Emission Of Partial Dislocations Research Articles

Transformation of Coherent Twin Boundary into Basal-Prismatic Boundary in HCP-Ti: A Molecular Dynamics Study.

The manufacturing process for wrought Ti alloys with the hexagonal close-packed (HCP) structure introduces a complicated microstructure with abundant intra- and inter-grain boundaries, which greatly influence performance. In the hexagonal close-packed (HCP) structure, two types of grain boundaries are commonly observed between grains with ~90° misorientation: the basal/prismatic boundary (BPB) and the coherent twin boundary (CTB). The mechanical response of the BPB and CTB under external loading was studied through molecular dynamic simulations of HCP-Ti. The results revealed that CTB undergoes transformation into BPB through the accumulation of twin boundary (TB) steps and subsequent emission of Shockley partial dislocations. When the total mismatch vector is close to the Burgers vector of a Shockley partial dislocation, BPB emits partial dislocations and further grows along the stacking faults. When a pair of CTBs are close to each other, severe boundary distortion occurs, facilitating the emission and absorption of partial dislocations, which further assists the CTB-BPB transformation. The present results thus help to explain the frequent observation of coexisting CTB and BPB in HCP alloys and further contribute to the understanding of their microstructure and property regulation.

Deformation behaviors in light of dislocation core characteristics with respect to the compositional-dependent misfit potentials of aluminum alloys

Dislocation core dominates dislocation mobility and mechanical properties of crystalline solids. To date, a complete landscape for describing dislocation with narrow core in metals like aluminum (Al) remains elusive, and thus deformation mechanisms of Al alloys are still unclear. This work investigates the dislocation core structure and deformation behaviors of Al alloyed with solutes X (X = Mg, Si, Cu, Zn, and Fe) within the framework of semi-discrete variational theory combined with first-principles calculations. Depending on the dislocation core characteristics, the deformation modes of Al alloys are governed by the compositional-dependent misfit potentials. Except for Fe, all other solutes in Al decrease the intrinsic SF energy, with considerable effects being Mg and Si. The deformation tends to occur via cross-slip of dislocations in Al alloyed with Mg, whereas Si additions can benefit to deform via emission of partial dislocations. Our investigation offers theoretical guidance for choosing solutes favorable to mechanical performances of Al alloys.

Atomic mixing mechanisms in nanocrystalline Cu/Ni composites under continuous shear deformation and thermal annealing

Molecular dynamics (MD) simulations are used to reveal the mechanisms of defect substructure evolution and atomic mixing in nanocrystalline Cu/Ni composites under severe shear deformation and subsequent thermal annealing. A continuous shear scheme of MD simulation utilizing an on-the-fly periodic boundary adjustment approach enables it to reach any large shear strain. The comprehensive evaluation on the evolution of dislocation structures at various strain states indicates partial dislocation-mediated plastic deformation and triple junction slide via partial dislocation nucleation and emission from triple junctions and grain boundaries. The analysis of atomic structures in unique mixing regions suggests that triple junction sliding can result in a long-range region of atomic mixing facilitated by net dislocation flux, which is the key mechanism of atomic mixing in nanocrystalline Cu/Ni composite under severe shear, while dislocation emissions at interfaces result in short-range mixing. It is also found that thermal annealing causes the evolution of non-equilibrium defect substructures which assists atomic mixing.

Unraveling the dynamics of stacking fault nucleation in ceramics: A case study of aluminum nitride

Stacking fault (SF), originating from the emission of partial dislocations, wields significant influence over the structural and physicochemical traits of ceramic materials. Yet, the intricate atomic dynamics driving SF nucleation remain obscured. Here, we introduce an improved methodology for computing the generalized stacking fault energy (GSFE) in ceramics, integrating uneven Degrees of Freedom (DOFs) for distinct lattice sites. This refinement has yielded substantial energy advantages over the traditional rigid shift method inherited from metallic systems. Our findings underscore that the relaxation of nonmetallic N atoms within the SF region is pivotal for achieving a more realistic SF simulation. This, in turn, unveils the involvement of N atom migration within the SF region between different aluminum tetrahedral sites during SF nucleation. By alleviating the energy barrier, this relaxation contrasts with previous simulations where nonmetallic elements remained more rigid. This work demonstrates the atomic dynamics of SF nucleation in ceramics and breaks the conventional wisdom of uniformly applying constraints for GSFE computations.

