Dislocation Locks Research Articles

Effects of elasticity and dislocation core structure on the interaction of dislocations with embedded CNTs in aluminium: An atomistic simulation study

The interaction between edge dislocations and carbon nanotubes (CNTs) embedded into an Al matrix is investigated. Both pristine and Ni-coated CNTs are considered. It is demonstrated that the embedded CNTs lead to a yield stress contribution equivalent to that of an array of non-shearable inclusions that are by-passed via the Orowan mechanism. The strain hardening mechanism associated with multiple dislocation passes shows, however, strong effects of atomistic core structure such as crystallographic and non-crystallographic cross slip of near-screw segments on the CNT-metal interface, dislocation lock formation and annihilation, jog formation and shedding of prismatic dislocation loops.

Enhancing the strength-ductility trade-off in a NiCoCr-based medium-entropy alloy with the synergetic effect of ultra fine precipitates, stacking faults, dislocation locks and twins

In this work, multi strengthening/deformation mechanisms were introduced into a Ni-rich Ni40Co32Cr20Al3.5Ti3Nb1.5 (at%) medium-entropy alloy. After thermo-mechanical procedures, the microstructure is characterized by ultra fine γ' particles (2–18 nm) and short stacking faults. The alloy has superior mechanical properties with gigapascal yield strength, excellent ductility and strong work hardening ability. The initial high strength is caused by the blocking effect of ultra fine γ' particles, dislocation locks (Lomer-Cottrell locks), solute atoms and grain boundaries to dislocation movement. TEM observations at different strains indicate that the increasing number of stacking faults, Lomer-Cottrell locks and deformation twins are the main reasons for the improvement of work hardening ability during deformation. Twinning and dislocation slip, including perfect dislocations and partial dislocations, are the main deformation mechanisms. Our findings show that the coordination of multi strength/toughing mechanisms may be an effect way to enhance the strength-ductility trade-off.

Structural transformation and strain localization at twin boundaries in Al0.4CoCrFeNi high-entropy alloy

Surface nanotribological properties and subsurface damage of Al0.4CoCrFeNi high-entropy alloy during the nano-scratching processes are investigated using molecular dynamics. The results show that the surface wear characteristics and scratching-caused surface damage significantly depend on the crystallographic orientation, spacing and inclination angle of the twin boundary. For the variation of crystallographic orientation, the largest friction coefficient belongs to the crystallographic orientation [001], indicating that the movement of the indenter in this substrate is most restricted. The microstructure evolution reveals the formation of Lomer-Cottrell and Hirth dislocation locks because of the distinctness of angle between various slip systems. Both the Hall–Petch and inverse Hall–Petch relationships are observed for the difference of twin boundary spacing, and the maximum indentation force is achieved with a tilt angle of 0° resulting from the various interactions among the dislocations and twin boundaries. The microstructure evolution and the atomic flow are greatly dependent on the spacing and the inclination angle of twin boundary, where the twin boundary migration is the significant factor. Furthermore, the surface morphology is distinct between workpieces due to the elastic recovery at the surface, nucleation and slipping of dislocation, which implies that the wear volume depends on the material microstructure.

Ultra-high-temperature creep behavior of a single-crystal nickel-based superalloy containing 6% Re/5% Ru

The creep behavior and influence factors of a nickel-based single-crystal superalloy containing 6% Re/5% Ru (mass fraction) at ultra-high-temperatures were studied through microstructural observations and ultra-high-temperature creep property measurements. The results indicated that the alloy's creep life at 1170 °C/110 MPa was 193 h. During steady-state creep, the alloy's deformation mechanism involved a large number of dislocations sliding in the γ matrix and climbing over the rafted γ' phases. With the increase in the content of refractory elements, the resistance of dislocation movement in the γ matrix increased. In the later stage of ultra-high-temperature creep, the dislocations shearing into the γ' phase could cross-slide from the {111} plane to the {100} plane to generate the Kear–Wilsdorf (K-W) dislocation locks. A greater number of K-W dislocation locks could inhibit the sliding and cross-sliding of dislocations to enhance the alloy resistance, which was one of the reasons for the smaller strain rate and better creep resistance. The larger effective stress in the necking area could initiate the dual-orientation slip of the dislocations, causing the rafted phase to twist. Crack initiation occurred in the kinking area. The initiated crack propagated along the direction perpendicular to the stress axis and expanded until fracture. This process was considered to be the alloy's deformation and damage mechanism during ultra-high-temperature creep. The Ru dissolved in γ' phase could replace Al atoms. The interactions of Ru with Re and W atoms in the high-concentration Re/Ru alloy caused more Re and W atoms to dissolve in the γ' phase, delaying elemental diffusion and depressing dislocation movement. This was the main reason for the retention of more K-W dislocation locks and the good resistance during creep at ultra-high temperatures.

