Fcc Metals Research Articles

Junction formation rates, residence times, and the rate of plastic flow in FCC metals

During plastic flow in metals, dislocations from slip systems with different glide planes collide to form junctions. After being in-residence within the dislocation network for some period of time, these junctions then break, thereby liberating the attached dislocation lines. In this work we use random forest discrete dislocation dynamics simulations to quantify the junction formation rate and junction residence time as a function of stress for all junction types in face-centered cubic metals. We then relate these quantities to the dislocation link-length distribution, which is found to exhibit an exponential form. This enables us to quantify the mean junction strength and also the slip system interaction coefficients. Finally, using the link-length model we obtain a flow rule for our systems which is physics-based with all parameters determined from DDD simulations. The insights here provide a path forward for a dislocation network theory of plastic flow based on the link-length distribution.

Thermal Shape Stability of fcc Metal Nanocrystals Synthesized with Faceted Nonequilibrium Shapes

Thermal Shape Stability of fcc Metal Nanocrystals Synthesized with Faceted Nonequilibrium Shapes

Cross slip based dynamic recovery during plane strain compression of aluminium and its role in preferential nucleation of the cube-oriented recrystallized grains

Understanding nucleation of the cube-oriented grains during static recrystallization of medium to high stacking fault energy (SFE) FCC metals is both scientifically and industrially important. Dynamic recovery due to weak Hirth junctions is considered as one of the plausible explanations for the nucleation advantage of the cube orientation. However, influence of cross slip based dynamic recovery on the nucleation propensity of the cube orientation has not been studied in detail. For this purpose, we explicitly incorporated stress dependent dynamic recovery of screw dislocations into a dislocation density based crystal plasticity model. The model is utilized for studying influence of cross slip based local dynamic recovery during deformation and recrystallization nucleation of aluminium. We show that dynamic recovery influences local dislocation density difference and disorientation with the neighbouring locations for the main rolling texture components. Next, we analysed nucleation propensity of these texture components using a stochastic nucleation model. Simulation results show that without explicit incorporation of the dynamic recovery of screw dislocations, the Goss orientation shows the highest nucleation propensity. On the other hand, incorporation of the cross slip based dynamic recovery enhances nucleation propensity of the cube component. Therefore, cross slip based dynamic recovery is one of the important mechanisms for nucleation of the cube-oriented recrystallized grains. These results provide new insights into deformation and recrystallization nucleation of medium to high SFE FCC metals.

Introducing vacancy defects by rapid quenching in FCC metal for deep learning micrograph dataset

Accurate detection of crystal defects from microstructure micrographs is essential for evaluating the mechanical and functional properties of metallic materials. Emerging deep learning techniques have great potential for application in crystal defects detection, but the excellent detection accuracy requires domain knowledge and micrograph datasets. Here we target to detect and distinguish nanoscale dislocation loops (DLs) and stacking fault tetrahedrons (SFTs) in transmission electronic microscope (TEM) micrographs. The micrograph datasets of crystal defects derive from copper prepared by rapid quenching and post-annealing. Applying standard U-Net architecture, the maximum Mean Intersection over Union (mIOUmax) of 3-class model is 54.29%, while it is 73.81% and 74.67% of DL 2-class and SFT 2-class model. Results suggest that excellent detection accuracy of deep learning model could be achieved by learning features from specific defect datasets. These findings provide a new strategy for accurate detection of crystal defects in the field of material engineering.

