Grain Boundary Plasticity Research Articles

A nucleation rate limited model for grain boundary creep

The motion of grain boundary (GB) dislocations, via a combination of glide and climb, mediates GB plasticity during processes such as creep and GB stress relaxation. Creep within the low stress regime, where non-Newtonian GB creep occurs, remains poorly understood. Models for GB dislocation nucleation rate limited creep, however, have not been sufficiently developed previously in the literature. Herein, a model for nucleation rate limited kinetics is developed and demonstrated to describe existing creep data reasonably well, using physically realistic materials properties as inputs to the model.

Atomistic dynamics of disconnection-mediated grain boundary plasticity: A case study of gold nanocrystals

• Representative nucleation, propagation and interaction dynamics of grain boundary (GB) disconnections are visualized at the atomic scale. • Sources and barriers of different disconnection nucleation dynamics in Σ11(113) GB are quantitatively compared. • The universal interlinks and transitions among different disconnection dynamics are systematically elucidated, which dominate GB migration. • A conceptual map of disconnection-mediated GB plasticity is established. Grain boundary (GB) mediated deformation is a vital contributor to the plasticity of polycrystalline materials, where the disconnection model has become a widely recognized approach to depict the GB dynamics. However, experimental understanding of the atomistic disconnection dynamics remains scarce. In this case study of gold nanocrystals, atomistic disconnection dynamics governing the shear-coupled migration of flat GBs have been systematically investigated via in situ transmission electron microscopy nanomechanical testing supported by molecular dynamics simulations. Specifically, the site-dependent nucleation, shear-driven propagation, and diverse interactions associated with distinct GB disconnections are systematically elucidated and quantitatively compared. Moreover, the disconnection-mediated GB plasticity proves to prevail among different tilt and mixed GBs in gold. Eventually, a conceptual map of disconnection-mediated GB dynamics is established, which would furnish a unified understanding of GB plasticity in metallic materials.

Effect of irradiation defects on the plastic slip of {112} grain boundary: Atomic scale study

Σ3{112} grain boundary (GB) is known to exhibit plastic slip in bcc metals, which overall contributes to the good ductility and toughness of such metals as iron, matrix element for the steels applied in nuclear industry. During nuclear operation, the neutron irradiation causes formation of point defects and their clusters which lead to the non-equilibrium mass transport and agglomeration of nanoscale defects inside grains and near the grain boundaries. Due to the effect of chemical segregation, GBs can encounter sessile obstacles preventing the GB plastic slip. Understanding the influence of the irradiation defects segregated at the grain boundaries is important for the development of structural steels for nuclear fusion and fission applications. The present paper studies the influence of the irradiation defects, such as Cr precipitates, He bubbles and voids on the plastic slip of the GB interface. The studied defects are found to act as strong obstacles to the glide of the elementary disconnections responsible for the shear-coupled GB migration. The interaction between a single elementary disconnection and a series of them provided by a source has been studied. As a result of the interaction with impenetrable defects the formation of residual defects such as loops is observed.

Improved load-bearing capacity of Mo-doped Ti-N coatings: Effects of Mo alloying and GB plasticity

Deformation mechanisms especially grain boundary(GB)-related plastic behaviors are essential for understanding and designing advanced hard coatings. This work systematically studied mechanical behaviors and the underlying interfacial mechanisms in Ti-Mo-N coatings, which is predicted to have both high hardness and toughness according to first-principle calculation. Mo-doped TiN coatings were prepared by multi-arc plating technique as a function of N 2 pressure. The as-deposited coatings demonstrate a bct-Ti 2 N + fcc-TiN dual-phase structure below 1.2 Pa, and single-fcc structure at above 1.2 Pa. Single-fcc phase Ti-Mo-N coatings show considerable higher hardness and load-bearing capacity compared to pure TiN coating due to alloying effect of Mo. Transmission electron microscope (TEM) study unveil that GB plays the key role in the deformation behaviors. The Mo-doped TiN coatings exhibit enhanced plastic deformation ability of GBs, including substantial plastic flow and twinning, which retard intergranular cracking that is geometry necessary during GB glide and rotation. While the appearance of the bct -Ti(Mo) 2 N phase leads to stress concentration at bct-fcc interface during deformation because of the large lattice misfit, considerably lowering load-bearing capacity. In addition, Ti-Mo-N coatings have lower friction coefficient (COF) and wear rate than TiN coating in reciprocal tribology tests, which is attributed to the higher hardness and toughness, as well as tribochemistry-induced solid lubricant effect. • Mo doping results in substantial hardening-toughening effect. • GB plastic behaviors play the critical role. • Enhanced GB plasticity leads to high load-bearing capacity. • bct-fcc interfaces cause stress concentration and embrittlement.