Modulating grain boundary-mediated plasticity of high-entropy alloys via chemo-mechanical coupling

Intrinsic chemical disorder and the subsequent interfacial roughening have posed formidable challenges in elucidating the grain boundary (GB)-mediated plasticity in high-entropy alloys (HEAs). Here using a self-propelling atomistic algorithm to probe the complex energy landscape of the CoCrFeMnNi HEAs, in conjunction with location-specific perturbations across GBs exposed to different environments, we investigate atomic-reconfiguration ensembles near GBs and their sensitivities to various chemo-mechanical conditions. Two distinct modes, collective and random, are discovered, and their partitions are dictated by multiple factors, including the activation energy window, external mechanical loading, and local compositions. Remarkably, Fe disproportionately promotes the collective events and facilitates the slip activities near GBs, while Cr atoms suppress the emission of partial dislocations from GBs. These findings imply promising solutions – via synergistic combination of microalloying, heat treatment, and mechanical loading – to selectively trigger desired plasticity modes at needed deformation stage, and hence to achieve an enhanced tunability of HEAs’ mechanical behaviors.

A Brief Review on Theoretical Models of Deformation Twinning at Locally Distorted Grain Boundaries

A brief review of the theoretical models which describe mechanisms of deformation twinning in nanocrystalline and ultrafine-grained materials is presented. In the framework of the models, formation of nanoscale deformation twins occurs at locally distorted grain boundaries that contain fragments being rich with grain boundary dislocations due to preceding severe plastic deformation processes. Within the review, mechanisms of deformation twinning at locally distorted grain boundaries represent (a) the consequent emission of partial dislocation; (b) the cooperative emission of partial dislocations; and (c) the generation of multiplane nanoscale shear.

Effect of twin spacing on the mechanical behavior and deformation mechanism of nanotwinned Al

The effect of twin spacing on the mechanical behavior and deformation mechanism of nanotwinned Al with high stacking fault energy (SFE) is investigated by molecular dynamics simulation, and the results are compared to nanotwinned Ni and Cu with different SFEs under the same simulation conditions. It is found that the nanotwinned Al with small grain size shows a strengthening-softening transition with the reduction of twin spacing, which is attributed to the transition of dislocation from inclined to twin boundaries (TBs) to parallel to TBs. Such observations are consistent with most nanotwinned metals with low SFE. However, the nanotwinned Al with large grain size shows a continuous strengthening with the decrease of twin spacing. It originates from the strain localization in grain boundaries, which decreases with the reduction of twin spacing, and few dislocations are activated in the tensile processes. The high ratio of stable SFE to unstable SFE in nanotwinned Al inhibits the partial dislocation emission, especially for the nanocrystalline structure with large grain size, which results in the different mechanical behaviors and deformation mechanisms in the nanotwinned Al.

Atomistic Simulation of Ultrasonic Welding of Copper

Molecular dynamics simulations of ultrasonic welding of two blocks of fcc copper containing asperities under the conditions of a constant clamping pressure and sinusoidal shear displacements were performed. Two different atomistic models of blocks were simulated: Model I with no misorientation between the lattices, and Model II with a special misorientation of 78.46°. Alternating shearing results in a plastic deformation of the interface layers and is accompanied by the emission of partial dislocations. Misorientation between the joined blocks contributes significantly to an interface sliding, interface migration, and pores healing during ultrasonic processing. A significantly larger increase in temperature occurs during shearing in Model II than in Model I. The applied pressure has almost no effect on the interface temperature in both studied models. The temperature increases almost up to maximum values after the first shear cycle, and then practically does not undergo changes in the next four cycles. The temperature at the interface in Model II is significantly higher than that in Model I. The change in the porosity of the interface and its structure are analyzed. The results obtained in the present work contribute to a deeper understanding of the processes occurring at the atomic level during ultrasonic welding of metals.

Deformation-induced concurrent formation of 9R phase and twins in a nanograined aluminum alloy

Twins and 9R phase (a structure containing a high density of stacking faults) have played important roles in simultaneously improving strength and strain hardening capacity of low-to-medium stacking fault energy (SFE, smaller than 50 mJ/m2) metallic materials. Due to the high SFE (166 mJ/m2) in pure face-centered-cubic Al and its insensitivity to most soluble alloying elements, concurrent introduction of deformation twins and 9R phase into an originally twin-free Al alloy is intuitively impossible. Here, we propose a strategy to simultaneously produce deformation twins and 9R structure in an Al-Mg alloy by nanostructuring grain sizes and alloying with Y. We demonstrate that plastic deformation of the nanograined Y-doped Al-Mg alloy can concurrently drive the dissociation of incoherent twin boundaries and direct emission of grain boundary Shockley partial dislocations, enabling the development of two different 9R structures in the nanograined Al-Mg-Y alloy. Our strategy combining grain size nanostructuring and SFE tuning presents a new microstructure design in strengthening bulk Al alloys by planar defects of deformation twins and 9R structure and/or stacking faults.

