Helium bubbles are known to form in nuclear reactor structural components when displacement damag... more Helium bubbles are known to form in nuclear reactor structural components when displacement damage occurs in conjunction with helium exposure and/or transmutation. If left unchecked, bubble production can cause swelling, blistering, and embrittlement, all of which substantially degrade materials and-moreover-diminish mechanical properties. On the mission to produce more robust materials, nanocrystalline (NC) metals show great potential and are postulated to exhibit superior radiation resistance due to their high defect and particle sink densities; however, much is still unknown about the mechanisms of defect evolution in these systems under extreme conditions. Here, the performances of NC nickel (Ni) and iron (Fe) are investigated under helium bombardment via transmission electron microscopy (TEM). Bubble density statistics are measured as a function of grain size in specimens implanted under similar conditions. While the overall trends revealed an increase in bubble density up to saturation in both samples, bubble density in Fe was over 300% greater than in Ni. To interrogate the kinetics of helium diffusion and trapping, a rate theory model is developed that substantiates that helium is more readily captured within grains in helium-vacancy complexes in NC Fe, whereas helium is more prone to traversing the grain matrices and migrating to GBs in NC Ni. Our results suggest that (1) grain boundaries can affect bubble swelling in grain matrices significantly and can have a dominant effect over crystal structure, and (2) an NC-Ni-based material can yield superior resistance to irradiation-induced bubble growth compared to an NC-Fe-based material and exhibits high potential for use in extreme environments where swelling due to He bubble formation is of significant concern.
We analyze the mechanisms underlying the deformation of nanovoids in Ta single crystals subjected... more We analyze the mechanisms underlying the deformation of nanovoids in Ta single crystals subjected to cyclic uniaxial deformation using numerical simulations. Boundary and cell-size effects have been mitigated by means of the Quasicontinuum (QC) method. We have considered ∼1 billion-atom systems containing 11.2nm voids. Two kinds of simulations have been performed, each characterized by a different boundary condition. First, we compress the material along the nominal [001] direction, resulting in a highly-symmetric configuration that results in high stresses. Second, we load the material along the high-index [ 4 8 19] direction to confine plasticity to a single slip system and break the symmetry. We find that the plastic response under these two conditions is strikingly different, the former governed by dislocation loop emission and dipole formation, while the latter is dominated by twinning. We calculate the irreversible plastic work budget derived from a loading-unloading cycle and identify the most relevant yield points. These calculations represent the first fully 3D, fully non-local simulations of any bcc metal using QC.
Thermally-activated 1 /2 111 screw dislocation motion is the controlling plastic mechanism at low... more Thermally-activated 1 /2 111 screw dislocation motion is the controlling plastic mechanism at low temperatures in body-centered cubic (bcc) crystals. Motion proceeds by nucleation and propagation of atomic-sized kink pairs susceptible of being studied using molecular dynamics (MD). However, MD's natural inability to properly sample thermally-activated processes as well as to capture {110} screw dislocation glide calls for the development of other methods capable of overcoming these limitations. Here we develop a kinetic Monte Carlo (kMC) approach to study single screw dislocation dynamics from room temperature to 0.5T m and at stresses 0 < σ < 0.9σ P , where T m and σ P are the melting point and the Peierls stress. The method is entirely parameterized with atomistic simulations using an embedded atom potential for tungsten. To increase the physical fidelity of our simulations, we calculate the deviations from Schmid's law prescribed by the interatomic potential used and we study single dislocation kinetics using both projections. We calculate dislocation velocities as a function of stress, temperature, and dislocation line length. We find that considering non-Schmid effects has a strong influence on both the magnitude of the velocities and the trajectories followed by the dislocation. We finish by condensing all the calculated data into effective stress and temperature dependent mobilities to be used in more homogenized numerical methods.
We develop a nodal dislocation dynamics (DD) model to simulate plastic processes in fcc crystals.... more We develop a nodal dislocation dynamics (DD) model to simulate plastic processes in fcc crystals. The model explicitely accounts for all slip systems and Burgers vectors observed in fcc systems, including stacking faults and partial dislocations. We derive simple conservation rules that describe all partial dislocation interactions rigurosuly and allow us to model and quantify cross-slip processes, the structure and strength of dislocation junctions and the formation of fcc-specific structures such as stacking fault tetrahedra. The DD framework is built upon isotropic non-singular linear elasticity, and supports itself on information transmitted from the atomistic scale. In this fashion, connection between the meso and micro scales is attained self-consistently with core parameters fitted to atomistic data. We perform a series of targeted simulations to demonstrate the capabilities of the model, including dislocation reactions and dissociations and dislocation junction strength. Additionally we map the four-dimensional stress space relevant for cross-slip and relate our findings to the plastic behavior of monocrystalline fcc metals.
We conduct dislocation dynamics (DD) simulations of Fe periodic single crystals under tensile loa... more We conduct dislocation dynamics (DD) simulations of Fe periodic single crystals under tensile load at several high strain rates and temperatures. The simulations are enabled by the recent development of temperature-dependent dislocation mobility relations obtained from atomistic calculations. The plastic evolution in the simulations is governed by rapid initial dislocation multiplication, followed by a saturation of the flow stress when the subpopulation of slow plastic carriers becomes stabilized by dislocation annihilation. Above 500 K, edge dislocations coexist with screw dislocations and contribute proportionaly to the value of the flow stress. The DD simulations are used to interpret shock-loading experiments in Fe in terms of the relative importance of different strengthening mechanisms. We find that in the 10 4 -to-10 6 s -1 strain rate regime, work hardening explains the hardening of shock-loaded bulk Fe crystals.
In body-centered cubic (bcc) crystals, 1 2 111 screw dislocations exhibit high intrinsic lattice ... more In body-centered cubic (bcc) crystals, 1 2 111 screw dislocations exhibit high intrinsic lattice friction as a consequence of their non-planar core structure, which results in a periodic energy landscape known as the Peierls potential, UP . The main features determining plastic flow, including its stress and temperature dependences, can be derived directly from this potential, hence its importance. In this Letter, we use thermodynamic integration to provide a full thermodynamic extension of UP for bcc Fe. We compute the Peierls free energy path as a function of stress and temperature and show that the critical stress vanishes at 700K, supplying the qualitative elements that explain plastic behavior in the athermal limit.
