Dislocation Nucleation Research Articles

Evolution of Symmetrical Grain Boundaries under External Strain in Iron Investigated by Molecular Dynamics Method

In the present work, the evolution of atomic structures and related changes in energy state, atomic displacement and free volume of symmetrical grain boundaries (GB) under the effects of external strain in body-centered cubic (bcc) iron are investigated by the molecular dynamics (MD) method. The results indicate that without external strain, full MD relaxations at high temperatures are necessary to obtain the lower energy states of GBs, especially for GBs that have lost the symmetrical feature near GB planes following MD relaxations. Under external strain, two mechanisms are explored for the failure of these GBs, including slip system activation, dislocation nucleation and dislocation network formation induced directly by either the external strain field or by phase transformation from the initial bcc to fcc structure under the effects of external strain. Detailed analysis shows that the change in free volume is related to local structure changes in these two mechanisms, and can also lead to increases in local stress concentration. These findings provide a new explanation for the failure of GBs in BCC iron systems.

Analysis of strain hardening behavior of a high-Mn TWIP steel using electron microscopy and cyclic stress relaxation

Electron microscopy and cyclic stress relaxation experiments were employed to study the dislocation-obstacle interactions and activated hardening mechanisms, during tensile straining of a high-Mn TWIP steel specimen, and to estimate the contributions of hardening mechanisms to the strain hardening behavior. Electron contrast channeling and high-resolution electron backscattered diffraction images showed that deformation twins and their twin boundaries affect the dislocation evolutions inside both the matrix and twin lamellae. High-resolution transmission electron microscopy suggested that twin boundaries (TBs) can be passed by dislocations through the dislocation multiplication mechanism resulting in the nucleation of sessile dislocations and alternated stacking faults at TBs. Consequently, high dislocation density was developed behind the TBs resulting in enhancement of the strain hardening rate through the composite hardening effect. Various thermally activated parameters determined through cyclic stress relaxation tests indicated that dislocation evolution, developed by TBs in terms of the composite effect, was the prevalent rate-controlling mechanism, imparting higher strengthening than TBs in terms of the dynamic Hall–Petch effect. Finally, the high nucleation tendency of Shockley partial dislocations at TBs was interpreted from the variation of the stress exponent and independent plastic strain rate, and also from the general stacking fault energy point of view.

Thermodynamics description of dynamic plasticity in metals

In metals, the plastic flow is initiated when externally-supplied energy exceeds limits imposed by energy barriers, where the word “exceeds” is of interest. The extra energy, called here activation energy, is responsible for the nucleation of dislocations and is shown to guide the plastic process toward the state of thermodynamic equilibrium. The macroscopic plastic flow comprises of a large number of discrete dislocation events. The events cause local disturbances in stresses and strains, are associated with micro-inertia forces, and their lifespan is short. It turns out that the micro-kinetic energy affects the plastic flow at all strain rates. However, in the extremes of dynamics, the magnified perturbations corrupt the flow patterns making the flow process increasingly chaotic. In here, the ideas are first discussed on the ground of dislocation-based considerations, and then, the concept of dynamic plasticity is formulated in the framework of the Helmholtz free energy and its dual potential, where the two energies complement each other.

Crystallinity and chain stem length dependence of yield kinetics in polyethylene

AbstractResearchers have long sought to predict the mechanical behavior of polyethylene from its microstructure. In particular, the yield strength and yield kinetics have been reported to be dependent on crystallinity and crystal thickness, but the relative importance of these two microstructural attributes has not been shown. In the present work, a series of microstructures was obtained through a combination of controlled quench rates from the melt and inclusion of various amounts of hexene comonomer. The yield strength for a wide range of strain‐rates was linearly dependent on the crystallinity, and independent of crystal thickness (chain stem length), both measured by Raman spectroscopy. Similarly, yield kinetics described by a Ree‐Eyring two‐process stress activated model showed linear dependence on crystallinity and no dependence on crystal thickness. The results of the present work call into question models of yield kinetics dependent on screw dislocation nucleation, which depend on crystal thickness.