Shock deformation and spallation of Cu bicrystals with (1 1 1) twist grain boundaries

The shock deformation and spallation of Cu bicrystals with (111) twist grain boundary (GB) of different misorientation angles are investigated by using molecular dynamics simulations. The selected GBs include low angle (LAGB), near twin (NTGB) and intermediate angle (IAGB) twist GBs, and the shock direction is along the GB normal. It is demonstrated that the bicrystals are of lower Hugoniot elastic limit than single crystal, as the twist GBs provide dislocation sources for deformation. At the shock compression stage, GB plasticity at the LAGBs and NTGBs is initiated via reaction and dissociation of GB dislocations, and can be triggered by elastic shock wave more readily than that at the IAGBs, on which dislocation emission is homogeneous and irrelevant to initial GB dislocations. At the tension stage, the incipient spall for different twist GBs occurs at an identical shock strength, but is of different details in dislocation and void evolutions. With varying shock strength and misorientation angle, spall strength of the Cu bicrystals depends on whether GB plasticity is produced at the compression stage, and the values are consistent with that of Cu single crystal under similar shock loading conditions.

From metallic glasses to nanocrystals: Molecular dynamics simulations on the crossover from glass-like to grain-boundary-mediated deformation behaviour

Nanocrystalline metals contain a large fraction of high-energy grain boundaries, which may be considered as glassy phases. Consequently, with decreasing grain size, a crossover in the deformation behaviour of nanocrystals to that of metallic glasses has been proposed. Here, we study this crossover using molecular dynamics simulations on bulk glasses, glass–crystal nanocomposites, and nanocrystals of Cu64Zr36 with varying crystalline volume fractions induced by long-time thermal annealing. We find that the grain boundary phase behaves like a metallic glass under constraint from the abutting crystallites. The transition from glass-like to grain-boundary-mediated plasticity can be classified into three regimes: (1) For low crystalline volume fractions, the system resembles a glass–crystal composite and plastic flow is localised in the amorphous phase; (2) with increasing crystalline volume fraction, clusters of crystallites become jammed and the mechanical response depends critically on the relaxation state of the glassy grain boundaries; (3) at grain sizes ≥10nm, the system is jammed completely, prohibiting pure grain-boundary plasticity and instead leading to co-deformation. We observe an inverse Hall–Petch effect only in the second regime when the grain boundary is not deeply relaxed. Experimental results with different grain boundary states are therefore not directly comparable in this regime.

Design of crystalline-amorphous nanolaminates using deformation mechanism maps

The deformation behavior of crystalline-amorphous nanolaminates is controlled by a confluence of mechanisms involving dislocation and grain boundary (GB) plasticity in the crystalline phase, shear transformation zone (STZ) plasticity and its localization into shear bands in the amorphous phase, and their coupling across the amorphous-crystalline interface. Leveraging molecular dynamics simulations, the influence of microstructural length scales on the mechanical behavior is quantified using deformation mechanism maps, which provide mechanistic insights into the scaling of mechanical properties with layer thickness ratio and grain size. We find that flow stress primarily scales with the relative phase fraction as deformation shifts toward STZ dominated plasticity with increasing amorphous layer thickness while the onset of plasticity is also influenced by grain size due to the role of GB plasticity in yielding. Toughness limiting mechanisms involve void formation at GBs and shear localization in the amorphous layer, where the former is attributed to dislocation-GB interactions during mixed mode deformation and the latter to dislocation slip bands biasing the process of shear localization. An Ashby plot representation combining flow stress with void and shear band localization factors demonstrates that configurations with microstructural length scales promoting cooperative strain accommodation through the coupling of dislocation, GB, and STZ plasticity exhibited limited strain localization while retaining a high flow stress, thus providing a mechanistic basis for microstructural design of crystalline-amorphous nanolaminates.