Under the anticipated operating conditions for demonstration magnetic fusion reactors beyond ITER... more Under the anticipated operating conditions for demonstration magnetic fusion reactors beyond ITER, structural and plasma facing materials will be exposed to unprecedented conditions of irradiation, heat flux, and temperature. While such extreme environments remain inaccessible experimentally, computational modeling and simulation can provide qualitative and quantitative insights into materials response and complement the available experimental measurements with carefully validated predictions. For plasma facing components such as the first wall and the divertor, tungsten (W) has been selected as the leading candidate material due to its superior hightemperature and irradiation properties, as well as for its low retention of implanted tritium. In this paper we provide a review of recent efforts in computational modeling of W both as a plasmafacing material exposed to He deposition as well as a bulk material subjected to fast neutron irradiation. We use a multiscale modeling approach -commonly used as the materials modeling paradigm-to define the outline of the paper and highlight recent advances using several classes of techniques and their interconnection. We highlight several of the most salient findings obtained via computational modeling and point out a number of remaining challenges and future research directions.
Using atomistic simulations of dislocation motion in Ni and Ni-Au alloys we report a detailed stu... more Using atomistic simulations of dislocation motion in Ni and Ni-Au alloys we report a detailed study of the mobility function as a function of stress, temperature and alloy composition. We analyze the results in terms of analytic models of phonon radiation and their selection rules for phonon excitation. We find a remarkable agreement between the location of the cusps in the σ-v relation and the velocity of waves propagating in the direction of dislocation motion. We identify and characterize three regimes of dissipation whose boundaries are essentially determined by the direction of motion of the dislocation, rather than by its screw or edge character.
Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as p... more Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as perfect interfaces that play a significant role in a variety of materials. Although the ability of CTBs in strengthening, maintaining the ductility and minimizing the electron scattering is well documented 1-3 , most of our understanding of the origin of these properties relies on perfect-interface assumptions. Here we report experiments and simulations demonstrating that as-grown CTBs in nanotwinned copper are inherently defective with kink-like steps and curvature, and that these imperfections consist of incoherent segments and partial dislocations. We further show that these defects play a crucial role in the deformation mechanisms and mechanical behaviour of nanotwinned copper. Our findings offer a view of the structure of CTBs that is largely different from that in the literature 2,4,5 , and underscore the significance of imperfections in nanotwinstrengthened materials. CTBs formed during growth, deformation or annealing exist broadly in many crystalline solids with low or medium stackingfault energies . The strengthening behaviour and other attractive properties of CTBs have been studied in nanotwinned metals (with an average twin spacing <100 nm; refs 7-9). One prevalent view is that CTB-strengthened materials have certain advantages over nanocrystalline or ultrafine-grained materials; that is, materials strengthened through traditional grain boundaries (GBs) that are considered incoherent and defective 10 . GBs not only scatter electrons, but can migrate and slide under shear stresses 11 , leading to a maximum in strength in nanocrystalline materials . In contrast, such migration/sliding mechanisms may not be operative in CTBs despite some reports of detwinning evidence and the observation of a similar maximum strength in a nanotwinned copper 3 (nt-Cu). Existing models widely assume perfect CTBs and rationalize flow softening due to CTB migrations and detwinning as caused by nucleation and motion of partial dislocations parallel to CTBs (ref. 4). These mechanisms are informative as long as CTB lengths are limited to the tens of nanometres typically used in molecular dynamics simulations . It still remains difficult through molecular dynamics simulations to validate the migrations/detwinning of the much longer CTBs seen in experiments (500 nm; ref. 3). There could be alternative mechanisms that are intricately related to the potential structures of CTBs and the characteristics of GBs, both of which are not accounted for in the literature. Recent studies of nanotwinned copper pillars without GBs revealed strong deformation anisotropy and a brittle-to-ductile transition behaviour (where CTBs are considered intrinsically brittle) 2 , suggesting that CTBs alone are not sufficient for increased plasticity despite their strong strengthening effect, and that a reasonable mix of GBs is helpful to mediate the plasticity and achieve high ductility. Experiments and simulations have frequently
The mechanical response of complex concentrated alloys (CCAs) deviates from that of their pure an... more The mechanical response of complex concentrated alloys (CCAs) deviates from that of their pure and dilute counterparts due to the introduction of a combinatorially sized chemical concentration dimension. Compositional fluctuations constantly alter the energy landscape over which dislocations move, leading to line roughness and the appearance of defects such as kinks and jogs under stress and temperature conditions where they would ordinarily not exist in pure metals and dilute alloys. The presence of such chemical defects gives rise to atomic-level mechanisms that fundamentally change how CCAs deform plastically at meso- and macroscales. In this article, we provide a review of recent advances in modeling dislocation glide processes in CCAs, including atomistic simulations of dislocation glide using molecular dynamics, kinetic Monte Carlo simulations of edge and screw dislocation motion in refractory CCAs, and phase-field models of dislocation evolution over complex energy landscapes...
We present a numerical methodology to compute the Nye-tensor fingerprints of dislocation loop abs... more We present a numerical methodology to compute the Nye-tensor fingerprints of dislocation loop absorption at grain boundaries (GBs) for comparison with TEM observations of irradiated polycrystals. Our approach links atomistic simulations of self-interstitial atom (SIA) prismatic loops gliding toward and interacting with GBs in body-centered cubic iron with experimentally extracted geometrically necessary dislocation (GND) maps to facilitate the interpretation of damage processes. The Nye-tensor analysis is strongly mesh-size dependent-corresponding to resolution-dependent TEM observations. The method computes GND fingerprints from discretized dislocation line segments extracted from molecular dynamics simulations of dislocation loops being absorbed at a GB. Specifically, we perform MD simulation of prismatic loops of two diameters and monitor the three stages of the absorption process: loop glide, the partial, and full absorption of the loops at a [1 0 0] symmetric tilt GB. These methods provide a framework for future investigations of the nature of defect absorption by grain boundaries under irradiation conditions.
