Dislocation Behavior Research Articles

Ductilization of single-phase refractory high-entropy alloys via activation of edge dislocation

Refractory high entropy alloys (RHEAs) are of particular interest for their superior high-temperature strength. However, most of RHEAs are suffered from the brittleness especially at room temperature which hindered their applications. Here, we choose TiZrHfNbx (x=0.4, 0.6, 0.8, 1) as a model alloy system in which all the alloys exhibited a single-phase BCC structure. With the increasing Nb content, the fracture strain significantly increased from 8.76% to 33.21% without sacrificing the overall yield strength. The dislocation interactions were systematically investigated through extensive experimental and theoretical approaches. The limited tensile ductility of TiZrHfNb0.4 was attributed to the long straight screw dislocations confined in a few slip planes. In sharp contrast, the dislocations in TiZrHfNb alloy are rather homogeneously distributed and the superior tensile ductility of TiZrHfNb alloy is due to the activation of edge dislocations which is rare in ordinary BCC structured alloys. The atomic scale physical origin of different dislocation configuration and behaviors can be attributed to the higher misfit volume and elastic asymmetry of TiZrHfNb alloy. The outcome of this research not only reveal a new ductilization mechanism but also provide a new pathway to design ductile RHEAs through regulation the atomic scale environment.

Tailoring strain rate sensitivity and creep behavior of CoCrNi multi-principal-element alloys by aging

While simulations predict that local chemical order in multi-principal-element alloys (MPEAs) renders anomalous dislocation behaviors and thereby varied strain rate sensitivity (SRS) and creep behaviors, the experimental demonstration of such an effect is still under exploration due to the challenges in the manipulation of local chemical order. Here we investigate SRS and creep behavior of CoCrNi MPEAs subjected to long-term aging at various temperatures using nanoindentation. Our results reveal that the SRS and creep resistance of the alloys enhance significantly with increasing aging temperatures, with the SRS notably surpassing that of elemental and solid solution counterparts. In particular, unusual magnitude of creep stress exponent is observed at room temperature and the possible mechanisms are discussed. We suggest that long-term aging could be a straightforward method to tailor the microstructural ordering and mechanical properties of MPEAs, aiming at bridging the gap between simulations and experiments.

A machine learning potential for simulation the dislocation behavior of magnesium

Accurate predictions of the dislocation behavior of magnesium (Mg) by molecular dynamics (MD) simulations are essential for studying the fundamental mechanisms of deformation and designing high plasticity Mg alloys. However, existing atomic potentials in MD simulation for Mg are not sufficiently quantitative for many dislocations-associated phenomena, such as stacking fault energy (SFE) and dislocation core structures. Here, by combining 468 density functional theory (DFT) calculated data points and a machine learning method, we create a broadly applicable deep learning potential (DLP) to study the dislocation behavior of Mg. We demonstrate that our DLP reproduces the SFE, lattice constants, elastic constants, and surface energies in reasonable agreement with experimental or DFT data. Furthermore, the DLP predicted basal 〈a〉, prismatic 〈a〉, pyramidal 〈c + a〉 dislocations all agree well with DFT results on dissociation distance and core structures. Importantly, the DLP has a superior performance on distinguishing the pyramidal I and II 〈c + a〉 screw dislocation core structures. Our results show that the DLP is suitable for investigating the dislocation behavior of Mg, making it valuable for future realistic atomistic studies of general deformation problems.

