Mobility Of Screw Dislocations Research Articles

Mechanical properties and deformation mechanisms of single crystal Mg micropillars subjected to high-strain-rate C-axis compression

The mechanical properties and deformation mechanisms of single crystal magnesium under c-axis quasi-static and high-strain rate compressions are investigated through in situ scanning electron microscope (SEM) experiments and post-mortem transmission electron microscope (TEM) characterization. The findings revealed that ductility and high rates of hardening are preserved for pillars as large as 15 μm. Furthermore, rate effects result in a mild increase in flow stress with plastic deformations controlled primarily by the slip of <a+c> type dislocations. Importantly and in contrast to other literature reports, plastic deformation occurs in the absence of twining. As the strain increases and plastic deformation exceeds about 4%, crystal rotation activates basal slip, <a> type dislocations, resulting in a more rate independent flow stress. TEM observation on micropillars compressed at a strain rate of 250/s, revealed the activation of {112‾2‾}<1‾1‾23> slip systems and high mobility of screw dislocations as major contributors to plastic strains in excess of 10% without fracture. These findings are relevant to the design of lightweight materials used in transportation systems, e.g., selection of material grain size. Moreover, the experimental data here reported provides the materials science community with a unique opportunity to validate discrete dislocation dynamics (DDD) formulations employed in multiscale design of materials.

Mobility of screw dislocations absorbed in symmetric tilt grain boundaries in Al

ABSTRACT The strength and ductility of polycrystalline metals are influenced by the interactions between dislocations and grain boundaries, particularly when the grain size is small. One possible reaction is the absorption of dislocations into the grain boundary structure; however, the mobility of the resulting grain boundary dislocations (GBDs) remains largely unexplored. Thus, the objective of this work is to determine the mobility of GBDs arising from the absorption of lattice screw dislocations into Σ3{111} and Σ11{113} <110> symmetric tilt grain boundaries (STGBs) in Al. Atomistic simulations reveal a reduction in Peierls stress of ∼80% and phonon damping coefficient of ∼65% for GBDs in the {111}<110> STGB compared to lattice screw dislocations. This significant mobility increase is caused by the complete dissociation of the absorbed dislocation. The Peierls stress of GBDs in the {113}<110> STGB is increased to almost 8 times that of the lattice screw dislocation due to the smaller interplanar spacing between slip planes. This work provides a structural justification for the absorption reactions and demonstrates that the mobility of an absorbed dislocation is highly sensitive to the structure of the host grain boundary. Ultimately, mobility laws are provided which can be used to model GBD motion in mesoscale simulations.

A unified model for ductile-to-brittle transition in body-centered cubic metals

Ductile-to-brittle transition (DBT) is a well-known phenomenon in body-centered-cubic (BCC) metals, intermetallics and semiconductor materials. A quantitative prediction of the DBT temperature, however, has so far remained intractable. Here, we propose a unified model based on the efficacy of dislocation multiplication as the controlling factor for DBT, with the dislocation source efficiency governed by the relative mobility of screw versus edge dislocations. The model successfully predicts the DBT temperature of iron, molybdenum and tungsten, and also covers the influence of grain size, initial dislocation density, and the multiplicity of dislocation sources. A comparison with experiments indicates that the model captures the key DBT features, providing new insight into the toughness of BCC metals.

Pre-stored edge dislocations-enabled pseudo-toughness in chromium

Pre-deformation is an effective mean to tune the ductile-to-brittle transition (DBT) temperature of body-centered cubic metals. However, due to complex microstructural evolution during pre-straining, altering not only dislocation density but also grain size, the mechanism of reduction in DBT temperature remains intriguing. Here, we designed two types of chromium samples, with similar grain size but different initial dislocation density, to uncover the underlying mechanisms that govern the variation in DBT. Small-punch, quasi-in situ crack propagation, and in-situ nanomechanical tests conducted over a wide temperature range revealed that pre-stored edge dislocations only can make pseudo-toughness, i.e., increasing deformability but the sample is still intrinsically brittle-prone to fragile cracking. Activation of the easy glide pre-stored edge dislocations could enhance the crack resistance at the initial stage of mechanical loading, which makes a left-shift of the DBT temperature, while the toughness/brittleness of chromium is still controlled by the relative mobility of screw versus edge dislocations.