Quantifying dislocation drag at high strain rates with laser-induced Microprojectile impact

As deformation rate increases, the thermally activated dislocation glide gives way to a continuous glide of dislocations governed by their interactions with phonons. Understanding the dislocation-phonon drag regime is critical for designing metallic materials for extreme deformations rates. However, it has proven challenging to study empirically, partly due to the resource intensive nature of the experimental approaches targeting this regime. Here, we develop an impression-based experimental approach combining laser-induced microprojectile impact (Hassani et al., 2020a) and spherical nanoindentation to characterize the dislocation-phonon drag regime. We also develop a physically based constitutive framework that, when integrated with our experimental measurements, can quantify the dislocation-phonon drag regime. We isolate the effect of dislocation-phonon drag by leveraging the similar deformation geometries and length scales but different operative mechanisms during spherical nanoindentation and microprojectile impact. We discuss mechanistic contributions to the plastic work for microprojectile impacts in a range of impact velocities producing strain rates up to 109 s−1. We also develop a deformation mechanism map focused on the transition from thermal activation to dislocation drag for a model FCC metal, copper.

Structure sensitivity in adsorbate-induced adatom formation on FCC transition-metal surfaces

Recent surface science discoveries reveal that adsorbates may induce in situ sub-nanometer cluster formation on transition-metal surfaces. To elucidate the structure sensitivity behind this phenomenon, we performed density functional theory calculations to construct an adatom formation energy database for eight fcc metals and 26 adsorbates commonly involved in catalytic reactions. We show that the adatom formation on (100) surfaces is generally easier than that on (111) surfaces. Many adsorbate/metal pairs exist, mostly on Pd, Ni, Rh, Pt, and Ir, for which adsorbates might induce adatom formation under near-ambient conditions on the (100) facet, but not on (111), highlighting the role of more open facets in adatom formation on metals intrinsically harder than Ag, Au, and Cu. Our study offers a new perspective towards understanding structure sensitivity in heterogeneous thermal- and electro-catalytic systems such as methane steam reforming, Fischer-Tropsch synthesis, and ammonia decomposition.

Critical assessment of hydrogen pipe diffusion at dislocations in metals

Hydrogen-dislocation interactions have always been a crucial problem in understanding and predicting the long-standing hydrogen embrittlement (HE) phenomenon in structural metals. Particularly with respect to hydrogen diffusion, dislocations have often been assumed with the ability to facilitate hydrogen transport via dislocation pipe diffusion (DPD). Yet the experimental and theoretical studies supporting DPD remain elusive and even controversial. In this work, hydrogen kinetics at dislocations in two representative metal systems, fcc Ni and bcc Fe, were systematically investigated combining comprehensive atomistic and kinetic activation relaxation technique simulations to reveal atomic details while enabling long-time (i.e., microsecond to second) hydrogen diffusion analysis. The dislocations were found to favor hydrogen trapping and provide regions of low migration barriers for hydrogen. However, these regions of low barriers only lead to localized fast short-circuit H diffusion, but do not translate into DPD over long time, which is attributed to fast hydrogen diffusion pathways along the dislocation line direction being periodically bound by high hydrogen migration barriers. Using the polyhedral structural units (PSUs) as an effective tool, we further quantitatively analyzed the correlation between hydrogen diffusion behaviors and local dislocation structures, illustrating the structural origin leading to local short-circuit diffusion and inhibition of DPD at dislocations. The new mechanistic insights gained not only are critical for understanding hydrogen-dislocation interaction, but also advances the general knowledge on hydrogen-microstructure interaction in structural metals.

Unifying the size effect observed in micropillar compression experiments

ABSTRACT Micropillar compression experiments show size effect, σp/μ = A(d/b)n , where σp , is the flow stress, μ is the resolved shear modulus, d is the pillar diameter and b is Burgers’ vector. With fcc metals n ≈ –0.67 and A ≈ 0.7; however, with bcc metals there is greater variation, with n closer to zero. Here we propose a different but similar empirical relation of σp/μ = σb/μ + A’(d/b)n’, where σb is a size independent resistance stress. In which case there must be a strong correlation between the original constants, A and n. This hypothesis is found to be true for the published data from a large number of bcc metals, ionic solids that possess the rock salt crystal structure, and some covalent bonded semiconductors. This correlation is shown to predict a universal power law with the exponent in the range, −1.0 < n’ < −0.5, and with A’ close to 1. These values are very similar to the empirical relation that can be used to describe the behaviour of fcc metals tested in micropillar compression with σb = 0. This universality of the empirical relation provides strong evidence for a common mechanism for the micropillar size effect across a range of materials.