Unraveling the Subsurface Damage and Material Removal Mechanism of Multi-Principal-Element Alloy FeCrNi Coatings During the Scratching Process

Multi-principal-element alloys (MPEAs) exhibit superior strength and good ductility. However, tribological properties of FeCrNi MPEAs remain unknown at nanoscale and complex environments. Here, we investigate the effects of scratching speed, depth, and temperature on microstructural and tribological characteristics of FeCrNi using molecular dynamics simulations combined with an elevated temperature tribological experiment. The scratching force experiences the increase stage, the undulated stage, and the stable stage due to chip formation. Compared to traditional alloy coatings, low force enhances the useful life. With increased speed, the friction coefficient decreases, agreeing with previous work. High speed impacting includes severe local plastic deformation, from dislocation to amorphization. As the scratching depth increases, the average scratch force and friction coefficient increases owing to material accumulation in front of the abrasive particles. The surface morphology and dislocation behavior are significantly different during the scratching process. In addition, we revealed a temperature-dependent friction mechanism. FeCrNi MPEAs have excellent wear resistance at an intermediate temperature, which is attributed to the high Cr content promoting the formation of the compact oxide layer. This work provides atomic-scale mechanistic insights into the tribological behavior of FeCrNi, and would be applied to the design of MPEAs with high performance.

Low-dose neutron irradiation effects on the elastoplastic deformation mechanisms of aluminum-doped gallium nitride under contact loading

The elastoplastic deformation mechanisms of irradiated aluminum (Al)-doped gallium nitride (GaN) under contact loading are investigated in this work using the nanoindentation simulations, which is of great significance for understanding the mechanical properties of the Al-doped GaN and guiding the design of durable and high-performance GaN-based devices. The mechanical behaviors of the Al-doped GaN with different doping concentrations are analyzed, including the indentation hardness, Young's modulus, elastic recovery rates, phase transformations, and stress distribution. It is found that Al doping increases their hardness, Young's modulus, and elastic recovery rates, and leads to an enlargement of the phase transformation regions, which is dominated by the high coordination number (CN) phase transformations. Furthermore, the effects of low-dose neutron irradiation on their elastoplastic deformation mechanisms are studied by triggering cascade collisions within the structure. When subjected to such irradiation, structural changes occur in the Al-doped GaN, their indentation hardness, Young's modulus, and elastic recovery rates increase remarkably, and its phase transformation mechanism is changed remarkably. The dislocation behaviors of the doped and undoped GaN are different under neutron irradiation. This study is important for capturing the mechanical stability and integrity of Al-doped GaN in an irradiation environment, as well as developing GaN-based devices with superior irradiation resistance.

A comprehensive theoretical scheme for understanding the core structure and the mechanical behavior of basal dislocations with solute atmosphere in Mg alloys

The basal dislocations play a key role in the deformation of hexagonal Mg alloys. However, its core structure and mechanical behavior have thus far not yet been understood adequately, largely owing to the fact that the basal dislocation, often decorated with segregated solute atmosphere and dissociated into two partials, cannot be modeled accurately. Significant discrepancies exist between theoretically calculated and experimentally observed structures for these dissociated dislocations. Here we present a comprehensive theoretical scheme to model them in the Mg alloys with alloying elements of Y, Zn, Al and Li, respectively. Firstly, solute–dislocation interactions were computed by appropriate first-principles calculations. Secondly, the statistic solute concentration at a dislocation is estimated by a Fermi–Dirac distribution. Finally, all these effects on dislocations are integrated into an improved two-dimensional Peierls–Nabarro model to predict their core structures and mechanical behaviors in Mg alloys. It is shown that although Zn, Al and Li atoms have little effect on basal dislocations, Y atoms can largely increase their dissociated width. Furthermore, when it moves from its ground state configuration, the dissociated dislocation can further be widened, such that its dissociated widths in a Mg–0.8Y (at%) alloy at 300 K can reach 12–36 nm, in rather good agreement with the experimentally observed values of 20–30 nm. Besides, the Peierls and yield stresses of Mg alloys shall increase with increasing added solute concentration, while they are decreased drastically with increasing the temperature from 300 to 800 K, but with the Mg-Y alloy as somewhat exception, which is again consistent with experimental observations.