Mobility of carbon-decorated screw dislocations in bcc iron

We investigate the mobility of screw dislocations decorated by carbon solutes in body-centred cubic iron based on density functional theory (DFT) calculations and transmission electron microscopy (TEM) observations of in-situ straining experiments. We focus on the high-temperature domain, between 500 and 800 K, where plasticity is controlled by the slow glide of decorated screw dislocations with a mobility similar to the Peierls mechanism existing below 300 K, at variance with the athermal regime observed at intermediate temperatures. We propose that due to the strong pinning of reconstructed dislocation lines by carbon solutes, dislocation glide occurs by the formation and migration of kinks, both controlled by the jumps of carbon atoms. We calculate the associated kink-pair nucleation and formation energies as well as the migration energies of isolated kinks and use these DFT values to predict the dislocation velocity as a function of temperature, applied stress and dislocation length. The validity of the mobility law is assessed by the comparison with dislocation velocities measured in TEM observations at different temperatures and local shear stresses. The quantitative agreement between the experimental measurements and the analytical model supports the proposed glide mechanism and the DFT values obtained for kink energies.

Influence of loading orientation on deformation localization of irradiated tungsten

The plasticity of tungsten in low or medium temperature regimes is dominated by the screw dislocation mobility, which is well known to exhibit a strong non-Schmid effect and loading orientation dependence. As a typical plasma-facing material, many questions regarding the role of these plasticity features on the irradiation degradation performance of tungsten remain unanswered. Considering that deformation localization is a continuing critical problem in the nuclear application, in the current work, a systematic set of crystal plasticity calculations is designed to quantify the degree of deformation localization in irradiated tungsten, which is found to be sensitive to the loading orientation. The underlying physical mechanisms are carefully analyzed, highlighting the important role of the number of active slip systems, the non-Schmid effect controlled by the evolution of local stress state, and the high resistance stress induced by irradiation defects. Based on the micro-scale physical mechanisms, two physical indicators are proposed to provide guidance for predicting the orientation-dependent deformation localization in irradiated tungsten, which agrees well with the simulation results.

Solid solution softening and hardening in binary BCC alloys

Solute-dislocation interaction energies are calculated using Density Functional Theory and the 2-D lattice Greens function approach for Al, Ti, Cr, Fe, Zr, Nb, Mo, Ru, Hf, Ta, W, Re solutes in BCC Nb, Ta, Mo, and W. The predictions are then used in an extension of the Rao-Suzuki model to analyze the experimentally observed solid solution strengthening and softening in Nb-X, Fe-X, Ta-X, Mo-X and W-X crystals, X= Mo, W, Ta, Si or Re. DFT predictions for the solute dislocation interaction energies for Fe-Si, taken from the literature are also analyzed. The original Rao-Suzuki model considers kink migration controlled mobility of screw dislocations in body-centered cubic (BCC) structures and is valid for high solute concentrations (>5 at%) as well as above a critical temperature. Here, we extend the Rao-Suzuki model to include kink-pair nucleation statistics in the presence of solutes which is expected to be the rate limiting process at low temperatures (<0.2Tmelt). Such an extension should be valid up to low concentration of solutes as well as fairly low temperatures. The key parameters required for the model are solute – ½[111] screw dislocation interaction energies in pure Nb, Ta, Mo or W. These parameters were derived in this paper using first principles calculations. Using the calculated interaction energies, the model satisfactorily reproduces experimentally observed solution strengthening data in Nb-Mo, Nb-W, Nb-Re crystals at temperatures from 77 to 600K, for solute concentrations in the range 8.5 to 19 at% for Mo, 6 to 15 at.% for W and 5 to 9 at.% for Re. Similar agreement is found for strengthening/softening data for Ta-W, Ta-Re, Ta-Nb, Fe-Si, Mo-Re and W-Re crystals in the temperature range 77 – 600K. We also explore sensitivity of these predictions to the variation in the solute-dislocation energies. In Ta-W, a 10% variation (±0.015 eV) produces a modest change in solution hardening, which is within experimental error. The refined Rao-Suzuki model can be used in designing high strength, multi-component BCC alloys for high temperature applications.

Spallation of polycrystalline tungsten using laser-induced shock at ultrahigh strain rates

Laser-induced shock technology was used to achieve the shock loading of polycrystalline tungsten (W) at an ultrahigh strain rate. The shock response and fracture behavior of W were investigated at different tensile strain rates. Unlike the intergranular fracture mode under quasistatic conditions, the transgranular–intergranular-mixed fracture mode of W was detected at high strain rates and the proportion of transgranular cleavage mode increased. The spall strength of W was positively correlated to the strain rate, reaching 5.86 GPa when the tensile strain rate reached 106 s−1 in the spall plane. The mobility of screw dislocations increased at high strain rates, and the simultaneous emission of screw dislocations improved the spall strength while accelerating the expansion of crack nucleation positions, inducing numerous transgranular fractures.