Atomic-scale observation of dynamic grain boundary structural transformation during shear-mediated migration.

Grain boundary (GB) structural change is commonly observed during and after stress-driven GB migration in nanocrystalline materials, but its exact atomic scale transformation has not been explored experimentally. Here, using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we observed the dynamic GB structural transformation stemming from reversible facet transformation and GB dissociation during the shear-mediated migration of faceted GBs in gold nanocrystals. A reversible transformation was found to occur between (002)/(111) and Σ11(113) GB facets, accomplished by the coalescence and detachment of [Formula: see text]-type GB steps or disconnections that mediated the GB migration. In comparison, the dissociation of (002)/(111) GB into Σ11(113) and Σ3(111) GBs occurred via the reaction of [Formula: see text]-type steps that involved the emission of partial dislocations. Furthermore, these transformations were loading dependent and could be accommodated by GB junctions. This work provides atomistic insights into the dynamic structural transformation during GB migration.

Molecular dynamics simulation of chemical short-range order strengthening in FCC FeNiCrCoAlx alloys

Chemical short-range order (CSRO) has been regarded as a possible strengthening method for multi-principal element alloys (MPEAs). In our work, Molecular Dynamics (MD) simulation method was used to study the strengthening effect and micro mechanism of CSRO on FCC FeNiCrCoAlx alloys under uniaxial tension. The results demonstrate that the strengthening effect of CSRO is consistent with the change of Al content. CSRO increases the stacking fault energy of FCC FeNiCrCoAlx alloys, reduces the twinning tendency of the alloy, and forms a stacking fault network and Lomer-Cottrell lock that hinder dislocation slip. Besides, CSRO also increases the critical shear stress required for the first Shockley partial dislocation emission. The strengthening effect of CSRO is significant at room and low temperatures. However, although CSRO alloy still has higher yield strength at high temperatures, there is no denser fault network and Lomer-Cottrell lock at the plastic stage.

Grain boundary relaxation induced ultrastrong-and-ductile bulk pure Ni

Both strength and ductility are essential for high-performance engineering structural materials, thus great endeavors have been invested to solve the strength-ductility trade-off of them during recent two decades. Here, we utilized grain boundary (GB) relaxation to circumvent the trade-off in bulk pure Ni through optimizing grain size when literatures tells that GB relaxation can improve strength but ductility. Both tensile strength and uniform elongation of Ni were elevated from 1450 MPa to 2013 MPa and from 2.33% to 5.17%, respectively. The combination of strength and ductility extends beyond the range established by the strongest bulk pure Ni known. Our investigation unraveled that the increased strength was produced by GB relaxation which remarkably mitigated GB softening produced by GB sliding, and that partial dislocation emission from GBs became the dominant plastic deformation mechanism. The dislocation activities inside the grains increased the strain hardening rate (θ). At the same time, enhanced probability of interaction between dislocations and GBs improved the strain rate sensitivity (m). It was the GB relaxation induced dislocation activities that improved θ and m, which stabilized the plastic deformation to enhance ductility. The present work offers a foundation for the ongoing of more advanced engineering structural materials with the advisable design of GB complexion.

Slip transmutation between basal [formula omitted] dislocation and [formula omitted] twinning across [formula omitted] twin boundary in titanium

Molecular dynamics simulations reveal slip transmutation mechanisms between basal 〈a〉 dislocations and {112¯1} twinning in titanium. When a co-planar pileup of basal 〈a〉 dislocations approaches (112¯4) coherent twin boundary (CTB), the 1st dislocation transmutes into a four-layer (112¯1) twin in the (112¯4) twin, forming a (112¯4)−(112¯1) double twin, and the 2nd and 3rd dislocations climb along the (112¯4) CTB and then transmute into twinning dislocations, thickening the (112¯1) secondary twin. When the (112¯1) secondary twin propagates and encounters the other (112¯4) CTB, a reverse transmutation happens associated with the nucleation and emission of partial dislocations.