Role of dislocation behavior during aging in high-temperature microstructural evolution and creep property of single crystal superalloys

Single-crystal superalloys of turbine blades suffer unexpected performance deterioration, even when manufactured with optimal γ/γ′ phase morphology. This study successfully reproduced this abnormal decrease in high-temperature creep properties by applying optimal aging treatments to a skillfully designed model single-crystal superalloy. Microstructural evidence indicates that interfacial dislocations deposited during aging processes weaken dislocation distribution anisotropy (DDA) in the steady creep stage and place γ′-topological inversion ahead of the competition between γ′-rafting and it, leading to an increase in the minimum creep rate. Through constitutive equations and inductive research, a linear negative correlation between the minimum creep rate and DDA was established. A new concept of “phase-interface freshness” is proposed as a quantitative characterization of interfacial dislocation deposition after preparation to reflect the ability to stimulate the beneficial DDA. These results help us further understand thermal processes such as aging, coating, and welding, providing theoretical support for the design and optimization of thermal processes.

Nanostructure and dislocation interactions in refractory complex concentrated alloy: From chemical short-range order to nanoscale B2 precipitates

Refractory complex concentrated alloys (RCCAs) have emerged as a promising class of structural materials, demonstrating exceptional mechanical performance in aggressive environments. However, the complex atomic environments, significant lattice distortion, and vast compositional space of RCCAs present challenges to understanding the mechanisms that govern structure-property relationships. In this study, we explore the dislocation mechanisms in three model quaternary RCCAs, namely Mo25Nb10Ta25W40 (at. %), Mo25Nb25Ta25W25, and Mo25Nb40Ta25W10 using large-scale atomistic simulations and machine learning based Spectral Neighbor Analysis Potential. Our atomistic simulations examine how the chemical composition and local ordering influence the mobility of both edge and screw dislocations, and how lattice distortion and diffuse anti-phase boundary energy (DAPBE) affect dislocation behaviors during nanostructural evolution. Notably, with the increase in Nb concentration in the model RCCAs, both DAPBE and lattice distortion are simultaneously enhanced as the chemical short-range order (CSRO) evolves into nanoscale B2 precipitates. This evolution results in high lattice distortion due to the lattice mismatch between B2 precipitates and the random matrix. Consequently, B2 nanoprecipitates provide a stronger pinning effect, hindering edge dislocation motion while promoting cross-slip of screw dislocations, leading to a reduced screw-to-edge ratio in slip resistance and mobility discrepancy. These findings offer valuable insights into dislocation behaviors and interactions with ordered precipitates, highlighting the importance of exploring non-equiatomic compositions and advancing beyond CSRO in RCCAs. This study has implications for optimizing alloy compositions and processing methods for superior performance in aggressive environments.

Temperature-dependent dynamics and dislocation behavior in nanoscale machining of FeCoNiCrAl high-entropy alloys: Molecular dynamics simulation

This study investigates the impact of machining parameters on forces, thermal dynamics, dislocation behavior, and crystalline structure changes in FeCoNiCrAl high-entropy alloys during nanoscale material removal using molecular dynamics (MD) simulations. Utilizing Embedded Atom Method (EAM) and Tersoff interaction potentials, simulations were performed at cryogenic (73 K) and room (293 K) temperatures. Novel findings reveal that at 73 K, cutting velocities below 200 m/s produced the highest forces along the [001] direction, whereas at 200 m/s, the peak force shifted to [100]. Increasing velocity decreased the force along [001], while [100] exhibited an inverse relationship. At 293 K, the force remained highest along [001] across all velocities. Notably, forces at 73 K were 1.82, 1.79, and 1.58 times higher than at 293 K for velocities of 100, 200, and 300 m/s, respectively. Dislocation density, particularly 1/6<112> (Shockley) dislocations, peaked under all machining conditions, with a slight initial decrease at 293 K before a significant drop with higher cutting speeds. At 293 K and a cutting speed of 100 m/s, dislocation densities for depths of 5, 10, 15, and 20 Å were approximately 1.04, 1.54, 1.17, and 1.14 times greater than those at 73 K, respectively.

