Face-centered Cubic Metals Research Articles

In-situ neutron diffraction study of deformation behavior of a multi-component high-entropy alloy

Deformation behavior of a high-entropy alloy (HEA) was investigated by in situ tensile deformation with neutron diffraction. It was found that the face-centered cubic (FCC) HEA alloy showed strong crystal elastic and plastic anisotropy, and the evolution of its lattice strains and textures were similar to those observed in conventional FCC metals and alloys. Our results demonstrated that, in spite of chemical complexity, the multi-component HEA behaved like a simple FCC metal and the deformation was caused by the motion of mixed dislocations.

Coupled Thermomechanical Modeling of Small Volume FCC Metals

The mechanical and thermal behavior of small volume metallic compounds on the fast transient time are addressed in this work through developing a thermodynamically consistent nonlocal framework. In this regard, an enhanced gradient plasticity theory is coupled with the application of the micromorphic approach to the temperature variable. The yield function of the VA–FCC (Voyiadjis Abed Face Centered Cubic) model based on the concept of thermal activation energy and the dislocations interaction mechanisms including nonlinear hardening is taken into consideration in the derivation. The effect of the material microstructural interface between two materials is also incorporated in the formulation with both temperature and rate effects. In order to accurately address the strengthening and hardening mechanisms, the theory is developed based on the decomposition of the mechanical state variables into energetic and dissipative counterparts which provided the constitutive equations to have both energetic and dissipative gradient length scales for the bulk material and the interface. Moreover, the nonlocal evolution of temperature is addressed by incorporating the microstructural interaction effect in the fast transient process using two time scales in the microscopic heat equation.

Restoration of face-centered cubic metals subjected to kinetic spraying

Deformation mode and restoration of face-centered cubic (FCC) metal (Al, Ni, and Cu) particles subjected to kinetic spraying (KS) were investigated. The FCC metal particles were accelerated to supersonic velocity by high pressure process gas, and collided with substrates or previously deposited coating layer. The high velocity impact of in-flight particles and their successive deposition leads to severe plastic deformation at ultra-high strain rate and the dissipation of heat energy from the plasticity. Accordingly, highly strained interface undergoes restoration to stabilize strained area during KS. Although Al, Ni, and Cu have equivalent slip systems {111} 〈110〉, the different physical and metallurgical properties of the FCC metals differentiate the deformation mode and lead to variations in static recovery and recrystallization rates. The deformation and restoration behavior of KS FCC metals are discussed, taking into account the physical and metallurgical factors such as stacking fault energy, dislocation mobility, diffusivity, and melting point.

Fatigue strength prediction for inhomogeneous face-centered cubic metal based on Vickers hardness

To find the Vickers hardness (HV) value for predicting the fatigue strength of inhomogeneous face-centered cubic (FCC) metals, HV tests were performed on SUH660 stainless steel. The results indicate that the intrinsic hardness distribution can be obtained from the HV distribution in test zones according to the Vickers hardness definition. The soft zone greatly affects the fatigue strength of an inhomogeneous FCC metal. Therefore, for another inhomogeneous FCC metal in which fatigue cracks initiate and propagate easily in the softest zone, the fatigue limit can be predicted using the mean HV value of the softest zone.

Nanoscale mechanisms of surface stress and morphology evolution in FCC metals under noble-gas ion bombardments

Here, we uncover three new nanoplasticity mechanisms, operating in highly stressed interstitial-rich regions in face-centred-cubic (FCC) metals, which are particularly important in understanding evolution of surface stress and morphology of a FCC metal under low-energy noble-gas ion bombardments. The first mechanism is the configurational motion of self-interstitials in subsonic scattering during ion bombardments. We have derived a stability criterion of self-interstitial scattering during ion embedding, which consistently predicts the possibility of vacancy- and interstitial-rich double-layer formation for various ion bombardments. The second mechanism is the growth by gliding of prismatic dislocation loops (PDLs) in a highly stressed interstitial-rich zone. This mechanism allows certain prismatic dislocations with their Burgers vectors parallel to the surface to grow in subway-glide mode (SGM) during ion bombardment. The SGM growth creates a large population of nanometre-sized prismatic dislocations beneath the surface. The third mechanism is the Burgers vector switching of a PDL that leads to unstable eruption of adatom islands during certain ion bombardments of FCC metals. We have also derived the driving force and kinetics for the growth by gliding of prismatic dislocations in an interstitial-rich environment as well as the criterion for Burgers vector switching, which consistently clarifies previously unexplainable experimental observations.

