Deformation Of Nickel Research Articles

Effect of grain boundaries and rigid inclusions on plasticity in nickel bicrystals containing helium bubbles and radiation-induced self-interstitial atom clusters

We use molecular dynamics to assess the effect of grain boundaries and rigid intergranular inclusions on plastic deformation in nickel (Ni) containing helium (He) bubbles and self-interstitial atom clusters. Our simulations show that plasticity in Ni bicrystals is relatively uniform, with no localized slip bands or nano-twins. We attribute this behavior to grain boundaries, which block dislocations and favor activation of new sources over persistent slip along a single plane. While there is no initiation of intergranular cracks, He bubbles at matrix/inclusion interfaces elongate along the tensile axis and migrate towards regions of high tension, potentially setting the conditions for formation of crack-like flaws via bubble coalescence. We discuss the implications of our work for understanding degradation of mechanical properties in Ni-base alloys in nuclear reactors.

Theoretical Analysis of the Distortion Tensors During the Formation Of 90° <110> Reorientation Nanobands in the Course of Plastic Deformation of Nickel on Bridgman Anvils

Based on the results of electron microscopy examinations, a theoretical study of the distortion tensors of strain localization nanobands with a 90° reorientation of the crystal lattice is performed. The analysis is carried out in the direct-plus-reverse model of FCC→BCC→FCC martensitic transformations in an approximation of the linear theory of small strains. Using the atomic model of martensitic transformations based on a system of cooperative thermal fluctuations of planes in the BCC lattice, the distortion tensors of the contraction and shear deformation modes are estimated and so is their contribution to the total distortion of the reversible martensitic transformation.

Probing the Effect of Hydrogen on Elastic Properties and Plastic Deformation in Nickel Using Nanoindentation and Ultrasonic Methods

Hydrogen effects on small-volume plasticity and elastic stiffness constants are investigated with nanoindentation of Ni-201 and sonic velocity measurements of bulk Ni single crystals. Elastic modulus of Ni-201, calculated from indentation data, decreases ~ 22% after hydrogen charging. This substantial decrease is independently confirmed by sonic velocity measurements of Ni single crystals; c44 decreases ~ 20% after hydrogen exposure. Furthermore, clear hydrogen–deformation interactions are observed. The maximum shear stress required to nucleate dislocations in hydrogen-charged Ni-201 is markedly lower than in as-annealed material, driven by hydrogen-reduced shear modulus. Additionally, a larger number of depth excursions are detected prior to general yielding in hydrogen-charged material, suggesting cross-slip restriction. Together, these data reveal a direct correlation between hydrogen-affected elastic properties and plastic deformation in Ni alloys.

Research of Stress Field Distribution in FCC-Single Crystal Samples in Compression

.Theoretical distribution of a stress field in a sample in the form of a rectangular parallelepiped in compression for an isotropic material was calculated using the method of finite elements. Calculations showed that the highest stress is observed at the top and front edges of a sample. There are areas of the tension stress on vertical edges in the area of sample tops. Shift anisotropy was determined by imposing of the distributed tension field in a sample on FCC crystal of a certain crystallographic orientation.Change of shift symmetry in single crystals for different crystallographic orientations of a compression axis was considered. It was established that a shift fragmentation in the parallel octahedral planes in the conditions of plastic deformation determines the process of low-symmetric shift deformation and maintenance of higher single crystal pseudo-symmetry. Connection of the obtained results and test data on heterogeneity of plastic deformation of nickel and aluminum single crystals is discussed.

Mechanical Aspects of Plastic Deformation of Nickel Based Superalloy

Variations of a true stress vs. true strain illustrate behaviour of materials during plastic deformation. Stress-strain relationship is generally evaluated by a torsion, compression and tensile tests. Results of these tests provide crucial information pertaining to the stress values which are necessary to run deformation process at specified temperature and cooling rate. Uniaxial compression tests at temperatures below the γ solvus were conducted on nickel based superalloy CMSX-4, to study the effect of temperature and strain rate on its flow stress. On the basis of received flow stress values activation energy of a high-temperature deformation process was estimated. Mathematical dependences (σpl -T i σpl - ἐ) and compression data were used to determine material constants. These constants allow to derive a formula that describes the relationship between strain rate, deformation temperature and true stress.

Influence of annealing on NiTi shape memory alloy subjected to severe plastic deformation

The amorphous phase with the retained nanocrystalline phase, the deformation bands and the amorphous bands coexist due to inhomogeneous plastic deformation in nickel–titanium shape memory alloy (NiTi SMA) subjected to severe plastic deformation (SPD) based on local canning compression. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to investigate microstructural evolution and phase transformation of NiTi sample subjected to SPD in the case of annealing for 2h at 300°C, 450°C and 600°C, respectively. Annealing at 300°C and 450°C leads to nanocrystallization of amorphous NiTi sample, while annealing at 600°C results in the coarse-grained NiTi sample, where the (001) martensite compound twins are found. The precipitation phases such as Ni4Ti3 and Ni3Ti are suppressed in NiTi sample formed as a result of crystallization of the amorphous structure and thus occur more easily in the deformation bands in the case of annealing. Martensitic phase transformation is influenced by the grain size and is suppressed with the decrease in the nanocrystalline grain size.