The hierarchical energy landscape of edge dislocation glide in refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) are considered as potential candidates for high-temperature applications, with the glide resistance of edge dislocations being a crucial factor in determining the high-temperature strength. However, the solid-solution strengthening mechanism of edge dislocations in RHEAs is not fully understood. The existing Labusch-type models mainly focus on the long-range interaction of solute atoms with the dislocation stress field, while there is little attention paid to the short-range interaction in the dislocation core region. Here, we conduct carefully designed atomic simulations to decouple the long-range and short-range interactions in a typical RHEA, NbMoTaW. Furthermore, the total change in solute-dislocation interaction energy is decomposed, and a hierarchical energy landscape is revealed, demonstrating that the short-range interaction at the core region gains more importance in the solid-solution strengthening of edge dislocations in NbMoTaW. Then, we determine the Larkin length, which signifies the transition from size-dependent to size-independent dislocation behavior. The activation barrier extracted from the simulation with the dislocation length above the Larkin length is incorporated into the crystal plasticity model, and the high-temperature yield strength is well predicted by the strengthening from edge dislocations. Our work provides deep insight into the solid-solution strengthening mechanism in random solution solids, elucidating the importance of the local atomic configuration around the dislocation core.

Microstructural evolution and dynamic mechanical behavior of PM 2195 Al-Li alloy with resistance to shear instability in high strain rate deformation

The risk of adiabatic shear instability is common in traditional alloys covering high-strength steels, copper alloys and aluminum alloys during high strain rate deformation. However, the research on shear instability of 2195 Al-Li alloy prepared by powder metallurgy (PM) at high strain rates is still blank. The purpose is to explore its microstructure evolution and dynamic compression behavior. In this work, the 2195 Al-Li alloy prepared by PM amazingly demonstrates excellent resistance to shear instability at high strain rates during Split-Hopkinson pressure bar (SHPB) experiment. The microstructural evolution of grains and T1 (Al2CuLi) precipitate phase, and dynamic compression behavior of PM 2195 Al-Li alloy at various strain rates from 3000 s−1 to 9000 s−1 are investigated in detail employing correlative electron-backscatter diffraction (EBSD) and transmission electron microscopy (TEM). During high strain rate deformation, the alloy undergoes grain fragmentation and rotation, increasing dislocation density around the grains. When the strain rate is from 3000 s−1 to 7500 s−1, the Kernel Average Misorientation (KAM) value increases, indicating intensified localized strain. At a strain rate of 9000 s−1, a small number of grains are surrounded by nanocrystals (around 200 nm) that undergo stress release, while the average size of micrometer-sized grains is 0.81 μm. As the strain rate increases, the T1 phases in the alloy extends, vibrates, and fractures before dissolving in the matrix, while high-rate deformation promotes dislocation entanglement and defects, leading to the decomposition or ordering of precipitated phases through rapid plastic deformation. By virtue of energy dissipation strategy via the formation of nanocrystallines and dislocation behavior induced by the dissolution of T1 phases, the "positive-positive" effect is observed. The PM 2195 Al-Li alloy undergoes no shear failure even at the strain rate of 9000 s−1, showing great prospects in future applications in dynamic-loading conditions.

Origin of the yield stress anomaly in L1[formula omitted] intermetallics unveiled with physically informed machine-learning potentials

The yield stress anomaly of L12 intermetallics such as Ni3Al, Ni3Ga, Co3(Al,W) is controlled by the so-called Kear–Wilsdorf lock (KWL), of which the formation and unlocking are governed by dislocation cross-slip. Despite the importance of this anomalous behavior in L12-strengthened alloys, microscopic understanding of the KWL is limited. Here, molecular dynamics simulations are conducted by employing a dedicated machine-learning interatomic potential derived via physically informed active learning. The potential facilitates modeling of the dislocation behavior in Ni3Al with near ab initio accuracy. KWL formation and unlocking are observed and analyzed. The unlocking stress demonstrates a pronounced temperature dependence, contradicting the assumptions of existing analytical models. A phenomenological model is proposed to effectively describe the atomistic unlocking stresses and extrapolate them to the macroscopic scale. The model is general and applicable to other L12 intermetallics. The acquired knowledge of KWLs provides a deeper understanding on the origin of the yield stress anomaly.