Atomistic simulations and theoretical modeling of dislocation slip and yield response of industrial tantalum alloys

Dislocation core structure, critical resolved shear stress (CRSS) and mobility of ½<111> screw and edge dislocations in BCC Ta-8%W alloy was studied using molecular statics and dynamics simulations at 5 and 300 K. Two EAM potentials are available to study the Ta-W system - Johnson-Zhou and Chen potentials. Both potentials give good estimates of edge dislocation mobility in Ta and Ta-8%W. However, the Johnson-Zhou potential predicts a considerably higher CRSS for screw dislocations in pure Ta as compared to experiments. Thus, only the Chen potential is used to study solid solution strengthening in Ta-8%W alloy. The CRSS for Ta-8%W alloy with Chen potential was estimated to be 600 MPa and 300 MPa for screw dislocations at 5 and 300 K, respectively. Edge dislocations exhibit twinning-anti twinning asymmetry in CRSS given the asymmetry in atomic disregistry and dislocation misfit across the glide plane. The simulation results for screw and edge dislocation mobility were found to be in good agreement with the CRSS values from the analytical model of Rao and Suzuki for screw dislocations and the model of Maresca and Curtin for edge dislocations. The study of stability and expansion of shear dislocation loops revealed that theoretical results for loop stabilizing stress from Scattergood and Bacon's equation agreed with results from direct atomistic simulations. The current work predicts that screw dislocations control the experimental yield behavior of the Ta-8at%W alloy at all temperatures. The CRSS for Ta-8%W alloy is a result of solid solution strengthening due to randomly dispersed W solutes in Ta.

Screw dislocation mobility in a face-centered cubic solid solution with short-range order

The effect of short-range ordering (SRO) on screw dislocation mobility in Ni-10%Al is investigated using molecular dynamics simulations. We find that the onset of SRO results in a change of the threshold stress as well as a decrease in the damping in the regime where the dislocation velocity and shear stress are linearly related. The presence of a diffuse anti-phase boundary (DAPB), a region of broken SRO formed from dislocation slip, decreases the stress threshold for dislocation motion and does not significantly change the linear damping at the temperatures studied. The results in this study suggest that screw dislocations in concentrated face-centered cubic alloys move faster in a DAPB than in a random alloy, due to both a decrease in threshold stress and damping in the linear regime. It also highlights the importance of differentiating the effect of SRO on mobility for mesoscale modeling of dislocations in such alloys.

Temperature and Stress Dependence of Screw Dislocation Mobility in Nb-V-Ta Alloys Using Kinetic Monte Carlo Simulations

In this work we present simulations of thermally-activated screw dislocation motion in Nb-Ta-V alloys for two distinct scenarios, one where kink propagation is solely driven by chemical energy changes, i.e., thermodynamic energy differences, and another one where a migration barrier of 1.0 eV is added to such changes. The simulations have been performed using a kinetic Monte Carlo model for screw dislocation kinetics modified for complex lattice-level chemical environments. At low stresses, we find that dislocation motion in the case with no barrier is controlled by long waiting times due to slow nucleation rates and extremely fast kink propagation. Conversely, at high stress, the distribution of sampled time steps for both kink-pair nucleation and kink propagation events are comparable, resulting in continuous motion and faster velocities. In the case of the 1.0-eV kink propagation energy barrier, at low stresses kink motion becomes the rate-limiting step, leading to slow dynamics and large kink lateral pileups, while at high stresses both kink pair nucleation and kink propagation coexist on similar time scales. In the end, dislocation velocities differ by more than four orders of magnitude between both scenarios, emphasizing the need to have accurate calculations of kink energy barriers in the complex chemical environments inherent to these alloys.

Molecular dynamics simulations of screw dislocation mobility in bcc Nb

The screw dislocation mobility in bcc Nb has been studied by molecular dynamics (MD) simulations at different strain rates and temperatures using an embedded-atom method (EAM) potential. Static properties of the screw dislocation, as determined with the EAM potential, are in agreement with previous density-functional-theory calculations. The elementary slip plane of the screw dislocation remains (110) for all studied strain rates (in the range 6.3 × 107–6.3 × 109 s−1) and temperatures (5 to 550 K). However, the consecutive cross-slip on different symmetry-equivalent (110) planes leads to an effective glide on (112) planes. It is demonstrated that the screw dislocation trajectories, velocities and waviness of the screw dislocation depend on the crystallographic indices, (110) or (112), of the maximum resolved shear stress plane. The waiting time for the start of the screw dislocation motion increases exponentially with decreasing strain rate, substantiating the necessity to apply in future accelerated MD techniques in order to compare with macroscopic stress-strain experiments.