Study on wear behavior of FeNiCrCoCu high entropy alloy coating on Cu substrate based on molecular dynamics

The high entropy alloy (HEA) is a potential material for coating because of its high hardness and wear resistance. In order to study the wear performance and protection ability of FeNiCrCoCu HEA coating on Cu substrate, the friction behavior of abrasive particle and material wear and damage were studied by molecular dynamics (MD). The cutting of depth and velocity of abrasive particle were used as variables for simulation. By analyzing the mechanical data of results, it was found that the friction coefficient of abrasive particle decreased gradually with the increase of the cutting of velocity. Dislocation extraction analysis (DXA) and Common neighbor analysis (CNA) were used to analyze the lattice structure and dislocations. By analyzing the above data, the wear and damage of the material was obtained, and the inhibition behavior of Hirth dislocation lock on dislocation nucleation was found. Cu substrate damage results showed that HEA coatings could effectively improve the wear resistance of the material surface. In the case of small cutting of depth, the high stress region in the material was concentrated below the abrasive particle, and the material subsurface damage would be intensified. This research can provide a theoretical basis for the application of HEA coatings.

Incorporating latent hardening in visco-plastic self-consistent framework for performing texture simulations

In the present work, the role of latent hardening on texture evolution at large strains is examined by embedding the latent hardening interaction matrix in the visco-plastic self-consistent framework. All possible interactions between different slip systems are allowed by specifying the 12 × 12 interaction matrix which consists of the relative strength of different dislocation junctions. In particular, the role of colinear junction which is the strongest among all dislocation locks on texture evolution is highlighted. It was observed that texture simulations in the latent hardening mode with a strong colinear lock provided excellent agreement with the experimentally measured textures, whereas isotropic hardening mode (12 × 12-unit matrix) failed to reproduce the experimental textures in a satisfactory manner.

Examination of inverse Hall-Petch relation in nanostructured aluminum alloys by ultra-severe plastic deformation

To have an insight into the occurrence of inverse Hall-Petch relationship in ultrafine-grained (UFG) aluminum alloys produced by severe plastic deformation (SPD), ultra-SPD (i.e. inducing several ten thousand shear strains via high-pressure torsion, HPT) followed by aging is applied to an Al-La-Ce alloy. Average nanograin sizes of 40 and 80 nm are successfully achieved together with strain-induced Lomer-Cottrell dislocation lock formation and aging-induced semi-coherent Al11(La,Ce)3 precipitation. Analysis of hardening mechanisms in this alloy compared to SPD-processed pure aluminum with micrometer grain sizes, SPD-processed Al-based alloys with submicrometer grain sizes and ultra-SPD-processed Al-Ca alloy with nanograin sizes reveals the presence of two breaks in the Hall-Petch relationship. First, a positive up-break appears when the grain sizes decrease from micrometer to submicrometer which is due to extra hardening by solute-dislocation interactions. Second, a negative down-break and softening occur by decreasing the grain sizes from submicrometer to nanometer which is caused by weakening the dislocation hardening mechanism with minor contribution of the inverse Hall-Petch mechanism. Detailed analyses confirm that nanograin formation is not necessarily a solution for extra hardening of Al-based alloys and other accompanying strategies such as grain-boundary segregation and precipitation are required to overcome such a down-break and softening.