Short range ordering and strengthening in CoCrNi medium-entropy alloy

The CrCoNi medium-entropy alloy is found to exhibit higher incipient plastic strength by slow cooling after the homogenization treatment. Compared with water quenching, this strengthening originates from the inconsistency of the residual stress and short-range orderings (SROs). By deconvolution of the density distribution of the critical shear stress for the first pop-in, two peaks corresponding to homogeneous and heterogeneous dislocation nucleation, respectively, are suggested. Combining the mean, areal fraction, and activation volume of the two peaks, the roles of residual stress and SROs were quantified. Here, for SRO that increases the critical shear stress for heterogeneous and homogeneous nucleation by 22.4% and 39.9%, the residual stress is negligible for the 2.1% strengthening of both. As a result, slow cooling promotes the formation of SRO in CrCoNi, which is of immense benefit to elevate its properties.

Effect of twin boundaries on the strength of body-centered cubic tungsten nanowires

Twin boundaries (TBs) are assumed to be obstacles to dislocation motion and increase the strength of metals. Here, we report the abnormal phenomenon that TBs reduce the strength of body-centered cubic (BCC) tungsten (W). [1–11]-oriented W nanowires with (121) twin planes and free of dislocations were fabricated by chemical vapor deposition. In situ tensile tests within the transmission electron microscope were performed on single-crystal and twinned W nanowires. The fracture strength of the twinned W nanowire was 13.7 GPa, 16% lower than that of the single-crystal W nanowire (16.3 GPa). The weakening mechanism of the TB was revealed by a combination of atomic-resolution characterizations and atomistic simulations. Twinned W nanowires failed by the early nucleation of a crack at the intersection of the TB with the surface. The standard strengthening mechanism by dislocation/TB interaction was not operative in W because the high Peierls barrier and stacking fault energy in W hinder dislocation nucleation and glide. These findings provide a new insight into the influence of TBs on the mechanical properties of BCC metals.

Atomistic Simulations of Ductile Failure in a b.c.c. High-Entropy Alloy

Ductile failure is studied in a bcc HfNbTaZr High-Entropy Alloy (HEA) with a pre-existing void. Using molecular dynamics simulations of uniaxial tensile tests, we explore the effect of void radius on the elastic modulus and yield stress. The elastic modulus scales with porosity as in closed-cell foams. The critical stress for dislocation nucleation as a function of the void radius is very well described by a model designed after pure bcc metals, taking into account a larger core radius for the HEA. Twinning takes place as a complementary deformation mechanism, and some detwinning occurs at large strain. No solid–solid phase transitions are identified. The concurrent effects of element size mismatch and plasticity lead to significant lattice disorder. By comparing our HEA results to pure tantalum simulations, we show that the critical stress for dislocation nucleation and the resulting dislocation densities are much lower than for pure Ta, as expected from lower energy barriers due to chemical complexity.

Phase Volume Fraction-Dependent Strengthening in a Nano-Laminated Dual-Phase High-Entropy Alloy.

A recently synthesized FCC/HCP nano-laminated dual-phase (NLDP) CoCrFeMnNi high entropy alloy (HEA) exhibits excellent strength–ductility synergy. However, the underlying strengthening mechanisms of such a novel material is far from being understood. In this work, large-scale atomistic simulations of in-plane tension of the NLDP HEA are carried out in order to explore the HCP phase volume fraction-dependent strengthening. It is found that the dual-phase (DP) structure can significantly enhance the strength of the material, and the strength shows apparent phase volume fraction dependence. The yield stress increases monotonously with the increase of phase volume fraction, resulting from the increased inhibition effect of interphase boundary (IPB) on the nucleation of partial dislocations in the FCC lamella. There exists a critical phase volume fraction, where the flow stress is the largest. The mechanisms for the volume fraction-dependent flow stress include volume fraction-dependent phase strengthening effect, volume fraction-dependent IPB strengthening effect, and volume fraction-dependent IPB softening effect, that is, IPB migration and dislocation nucleation from the dislocation–IPB reaction sites. This work can provide a fundamental understanding for the physical mechanisms of strengthening effects in face-centered cubic HEAs with a nanoscale NLDP structure.