Electric current-induced directional slip of dislocation and grain boundary ordering

The evolution of lattice dislocations in metals can be induced by electric current. However, the effect of electric current on the interaction between lattice dislocations and grain boundaries (GBs) remains unclear. Herein, we investigate the evolution of lattice dislocations and grain boundary dislocations (GBDs) induced by electric current in M50 steel via in-situ TEM characterization, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. Our findings reveal that electric current induces the directional slip of lattice dislocations towards GBs, while the reduction of GB contrast is attributed to the decomposition and reorganization of GBDs. DFT calculations demonstrate that electric current enhances the force on atoms in defect regions. These in-situ TEM observations are well reproduced by MD simulations incorporating electric current, which include a virtual temperature rise at defect regions to reflect the electric current force on defect atoms. Based on these observations, we propose a novel GB accommodation model under electric current. These results provide valuable insights into the mechanisms by which electric current tailors lattice dislocation behavior and enhances GBDs ordering in metallic materials.

Interactions between shuffle-set (001) dislocation and (111) twin boundary in nanotwinned diamond

Recent studies have revealed unprecedented fracture toughness in synthesized nanotwinned diamond (nt-diamond) due to the obstruction of dislocations by twin boundary. However, the atomic-scale interactions between dislocations and twin boundary in diamond remain elusive and unclear. In this work, we take advantage of the recently published interatomic potential for diamond's dislocation activity to study the interactions between (001) dislocation and (111) twin boundary in molecular dynamics (MD) simulations. The twin boundaries show obvious hindrance for (001) dislocation. In the penetrations, the shuffle-set dislocation will climb to the upper gliding plane and leave a vacancy tube behind. With the gradual increase of applied shear stress, the vacancy left on the twin boundary could launch another shuffle-set dislocation in the opposite direction in matrix. The mechanism revealed in this work show obvious difference between the interaction between twinning and dislocation in metals. This study will provide a fundamental understanding of the dislocation behaviors and their strengthening and toughing mechanisms in nt-diamond materials and the related materials.

Influence of hydrogen on the elastic properties and dislocation behavior in Fe–Cr and Fe–Ni alloys by DFT calculations

The effect of interstitial hydrogen on the elastic properties of bcc Fe, bcc Fe–Cr, and bcc Fe–Ni was investigated using density functional theory calculations. Our results indicate that the elastic moduli decrease linearly with increasing hydrogen concentration. The consequences of hydrogen for the mechanical properties of bcc Fe, bcc Fe–Cr, and bcc Fe–Ni were analyzed, considering various factors such as the ideal shear stress, Peierls stress, number of dislocation pile-ups, and critical crack growth lengths. At the same hydrogen concentration, compared to the bcc Fe and bcc Fe–Ni systems, fewer dislocation pile-ups and shorter critical crack growth lengths can facilitate the nucleation and propagation of cracks in the bcc Fe–Cr system. Finally, we propose a mechanism to explain the influence of Cr and Ni on hydrogen embrittlement.

Elastodynamic behaviors of steady moving straight dislocation within thin nano film

The elastodynamic dislocation behaviors are of great interest for understanding the performances of structural alloys under intense dynamic loading conditions. The formation, propagations, and interactions of dislocations (such as injected dislocation, accelerating dislocation, steady moving dislocation at high constant speed) are quite different from static dislocations. For steady-moving dislocation within the isotropic infinite medium, the effects of surface and interface on steady-moving dislocations within limited space are still known. In this paper, we investigate the elastodynamic image stress simulation of steady moving dislocation within film of limited thickness at constant speed using Eigenstrain theory, Lorentz transformation, and steady dynamic equilibrium equations. We propose an efficient solution method that involves complex Fourier series, transforming partial differential equations into ordinary differential equations, and ultimately into a set of algebraic equations in spectral space. The effects of dislocation speed and position near the free surface on the image stress of steady-moving climbing and gliding dislocations within the thin film are examined. The results show that relativistic effects are significant for certain dislocation configurations and stress components, whereas other stress components are less sensitive to relativistic effects near the transonic speed region.