Mechanism of brittle-to-ductile transition in tungsten under small-punch testing

Exceptional high brittle-to-ductile transition temperature (BDTT) is a critical issue that limits the application of tungsten as structural materials. The controlling factors in BDT have been the focus of debate in past decades. In this work, small-punch testing were performed in a wide temperature range to explore the mechanism of BDT in as-rolled tungsten. A transition in failure mode from cleavage cracking to delamination fracture with increasing temperature marks the BDT of the as-rolled tungsten. Numerous (001) planar triangle-shaped shear tongues are produced on fracture surface. The number density and distribution of shear tongues show a clear temperature and orientation dependence. Analysis of the slip line angle on the shear tongues reveals that profuse dislocation activities on the {110}, {112} and {123} planes at and above BDTT. Temperature and orientation dependent slip activities are decisive factors controlling the BDT. Below BDTT, fewer freely moved edge dislocations are insufficient to blunt the crack tip, thus as-rolled tungsten displays brittleness. Once above the BDTT, the mobility of screw dislocations is increased, and a large fraction of mixed dislocations glide could effectively suppress cracking and make as-rolled tungsten ductile.

Thermally-activated dislocation mobility in bcc metals: An accelerated molecular dynamics study

Plastic deformation in metals is controlled by dislocation density and mobility. In bcc metals the mobility of screw dislocations, which takes place by temperature- and stress-driven nucleation of critical kink-pairs, is most essential for deformation. However, the critical resolved shear stresses at low temperatures, as determined from molecular dynamics (MD) simulations performed at constant strain rate, are typically 2–3 times larger than the yield stresses measured experimentally. Here, an accelerated MD procedure is developed and employed to investigate the onset of dislocation mobility in the prototypical system bcc Nb. The method combines constant strain and temperature MD with hyperdynamics, using a bond-boost potential. We demonstrate, with a careful statistical analysis, that the method enables nucleation of critical kink-pairs and the determination of the Gibbs energy of kink-pair formation from accelerated MD simulations at experimentally-measured shear stresses.

Relative mobility of screw versus edge dislocations controls the ductile-to-brittle transition in metals.

Body-centered cubic metals including steels and refractory metals suffer from an abrupt ductile-to-brittle transition (DBT) at a critical temperature, hampering their performance and applications. Temperature-dependent dislocation mobility and dislocation nucleation have been proposed as the potential factors responsible for the DBT. However, the origin of this sudden switch from toughness to brittleness still remains a mystery. Here, we discover that the ratio of screw dislocation velocity to edge dislocation velocity is a controlling factor responsible for the DBT. A physical model was conceived to correlate the efficiency of Frank-Read dislocation source with the relative mobility of screw versus edge dislocations. A sufficiently high relative mobility is a prerequisite for the coordinated movement of screw and edge segments to sustain dislocation multiplication. Nanoindentation experiments found that DBT in chromium requires a critical mobility ratio of 0.7, above which the dislocation sources transition from disposable to regeneratable ones. The proposed model is also supported by the experimental results of iron, tungsten, and aluminum.

Temperature dependent plasticity in BCC micropillars

Recent experiments in micron- or sub-micron metallic pillars have displayed size dependent plasticity. Dislocation dynamics (DD) simulations can be a useful tool in studying dislocation-mediated plasticity, which is widely believed to contribute to observed size effects at small length scale. In particular, the plasticity in body-centered cubic (BCC) metals is known to have a strong temperature dependence, which stems from the thermally activated motion of screw dislocations. In this study, we develop a DD model based on the atomistic characterization of dislocation mobility and potential source mechanisms to investigate the temperature dependent plasticity in BCC micropillars. Our models show the dislocation source mechanism changes with respect to temperature due to the change in mobility of screw dislocations, and these results are compared with experimental results and theoretical models. In addition, the size dependence increases with temperature, which agrees with recent experimental observations.