Thickness-dependent physical and nanomechanical properties of [formula omitted] thin films

A set of undoped AlxGa1−xN epilayers with different thicknesses were grown on (0001) sapphire substrates using metal-organic chemical vapor deposition (MOCVD) technique, using the same gas-phase composition of trimethylaluminum (TMAl) and trimethylgallium (TMGa). Giving that the first growth stage is crucial for the successful fabrication of AlxGa1-xN epilayers; thus, SiN nano-mask, as an alternative method to nanoimprint lithography (NIL) which is used for patterning substrates, was deposited on sapphire substrate. Indeed, SiN treatment initiates the three-dimensional (3D) growth mode to efficiently decrease the threading dislocation density. It was found that depending on the film surface coalescence degree, the physical and nanomechanical properties are thoroughly dependent. As the non-coalescent surface is present, the porosity effects are dominant. As soon as the film thickness attains the surface coalescence, the Al incorporation is enhanced and convoyed with it point defects. Despite the decrease of the threading dislocations along with the film thickness evolution, it was observed that the physical properties are slightly decreased and the nanomechanical properties are improved. This is likely related to point defect interaction with nucleated dislocations. Moreover, the defects did not severally affect the plastic deformation capacity (ductility) of AlxGa1-xN films, where high indentation force was applied and no failure was revealed. Therefore, the balance between AlxGa1−xN physical and nanomechanical performances devotes to manufacture a wider range of efficient UV devices.

Phase field crystal simulation of gap healing at nanoscale

The phase field crystal method is used to simulate the healing process of the central gap of three-dimensional bcc crystal material under compressive strain at the atomic level. It is found that during the healing process of the central gap, the gap protrudes at both ends of it, leading to dislocation nucleation and vacancy formation. Through the mechanism of dislocation nucleation and dislocation emission, the thickness of gap is reduced layer by layer, and finally the connection and closure of the lattice atoms on up and down surface of the gap are achieved, and the surface healing of the central gap is realized. According to the sharpening and passivation mechanism of the lattice atomic planes at both ends of the gap, the elliptic shape gap is approximated to calculate and analyze the influence of the change of stress intensity factor during the gap healing, and the critical condition of the gap dislocation emission is determined.

Transient dislocation emission from the nanosized interface of Cu–Ag composite under the coupled thermal-mechanical shock: Molecular dynamics simulations study

In this study, we use molecular dynamics to simulate the deformation of Cu–Ag nanocomposite under the coupled thermal-mechanical shock. We compare the dislocation evolution under different loading conditions by changing the strain rate and the heating rate applied to the nanocomposite. The result indicates that for a given strain rate with increasing heating rates, the dislocation density decreases, and so does the interaction between dislocations. The resistance encountered by the dislocation movement decreases as the dislocation movement mode shifts from dislocation slip to climb. The plastic deformation mechanism transforms from dislocation movement to the amorphization of the whole composite, resulting in the disruption of the interfacial dislocation network. These factors conspire to cause a reduction in yield stress. For a given heating rate with increasing strain rates, owing to the strain rate effect and the temperature difference during the deformation process, the yield stress increase. We can conclude that due to the difference in dislocation density, dislocation movement mode, and the plastic deformation mechanism, the dislocation nucleation stress and the yield stress of composite decrease with the strain rate decreases and the heating rate increases (which involves the effect of temperature difference).

Grain structure and cracks behaviors of tungsten with different geometrical shapes and support structure prepared by laser powder bed fusion

In order to explore the application of tungsten (W) fabricated by laser powder bed fusion (LPBF), W with different geometrical shapes and different support structures were prepared, and the corresponding microstructural characteristics were investigated. Coarse columnar grains and fine grains exist simultaneously in the cylinder or bulk W specimens fabricated by bi-directional scanning. The single-directional scanning used in wall-shape specimen fabrication facilitates the epitaxial growth of W grains, forming regular elongated columnar grains. Cracks were found in all the specimens, the formation of cracks in W during LPBF is determined by a combination of the localized stress distribution and the intrinsic strength/ductility of the W phase. Cracks are concentrated at the grain boundaries where the coarse columnar grain and fine grains are adjacent. With the specimen size increased from cylinder (slender cylinder, squat cylinder), wall (thin wall) to the bulk specimen (with 0–2 mm support), more cracks were found due to higher residual stress. Cracks could relieve the residual stress dramatically, which leads to the decrease of the dislocation density, therefore, with the increase in specimen size, both low angle grain boundaries (LAGBs) and the geometrically necessary dislocation (GND) density decrease. Adding support structure in bulk W could decrease the cooling rate and increase the cooling time. Low cooling rate can reduce the stress level as well as the generation of cracks. Long cooling time could provide sufficient time for dislocation nucleation and movement to form LAGBs structure. Therefore, there are fewer cracks and higher GNDs density/LAGBs portion inside the W phase with a higher support structure.

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.