Atomic-scale simulations of material behaviors and tribology properties for FCC and BCC metal films

The material behaviors and tribology properties of face-centered cubic (FCC) and body-centered cubic (BCC) metal films under nanocontact with a scanning probe tip are studied using molecular dynamics simulations. The results clearly show that for a given indentation depth, the required indentation force increases with atomic bonding energy. During scratching, the chips (removed atoms) pile up in front of the probe tip due to adhesion. Most of the chips behind the probe tip disappear due to elastic relaxation and elastic recovery. A slip system clearly occurs in the <110> and <111> directions for FCC and BCC metal films, respectively. A material with higher atomic bonding energy exhibits a larger normal force, a larger friction force, and a lower friction coefficient.

Indentation size effect in FCC metals: An examination of experimental techniques and the bilinear behavior

Abstract

Analysis on the Twinning of FCC Metals by EBSD

For the technology of EBSD, the twinning of metals was described in the form of rotation angle combined with rotation axis, while the twinning of metals was usually described in the form of twinning plane combined with twinning direction. In this report, the corresponding relationship between the two description forms of twinning of face-centered cubic (FCC) metals has been built, based on this relationship, the twinning plane and twinning direction of FCC metals can be determined by EBSD. As the practical application of this relationship above, the twinning variants of two kinds of Ni based superalloys were analyzed.

Modelling Static Recrystallisation Textures Using a Coupled Crystal Plasticity-Phase Field Technique

An integrated crystal plasticity-phase field model has been developed to simulate the static recrystallisation textures of both Face-Centred Cubic (FCC) and Body-Centred Cubic (BCC) metals. Nucleation sites are determined using the Orientation Dependent Recovery (ODR) theory. Both the interface mobility and the grain boundary energy are set to be dependent on mis-orientation angles in the simulations. A pre-deformed microstructure without a particular texture is generated using a Monte Carlo simulation. Plane strain compression textures before recrystallisation are predicted by a Crystal Plasticity Finite Element (CPFE) model showing a good agreement with the typical experimental rolling textures. It is shown that the typical recrystallisation textures for FCC and BCC metals can be simulated correctly using a Phase Field (PF) method by choosing appropriate critical values for the nucleation criterion. A comparison between the two different nucleation criteria based on the ODR theory or the stored energy is also presented.

Edge dislocation core structures in FCC metals determined from ab initio calculations combined with the improved Peierls–Nabarro equation

We have employed the improved Peierls–Nabarro (P–N) equation to study the properties of 1/2⟨110⟩ edge dislocation in the {111} plane in face-centered cubic (FCC) metals Al, Cu, Ir, Pd and Pt. The generalized-stacking-fault energy surface entering the equation is calculated by using first-principles density functional theory (DFT). The accuracy of the method has been tested by calculating the values for various stacking fault energies that favorably compare with previous theoretical and experimental results. The core structures, including the core widths of the edge and screw components, and dissociation behavior have been investigated. The dissociated distance between two partials for Al in our calculation agrees well with the values obtained from numerical simulation with DFT and molecular dynamics simulation, as well as experiment. Our calculations show that it is preferred to create partial dislocations in Cu, and easily observed full dislocations in Al, Ir, Pd and especially Pt.

Mechanical Behavior and Size Sensitivity of Nanocrystalline Nickel Wires Using Molecular Dynamics Simulation

The mechanisms of deformation and failure in face-centered cubic (FCC) nickel nanowires subjected to uniaxial tensile loading are investigated using molecular dynamics (MD) simulation, and the size effect on mechanical properties of FCC metal nanowires is studied. Simulation reveals that the surface free energy has great influence on the deformation and failure mechanism of metal nanowires. As a result of free surfaces and their reconstruction, the surface atoms depart from the perfect crystal lattice positions, leading to the appearance of nanocavities on the surfaces that are exposed to external load. The deformation process of nanowires undergoes expansion and connection of nanocavities from surface into inner lattices. Slip occurs during the deformation process, which is consistent with experimental phenomena. Elastic stiffness, yield, and fracture strength of nickel nanowires with various cross-sectional sizes are obtained, and the size effect on these mechanical properties is further analyzed. Based...