Thermally activated dislocation mechanism in Mo studied by indentation, compression and impact testing

Body-centered cubic metals like molybdenum and tungsten are interesting structural materials for high-temperature applications. These metals, are however, brittle at low homologous temperature, caused by the limited mobility of screw dislocations. In this study, the thermally activated deformation mechanisms in bcc Mo have been investigated using strain rate jump nanoindentation and compression tests as well as Charpy V-notch impact testing. The material shows a significant softening with increasing temperature and a maximum in strain rate sensitivity is found at the critical temperature, before decreasing again in the ductile regime. The activation volume, however, showed a distinct increase from about 5 b3 at the onset of the brittle to ductile transition temperature. Here we propose to use temperature-dependent nanoindentation strain rate jump testing and the activation volume as a complementary approach to provide some indication of the brittle to ductile transition temperature of bcc metals.Graphic

Temperature and composition dependent screw dislocation mobility in austenitic stainless steels from large-scale molecular dynamics

Extensive molecular dynamics simulations are performed to determine screw dislocation mobility in austenitic Fe0.7NixCr0.3-x stainless steels as a function of temperature ranging from 100 to 1300 K, resolved shear stress from 30 to 140 MPa, and Ni composition from 0.0 to 30.0 at%. These mobility data are fitted to a linear mobility law with a nonzero stress offset, referred to as the threshold stress. We find that both the linear drag coefficient and the threshold stress increase with Ni composition. The drag coefficient increases with temperature, whereas the threshold stress decreases with temperature. Based on these calculations, we determine fitting functions for the linear solute drag coefficient as a function of temperature and composition. The mobility laws determined in this study may serve to inform dislocation dynamics simulations pertinent to dislocation network evolution at elevated temperatures for a wide composition range of austenitic stainless steels.

Oxygen solutes induced anomalous hardening, toughening and embrittlement in body-centered cubic vanadium

Vanadium (V) is sensitive to minute quantity of oxygen interstitials, which induce pronounced hardening and embrittlement. Here, we utilize oxygen to synthesize V solid solutions in order to reveal the mechanism of oxygen solutes induced hardening. With increasing of oxygen solute concentrations, the fracture modes of V samples transform from dimple, to a mixture of dimple and cleavage, and to a fully transgranular cleavage. High density of dislocations and dislocation debris are produced in strained samples. The mobility of screw dislocations is reduced and the dislocation cross-slip events are promoted by oxygen solutes. In addition to oxygen solution hardening, the generation of high density of oxygen-vacancy complexes plays a dominant role in the strengthening. High quantity of loop-shaped dislocation debris are direct evidence for the formation of oxygen-vacancy complexes. Profuse oxygen-vacancy complexes trap dislocations, promote cross-slips and assist dislocation storage, thus give rise to a superior combination of strengthening, strain hardening, and ductility in V with 1.0 at% of oxygen. Once beyond a critical oxygen concentration (>1.6 at%), V shows catastrophic brittle failure due to the exceptional high density of oxygen-vacancy complexes. These findings provide insight to design high performance refractory metals utilizing oxygen solutes.

Probabilistic and constitutive models for ductile-to-brittle transition in steels: A competition between cleavage and ductile fracture

We propose a probabilistic model coupling with the temperature dependent constitutive relationship to describe the competition between the cleavage and ductile void failure of ferritic/martensitic steels with irradiation effects. It is found that both the material deformation and failure modes exhibit significant temperature dependence. Regarding the material deformation, two regimes have been found for the ductile-to-brittle transition (DBT). At low temperature, the flow stress is controlled by the mobility of screw dislocations. The high-temperature regime is about the thermally activated jog drag of dislocations. The failure modes depend on the carbide precipitate and temperature. Cleavage micro-cracks initiate at the carbide sites at low temperature giving rise to brittle behaviors. At high temperature, void nucleation evolving from the carbide precipitate suppresses the cleavage nucleation and propagation, and leads to good ductility. It is demonstrated that the competition between the cleavage fracture and ductile void failure is the major mechanism for the DBT. Our probabilistic model successfully predicts the temperature-dependent fracture toughness and ductile-to-brittle transition temperature (DBTT). Besides temperature, irradiation has a significant effect on the DBT of steels. Upon irradiation, the steels exhibit both irradiation hardening and irradiation embrittlement, i.e., rise in yield and flow stresses, and increase on DBTT. The main reason for these irradiation effects is that the irradiation-induced hardening increases the flow stress, and the plasticity localization weakens the overall hindering effect from ductile void growth on the cleavage nucleation and propagation, promoting the development of cleavage. Our probabilistic model takes into account the cleavage nucleation-propagation process and the influence of ductile void growth simultaneously, and it gives a sound explanation for the fundamental relation between the DBTT and mechanical properties of metallic materials.