In this work, we study vacancy energetics in the equiatomic Nb-Mo-Ta-W alloy, especially vacancy ... more In this work, we study vacancy energetics in the equiatomic Nb-Mo-Ta-W alloy, especially vacancy formation and migration energies, using molecular statics calculations based on a spectral neighbor analysis potential specifically developed for Nb-Mo-Ta-W. We consider vacancy properties in bulk environments as well as near edge dislocation cores, including the effect of short-range order (SRO) by preparing supercells through Metropolis Monte-Carlo relaxations and temperature on the calculation. The nudged elastic band (NEB) method is applied to study vacancy migration energies. Our results show that both vacancy formation energies and vacancy migration energies are statistically distributed with a wide spread, on the order of 1.0 eV in some cases, and display a noticeable dependence on SRO. We find that, in some cases, vacancies can form with very low energies at edge dislocation cores, from which we hypothesize the formation of stable ‘superjogs’ on edge dislocation lines. Moreover, ...
Helium bubbles are known to form in nuclear reactor structural components when displacement damag... more Helium bubbles are known to form in nuclear reactor structural components when displacement damage occurs in conjunction with helium exposure and/or transmutation. If left unchecked, bubble production can cause swelling, blistering, and embrittlement, all of which substantially degrade materials and—moreover—diminish mechanical properties. On the mission to produce more robust materials, nanocrystalline (NC) metals show great potential and are postulated to exhibit superior radiation resistance due to their high defect and particle sink densities; however, much is still unknown about the mechanisms of defect evolution in these systems under extreme conditions. Here, the performances of NC nickel (Ni) and iron (Fe) are investigated under helium bombardment via transmission electron microscopy (TEM). Bubble density statistics are measured as a function of grain size in specimens implanted under similar conditions. While the overall trends revealed an increase in bubble density up to s...
lthough the mechanisms underpinning Hall-Petch hardening and softening in nanocrystalline (NC) an... more lthough the mechanisms underpinning Hall-Petch hardening and softening in nanocrystalline (NC) and nanotwinned (NT) materials have been investigated separately for several decades , an interesting scenario may arise when a metal is strengthened by both grain boundaries (GBs) and twin boundaries (TBs); that is, in a new class of NC and NT materials (we term them nanocrystalline-nanotwinned materials (NNTs) from here on). The existing theories suggest drastically different Hall-Petch breakdown mechanisms in these two types of material; that is, the former is caused by GB sliding 3 and the latter by dislocation-nucleation governed plasticity 6 . To date, the atomic softening mechanisms have been garnered by molecular dynamics (MD) simulations of materials containing nanosized grains (≤70 nm) . It has been a longstanding challenge to experimentally verify or disapprove these mechanisms due to the technological difficulty of synthesizing nanostructured materials with microstructures that are as ideal and infinitesimal as those used in these models. For pure NT metals, the grain sizes (d) of assynthesized materials are typically larger than 100 nm (refs. 5,14,15 ), precluding experimental investigations of the truly NC region. It thus remains scientifically significant to experimentally interrogate the strength behaviour in NT materials with d well below 100 nm, where GBs and TBs could become competing mechanisms. In essence, the strengthening and softening behaviour in NNT metals is largely unknown at present. We choose silver (Ag) as a model material system to investigate the Hall-Petch strengthening and softening in NNT-materials. Ag is a low stacking-fault energy metal (~16 mJ m -2 ) 16 and known to form copious growth nanotwins during magnetron sputtering processes . However, the smallest d achieved using this method is ~150 nm (ref. ); that is, outside the NC region. Heavily alloying is a common strategy to reducing d but inevitably complicates the fundamental mechanism studies due to the hardening effects of solutes , or a change of stacking-fault energies in Ag caused by alloying that alters deformation mechanisms. For these reasons, we developed a microalloying (or doping) strategy by carefully selecting Cu as the primary impurity-a solute that is predicted by our own simulations to have no solid-solution strengthening effect in Ag when its content is below 3.0 wt% (Supplementary Information and Supplementary Fig. ). Neither will Cu affect the stacking-fault energy of Ag at a concentration <1.0 wt%. Moreover, Cu atoms are ~12% smaller than Ag ones, and Ag-Cu is an immiscible system, which facilitates the segregation of Cu into high-energy interface sites such as GB and TB defects 15 . We have successfully fabricated a series of Cu-and Al-microalloyed (<1.0 wt%) NNT-Ag samples (Supplementary Table ). The strongest material has an average d of 49 ± 15 nm and twin spacing λ = 3.6 ± 1.5 nm. We find that these materials retain a continuous Hall-Petch strengthening behaviour down to the smallest twin size, which markedly differs from the projected Hall-Petch breakdown from ref. . We performed atomic simulations with comparable d value to experiments and varying λ. In contrast to popular belief, three Hall-Petch strengthening and softening regions are uncovered. We observe that the softening behaviour is fundamentally different between 'ideal' materials with perfect TBs and 'real world' materials that contain TB defects. A strength plateau region is observed for pure NT-Ag and Al-microalloyed NNT-Ag, which qualitatively agrees with our simulation predictions. Large-scale hybrid Monte Carlo (MC) and MD simulations involving a total of 2.3 billion atoms were used to study ten λ
Nanocrystalline materials have useful mechanical properties: superplasticity, increased hardness,... more Nanocrystalline materials have useful mechanical properties: superplasticity, increased hardness, etc. BUT Can we use nanocrystalline materials in environments with "extreme" radiation doses (fusion reactors, space applications)? keV bombardment of nanocrystals Can we design of improved radiation-resistant materials? 20 keV, 12 nm grains BULK: ZM. Samaras et al. [PRL 88 (2002) 125505] and W. Voegeli et al. [NIMB 202 (2003) 230], found that irradiation can lead to grain size changes and that grain boundaries absorb most point defects. No radiation damage studies to the best of our knowledge. Ion bombardment è localized "shocks" è behavior of materials under extreme pressure/temperature conditions. Can we create new, better materials (new phases, metastable states, etc.) using high-pressure loading? fast ion H 2 O MeV-GeV ions: (fission) track evolution Two main "competing" models Coulomb Explosion Thermal Spikes Fleisher, Price and Walker,
In body-centered cubic (bcc) crystals, 1 2 111 screw dislocations exhibit high intrinsic lattice ... more In body-centered cubic (bcc) crystals, 1 2 111 screw dislocations exhibit high intrinsic lattice friction as a consequence of their non-planar core structure, which results in a periodic energy landscape known as the Peierls potential, UP . The main features determining plastic flow, including its stress and temperature dependences, can be derived directly from this potential, hence its importance. In this Letter, we use thermodynamic integration to provide a full thermodynamic extension of UP for bcc Fe. We compute the Peierls free energy path as a function of stress and temperature and show that the critical stress vanishes at 700K, supplying the qualitative elements that explain plastic behavior in the athermal limit.
Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as p... more Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as perfect interfaces that play a significant role in a variety of materials. Although the ability of CTBs in strengthening, maintaining the ductility and minimizing the electron scattering is well documented 1-3 , most of our understanding of the origin of these properties relies on perfect-interface assumptions. Here we report experiments and simulations demonstrating that as-grown CTBs in nanotwinned copper are inherently defective with kink-like steps and curvature, and that these imperfections consist of incoherent segments and partial dislocations. We further show that these defects play a crucial role in the deformation mechanisms and mechanical behaviour of nanotwinned copper. Our findings offer a view of the structure of CTBs that is largely different from that in the literature 2,4,5 , and underscore the significance of imperfections in nanotwinstrengthened materials. CTBs formed during growth, deformation or annealing exist broadly in many crystalline solids with low or medium stackingfault energies . The strengthening behaviour and other attractive properties of CTBs have been studied in nanotwinned metals (with an average twin spacing <100 nm; refs 7-9). One prevalent view is that CTB-strengthened materials have certain advantages over nanocrystalline or ultrafine-grained materials; that is, materials strengthened through traditional grain boundaries (GBs) that are considered incoherent and defective 10 . GBs not only scatter electrons, but can migrate and slide under shear stresses 11 , leading to a maximum in strength in nanocrystalline materials . In contrast, such migration/sliding mechanisms may not be operative in CTBs despite some reports of detwinning evidence and the observation of a similar maximum strength in a nanotwinned copper 3 (nt-Cu). Existing models widely assume perfect CTBs and rationalize flow softening due to CTB migrations and detwinning as caused by nucleation and motion of partial dislocations parallel to CTBs (ref. 4). These mechanisms are informative as long as CTB lengths are limited to the tens of nanometres typically used in molecular dynamics simulations . It still remains difficult through molecular dynamics simulations to validate the migrations/detwinning of the much longer CTBs seen in experiments (500 nm; ref. 3). There could be alternative mechanisms that are intricately related to the potential structures of CTBs and the characteristics of GBs, both of which are not accounted for in the literature. Recent studies of nanotwinned copper pillars without GBs revealed strong deformation anisotropy and a brittle-to-ductile transition behaviour (where CTBs are considered intrinsically brittle) 2 , suggesting that CTBs alone are not sufficient for increased plasticity despite their strong strengthening effect, and that a reasonable mix of GBs is helpful to mediate the plasticity and achieve high ductility. Experiments and simulations have frequently
Abstract Atomistic simulations have shown that a screw dislocation in body-centered cubic (BCC) m... more Abstract Atomistic simulations have shown that a screw dislocation in body-centered cubic (BCC) metals has a complex non-planar atomic core structure. The configuration of this core controls their motion and is affected not only by the usual resolved shear stress on the dislocation, but also by non-driving stress components. Consequences of the latter are referred to as non-Schmid effects. These atomic and micro-scale effects are the reason slip characteristics in deforming single and polycrystalline BCC metals are extremely sensitive to the direction and sense of the applied load. In this paper, we develop a three-dimensional discrete dislocation dynamics (DD) simulation model to understand the relationship between individual dislocation glide behavior and macro-scale plastic slip behavior in single crystal BCC Ta. For the first time, it is shown that non-Schmid effects on screw dislocations of both {110} and {112} slip systems must be implemented into the DD models in order to predict the strong plastic anisotropy and tension–compression asymmetry experimentally observed in the stress–strain curves of single crystal Ta. Incorporation of fundamental atomistic information is critical for developing a physics-based, predictive meso-scale DD simulation tool that can connect length/time scales and investigate the underlying mechanisms governing the deformation of BCC metals.
Dislocation mobility -the relation between applied stress and dislocation velocity-is an importan... more Dislocation mobility -the relation between applied stress and dislocation velocity-is an important property to model the mechanical behavior of structural materials. These mobilities reflect the interaction between the dislocation core and the host lattice and, thus, atomistic resolution is required to capture its details. Because the mobility function is multiparametric, its computation is often highly demanding in terms of computational requirements. Optimizing how tractions are applied can be greatly advantageous in accelerating convergence and reducing the overall computational cost of the simulations. In this paper we perform molecular dynamics simulations of Vi (111) screw dislocation motion in tungsten using step and linear time functions for applying external stress. We find that linear functions over time scales of the order of 10-20 ps reduce fluctuations and speed up convergence to the steady-state velocity value by up to a factor of two.
We conduct dislocation dynamics (DD) simulations of Fe periodic single crystals under tensile loa... more We conduct dislocation dynamics (DD) simulations of Fe periodic single crystals under tensile load at several high strain rates and temperatures. The simulations are enabled by the recent development of temperature-dependent dislocation mobility relations obtained from atomistic calculations. The plastic evolution in the simulations is governed by rapid initial dislocation multiplication, followed by a saturation of the flow stress when the subpopulation of slow plastic carriers becomes stabilized by dislocation annihilation. Above 500 K, edge dislocations coexist with screw dislocations and contribute proportionaly to the value of the flow stress. The DD simulations are used to interpret shock-loading experiments in Fe in terms of the relative importance of different strengthening mechanisms. We find that in the 10 4 -to-10 6 s -1 strain rate regime, work hardening explains the hardening of shock-loaded bulk Fe crystals.