Dynamic recrystallization in face-centered cubic particles during high-velocity impacts

In this work, dynamic recrystallization of nano- and micro-particles is investigated with high-velocity impact simulations. Molecular dynamics simulations reveal that upon a critical velocity, face-centered cubic particles recrystallize due to a synchronous emission of Shockley partial dislocations. Dislocations interaction generates a complex network, ultimately leading to recrystallization of high misorientation angle grains. Since pressure wave decays with the distance from the impacted surface, so does the grain size, generating a new microstructure with spatial gradients within the particles. We show that this behavior is universal for a set of six representative face-centered cubic materials. Below the critical velocity, twinning dominates the deformation mechanisms. The atomistic results are then extrapolated to microscopic size particles relevant in the context of some additive manufacturing techniques using a continuum dislocation density model. The severe plastic deformation experienced by the microparticles results in large dislocation densities and recrystallized grains of varying sizes. These findings are then compared with experimental observations of previous reported microparticles impacts, finding good agreements between experiments and simulations. Our work provides a quantitative analysis of the dynamic recrystallization of particles during high-impact velocities. It also suggests the possibility of tailoring their microstructure by controlling their size, orientation, and impact velocity.

Deformation Mechanisms of FCC-Structured Metallic Nanocrystal with Incoherent Twin Boundary

Incoherent twin boundaries (ITBs) can significantly affect the mechanical properties of twin-structured metals. However, most previous studies have focused on the deformation mechanism of the coherent twin boundary (CTB), and metals with ITB-accommodated plasticity still require further investigation. In this study, deformation mechanisms of FCC-structured nanocrystal metals with ITBs were investigated using molecular dynamic (MD) simulations. We revealed that three deformation mechanisms occur in metals with ITBs. The first type of deformation was observed in Au, where the plasticity is governed by partial dislocation intersections with CTBs or reactions with each other to form Lomer–Cottrell (L–C) locks. In the second type, found in Al, the deformation is governed by reversible ITB migration. The third type of deformation, in Ni and Cu, is governed by partial dislocations emitted from the ITB or the tips of the stacking faults (SFs). The observed L–C lock formation, as well as the reversible ITB migration and partial dislocation emission from the tips of SFs, have rarely been reported before.

Study on the evolution mechanism of subsurface defects in nickel-based single crystal alloy during atomic and close-to-atomic scale cutting

Nickel-based single crystal alloys are widely used in aerospace industry. From the point of view of defect evolution and energy change, the evolution mechanism of subsurface defects in the atomic and close-to-atomic scale (ACS) cutting single crystal nickel-based alloy with silicon nitride tool was studied, combining with the molecular dynamics method. The mixed potential function was used to describe the force interaction between atoms in the ACS cutting system. The process of nucleation, propagation and transformation of subsurface defects were analyzed. Based on the dislocation emission model, the relationship between dislocation slip length and change of energy was described. The variety, quantity and area of subsurface defect changes caused by the change of cutting speed, crystal orientation and cutting depth were discussed. The result shows that increasing cutting speed can effectively reduce the number and area of subsurface defects and improve the quality of machined parts. The larger the cutting speed, the larger the chip volume. With the increase of cutting depth, the number of stacking fault tetrahedrons in the subsurface area of workpiece increases. When crystal orientation 010 00 1 ¯ is used for cutting, more defects will be produced in the subsurface region. When crystal orientation are 111 10 1 ¯ and 011 01 1 ¯ , the dislocation emission angle is π /2, it is the most favorable for the emission of partial dislocations and easy to remove materials. The research content provides important references for the optimization of machining parameters and improvement of processing quality for the ultraprecision cutting of nickel-based alloy.

Shear-strain induced structural relaxation of Cu Σ3 [110](112) symmetric tilt grain boundary: The role of foreign atoms and temperature

Grain boundaries (GBs) relaxation is a promising and effective strategy to improving GB stability or stabilizing nanocrystalline metals. However, previous studies mainly focused on nanocrystalline pure metals and GB behaviors therein, without considering the role of foreign atoms such as impurity or alloying atoms in GB relaxation. In this work, the shear-strain induced structural relaxation of pure Cu Σ3 [110](112) symmetric tilt GBs (STGBs), and the effects of foreign elements (Fe and Ni) and temperature on the GB relaxation were investigated in detail by molecular dynamics method. The results show that shear strain can trigger the structural relaxation of pure, Fe- and Ni-containing Cu GBs by the emission of Shockley partial dislocations from Cu GBs. Both Fe and Ni have impediment effects on the shear-strain induced GB relaxation, though the content of Fe or Ni atom (0.00165 at.%) is quite low in the GB model. The temperature cannot trigger GB relaxation independently within the considered temperature range, but play a positive role in the shear-strain induced structural relaxations of pure, Fe- and Ni-containing Cu Σ3 [110](112) STGBs. Our work might gain new insights into the mechanically induced GB relaxation in nanocrystalline copper and could be beneficial for improving the stability of Cu GBs.