Strengthening behavior and microstructure of 2195 Al-Cu-Li alloy under cycling loading

Cyclic strengthening process, as a newly developed aluminum alloy strengthening technique, has garnered significant attention due to its advantages of rapidity, efficiency, and superior mechanical properties. This study investigated the effects of strain amplitude and loading frequency on cyclic strengthening, including mechanical properties and microstructural evolution. At room temperature, the cyclic strengthening effect of the 2195 alloy increases with increasing strain amplitude, while the influence of loading frequency is pronounced within the elastic range but nearly negligible within the plastic range. Microstructural analysis reveals that cyclic strengthening behavior within the elastic range is attributed to the precipitation of clusters, whereas within the plastic range, it is attributed to the precipitation of clusters and dislocation multiplication. Unlike other plastic deformations, cyclic plastic loading generates dislocations primarily in the form of dislocation loops. The dynamic precipitation of clusters and the diffuse distribution of dislocation loops significantly enhance the alloy's strength with minimal sacrifice of elongation. Experimental investigations on the influence of loading frequency on these precipitation behaviors demonstrate a positive correlation with dislocation generation and a negative correlation with cluster precipitation. Comparison of specimens with different strain amplitudes reveals that larger strain amplitudes provide more energy for cluster precipitation, thereby promoting precipitation. However, excessively large strain amplitudes lead to the annihilation of vacancies due to the introduction of more dislocations, thus inhibiting cluster precipitation. This indicates that the precipitation of clusters during cyclic deformation is profoundly influenced by dislocation behavior. This study deepens the understanding of cyclic strengthening mechanisms and provides support for subsequent development of cyclic strengthening processes in other materials design.

Investigation of Dislocation Behaviors in 4H-SiC Substrate during Post-Growth Thermal Treatment

Dislocation behaviors after post-growth thermal treatment were investigated by X-ray topography and KOH etching. Generation of prismatic dislocations were observed in X-ray topography, and density of basal plane dislocations (BPDs) increases with annealing temperature and radial temperature gradient. Distribution of newly generated BPDs in the wafer after thermal treatment is correlated to the resolved shear stress arising from radial temperature gradient.

Evaluation of Basal Plane Dislocation Behavior near Epilayer and Substrate Interface

An essential silicon carbide (SiC) manufacturing procedure for eliminating bipolar degradation in a SiC device is the detection of the basal plane dislocation (BPD) causing the phenomenon. In this work, we employed the mirror electron microscope (MEM) technique, which has higher resolution than photoluminescence. The MEM provided results for the detection of short BPDs without conversion to threading edge dislocation at the epi/sub interface. In addition, a considerable number of short BPDs were observed in the epilayer grown with the improved method, and the conversion ratio around the buffer layer could be derived.

Elucidation of the trigger for trenching around Al6(Fe, Mn) on AA5083 aluminum alloy in diluted synthetic seawater

A small area with a large Al6(Fe, Mn) particle and a large Mg2Si particle was prepared on AA5083, and the dislocation and corrosion behaviors were compared with those of small areas containing only Mg2Si and Al6(Fe, Mn) particles in 100-times diluted synthetic seawater. Mg2Si discolored in both cases upon immersion, but discoloration and trenching were observed for Al6(Fe, Mn) only when Mg2Si was on the same area. The discoloration of Al6(Fe, Mn) started simultaneously with the dissolution of Mg2Si. The role of Mg2Si in the discoloration of Al6(Fe, Mn) particles and the trench formation mechanism was discussed.