Chemical short-range ordering regulated dislocation cross slip in high-entropy alloys

High-entropy alloys (HEAs) have received extensive interest owing to their unusual mechanical properties as a result of the extreme compositional complexity. Understanding the dislocation behavior under the influence of chemical disorder, especially chemical short-range ordering (SRO), which is commonly found in HEAs, is fundamental to the rational design of high-performance alloy systems. Here, we systematically investigate the effect of disorder and SRO on the regulation mechanism of dislocation nucleation and cross-slip in HEAs through a combination of Monte Carlo methods, molecular dynamic (MD) simulations, and theoretical analysis. Our results demonstrate that the incipient plasticity of HEAs under nanoindentation is mainly controlled by the nucleation and cross-slip of Shockley partial dislocation loops, which are found to be highly dependent on SRO. In contrast to traditional strengthening, SRO leads to a significant increase in the stable nucleation radius of the dislocation loop and cross-slip resistance. Strengthening is then achieved due to the extension of the dislocation nucleation period and the high resistance for dislocation cross-slip. Aided by the transition path analysis and theoretical model, we show that the local compositional changes originating from SRO, specifically for strong Co-Cr pairs, are the dominant factors governing the dislocation properties and accordingly, the strengthening effects. Finally, the SRO strengthen effect is further validated against different alloy systems with different SRO types. These findings suggest that the elasticity and strength of HEAs can be improved through the prolonging of the nucleation period of dislocations resulting from SRO, dictating an avenue for enhancing the mechanical properties of HEAs. • The incipient plasticity of HEAs under nanoindentation is controlled by the nucleation and cross-slip of partial dislocation loops. • Strong SRO increases the nucleation energy barriers and the stable nucleation length is longer simultaneously. • Dislocation cross-slip is inhibited by SRO. • SRO increase the critical transition radius from dislocation slip to cross slip. • Strength originating from the interaction between solute and dislocation is enhanced by strong SRO.

Inhibiting the inverse Hall-Petch behavior in CoCuFeNiPd high-entropy alloys with short-range ordering and grain boundary segregation

The mechanical implications of short-range ordering (SRO) and grain boundary (GB) segregation in the inverse Hall-Petch behavior of nanocrystalline CoCuFeNiPd high-entropy alloys (HEAs) were studied using hybrid Monte Carlo (MC)/Molecular Dynamics (MD) simulations. Results show that the presences of SRO and GB segregation inhibit the grain size softening (inverse Hall-Petch relation), leading to grain-size independence of the high flow stress. We find that the dislocation nucleation at triple junctions is suppressed by the SRO due to the increased stacking fault energy, which attenuates the dislocation activities. Furthermore, the strain localization at GBs is intensified by the GB segregation due to the increased GB energy, which facilitates the GB activities and glass-like deformation. The GB-governed plasticity associated with glass-like deformation leads to suppression of the inverse Hall-Petch effect. These findings may provide useful strategies for the design of high-performance HEAs by tuning the GB composition and SRO structure.

A coarse-grained concurrent multiscale method for simulating brittle fracture

This paper proposed a concurrent multiscale method for modeling crack propagation in brittle solids. The method couples the continuum finite element domain with a recently-developed coarse-grained atomistic potential, which has been developed to simulate crack nucleation and propagation within the molecular dynamics domain. The main advantage of this potential is to suppress dislocation nucleation from the crack tip which significantly eases the coupling procedure. In this multiscale framework, complexities like large deformation, crack nucleation and propagation are simulated in the atomistic domain while the rest such as elastic wave propagation and boundary conditions are modeled in the continuum domain. As a result, the atomistic area will be limited around the cracking zone which increases the computational efficiency. Two examples of edge crack propagation and sliding surfaces in contact are presented and compared with the fully atomistic models to verify and check the efficiency of the proposed multiscale method. The comparison shows that the proposed method can accurately capture the crack formation while the computation cost reduces significantly.