Recent Developments on the Microstructural Effects Caused by Small-Charge Explosions in FCC Alloys

Metals exposed to small charge explosions, even in absence of overall deformation, show characteristic and permanent microstructural features, that can be related to blast wave properties, e.g. to the charge mass and the charge-to-target distance. In particular, Face Centered Cubic (FCC) alloys with low stacking fault energy may exhibit mechanical twinning due to the high strain rate caused by an explosion, even if in slower processes they mainly deform by slip. In some forensic science investigations, these crystallographic modifications, and particularly the occurrence of twinning, may be among the few remaining clues of a small charge explosion, and may be useful to hypothesize the nature and location of the charge. A wide experimental campaign was performed to correlate the blast wave properties with the ensuing modifications of FCC metal targets, and to investigate the microscopic deformation mechanisms leading to these modifications. In particular, it was attempted to identify the threshold conditions (charge-to-target distance, charge mass, and hence applied stress) that yield barely detectable microstructural modifications, and to study the transition from slip to twinning. FCC metal alloys, with low (α-brass, stainless steel), intermediate (copper, gold alloy), or high (aluminum alloy) stacking fault energy, were exposed to blast waves (caused by 50 or 100 g plastic explosive charges located at increasing charge-to-target distances) and then analyzed by X-ray diffraction, optical microscopy, scanning electron microscopy, and electron backscattered diffraction imaging. A comprehensive review of the most significant findings of the whole research, together with theoretical considerations on the slip and twinning deformation mechanisms, is here presented.

Grain Refinement Efficiency in Equal Channel Angular Extrusion of FCC Metals Inferred from Crystal Plasticity Simulations of Slip Activities

Equal channel angular extrusion (ECAE) is a relatively new technique to produce ultrafine-grained materials by severe plastic deformation. Its efficiency of grain refinement varies with the processing route, i.e. the billet rotation () about its longitudinal axis between successive passes. The influence of processing route can not be fully explained by existing theories that consider only the macroscopic deformation features. In this study, the mesoscopic deformation behavior during multi-pass ECAE of face-centered cubic (FCC) metals was simulated using a visco-plasticity self-consistent (VPSC) polycrystal model and assuming simple shear deformation in each pass. It is shown that the slip activities vary significantly at the transitions between successive passes, depending on the die angle and processing route. The efficiencies of grain refinement in the different cases can be well correlated to the contribution of slip systems newly activated in a subsequent pass. The grain refinement is more efficient when such contributions are higher, such as in route B ( = 90) with a 90 die or route A ( = 0) with a 120 die. These crystal plasticity simulations provide insights into the efficiency of grain refinement during severe plastic deformation with strain path changes.

On the nucleation of partial dislocations in nanoparticles

Nanocrystalline face-centred-cubic (FCC) materials show unique mechanical properties. In this regard, an interesting but controversial discovery is that nanocrystalline FCC materials with high stacking-fault energy (SFE), such as aluminum (Al), exhibit a novel mechanism of deformation. In particular, both molecular dynamic simulations and high-resolution transmission electron microscopy observations show that for nanocrystalline Al mechanical twinning plays an important role in the deformation process. Yet these results are surprising, as the presence of partial dislocations in high SFE FCC metals are expected to be unstable. The purpose of this article is to offer a plausible explanation for the occurrence of stacking faults and deformation twins in nanocrystalline Al. A simple model is developed for the nucleation of both perfect and partial dislocation half-loops in individual single-crystal nanoparticles of Al and copper (Cu) with different sizes subjected to shear stress. The model shows that as the size of the nanoparticles decreases, partial dislocations are more likely to occur. Moreover, for a constant applied stress, there is a critical size for which the nucleation of partial dislocations occurs is more probable, compared to that of perfect dislocations.

기계적 변형하에서 금속재료의 표면응력 계산

We calculate the variation of the surface stresses according to uniaxial and biaxial strains in face-centered cubic (FCC) metals. In our study, three mainly observed free surfaces of seven representative FCC metals are considered. Employed method is molecular mechanics, in which the interaction of atoms is described by empirical interatomic potentials. As uniaxial strain increases to tensile direction, the surface stresses on {100} and {110} free surfaces decrease monotonously, while those on {111} surface increase. These tendencies are the same regardless of the species of metals and interatomic potentials employed. However, when the system is under biaxial strain, surface stresses change different according to the surface directions, the species of metals, and even interatomic potentials. On {100} and {111} surfaces, heavy metals (Pt, Au) show the opposite variation to light metals (Ni, Cu). In the cases of Pd and Ag, the surface stresses reveal the opposite tendency, depending on interatomic potentials used.

