarse-grained nickel coatings under lubricated conditions. This was partly attributed to the high hardness of the NC nickel coating and L-B408 ionic liquid formed tribochemical reaction films under NC nickel coatings.

Deformation Twinning in Pure Nickel Induced by a High-Current Pulsed Electron Beam

Recent experimental evidence has shown that face-centered-cubic materials that are not normally associated with deformation twinning will twin during moderate high-current pulsed electron beam (HCPEB) processing. In this paper, we present an experimental investigation of deformation twinning in pure multi-crystalline nickel exposed to HCPEB irradiation from a Nadezhda-2 HCPEB device. The samples, which were irradiated with various numbers of pulses, were examined. The formation of a large number of twin lamellae on the treated surface was observed in the regions without surface melting. Moreover, a large strain located at the twin lamellae deformation was found. Transmission electron microscope analysis revealed that the deformation twinning was indeed triggered during the HCPEB irradiation. This suggests that the very high stresses and superfast strain rates induced via the rapid heating and cooling caused by the HCPEB irradiation lead to deformation twinning in coarse-grained nickel.

Isothermal oxidation kinetics of nanocrystalline and coarse grained nickel: Experimental results and theoretical approach

In this research, isothermal oxidation kinetics of nanocrystalline (NC) nickel prepared by electroplating as well as coarse grained (CG) nickel was investigated. Oxidation tests were carried out in static air for various durations at 700 °C, 800 °C and 900 °C. The structure of oxidized specimens was characterized by means of SEM and XRD. Moreover, a theoretical model based on Atkinson approach as well as Maxwell Equation was proposed. The derived model was checked by fitting the theoretical curves to the experimental data using Matlab software. Good agreement between theoretical model and obtained experimental results as well as reported data was observed. The results indicated that the weight gain per unit area (w = ∆m/A) of NC samples was considerably higher than the CG nickel. Moreover, the decrease in the parabolic growth rate constant (k p) of NC nickel by increasing the oxidation time was sharper than the microcrystalline samples. In spite of the overall data of log (k p), the logarithms of k p values corresponding to a given w was a linear function of the reciprocal temperature resulted an activation energy of ~ 108 kJ/mol for oxide layer growth on NC nickel as well as CG samples. In addition, the values for the activation energy for grain boundary diffusion of Ni in NiO layer which are determined from the fitting parameters for NC and CG nickel, both were close to that for the growth of NiO layer.

Curie Transition of NC Nickel by Mechanical Spectroscopy and Magnetization Study

Mechanical spectroscopy measurement is performed to study the internal friction of nanocrystalline (NC) nickel with an average grain size of 23 nm from room temperature to 610 K. An internal friction peak is observed at about 550 K, which corresponds to the Curie transition process of the NC nickel according to the result of magnetization test. Moreover, the fact that the Curie temperature of NC nickel is lower than that of coarse-grained nickel is explained by an analytical model based on the weakening of cohesive energy.

A Comparative Study on the Tribological Behavior of Nanocrystalline Nickel and Coarse-grained Nickel Coatings under Ionic Liquid Lubrication

Tribological properties of nanocrystalline (NC) nickel coating and coarse-grained nickel coating were evaluated using a ball-on-flat sliding tester. Results indicated that the NC nickel coating had excellent tribological properties. This was partly attributed to the high hardness of the NC nickel coating and the tribochemical interactions between rubbing pairs and ionic liquid.

Comparing the Role of Dislocations during Plastic Deformation of Nanocrystalline Nickel and Coarse-Grained Nickel during Cold Rolling

AbstractWe investigated plastic deformation of fully dense electrodeposited nanocrystalline (NC) Ni with an average grain size of 30-40nm. We studied the microstructural evolution during cold rolling of NC Ni to a reduction in thickness up to 76% (true strain equivalent ∼1.42). We determined changes in texture, grain morphology, grain boundary character and grain sizes as a function of cold rolling strain, using X-ray diffraction and transmission electron microscopy TEM. We compared our results of the NC Ni with our own results for cold rolled coarse grained (CG) Ni. Differences and similarities in deformation behavior are discussed with respect to well documented findings in the literature.

Effect of nanocrystalline grain size on the electrochemical and corrosion behavior of nickel

Nanocrystalline nickel of different grain sizes (8–28 nm) was produced by electrodeposition using Watt's bath. Saccharine addition to the bath and pulsed current deposition were effective in lowering the grain size of the deposits. The grain size and microstrain of deposits was determined by X-ray diffraction analysis. The microhardness of nanocrystalline Ni ranged between 572 and 724 kg/mm 2. The electrochemical behavior of nanocrystalline Ni was evaluated in 1 mol/l H 2SO 4 and compared with that of coarse-grained nickel. All the nickel samples exhibited active–passive potentiodynamic polarization behavior. The zero current potential, passive current density and breakdown potential generally increased with decrease in grain size. The increased passive current density for nanocrystalline nickel confirmed the defective nature of passive film that forms on nanocrystalline nickel. The tendency for localized corrosion was lower in case of nanocrystalline nickel as indicated by increased breakdown potential. Tafel and linear polarization tests revealed that the corrosion rate of freshly exposed surfaces of Ni decreased with grain size, thereby indicating greater hindrance to anodic dissolution in nanocrystalline Ni. The magnitude of compressive microstrain in the Ni deposits increased with decrease in grain size.