Effects of microstructure and temperature on the mechanical properties of nanocrystalline CoCrFeMnNi high entropy alloy under nanoscratching using molecular dynamics simulation

In this study, molecular dynamics is used to examine mechanical properties and microstructure evolution of nanocrystalline high-entropy alloy (CoCrFeMnNi) under nanoscratching load. Special attentions are paid to the effects of crystal structure, temperature, grain size and twin boundary spacing on the deformation mechanisms and microstructure evolution. The P-h curves show that the maximum indentation force is in reduced order with single-crystal, nanotwin polycrystalline and polycrystalline structures. Specifically, for the single-crystal structure, the amorphization caused by the increment of temperature hinders the movements of dislocations and stacking faults. For the polycrystalline and nanotwin polycrystalline structures, the inverse Hall–Petch relation is observed due to that the interaction between the dislocations and grain boundary (GB) / twin boundary (TB). For the nanotwin polycrystalline structure, the migration of twin boundary is observed during the process of nanoindentation. Furthermore, Hirth and Lomer-Cottrell dislocation locks, secondary twins and FCC→HCP phase transformation are also found in simulations.

Constant twist rate response of symmetric and asymmetric Σ5 aluminium tilt grain boundaries: molecular dynamics study of deformation processes

Room temperature torsional deformation behaviour of bicrystal aluminium specimens (corresponding to Σ5 family) is studied by the application of constant twist rate using molecular dynamics simulations by employing the (embedded atom method) EAM potential. To simulate the torsional loading, periodic boundary condition is employed in x-direction, whereas shrink-wrapped boundary condition is applied in y- and z-directions. A constant twist rate of 1°/ps has been applied at either end of the samples in opposite directions. The variation of potential energy as a function of relative rotation between the ends of the specimen is recorded. The evolution of different types of dislocation and characteristics behaviour due to interactions among dislocations during deformation has been analysed. The populations of Shockley partial dislocations, dislocation loops, dislocation locks, and dislocation junctions increase as the torsional deformation progresses. The structural variations occurred in the specimen are explained with the aid of defect evolutions (such as vacancies population and amorphous atoms percentage as a function of torsion angle), and the grain boundary migration and grain rotation processes are described with atomic snapshots. Additionally, we have tried to correlate trends between grain boundary energy and various deformation mechanisms.

The role of pyramidal 〈c + a〉 dislocations in the grain refinement mechanism in Ti-6Al-4V alloy processed by severe plastic deformation

This study focuses on the role of pyramidal 〈c + a〉 dislocations in the grain refinement mechanism in the Ti-6Al-4V alloy with an initial {112¯0}<101¯0>rolling texture. A large number of pyramidal 〈c + a〉 dislocations were activated in the sample subjected to the severe shot peening process. Two important roles of pyramidal 〈c + a〉 dislocations were discovered. First, pyramidal 〈c + a〉 slip coordinates the large c-axis strain, thereby achieving generalized plastic flow, especially in nanograins. Second, the unique low-angle grain boundaries (LAGBs) with basal-pyramidal dislocation locks (prismatic 〈c〉 and prismatic 〈c + a〉 dislocations) were produced for the first time by pyramidal 〈c + a〉 interacting with basal 〈a〉 dislocations. This unique low-energy boundary greatly enhances the stability of the strain-induced grain boundary and dislocation density (~6.6 × 1015m − 2 in nanograins). The grain refinement process contains three types of subdivision modes: (I) dislocation walls with pyramidal 〈c + a〉 dislocations in coarse grains; (II) basal 〈a〉 intersecting with prismatic 〈a〉 dislocations in coarse grains; and (III) basal 〈a〉 intersecting with pyramidal 〈c + a〉 dislocations in coarse grains, ultrafine-grains and nanograins. The occurrence of slip modes depends on the initial texture and texture evolution during dynamic recrystallization. Besides, Hall-Petch breakdown at the nanoscale was found and is attributed to the decreasing critical resolved shear stress of pyramidal 〈c + a〉 slip at the nanoscale. This study provides a new approach for the design of stable nanostructured hexagonal close-packed metals by the unique LAGBs with basal-pyramidal dislocation locks.

Dynamic mechanical responses of the Al0·1CoCrFeNi high entropy alloy at cryogenic temperature

In this work, dynamic compression experiments are carried-out at both room temperature (RT = 298 K) and liquid nitrogen temperature (LNT = 77 K) to systematically investigate the dynamic mechanical responses of the Al0·1CoCrFeNi high entropy alloy (HEA). The yield strength significantly increases with the increasing strain rate. Even higher yield strengths are revealed when the temperature decreases from 298 K to 77 K. At cryogenic temperature, abundant nanoscale deformation twins are observed due to lower stacking fault energy (SFE). By introducing new interfaces in the grains and decreasing the mean free path of dislocations during deformation, the high density deformation twins that formed during dynamic deformation at 77 K lead to a higher degree of strain hardening rate. In addition, the low SFE results in widely dissociations and reactions of dislocations, which lead to the formation of Lomer-Cottrell (L-C) and Lomer dislocation locks. These immobile dislocation locks can act as both dislocation sources and barriers, which can also contribute to the outstanding strain hardening capability of the Al0·1CoCrFeNi HEA upon dynamic deformation at 77 K.