A Study on the Uniaxial Tension of FCC Metals at Nano Level Using MD

Molecular Dynamics (MD) are now having orthodox means for simulation of matter in nano-scale. It can be regarded as an accurate alternative for experimental work in nano-science. In this paper, Molecular Dynamics simulation of uniaxial tension of some face centered cubic (FCC) metals (namely Au, Ag, Cu and Ni) at nano-level have been carried out. Sutton-Chen potential functions and velocity Verlet formulation of Noise-Hoover dynamic as well as periodic boundary conditions were applied. MD simulations at different loading rates and temperatures were conducted, and it was concluded that by increasing the temperature, maximum engineering stress decreases while engineering strain at failure is increasing. On the other hand, by increasing the loading rate both maximum engineering stress and strain at failure are increasing.

Thermodynamic Consistent Formulations of Viscoplastic Deformations in FCC Metals

A general consistent thermodynamic framework for small strain thermoviscoplastic deformations of face-centered cubic (FCC) metals is presented in this study. An appropriate and consistent Helmholtz free energy definition is incorporated, after considering the strain rate effect imbedded through the hardening definition, in deriving the proposed three-dimensional kinematical model. Microstructural physically based thermal and athermal yield function definitions (von Mises type) are utilized in this work for dynamic and static deformations of FCC metals. A length scale parameter introduced implicitly through the viscosity parameter is related to the waiting time of dislocations at an obstacle. The role of material dependence in setting the character of the governing equations is illustrated in the context of a simple uniaxial tensile problem in order to check the effectiveness and the performance of the proposed framework and its finite-element implementation. Results obtained for OFHC copper at low and high strain rates and temperatures show, generally, good comparisons with experimental results.

Effect of Microstructure on the Mirror-Like Surface Quality of FCC and BCC Metals

The microstructure of machined metals changes near the tool-affected zone. This paper presents some new results concerning mirror-like surface cutting of aluminum, copper and tungsten. The microstructure of aluminum and copper represents polycrystalline mild metals with face centered cubic (FCC) crystal structure. Examination of a mirror-like surface by optical microscopy, scanning electron microscopy (SEM), electron backscattered diffraction and atomic force microscopy revealed that grain boundaries and twin boundaries were present, which separates two domains for different crystal orientation. The Young's modulus that depends on orientation can change considerably on these boundaries and, consequently, the value of elastic deformation of the layer under the machined surface. This effect modified the roughness. Ultraprecision machining of tungsten, which is a body centered cubic (BCC) metal, proved useless using diamond and/or boron-nitride tools. Since tungsten is a very brittle metal at room temperature, its ductile to brittle transition temperature is much higher. Therefore, in contrast to normal cutting, the material that is incapable of plastic deformation will cause brittle fracture of the chip and bad surface quality.

Dependence of stresses and strain energies on grain orientations in FCC metal films

A thin polycrystalline film tightly bonded to a thick substrate of different thermal expansion coefficients will experience thermal stresses when the temperature is changed during post-processing such as annealing. Calculations of these stresses and the corresponding strain energies for grains having various crystallographic orientations ( h k l) relative to the surface of the film were made for a polycrystalline film composed of the face-centered cubic (FCC) metal Cu, Al, Ag, Au, Ni, Pb and Th, respectively. The abnormal growth of (1 0 0)-oriented grains and change in texture from (1 1 1) to (1 0 0) in attached FCC films after annealing have been explained satisfactory from the minimization of the strain energy.

Observation and analysis of defect cluster production and interactions with dislocations

The current understanding of defect production fundamentals in neutron-irradiated face centered cubic (FCC) and body centered cubic (BCC) metals is briefly reviewed, based primarily on transmission electron microscope observations. Experimental procedures developed by Michio Kiritani and colleagues have been applied to quantify defect cluster size, density, and nature. Differences in defect accumulation behavior of irradiated BCC and FCC metals are discussed. Depending on the defect cluster obstacle strength, either the dispersed barrier hardening model or the Friedel–Kroupa–Hirsch weak barrier model can be used to describe major aspects of radiation hardening. Irradiation at low temperature can cause a change in deformation mode from dislocation cell formation at low doses to twinning or dislocation channeling at higher doses. The detailed interaction between dislocations and defect clusters helps determine the dominant deformation mode. Recent observations of the microstructure created by plastic deformation of quenched and irradiated metals are summarized, including in situ deformation results. Examples of annihilation of stacking fault tetrahedra by gliding dislocations and subsequent formation of mobile superjogs are shown.