Surface energy calculation for some transition metal using the GEAM

The three low index surface energies of fcc metals Cu, Ag, and Au are here calculated using the generalized embedded-atom method (GEAM). The low-index surface energies {, and }, with having the lowest and having the highest energy value. The anticipated values accord well with the experimental values.

Activation entropy and enthalpy of creep in pure metals

Previously published results on the power law creep of a selection of fcc, bcc and hcp pure metals are discussed using a modified Stokes – Einstein equation originally describing Newtonian viscosity of spherical particles in a fluid. Activation energy is modified to include terms representing activation enthalpy, activation entropy and the effect of strain rate. It is similar to the Dorn equation for power law creep but with strain rate rather than stress as the independent variable. Curves of activation energy vs shear rate are linear rather than having a discontinuity as have those with stress as independent variable. All metals have linear curves of activation energy at zero rate vs temperature, the slope of which reflects the activation entropy and is similar for all metals. Intercepts which represent activation enthalpy are dependent on lattice structure.

A microdamage model for FCC single crystals considering a mixed failure mechanism of slip and cleavage

In this study, a microdamage model is newly proposed to predict the failure process of FCC single crystals. Two micro damage mechanisms, slip and cleavage, are both involved in the model. A microscopic damage variable is first defined at the slip plane scale, and a damage coupled crystal plasticity constitutive equation in a co-rotational tensor framework is then established. A damage driving force consisting of generalized energies related to the resolved shear stress and the positive resolved normal stress is proposed to reflect the mixed failure mechanism of FCC single crystals induced by slip and cleavage. The damage evolution equation is then derived in the framework of thermodynamics. An explicit algorithm for the damage coupled crystal plasticity finite element method is given. The proposed microdamage model is utilized to predict the slip-dominant fracture behavior of a single crystal copper and the cleavage-dominant fracture behavior of a nickel-based single crystal alloy. The damage evolution process of slip planes is given and the influence mechanism is revealed. The calculated results agree well with the experimental data, which verifies the good capability of the proposed model in describing and predicting the multi failure processes of FCC metals. Finally, a 2D fracture simulation of a single crystal notched specimen under shear loading is conducted, and the effect of damage parameters on the fracture modes is also discussed.

A new analytical surface energy model for arbitrary (h k l) planes in BCC and FCC metals

A new analytical surface energy model (Eq. 9) that is free of adjustable parameters has been developed to accurately predict the surface energies of arbitrary (h k l) planes in both BCC and FCC ground-state pure metals at 0 K, using only (100), (110) and (111) surface energies as input. The formulation is established by expanding the original broken-bond model up to the 3rd nearest neighbor shell and considering the surface relaxation effect. In addition, this model consists of only one single formula per crystal structure, overcoming the geometric complexities in previous models in which multiple equations are needed for different surface groups. The analytical model has been validated through the comparison with 4357 independent surface energies. These data include the energies of 517 unique surfaces per metal in Fe, Cr, W, Cu, Ni, and Al (a total of 3102 surface energies) that are calculated through our molecular dynamics simulations, and excellent agreement is obtained. The validation data also include 1255 independent surface energies in literature or online database for up to 46 BCC and FCC metals that were calculated by either empirical potentials or density functional theory. The agreement is still very good for most of the data, demonstrating the robustness of our new model.