In situ atomic-scale observation of dislocation behaviors in twin-structured Pt nanocrystals

The deformation mechanisms of twin-structured metallic materials have attracted great interest. Though previous theoretical predictions have suggested that the repulsive force of the twin boundary (TB) can significantly affect the deformation of twin-structured metals, it remains unclear whether this prediction applies to experimental conditions. In this paper, the atomic-scaled deformation process of twin-structured Pt nanocrystals was in situ observed using our home-made device in a high-resolution transmission electron microscope. We have shown that the plastic deformation of the twin-structured Pt nanocrystals was governed by full dislocation generation as well as Lomer dislocation (LD) lock formation and destruction. After LD locks were destructed, these full dislocations tended to move towards the surface of the nanocrystals. The findings revealed that due to the ultra-high repulsive force of TB on dislocation, there was no dislocation-TB reaction during the deformation. These findings can enrich our understanding of the dislocation behaviors of twin-structured nanocrystals.

Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals

ABSTRACTUltra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300 K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior.

Monotonic and cyclic plastic deformation behavior of nanocrystalline gold: atomistic simulations.

Monotonic and cyclic plastic deformation behavior of nanocrystalline gold was investigated at room temperature using molecular dynamics simulation. Yielding starts in nanocrystalline gold by initiation of 1/6 〈1 1 2〉 Shockley partial dislocation from the grain boundary. A stacking fault is created between the grain boundary and moved Shockley partial dislocation. With the progression of further deformation, increase in dislocation density inside the grain, the formation of dislocation locks such as Stair-rod 1/6 〈1 1 0〉 and Hirth 1/3 〈1 0 0〉 dislocations, generation and removal of stacking fault tetrahedron are observed. Grain coarsening and a rise in the number of dislocation inside the grains were noted with an increasing number of cycles.

Dislocation Interaction and V-Shaped Growth of the Distorted Structure During Nanoindentation of Cu20Ni20Al20Co20Fe20 (high-entropy alloy)-Coated Copper: A Molecular Dynamics Simulation-Based Study

In this paper, the deformation behavior of Cu20Ni20Al20Co20Fe20 high-entropy alloy-coated single-crystal Cu substrate which undergoes nanoindentation has been investigated under molecular dynamic simulation with embedded-atom method potential. The dynamic structural evolutions under nanoindentation are presented using centrosymmetry parameter analysis, common neighbor analysis and radial distribution function plots. In the initial level of nanoindentation, the interface deformation is greatly confronted by the confined V-shaped growth of the distorted structure. But the sudden discrete dislocation burst account for avalanche break-down in the interface layer, which further get influenced by the evolution of multiple dislocation nodes that is significantly governed by core spreading, extended misfit dislocation generation and relative rotation. In the meanwhile, the subsequent generation of dislocation locks, complicated multiple dislocation loops, dislocation junctions and limited cross-slip in wide stacking faults (SFs) hasten the work hardening and in turn slows down the deformation progress. On the other hand, the intermediate appearance of narrow SFs and slip bands significantly reduces the work hardening rate that increases the optimum fracture strain value of the specimen. Moreover, the overall increase in dislocation density and dislocation length leads to a significant growth in dislocation sources which leads to forest hardening in the later stage.