Effect of Strain Rate on Evolution of the Deformation Microstructure and Texture in Polycrystalline Copper and Nickel

The evolution of crystallographic texture in polycrystalline copper and nickel has been studied. The deformation texture evolution in these two materials over seven orders of magnitude of strain rate from 3 × 10−4 to ~2.0 × 10+3 s−1 show little dependence on the stacking fault energy (SFE) and the amount of deformation. Higher strain rate deformation in nickel leads to weaker \( \left\langle {101} \right\rangle \) texture because of extensive microband formation and grain fragmentation. This behavior, in turn, causes less plastic spin and hence retards texture evolution. Copper maintains the stable end \( \left\langle {101} \right\rangle \) component over large strain rates (from 3 × 10−4 to 10+2 s−1) because of its higher strain-hardening rate that resists formation of deformation heterogeneities. At higher strain rates of the order of 2 × 10+3 s−1, the adiabatic temperature rise assists in continuous dynamic recrystallization that leads to an increase in the volume fraction of the \( \left\langle {101} \right\rangle \) component. Thus, strain-hardening behavior plays a significant role in the texture evolution of face-centered cubic materials. In addition, factors governing the onset of restoration mechanisms like purity and melting point govern texture evolution at high strain rates. SFE may play a secondary role by governing the propensity of cross slip that in turn helps in the activation of restoration processes.

Relaxation Structure Formation in Deformation of Nickel

The purpose of this study was to quantitatively investigate the relaxation structure formation in polycrystalline nickel deformed by rolling at room temperature. The relaxation structures were examined on polycrystalline nickel with a purity of 99.99% Ni, deformed by rolling. The sub-structure of nickel after rolling to 50-80% was examined by the TEM and X-ray analysis methods by line profile analysis. It was revealed the presence of three qualitatively different types of structural elements: microbands of a macroscopic length; cells of different shapes and dimensions; micrograins less than 80 nm in size. The normalized curves of distribution of cells dimensions coincide, which suggests a scale invariance of these parameters. On the other hand, addition of normalized dimensions of micrograins to statistical dependence of dimensions of the cells leads to distortion of the dependence. The mechanisms of formation of the cells and micrograins to be different: micrograins were shown are nuclei of primary recrystallization.

Deuterium trapping in irradiated nickel

The effect of cold plastic deformation in nickel on the deuterium segregation induced by ion irradiation has been studied by the nuclear-reaction method. The irradiation and discrete measurements have been performed at room temperature and coordinated in time due to the use of a beam of accelerated deuterons with an energy of 700 keV in the reaction D (d,p) T. An analysis of the experimental dose dependences of the accumulation of implanted deuterium and of the results of calculation of the process of deuterium implantation using an SRIM program made it possible to reveal the basic laws of the deuterium segregation. It has been established that deuterium trapping in the irradiated volume occurs into initial traps (deformation-induced vacancies) and radiation-induced traps (vacancies and their clusters). The dislocations at room temperature are not efficient traps of deuterium. Dose ranges of deuterium trapping by different-type traps have been revealed, and a model of the formation and evolution of the deuterium segregates in nickel has been put forward.

The effect of deuterium and tritium on formation and annealing of vacancy‐type defects in deformed nickel

AbstractPure nickel and nickel loaded with deuterium or tritium were examined during deformation at 270–300 K to 3–45% and subsequent annealing up to 850 K using the measurements of the electrical resistivity and positron annihilation spectroscopy. Three‐dimensional vacancy clusters were formed during deformation in nickel loaded with the hydrogen isotopes unlike in pure nickel. Vacancy clusters decorated with deuterium or tritium atoms created at room temperature, i.e. below a temperature at which monovacancies, are mobile in nickel. Vacancy clusters decorated with deuterium atoms have two types of atomic configurations or degrees of decoration with the binding energy of 0.73 eV and 1.05 eV, respectively. Only one configuration having the binding energy of 1.05 eV was detected for vacancy clusters decorated with tritium atoms. Possible reasons for different behaviors of vacancy clusters in T‐ and D‐loaded nickel are also discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

HRTEM and STEM HAADF Study of Microtwining During Creep Deformation in Nickel Based Superalloy Rene 104

HRTEM and STEM HAADF Study of Microtwining During Creep Deformation in Nickel Based Superalloy Rene 104

Mechanism and kinetics of coagulation of γ′-phase in nickel alloys at temperatures of hot plastic deformationin nickel alloys at temperatures of hot plastic deformation

It is known that hot plastic deformation of hard-to-deform complexly alloyed heat-resistant nickel alloys is easier to conduct at temperatures of the double-phase region than at those of the single-phase region. In heating such alloys before deformation and in the process of deformation cooling of them the amount of strengthening γ′-phase and its sizes and distribution change, which exerts a substantial effect on their susceptibility to hot plastic deformation in the double-phase range. For this reason, knowledge of the laws of variation of the amount, dispersity, and distribution of the strengthening γ′-phase in heating to the deformation temperature is very important, because it makes it possible to control these processes.