Helium bubbles are known to form in nuclear reactor structural components when displacement damag... more Helium bubbles are known to form in nuclear reactor structural components when displacement damage occurs in conjunction with helium exposure and/or transmutation. If left unchecked, bubble production can cause swelling, blistering, and embrittlement, all of which substantially degrade materials and-moreover-diminish mechanical properties. On the mission to produce more robust materials, nanocrystalline (NC) metals show great potential and are postulated to exhibit superior radiation resistance due to their high defect and particle sink densities; however, much is still unknown about the mechanisms of defect evolution in these systems under extreme conditions. Here, the performances of NC nickel (Ni) and iron (Fe) are investigated under helium bombardment via transmission electron microscopy (TEM). Bubble density statistics are measured as a function of grain size in specimens implanted under similar conditions. While the overall trends revealed an increase in bubble density up to saturation in both samples, bubble density in Fe was over 300% greater than in Ni. To interrogate the kinetics of helium diffusion and trapping, a rate theory model is developed that substantiates that helium is more readily captured within grains in helium-vacancy complexes in NC Fe, whereas helium is more prone to traversing the grain matrices and migrating to GBs in NC Ni. Our results suggest that (1) grain boundaries can affect bubble swelling in grain matrices significantly and can have a dominant effect over crystal structure, and (2) an NC-Ni-based material can yield superior resistance to irradiation-induced bubble growth compared to an NC-Fe-based material and exhibits high potential for use in extreme environments where swelling due to He bubble formation is of significant concern.
We analyze the mechanisms underlying the deformation of nanovoids in Ta single crystals subjected... more We analyze the mechanisms underlying the deformation of nanovoids in Ta single crystals subjected to cyclic uniaxial deformation using numerical simulations. Boundary and cell-size effects have been mitigated by means of the Quasicontinuum (QC) method. We have considered ∼1 billion-atom systems containing 11.2nm voids. Two kinds of simulations have been performed, each characterized by a different boundary condition. First, we compress the material along the nominal [001] direction, resulting in a highly-symmetric configuration that results in high stresses. Second, we load the material along the high-index [ 4 8 19] direction to confine plasticity to a single slip system and break the symmetry. We find that the plastic response under these two conditions is strikingly different, the former governed by dislocation loop emission and dipole formation, while the latter is dominated by twinning. We calculate the irreversible plastic work budget derived from a loading-unloading cycle and identify the most relevant yield points. These calculations represent the first fully 3D, fully non-local simulations of any bcc metal using QC.
Thermally-activated 1 /2 111 screw dislocation motion is the controlling plastic mechanism at low... more Thermally-activated 1 /2 111 screw dislocation motion is the controlling plastic mechanism at low temperatures in body-centered cubic (bcc) crystals. Motion proceeds by nucleation and propagation of atomic-sized kink pairs susceptible of being studied using molecular dynamics (MD). However, MD's natural inability to properly sample thermally-activated processes as well as to capture {110} screw dislocation glide calls for the development of other methods capable of overcoming these limitations. Here we develop a kinetic Monte Carlo (kMC) approach to study single screw dislocation dynamics from room temperature to 0.5T m and at stresses 0 < σ < 0.9σ P , where T m and σ P are the melting point and the Peierls stress. The method is entirely parameterized with atomistic simulations using an embedded atom potential for tungsten. To increase the physical fidelity of our simulations, we calculate the deviations from Schmid's law prescribed by the interatomic potential used and we study single dislocation kinetics using both projections. We calculate dislocation velocities as a function of stress, temperature, and dislocation line length. We find that considering non-Schmid effects has a strong influence on both the magnitude of the velocities and the trajectories followed by the dislocation. We finish by condensing all the calculated data into effective stress and temperature dependent mobilities to be used in more homogenized numerical methods.
We develop a nodal dislocation dynamics (DD) model to simulate plastic processes in fcc crystals.... more We develop a nodal dislocation dynamics (DD) model to simulate plastic processes in fcc crystals. The model explicitely accounts for all slip systems and Burgers vectors observed in fcc systems, including stacking faults and partial dislocations. We derive simple conservation rules that describe all partial dislocation interactions rigurosuly and allow us to model and quantify cross-slip processes, the structure and strength of dislocation junctions and the formation of fcc-specific structures such as stacking fault tetrahedra. The DD framework is built upon isotropic non-singular linear elasticity, and supports itself on information transmitted from the atomistic scale. In this fashion, connection between the meso and micro scales is attained self-consistently with core parameters fitted to atomistic data. We perform a series of targeted simulations to demonstrate the capabilities of the model, including dislocation reactions and dissociations and dislocation junction strength. Additionally we map the four-dimensional stress space relevant for cross-slip and relate our findings to the plastic behavior of monocrystalline fcc metals.
We conduct dislocation dynamics (DD) simulations of Fe periodic single crystals under tensile loa... more We conduct dislocation dynamics (DD) simulations of Fe periodic single crystals under tensile load at several high strain rates and temperatures. The simulations are enabled by the recent development of temperature-dependent dislocation mobility relations obtained from atomistic calculations. The plastic evolution in the simulations is governed by rapid initial dislocation multiplication, followed by a saturation of the flow stress when the subpopulation of slow plastic carriers becomes stabilized by dislocation annihilation. Above 500 K, edge dislocations coexist with screw dislocations and contribute proportionaly to the value of the flow stress. The DD simulations are used to interpret shock-loading experiments in Fe in terms of the relative importance of different strengthening mechanisms. We find that in the 10 4 -to-10 6 s -1 strain rate regime, work hardening explains the hardening of shock-loaded bulk Fe crystals.
In body-centered cubic (bcc) crystals, 1 2 111 screw dislocations exhibit high intrinsic lattice ... more In body-centered cubic (bcc) crystals, 1 2 111 screw dislocations exhibit high intrinsic lattice friction as a consequence of their non-planar core structure, which results in a periodic energy landscape known as the Peierls potential, UP . The main features determining plastic flow, including its stress and temperature dependences, can be derived directly from this potential, hence its importance. In this Letter, we use thermodynamic integration to provide a full thermodynamic extension of UP for bcc Fe. We compute the Peierls free energy path as a function of stress and temperature and show that the critical stress vanishes at 700K, supplying the qualitative elements that explain plastic behavior in the athermal limit.