Structural characterization of the [formula omitted] twin boundary and the corresponding stress accommodation mechanisms in pure titanium

{112-2} compression twin plays an important role in accommodating deformation along the c axis of HCP metals. However, the studies on the interface structure of {112-2} twin boundary (TB), especially the twin tip, and the corresponding local stress release mechanism are still limited. This work studied the interface characters of {11 2- 2} TB of a deformed pure titanium by transmission electron microscope. The {11 2- 2} TB presented serrated character, consisting of coherent twin boundary (CTB) and (0002)T // (112-2-)M or (112-2-)T // (0002)M steps, and the twin tip was fully composed of the basal-pyramidal (BPy) and pyramidal-basal (PyB) steps. The twin tip presents asymmetric morphology and the step height at the twin tip is much larger than that away from the twin tip. Two types of local stress accommodation mechanisms were observed: zonal dislocation emission and hexagonal close packed structure to face-centered cubic structure transformation. The zonal dislocation was produced by the dissociation of the 1/3 < 11 2-3- ><c + a> dislocation that was nucleated at the twin tip and the phase transformation was introduced by emission of Shockley partial dislocations from the {11 2- 2} twin boundary.

Effect of Cr on the generalized stacking fault energy of impure doped Ni (111) surface: a first-principles study

In the present work, a systematic analysis of the microscopic plastic deformation mechanism and mechanical properties of interstitial impurities (H, O, N, S, and P) and Ni doped, or not doped, with Cr, was conducted based on generalized stacking fault energy curves generated via first-principles calculations. Focus has been put on the effects of Cr on plastic deformation for Ni doped interstitial impurities, upon the GPFE curve, with the aim to investigate the effects of Cr on the deformation mode and mechanical properties for doped Ni systems. It is found that a solid solution of Cr caused the tendency of partial dislocation in Ni. The evaluation of the Rice criterion reveals that Cr tends to decrease the ductility in Ni, and it cannot reverse interstitial H promoting the probability of cleavage fracture in Ni, while increases the ductility of O, P and S doped Ni, particular in O doped Ni, due to increasing the value of ductility D remarkably, so possibly changes the tendency of cleavage fracture. Besides, the solid solution of Cr is beneficial in promoting the dissociation of dislocation into fragments more easily in Ni, and enhances dislocation nucleation, while O, N, and S impurities have a slower rate of partial dislocation emission in Ni when interacting with Cr. Furthermore, Cr promotes the probability of twinning in Ni, and probably switches the deformation mechanism of H doped Ni from dislocation mediated slipping to twinning. Our study provides important insights toward the understanding and control of dislocation dynamics in doped Ni.

Molecular Dynamics as a Means to Investigate Grain Size and Strain Rate Effect on Plastic Deformation of 316 L Nanocrystalline Stainless-Steel.

In the present study, molecular dynamics simulations were employed to investigate the effect of strain rate on the plastic deformation mechanism of nanocrystalline 316 L stainless-steel, wherein there was an average grain of 2.5–11.5 nm at room temperature. The results showed that the critical grain size was 7.7 nm. Below critical grain size, grain boundary activation was dominant (i.e., grain boundary sliding and grain rotation). Above critical grain size, dislocation activities were dominant. There was a slight effect that occurred during the plastic deformation mechanism transition from dislocation-based plasticity to grain boundaries, as a result of the stress rate on larger grain sizes. There was also a greater sensitive on the strain rate for smaller grain sizes than the larger grain sizes. We chose samples of 316 L nanocrystalline stainless-steel with mean grain sizes of 2.5, 4.1, and 9.9 nm. The values of strain rate sensitivity were 0.19, 0.22, and 0.14, respectively. These values indicated that small grain sizes in the plastic deformation mechanism, such as grain boundary sliding and grain boundary rotation, were sensitive to strain rates bigger than those of the larger grain sizes. We found that the stacking fault was formed by partial dislocation in all samples. These stacking faults were obstacles to partial dislocation emission in more sensitive stress rates. Additionally, the results showed that mechanical properties such as yield stress and flow stress increased by increasing the strain rate.