Deformation mechanisms and analysis of a single crystal nickel-based superalloy during tensile at room temperature

By means of measuring the yield and ultimate tensile strength at room temperature, microstructure observation and contrast analysis of dislocation configuration, the deformation features of a single crystal Ni-based superalloy during tensile are investigated. The results show that the ultimate tensile strength (σb) and yield strength (σ0.2) of the single crystal nickel-based superalloy at room temperature are measured to be 1023 MPa and 958 MPa, respectively. During tensile at RT, the <110> super-dislocations in γ′ phase may decompose on the {111} plane to form only configuration of (1/2)<110> partials plus APB. no configuration of (1/3)<112> partials plus SISF in the alloy is attributed to the dislocation decomposing on {111} plane to form the APB needing a smaller energy than to form the SISF. And the reason of decomposed width with about 20–30 nm keeping in the alloy is attributed to the lower stacking fault energy of alloy and a few deformed heat released during tensile at RT. The <110> super-dislocation in γ′ phase may cross-slip form the {111} to {100} plane to form the K-W locks during tensile at RT, no decomposed feature in K-W locks is attributed to the dislocation decomposing on {100} plane needing a bigger energy. The <110> super-dislocation of shearing into γ′ phase may be also decomposed on the {111} plane to form the Giamei dislocation locks consisting of (1/3)<112> partial loops plus SISF, which is attributed to the combined role of applying a big normal stress and a few deformed heat during tensile.

Low and room temperatures tensile properties of a nanoprecipitate-strengthened (FeCoCr)40Ni40Al10Cu10 high-entropy alloy

We report the tensile properties and microstructural evolution of a high entropy alloy (FeCoCr)40Ni40Al10Cu10 deformed at low temperature (233K) and room temperature (293 K). The alloy presented a superior combination of tensile strength (~1200 MPa) and elongation (~36%) at 233 K than at 293 K. Transmission electron microscopy revealed that the enhanced tensile properties at low temperature were due to deformation twinning, the formation of immobile Lomer-Cottrell dislocation locks, and the high density coherent nano-precipitates.

Effect of twist boundary angle on deformation behavior of 〈1 0 0〉 FCC copper nanowires

Uniaxial tensile deformation behavior of nano-scale bicrystal copper nanowire with different twist grain boundary which comprises several twist angles is investigated via molecular dynamics simulation to obtain the influence of twist angle on deformation mechanism. The grain boundary twist angle has a significant effect on mechanisms of tensile deformation. Highest amount of twinning activity and creation of staking faults, formation of dislocation locks such as Stair-rod 1/6 〈1 1 0〉 dislocation, and Hirth1/3 〈0 1 0〉 dislocation are witnessed during tensile deformation of Cu nanowire with twist grain boundary angle of ±15°, whereas amount of twinning and creation of staking faults are comparatively less, and dislocation locks are not evident for Cu nanowire with twist grain boundary angle of ±30°.

Intersectant coherent twin boundaries governed strong strain hardening behavior in nanocrystalline Cu

Introducing parallel coherent twin boundaries (p-CTBs) is a high effective approach to material strengthening. However, it has been widely verified that the strengthening effect of p-CTBs reaches saturation at a critical twin lamellar thickness. In this work, we demonstrate by experiments and molecular dynamic simulations that the novel intersectant coherent twin boundaries (i-CTBs) involving Lomer-Cottrell (L-C) dislocation locks trigger off a strong strain hardening on nanostructured metal, which exceeds the strength induced by p-CTBs strengthening. A transition from strain burst to dislocation multiplication occurs as the p-CTBs turn into i-CTBs in nanoscaled single crystals. The i-CTBs with different orientations can significantly promote the formation of L-C dislocation locks inside a nanocrystal, reflecting an intrinsic correlation between the i-CTBs and the L-C dislocation locks. The unique strain hardening primarily originates from the synergistic strengthening effect of the i-CTBs blocking and the L-C locks pinning on the glissile dislocation slip. These findings provide the impetus for a new strategy to design high strength and high plasticity crystalline materials.

Transmission electron microscopy characterization of dislocation structure in a face-centered cubic high-entropy alloy Al0.1CoCrFeNi

Structural characterization of dislocations and dislocation reactions in a face-centered cubic high entropy alloy was conducted using the state-of-the-art spherical aberration corrected transmission electron microscopy. We experimentally measured the stacking fault energy of the high entropy alloy from the atomic images and diffraction contrast of dislocation cores. The low stacking fault energy results in widely dissociated dislocations and extensive dislocation reactions, which leads to the formation of immobile Lomer and Lomer-Cottrell dislocation locks. These dislocation locks act as both dislocation barriers and sources and are responsible for the significant work hardening with a large hardening rate in the alloy. Based on the atomic-scale characterization and classical dislocation theory, a simple equation was derived to describe the work hardening behavior of the high entropy alloy in the early-stage of plastic deformation.