Under the anticipated operating conditions for demonstration magnetic fusion reactors beyond ITER... more Under the anticipated operating conditions for demonstration magnetic fusion reactors beyond ITER, structural and plasma facing materials will be exposed to unprecedented conditions of irradiation, heat flux, and temperature. While such extreme environments remain inaccessible experimentally, computational modeling and simulation can provide qualitative and quantitative insights into materials response and complement the available experimental measurements with carefully validated predictions. For plasma facing components such as the first wall and the divertor, tungsten (W) has been selected as the leading candidate material due to its superior hightemperature and irradiation properties, as well as for its low retention of implanted tritium. In this paper we provide a review of recent efforts in computational modeling of W both as a plasmafacing material exposed to He deposition as well as a bulk material subjected to fast neutron irradiation. We use a multiscale modeling approach -commonly used as the materials modeling paradigm-to define the outline of the paper and highlight recent advances using several classes of techniques and their interconnection. We highlight several of the most salient findings obtained via computational modeling and point out a number of remaining challenges and future research directions.
Using atomistic simulations of dislocation motion in Ni and Ni-Au alloys we report a detailed stu... more Using atomistic simulations of dislocation motion in Ni and Ni-Au alloys we report a detailed study of the mobility function as a function of stress, temperature and alloy composition. We analyze the results in terms of analytic models of phonon radiation and their selection rules for phonon excitation. We find a remarkable agreement between the location of the cusps in the σ-v relation and the velocity of waves propagating in the direction of dislocation motion. We identify and characterize three regimes of dissipation whose boundaries are essentially determined by the direction of motion of the dislocation, rather than by its screw or edge character.
Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as p... more Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as perfect interfaces that play a significant role in a variety of materials. Although the ability of CTBs in strengthening, maintaining the ductility and minimizing the electron scattering is well documented 1-3 , most of our understanding of the origin of these properties relies on perfect-interface assumptions. Here we report experiments and simulations demonstrating that as-grown CTBs in nanotwinned copper are inherently defective with kink-like steps and curvature, and that these imperfections consist of incoherent segments and partial dislocations. We further show that these defects play a crucial role in the deformation mechanisms and mechanical behaviour of nanotwinned copper. Our findings offer a view of the structure of CTBs that is largely different from that in the literature 2,4,5 , and underscore the significance of imperfections in nanotwinstrengthened materials. CTBs formed during growth, deformation or annealing exist broadly in many crystalline solids with low or medium stackingfault energies . The strengthening behaviour and other attractive properties of CTBs have been studied in nanotwinned metals (with an average twin spacing <100 nm; refs 7-9). One prevalent view is that CTB-strengthened materials have certain advantages over nanocrystalline or ultrafine-grained materials; that is, materials strengthened through traditional grain boundaries (GBs) that are considered incoherent and defective 10 . GBs not only scatter electrons, but can migrate and slide under shear stresses 11 , leading to a maximum in strength in nanocrystalline materials . In contrast, such migration/sliding mechanisms may not be operative in CTBs despite some reports of detwinning evidence and the observation of a similar maximum strength in a nanotwinned copper 3 (nt-Cu). Existing models widely assume perfect CTBs and rationalize flow softening due to CTB migrations and detwinning as caused by nucleation and motion of partial dislocations parallel to CTBs (ref. 4). These mechanisms are informative as long as CTB lengths are limited to the tens of nanometres typically used in molecular dynamics simulations . It still remains difficult through molecular dynamics simulations to validate the migrations/detwinning of the much longer CTBs seen in experiments (500 nm; ref. 3). There could be alternative mechanisms that are intricately related to the potential structures of CTBs and the characteristics of GBs, both of which are not accounted for in the literature. Recent studies of nanotwinned copper pillars without GBs revealed strong deformation anisotropy and a brittle-to-ductile transition behaviour (where CTBs are considered intrinsically brittle) 2 , suggesting that CTBs alone are not sufficient for increased plasticity despite their strong strengthening effect, and that a reasonable mix of GBs is helpful to mediate the plasticity and achieve high ductility. Experiments and simulations have frequently
The mechanical response of complex concentrated alloys (CCAs) deviates from that of their pure an... more The mechanical response of complex concentrated alloys (CCAs) deviates from that of their pure and dilute counterparts due to the introduction of a combinatorially sized chemical concentration dimension. Compositional fluctuations constantly alter the energy landscape over which dislocations move, leading to line roughness and the appearance of defects such as kinks and jogs under stress and temperature conditions where they would ordinarily not exist in pure metals and dilute alloys. The presence of such chemical defects gives rise to atomic-level mechanisms that fundamentally change how CCAs deform plastically at meso- and macroscales. In this article, we provide a review of recent advances in modeling dislocation glide processes in CCAs, including atomistic simulations of dislocation glide using molecular dynamics, kinetic Monte Carlo simulations of edge and screw dislocation motion in refractory CCAs, and phase-field models of dislocation evolution over complex energy landscapes...
We present a numerical methodology to compute the Nye-tensor fingerprints of dislocation loop abs... more We present a numerical methodology to compute the Nye-tensor fingerprints of dislocation loop absorption at grain boundaries (GBs) for comparison with TEM observations of irradiated polycrystals. Our approach links atomistic simulations of self-interstitial atom (SIA) prismatic loops gliding toward and interacting with GBs in body-centered cubic iron with experimentally extracted geometrically necessary dislocation (GND) maps to facilitate the interpretation of damage processes. The Nye-tensor analysis is strongly mesh-size dependent-corresponding to resolution-dependent TEM observations. The method computes GND fingerprints from discretized dislocation line segments extracted from molecular dynamics simulations of dislocation loops being absorbed at a GB. Specifically, we perform MD simulation of prismatic loops of two diameters and monitor the three stages of the absorption process: loop glide, the partial, and full absorption of the loops at a [1 0 0] symmetric tilt GB. These methods provide a framework for future investigations of the nature of defect absorption by grain boundaries under irradiation conditions.