Microstructure and Microtexture Evolution During High Temperature Low Cycle Fatigue of Nickel

The development of microstructure, texture, misorientation and grain boundary character during cyclic deformation of Nickel at elevated temperatures was investigated. At high numbers of cycles grain boundaries align with the maximum shear stress. Texture and microstructure change significantly and low ∑CSL boundaries with (111) rotation axes emerge. The Schmid factor increases with progressing deformation but Sachs- and Taylor-type deformation modelling fail to predict the actual texture change. It is proposed to attribute the observed texture and microstructure development to an interaction of deformation mechanisms with grain boundary motion.

Effects of hydrogen and carbon on thermally activated deformation in nickel

Deformation experiments were performed to determine the effects of hydrogen and carbon on the activation parameters for dislocation slip in nickel. The techniques used were isothermal stress relaxation and differential temperature measurements. These methods allowed determination of the activation enthalpy and activation area for dislocation motion in nickel and the effects of hydrogen and carbon on these parameters. The results show that hydrogen increases the dislocation mobility in Ni and in NiC alloys by reducing the activation enthalpy for dislocation motion, while carbon reduces the dislocation mobility by increasing the activation enthalpy. Hydrogen solutes decrease the activation area for dislocation motion in both pure Ni and in NiC alloys.

Low-temperature and high-strain-rate deformation of nickel and nickel-carbon alloys and analysis of the constitutive behavior according to an internal state variable model

Analysis of the strain-rate sensitivity of the yield stress and the strain-rate sensitivity of strain hardening in annealed and quasi-statically and dynamically prestrained Ni270, Ni-510 wt ppm (0.25 at.%) C, and Ni-1900 wt ppm (0.92 at.%) C is presented. Measurements in compression are analyzed according to an internal state variable model where for a given obstacle type the state variable represents the stress required for deformation at 0 K. State variables are defined for dislocations interacting with interstitial carbon atoms and for dislocations interacting with other dislocations. The application of a power law to combine the contributions of separate obstacles is investigated. Evolution of the mechanical threshold stress characterizing dislocation/dislocation interactions is analyzed using a modified Voce equation. The Stage II hardening rate is shown to be insensitive to the carbon concentration, while the hardening at large strains is strongly dependent on the carbon concentration. Dynamic strain aging is found to influence both the temperature and strain-rate dependent yield stress and the hardening rate in the nickel-carbon alloys. Deformation microstructures are characterized using transmission electron microscopy, and the measured dislocation cell size is correlated with the internal state variable characterizing dislocation/dislocation interactions.

The Effects of Prestrain at High Temperatures on the Early Stage of Creep Deformation in Nickel and Nimonic 75

The Effects of Prestrain at High Temperatures on the Early Stage of Creep Deformation in Nickel and Nimonic 75

Grain boundary deformation in neutron irradiated nickel

The authors studied grain boundary deformation in nickel irradiated with neutrons in a VVR-K reactor and mechanically tested at different temperatures and various neutron fluences. The degree of radiation embrittlement was found to be significant. Results show that the general nature of intergranular displacement occurred in the form of relative sliding and mutual rotation of grains. Neutron irradiation was also found to suppress grain boundary migration. These conditions are assumed to cause the formation of grain boundary microcracks during high temperature tensile deformation. The microcracks increase with increasing deformation and merge together to form a potential main crack leading to ductility loss and intergranular fracture.

Hydrogen transport during deformation in nickel: Part II. Single crystal nickel

Direct evidence for dislocation transport of hydrogen was observed in nickel single crystal slices using the electrochemical permeation technique modified to allow for simultaneous deformation. The hydrogen flux increased in the easy glide region of deformation even after accounting for the effects of a decrease in specimen thickness. At a fast strain rate the only explanation for this increase is dislocation transport.

Hydrogen transport during deformation in nickel: Part I. Polycrystalline nickel

The electrochemical permeation technique was used to study the effects of deformation on the steady state flux of hydrogen in polycrystalline nickel 270. The hydrogen flux change was found to depend strongly on strain rate. At fast strain rates, the hydrogen flux decreased due to dynamic trapping by newly created dislocations. At slow strain rates, new traps were created more slowly with time and the lattice had a chance to be refilled with hydrogen from the charging surface. Lattice refilling masks the trapping effect so that less of a decrease in hydrogen flux was observed. Under the conditions of total lattice refilling, a decrease in specimen thickness and an increase in input concentration resulted in an increase in the permeation flux. No evidence of dislocation transport was observed in polycrystalline nickel.

Grain-boundary deformation in nickel irradiated by α-particles

The results of a study of grain-boundary deformation in Ni doped with 10−6 at% He tested in the temperature range 20 to 1000 °C are given. Compared to non-irradiated metals more intense deformation is observed in the samples irradiated by α-particles. The data show the predominant role of grain-boundary deformation in the high-temperature radiation embrittlement process. [Russian Text Ignored].