Core shift controls grain boundary energy scaling in Cu and Al

Grain boundary energies in different elements are correlated. The proportional scaling constants relating the energies of crystallography-equivalent boundaries in any two f.c.c. elements are nearly constant, with the notable exception of aluminum where these constants are known to vary significantly. However, the origins of the exceptional behavior of aluminum are not understood. Previously, we reported that for fcc metals there is a preference for tilt boundaries to shift their tilt axis across the (11¯0) plane towards [112] and to ultimately form low energy [112] core shifted boundaries (CSBs). By comparing grain boundary energies in copper and aluminum with different tilt axes in (11¯0) plane, we now report the existence of a well-defined scaling behavior for the case of low angle boundaries. In contrast, the scaling constant for high angle boundaries is essentially fixed regardless of their tilt axis shift. This results in a gradual change in the scaling constants from low angle to high angle boundaries, which is responsible for the apparent exceptional scaling behavior found in aluminum. An analysis of structure evolution during core shifting points to the significance of boundary-core dissociation, a form of correlated relaxation of individual atoms at boundaries, in controlling the scaling of boundary energies.

Universal trends in computed grain boundary energies of FCC metals

We use a simple parameterization of the Embedded Atom Method (EAM), termed EAM-X, to explore the dependence of grain boundary (GB) energies in FCC metals on EAM parameters. We study a set of GBs with various geometries and calculate their energies, scanning through EAM-X parameter space. We find that variations in GB energy with EAM parameters can be larger than variations due to GB geometry. The atomistic data are used to determine a fit of the GB energy in EAM parameter space, which can be used to obtain boundary energies in real FCC elements without the need to perform new atomistic calculations. Our results provide broad confirmation of a proposed correlation between calculated GB energy and shear modulii among FCC metals. Our work also highlights the need to consider sensitivity to details of empirical potentials when performing quantitative studies of GB physics.

An electroplastic constitutive model of fcc metals and their alloys under high current density

The mechanical behavior of fcc (face center cubic) metals and their alloys such as copper-based and aluminum-based materials changes obviously under high current density, but there is a lack of suitable constitutive model to describe the electroplastic behavior of these materials, which limits the accuracy of the mechanical response prediction in applications, such as electromagnetic launch. In this paper, a novel nonlinear constitutive model for electroplastic behavior under the high current density of fcc metals and their alloys is proposed. The model proposed is based on the theory of dislocation evolution, energy transfer theory and electron transport theory and takes into account the effects of high current density on dislocation density, strain rate and resistivity. Considering the evolution of forward and reverse dislocations caused by pulsed current, the relationship between the evolution rate of reverse dislocation and strain is established. Due to the sensitivity of the electroplastic effect to strain rate, the strain rate under electric current is obtained by combining the theory of dislocation thermal activation and energy transfer. According to the concept of thermal resistance and dislocation resistance, the relationship between resistance and current density is derived. The quantitative results demonstrate that this model can well capture the flow stress softening under high current density (>3000 A/mm2) and the transient hardening after current removal because of the consideration of a more suitable strain rate and dislocation evolution. The critical current density that has a significant effect on the strain rate and the current density corresponding to the subsection point of the resistivity are quantitatively obtained. Especially, it is shown that the evolution rate of the reverse dislocation decreases nonlinearly with the increase of strain after the current is removed. These results will be helpful to study the mechanical behavior of the rail during electromagnetic launch and further optimize the design of electromagnetic launch device.

Origin of the high propensity for nanoscale deformation twins in CrCoNi medium-entropy alloy

Single-phase face-centered cubic (fcc) high/medium-entropy alloys (H/MEAs) exhibit a much higher tendency to form nanoscale deformation twins than conventional fcc metals with similar low stacking fault energies (SFEs). This extraordinary propensity for nanotwin formation in H/MEAs cannot therefore be explained by their low SFEs alone. Here, using in situ compression tests of CrCoNi in comparison with Ag nanopillars inside a transmission electron microscope, we found that in the CrCoNi MEA, a high density of nanoscale twins continuously formed with an average thickness of 4.6 nm. In contrast, for similar experiments on Ag with almost identical SFE, following the nucleation of a few twins, they could further thicken to above one hundred nanometers by twin boundary migration. Molecular dynamics calculations indicated that in the highly-concentrated CrCoNi solid solution, the magnitude of the energy barriers for nucleating a stacking fault as a twin precursor in the pristine lattice and for the thickening of an existing twin both span a wide range and largely overlap with each other. Therefore, twin thickening through successive addition of atomic layers is prone to discontinuation, giving way to the nucleation of new twins at other sites where a lower energy barrier is encountered for partial-dislocation mediated fault formation.