In this work, we study vacancy energetics in the equiatomic Nb-Mo-Ta-W alloy, especially vacancy ... more In this work, we study vacancy energetics in the equiatomic Nb-Mo-Ta-W alloy, especially vacancy formation and migration energies, using molecular statics calculations based on a spectral neighbor analysis potential specifically developed for Nb-Mo-Ta-W. We consider vacancy properties in bulk environments as well as near edge dislocation cores, including the effect of short-range order (SRO) by preparing supercells through Metropolis Monte-Carlo relaxations and temperature on the calculation. The nudged elastic band (NEB) method is applied to study vacancy migration energies. Our results show that both vacancy formation energies and vacancy migration energies are statistically distributed with a wide spread, on the order of 1.0 eV in some cases, and display a noticeable dependence on SRO. We find that, in some cases, vacancies can form with very low energies at edge dislocation cores, from which we hypothesize the formation of stable ‘superjogs’ on edge dislocation lines. Moreover, ...
Helium bubbles are known to form in nuclear reactor structural components when displacement damag... more Helium bubbles are known to form in nuclear reactor structural components when displacement damage occurs in conjunction with helium exposure and/or transmutation. If left unchecked, bubble production can cause swelling, blistering, and embrittlement, all of which substantially degrade materials and—moreover—diminish mechanical properties. On the mission to produce more robust materials, nanocrystalline (NC) metals show great potential and are postulated to exhibit superior radiation resistance due to their high defect and particle sink densities; however, much is still unknown about the mechanisms of defect evolution in these systems under extreme conditions. Here, the performances of NC nickel (Ni) and iron (Fe) are investigated under helium bombardment via transmission electron microscopy (TEM). Bubble density statistics are measured as a function of grain size in specimens implanted under similar conditions. While the overall trends revealed an increase in bubble density up to s...
lthough the mechanisms underpinning Hall-Petch hardening and softening in nanocrystalline (NC) an... more lthough the mechanisms underpinning Hall-Petch hardening and softening in nanocrystalline (NC) and nanotwinned (NT) materials have been investigated separately for several decades , an interesting scenario may arise when a metal is strengthened by both grain boundaries (GBs) and twin boundaries (TBs); that is, in a new class of NC and NT materials (we term them nanocrystalline-nanotwinned materials (NNTs) from here on). The existing theories suggest drastically different Hall-Petch breakdown mechanisms in these two types of material; that is, the former is caused by GB sliding 3 and the latter by dislocation-nucleation governed plasticity 6 . To date, the atomic softening mechanisms have been garnered by molecular dynamics (MD) simulations of materials containing nanosized grains (≤70 nm) . It has been a longstanding challenge to experimentally verify or disapprove these mechanisms due to the technological difficulty of synthesizing nanostructured materials with microstructures that are as ideal and infinitesimal as those used in these models. For pure NT metals, the grain sizes (d) of assynthesized materials are typically larger than 100 nm (refs. 5,14,15 ), precluding experimental investigations of the truly NC region. It thus remains scientifically significant to experimentally interrogate the strength behaviour in NT materials with d well below 100 nm, where GBs and TBs could become competing mechanisms. In essence, the strengthening and softening behaviour in NNT metals is largely unknown at present. We choose silver (Ag) as a model material system to investigate the Hall-Petch strengthening and softening in NNT-materials. Ag is a low stacking-fault energy metal (~16 mJ m -2 ) 16 and known to form copious growth nanotwins during magnetron sputtering processes . However, the smallest d achieved using this method is ~150 nm (ref. ); that is, outside the NC region. Heavily alloying is a common strategy to reducing d but inevitably complicates the fundamental mechanism studies due to the hardening effects of solutes , or a change of stacking-fault energies in Ag caused by alloying that alters deformation mechanisms. For these reasons, we developed a microalloying (or doping) strategy by carefully selecting Cu as the primary impurity-a solute that is predicted by our own simulations to have no solid-solution strengthening effect in Ag when its content is below 3.0 wt% (Supplementary Information and Supplementary Fig. ). Neither will Cu affect the stacking-fault energy of Ag at a concentration <1.0 wt%. Moreover, Cu atoms are ~12% smaller than Ag ones, and Ag-Cu is an immiscible system, which facilitates the segregation of Cu into high-energy interface sites such as GB and TB defects 15 . We have successfully fabricated a series of Cu-and Al-microalloyed (<1.0 wt%) NNT-Ag samples (Supplementary Table ). The strongest material has an average d of 49 ± 15 nm and twin spacing λ = 3.6 ± 1.5 nm. We find that these materials retain a continuous Hall-Petch strengthening behaviour down to the smallest twin size, which markedly differs from the projected Hall-Petch breakdown from ref. . We performed atomic simulations with comparable d value to experiments and varying λ. In contrast to popular belief, three Hall-Petch strengthening and softening regions are uncovered. We observe that the softening behaviour is fundamentally different between 'ideal' materials with perfect TBs and 'real world' materials that contain TB defects. A strength plateau region is observed for pure NT-Ag and Al-microalloyed NNT-Ag, which qualitatively agrees with our simulation predictions. Large-scale hybrid Monte Carlo (MC) and MD simulations involving a total of 2.3 billion atoms were used to study ten λ
Nanocrystalline materials have useful mechanical properties: superplasticity, increased hardness,... more Nanocrystalline materials have useful mechanical properties: superplasticity, increased hardness, etc. BUT Can we use nanocrystalline materials in environments with "extreme" radiation doses (fusion reactors, space applications)? keV bombardment of nanocrystals Can we design of improved radiation-resistant materials? 20 keV, 12 nm grains BULK: ZM. Samaras et al. [PRL 88 (2002) 125505] and W. Voegeli et al. [NIMB 202 (2003) 230], found that irradiation can lead to grain size changes and that grain boundaries absorb most point defects. No radiation damage studies to the best of our knowledge. Ion bombardment è localized "shocks" è behavior of materials under extreme pressure/temperature conditions. Can we create new, better materials (new phases, metastable states, etc.) using high-pressure loading? fast ion H 2 O MeV-GeV ions: (fission) track evolution Two main "competing" models Coulomb Explosion Thermal Spikes Fleisher, Price and Walker,
In body-centered cubic (bcc) crystals, 1 2 111 screw dislocations exhibit high intrinsic lattice ... more In body-centered cubic (bcc) crystals, 1 2 111 screw dislocations exhibit high intrinsic lattice friction as a consequence of their non-planar core structure, which results in a periodic energy landscape known as the Peierls potential, UP . The main features determining plastic flow, including its stress and temperature dependences, can be derived directly from this potential, hence its importance. In this Letter, we use thermodynamic integration to provide a full thermodynamic extension of UP for bcc Fe. We compute the Peierls free energy path as a function of stress and temperature and show that the critical stress vanishes at 700K, supplying the qualitative elements that explain plastic behavior in the athermal limit.
Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as p... more Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as perfect interfaces that play a significant role in a variety of materials. Although the ability of CTBs in strengthening, maintaining the ductility and minimizing the electron scattering is well documented 1-3 , most of our understanding of the origin of these properties relies on perfect-interface assumptions. Here we report experiments and simulations demonstrating that as-grown CTBs in nanotwinned copper are inherently defective with kink-like steps and curvature, and that these imperfections consist of incoherent segments and partial dislocations. We further show that these defects play a crucial role in the deformation mechanisms and mechanical behaviour of nanotwinned copper. Our findings offer a view of the structure of CTBs that is largely different from that in the literature 2,4,5 , and underscore the significance of imperfections in nanotwinstrengthened materials. CTBs formed during growth, deformation or annealing exist broadly in many crystalline solids with low or medium stackingfault energies . The strengthening behaviour and other attractive properties of CTBs have been studied in nanotwinned metals (with an average twin spacing <100 nm; refs 7-9). One prevalent view is that CTB-strengthened materials have certain advantages over nanocrystalline or ultrafine-grained materials; that is, materials strengthened through traditional grain boundaries (GBs) that are considered incoherent and defective 10 . GBs not only scatter electrons, but can migrate and slide under shear stresses 11 , leading to a maximum in strength in nanocrystalline materials . In contrast, such migration/sliding mechanisms may not be operative in CTBs despite some reports of detwinning evidence and the observation of a similar maximum strength in a nanotwinned copper 3 (nt-Cu). Existing models widely assume perfect CTBs and rationalize flow softening due to CTB migrations and detwinning as caused by nucleation and motion of partial dislocations parallel to CTBs (ref. 4). These mechanisms are informative as long as CTB lengths are limited to the tens of nanometres typically used in molecular dynamics simulations . It still remains difficult through molecular dynamics simulations to validate the migrations/detwinning of the much longer CTBs seen in experiments (500 nm; ref. 3). There could be alternative mechanisms that are intricately related to the potential structures of CTBs and the characteristics of GBs, both of which are not accounted for in the literature. Recent studies of nanotwinned copper pillars without GBs revealed strong deformation anisotropy and a brittle-to-ductile transition behaviour (where CTBs are considered intrinsically brittle) 2 , suggesting that CTBs alone are not sufficient for increased plasticity despite their strong strengthening effect, and that a reasonable mix of GBs is helpful to mediate the plasticity and achieve high ductility. Experiments and simulations have frequently
Abstract Atomistic simulations have shown that a screw dislocation in body-centered cubic (BCC) m... more Abstract Atomistic simulations have shown that a screw dislocation in body-centered cubic (BCC) metals has a complex non-planar atomic core structure. The configuration of this core controls their motion and is affected not only by the usual resolved shear stress on the dislocation, but also by non-driving stress components. Consequences of the latter are referred to as non-Schmid effects. These atomic and micro-scale effects are the reason slip characteristics in deforming single and polycrystalline BCC metals are extremely sensitive to the direction and sense of the applied load. In this paper, we develop a three-dimensional discrete dislocation dynamics (DD) simulation model to understand the relationship between individual dislocation glide behavior and macro-scale plastic slip behavior in single crystal BCC Ta. For the first time, it is shown that non-Schmid effects on screw dislocations of both {110} and {112} slip systems must be implemented into the DD models in order to predict the strong plastic anisotropy and tension–compression asymmetry experimentally observed in the stress–strain curves of single crystal Ta. Incorporation of fundamental atomistic information is critical for developing a physics-based, predictive meso-scale DD simulation tool that can connect length/time scales and investigate the underlying mechanisms governing the deformation of BCC metals.
Dislocation mobility -the relation between applied stress and dislocation velocity-is an importan... more Dislocation mobility -the relation between applied stress and dislocation velocity-is an important property to model the mechanical behavior of structural materials. These mobilities reflect the interaction between the dislocation core and the host lattice and, thus, atomistic resolution is required to capture its details. Because the mobility function is multiparametric, its computation is often highly demanding in terms of computational requirements. Optimizing how tractions are applied can be greatly advantageous in accelerating convergence and reducing the overall computational cost of the simulations. In this paper we perform molecular dynamics simulations of Vi (111) screw dislocation motion in tungsten using step and linear time functions for applying external stress. We find that linear functions over time scales of the order of 10-20 ps reduce fluctuations and speed up convergence to the steady-state velocity value by up to a factor of two.
We conduct dislocation dynamics (DD) simulations of Fe periodic single crystals under tensile loa... more We conduct dislocation dynamics (DD) simulations of Fe periodic single crystals under tensile load at several high strain rates and temperatures. The simulations are enabled by the recent development of temperature-dependent dislocation mobility relations obtained from atomistic calculations. The plastic evolution in the simulations is governed by rapid initial dislocation multiplication, followed by a saturation of the flow stress when the subpopulation of slow plastic carriers becomes stabilized by dislocation annihilation. Above 500 K, edge dislocations coexist with screw dislocations and contribute proportionaly to the value of the flow stress. The DD simulations are used to interpret shock-loading experiments in Fe in terms of the relative importance of different strengthening mechanisms. We find that in the 10 4 -to-10 6 s -1 strain rate regime, work hardening explains the hardening of shock-loaded bulk Fe crystals.
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Papers by Jaime Marian