Atomistic simulations of mechanical response of a heterogeneous fcc/bcc nanolayered composite

Molecular dynamics simulations are performed to study the mechanical properties and deformation mechanisms of a heterogeneous face-centered cubic/ body-centered cubic Cu/Ta nanolayered composite under uniaxial tension and compression. The results show that the stress–strain curves exhibit two main yield points in tension while only one yield point during compression, and the deformation primarily experiences three stages. The first stage is linearly elastic at small strains, followed by the nucleation and propagation of dislocations and stacking faults in the Cu layers, and eventually the Ta layers yield to plastic deformation. The yield of the specimen is mainly determined by the dislocation evolution in the hard phase (i.e. Ta layers), which leads to a sharp drop in the stress–strain curve. We show that the heterogeneous nanolayered composite exhibits a good deformation compatibility during compression but an obvious deformation incompatibility between Cu and Ta layers in tension. The temperature effect is also systematically investigated. It is revealed that the yield of the specimen at higher temperature depends only on the dislocation evolution in the thick Ta layers, and the yield strengths in tension and compression both decrease with the increasing temperature. In particular, our computations show that high temperature can significantly suppress the dislocation activities in the Cu layers during deformation, which results in a lower dislocation density of the Cu layers compared with that of the Ta layers and thus causing an incompatible fashion among the constituent layers.

Interface-related deformation phenomena in metallic glass/high entropy nanolaminates

The deformation behavior of metallic glass (MG)-high entropy alloy (HEA) nanolaminate is explored through molecular dynamics simulations using nanolaminate models of FeCoCrNiAl1.7 MG and FeCoCrNiAl0.5 HEA. Quantitative analysis of the atomic strain and dislocation dynamics among competing and cooperative deformation mechanisms is accomplished through the implementation of uniaxial tensile deformation. The combination of glassy and crystalline nanolayers biases the plastic deformation to regions near the glass-crystalline interface at lower strains, which lowers the activation barrier for the onset of dislocation nucleation and propagation. With increasing applied strain, dislocations are absorbed into the amorphous plate via slip transfer across the glass-crystalline interface, in turn triggering the activation of homogeneously distributed shear transformation zones (STZs) in amorphous plate. The competitive deformation mechanism suppresses the formation of localized shear bands and increases the resistance to dislocation motion, thereby promoting enhanced ductility in MG-HEA nanolaminates. Additionally, due to the high strength of the HEA, the laminate structures exhibit a much higher strength than conventional MG-crystalline laminates. The combination of high strength HEAs and MGs and the complex deformation behavior may overcome the typical strength-ductility trade-off and make MG-HEA laminates promising candidates for a variety of structural and functional applications.

Investigation of microstructure and strengthening mechanism in CNTs reinforced Mo-based composites

The carbon nanotubes (CNTs) reinforced Mo-based composites were fabricated by an efficient powder metallurgy method, including acid treatment, wet dispersion, ball milling (BM) and spark plasma sintering (SPS). The sintered Mo-based composites possess high relative density (99.1%) and compressive yield strength (970 ± 10 MPa). Acid treatment and wet dispersion effectively address the problems of the agglomeration and subsequent uniform distribution of the CNTs in the composite powders. After BM and SPS, ultrafine-grained composites with a grain size of 1.84 μm and a nanoparticle size of less than 200 nm were obtained. A mechanism for purification and strengthening is proposed. On the one hand, Hf and HfC can adsorb nearby oxygen impurities and purify the Mo matrix. On the other hand, the incoherent interface between Mo and nanoparticles acts as primary dislocation nucleation and prevents the propagation of secondary dislocations, which in turn leads to strain hardening.

Deformation and damage characteristics of copper/honeycomb-graphene under shock loading

Three-dimensional graphene networks have been promising structures for improving the mechanical properties of metallic composites. This work investigated the deformation and damage characteristics of a single-crystalline and polycrystalline copper matrix with a honeycomb graphene network based on atomistic simulations. The graphene interface exerts a minor influence on the shock Hugoniot relation compared with that of the crystalline orientation of the copper matrix. Shock-induced dislocation nucleation occurs at the junction of the graphene interface and then propagates along the (111) plane of the copper matrix, and the graphene interface can be penetrated by copper atoms under strong shock. The graphene interface will dramatically reduce wave transmission and convert some shock energy into heat at the grain boundary, forming local hot spots and disordered regions. With shock wave releasing to the tensile stage, the graphene interface undergoes wrinkling, folding, and even fracture. Differing from the classical spall, the composites easily spall at the graphene interface under the release wave due to the weak Cu-C interfacial interaction, which significantly reduces the damage degree inside grains and leads to a special failure mode of grain flying. In addition, this fracture dominated by interface separation will produce a complex fracture interface and lead to a nonuniform distribution of spall strength on the fracture surface, with obvious local spallation waves, which makes the estimation of spall strength based on the acoustic approximation method difficult. The average strength of the graphene/single-crystalline copper composite is close to that of polycrystalline copper but higher than that of the graphene/polycrystalline copper composite.