Strain rate effect on plastic deformation of nanocrystalline copper investigated by molecular dynamics

The strain rate effect on the plastic deformation of nanocrystalline copper with mean grain sizes in the range of 3.8–27.3nm has been investigated by using molecular dynamics simulation. The simulated results indicate that the critical mean grain size corresponding to the transition of plastic deformation mechanism is little influenced by the strain rate in the strain-rate range of 1×107–1×1010s−1. The simulated grain-size dependence of the strain rate sensitivity for strain rate below 1×108s−1 is in agreement with the experimental results of nanocrystalline copper reported in literatures. The strain rate sensitivity values for the simulated samples with mean grain sizes of 3.8 and 5.5nm are 0.073 and 0.065 respectively. These results reveal that the stress-driven grain-boundary plastic deformation mechanisms such as grain-boundary sliding and migration are not as sensitive to strain rate as that expected for the thermally assisted mechanisms. Furthermore it is found that if the stacking faults act as obstacles to the motion of partial dislocations the strain rate sensitivity will increase.

Shock response of copper bicrystals with a ∑3 asymmetric tilt grain boundary

Molecular dynamics (MD) simulations were performed to examine the behavior of the asymmetric ∑3〈110〉(110)1/(114)2 grain boundary (GB) in copper bicrystals under shock loading, with particular regard to the GB effect on the evolution of free surface velocity, plasticity and void nucleation and growth. Shock loadings with a direction perpendicular to the GB were employed by using the typical flyer plate and target configuration at three different velocities of 0.375, 0.5 and 0.75km/s. Our results showed that due to the existence of the GB, the free surface velocity of copper bicrystal exhibited an inclined plastic plateau in its historical curve when compared to the monocrystal. Shock front of bicrystal was found to become wider than that of the monocrystal since GB plasticity was triggered when the elastic wave crossed the GB. Spall strength also was lower in bicrystal than monocrystal loading at the same impact velocity. GB sliding and grain rotation under the shock wave were also observed, which induced the 180° phase shift of Vy, the relaxation of shear stress and the increase of temperature at the GB region. It was also found that due to the weakening effect of GB and the stress concentration, voids tend to nucleate and grow along the GB.

Shock-induced consolidation and spallation of Cu nanopowders

A useful synthesis technique, shock synthesis of bulk nanomaterials from nanopowders, is explored here with molecular dynamics simulations. We choose nanoporous Cu (∼11 nm in grain size and 6% porosity) as a representative system, and perform consolidation and spallation simulations. The spallation simulations characterize the consolidated nanopowders in terms of spall strength and damage mechanisms. The impactor is full density Cu, and the impact velocity (ui) ranges from 0.2 to 2 km s−1. We present detailed analysis of consolidation and spallation processes, including atomic-level structure and wave propagation features. The critical values of ui are identified for the onset plasticity at the contact points (0.2 km s−1) and complete void collapse (0.5 km s−1). Void collapse involves dislocations, lattice rotation, shearing/friction, heating, and microkinetic energy. Plasticity initiated at the contact points and its propagation play a key role in void collapse at low ui, while the pronounced, grain-wise deformation may contribute as well at high ui. The grain structure gives rise to nonplanar shock response at nanometer scales. Bulk nanomaterials from ultrafine nanopowders (∼10 nm) can be synthesized with shock waves. For spallation, grain boundary (GB) or GB triple junction damage prevails, while we also observe intragranular voids as a result of GB plasticity.