Crystal-orientation-dependent physical sputtering from four elemental metals

Physical sputtering and its dependence on crystal orientation is well-known, but crystallinity of the target is neither commonly considered in simulations nor when publishing experimental values for its yield. Due to the recent development of detector technology and therefore of crystal orientation mapping, a measuring technique has been established to obtain the physical sputtering yield for an extensive number of orientations of polycrystalline specimens. In this work, yields for two bcc metals (W and Mo) and two fcc metals (Cu and Pt) for impacting Ga ions with 30 keV and around 5 keV energy were determined experimentally and are compared with molecular dynamics (MD) simulations in a recently established two level approach and with binary collision approximation (BCA) simulations. The agreement between experimental yields and simulations is excellent regarding distribution in the angular space and reasonable regarding absolute values. As expected, the variation in angular space among the materials with the same crystal lattice structure is minor.

Role of grain boundary crystallography on void growth in FCC metals

The growth of an isolated spherical void at and near a grain boundary or a triple junction is studied by means of full-field crystal plasticity simulations with explicit representation of the void. To examine the fundamental aspects of void/grain boundary interactions in these cases, a three-dimensional large-strain elasto-viscoplastic fast-Fourier transform (LS-EVP-FFT) model with axisymmetric tensile periodic boundary conditions was developed. We employ this model to reveal the role of crystallography (grain orientations, grain boundary misorientation and inclination angle with respect to the largest component of the applied stress) and type of loading on void growth. It was found that grain orientation was the primary crystallographic feature governing void growth in strain-rate controlled simulations. For strains small enough to not significantly alter the local geometry, the overall growth of the void in bicrystal boundaries was not strongly affected by grain boundary inclination angle. The relative amount of void growth experienced within each grain in a bicrystal or tri-crystal was strongly dependent on the grain boundary inclination angle. We found a subset of orientations that exhibit faster void growth in bicrystal boundaries than inside the constituent single crystals. Last, we show that when placed primarily in one grain but near the grain boundary, void growth was similar to void growth when placed at the grain boundary. The growing void remained in the original grain and did not grow into the other grain, regardless of crystallographic character.

Temperature- and internal structural size-dependent strength of nanotwinned face-centered cubic metals

The Nanotwin structure exhibits great potential as a strengthening medium, with its effectiveness influenced by both internal structural size and temperature. In this work, we developed a strength model that incorporates the dependence on temperature and internal structural size. This model can predict the strength of nanotwinned (NT) FCC metals with intracrystalline or intercrystalline twin boundaries featuring different internal structural sizes at varying temperatures at the inverse Hall-Petch stage. The proposed prediction model has significant importance for enhancing our understanding of the mechanical behavior exhibited by NT metals and facilitating the design of super-strong materials that can withstand extreme conditions.

Modulating strengthening ability of layer-grained fcc metals with distinct stacking fault energy: Molecular dynamics simulation

The limited ductility of nanocrystalline metals severely hinders their practical applications due to the absence of strain hardening. In this study, we observed strain hardening in layer-grained models through molecular dynamics simulations. Our quantitative analysis demonstrated that grain boundary deformation induced strain hardening, while dislocation activities led to strain softening. This finding is fundamentally different from previous assumptions that dislocation activities cause strain hardening, whereas grain boundary deformation can lead to strain softening. The competition between grain boundary deformation and dislocation activities determines the strain hardening behaviors in nanocrystalline metals. By modifying the stacking fault energy and tailoring lamellar grains, it is possible to increase the proportion of grain boundary deformation and achieve good ductility in strong nanocrystalline metals. Such metallic materials with high strength and good ductility, hold great potential for a wide range of industrial and engineering applications.