Partial Dislocation Slip Research Articles

Prediction of the threshold load of dislocation emission in silicon during nanoscratching

This paper establishes an analytical model to predict the normal threshold load that causes the emission of partial dislocations in silicon during nanoscratching. In the modeling, the deformation mechanisms and the sequence of microstructural changes already explored by experiment and molecular dynamics will be exactly followed; that is, with increasing the normal load of nanoscratching, phase transformation first takes place, followed by partial dislocation emission from the interface between the phase transformed zone and the original crystalline silicon when the scratching load reaches its threshold. The model postulates that the emission process represents the generation of a dipole of Shockley partial dislocations. One partial dislocation of the dipole, located at the interface, is considered immobile, while the other partial dislocation moves into the bulk of the crystalline silicon. The mobile partial dislocation slips along a crystallographic plane, and a stacking fault is formed in its wake. Based on the above, the threshold normal scratching load for the emission of a partial dislocation is determined by means of the energy criterion. The influence of the indenter geometry and the location of dislocation nucleation on the threshold normal scratching load is then investigated. Compared with the deformation of silicon under nanoindentation, the present study concludes that the threshold load under scratching is always smaller, and that a sharp indenter leads to a relatively smaller threshold load. The model prediction is well verified by scratching experiments.

A surface stacking fault energy approach to predicting defect nucleation in surface-dominated nanostructures

We present a surface stacking fault (SSF) energy approach to predicting defect nucleation from the surfaces of surface-dominated nanostructure such as FCC metal nanowires. The approach leads to a criterion that predicts the initial yield mechanism via either slip or twinning depending on whether the unstable twinning energy or unstable slip energy is smaller as determined from the resulting SSF energy curve. The approach is validated through a comparison between the SSF energy calculation and low-temperature classical molecular dynamics simulations of copper nanowires with different axial and transverse surface orientations, and cross sectional geometries. We focus on the effects of the geometric cross section by studying the transition from slip to twinning previously predicted in moving from a square to rectangular cross section for 〈100〉/{100} nanowires, and also for moving from a rhombic to truncated rhombic cross sectional geometry for 〈110〉 nanowires. We also provide the important demonstration that the criterion is able to predict the correct deformation mechanism when full dislocation slip is considered concurrently with partial dislocation slip and twinning. This is done in the context of rhombic 〈110〉 aluminum nanowires which do not show a tensile reorientation due to full dislocation slip. We show that the SSF energy criterion successfully predicts the initial mode of surface-nucleated plasticity at low temperature, while also discussing the effects of strain and temperature on the applicability of the criterion.

Shock response of nanotwinned copper from large-scale molecular dynamics simulations

A series of large-scale molecular dynamics simulations have been performed to investigate the shock response of nanotwinned (NT) Cu, including shock-induced plasticity, strength behind the shock front, and spall behaviors. In this study, two configurations were investigated at an impact velocity of 600m/s, i.e., the practical NT polycrystalline Cu with an average grain size of 10 nm and the simple NT single-crystalline Cu with an impact direction of [11 (2) over bar]. In the NT polycrystalline Cu, the average flow stress behind the shock front first increases with decreasing twin-boundary spacing (TBS), reaching a maximum at a critical TBS, and then decreases as the TBS become even smaller. This trend of the average flow stress with decreasing TBS is due to two competitive dislocation activities under shock loading, with one being inclined to the twin boundaries (the dislocation-twin boundary intersecting) and the other parallel to the twin boundaries (detwinning with twin-boundary migration). Since voids always nucleate near the grain boundary (GB) junctions and then grow along the GBs to create spallation, no apparent correlation between the spall strength and TBS is observed in the NT polycrystalline Cu. However, the spall strengths of the NT single-crystalline Cu are found to increase with decreasing TBS. Two partial dislocation slips initiated from each twin boundary create voids at the intersections between the partial dislocation slips and twin boundaries. The smaller TBSs result in a larger number of twin boundaries and provide more nucleation sites for voids, requiring a higher tensile stress to create spallation in the NT single-crystalline Cu. These findings should provide insights for understanding the deformation physics of the NT metals subjected to shock loading.

Influence of structure transition on plastic behaviors of iron based ordered alloys

The influence of ordered structure on the dislocation configuration, structure of anti-phase domain boundary, partial dislocation slips, etc. are analyzed in the background of promoting the plasticity of iron based ordered solid solutions with second-order phase transformation. The principles of deformation softening and annealing hardening in ordered solid solutions are discussed because of deformation induced structure disordering. It is concluded that the independent slip ability of the partial dislocations and the corresponding low temperature plasticity of ordered solid solutions could be promoted obviously by proper alloying effects, which reduces the anti-phase domain boundary energy, or by maintaining the disordering state into the low temperature range. The similar principles could be also used to modify the low temperature plasticity of other metal based ordered solid solutions.

Deformation and spallation of shocked Cu bicrystals withΣ3 coherent and symmetric incoherent twin boundaries

We perform molecular dynamics simulations of Cu bicrystals with two important grain boundaries (GBs), \ensuremath{\Sigma}3 coherent twin boundaries (CTB), and symmetric incoherent twin boundaries (SITB) under planar shock wave loading. It is revealed that the shock response (deformation and spallation) of the Cu bicrystals strongly depends on the GB characteristics. At the shock compression stage, elastic shock wave can readily trigger GB plasticity at SITB but not at CTB. The SITB can induce considerable wave attenuation such as the elastic precursor decay via activating GB dislocations. For example, our simulations of a Cu multilayer structure with 53 SITBs (\ensuremath{\sim}1.5-\ensuremath{\mu}m thick) demonstrate a \ensuremath{\sim}80$%$ elastic shock decay. At the tension stage, spallation tends to occur at CTB but not at SITB due to the high mobility of SITB. The SITB region transforms into a threefold twin via a sequential partial dislocation slip mechanism, while CTB preserves its integrity before spallation. In addition, deformation twinning is a mechanism for inducing surface step during shock tension stage. The drastically different shock response of CTB and SITB could in principle be exploited for, or benefit, interface engineering and materials design.

Atomic layer deposition of ZnO/Al2O3/ZrO2 nanolaminates for improved thermal and wear resistance in carbon-carbon composites

Carbon-carbon composites (CCCs) are well recognized structural materials which are generally known for their high strength and thermal conductivity, low density, and open cell structure. They have attracted great interest for aerospace applications, such as bushings used in jet engines, and potentially in future hypersonic aircraft wing structures, which function in an oxidizing environment. The enhancement in thermal (oxidation) and wear resistance of CCCs is desirable since carbon-based materials start to oxidize around 400 °C and uncoated bushings are susceptible to wear. To this end, thermodynamically stable protective oxide nanolaminates (ZnO/Al2O3/ZrO2) have been deposited by atomic layer deposition (ALD) to infiltrate porous CCCs and graphite foams in order to improve their thermal stability and sliding wear resistance. It was determined with x-ray tomography imaging and energy dispersive x-ray spectroscopy mapping that ALD ZnO/Al2O3/ZrO2 nanolaminates and baseline ZrO2 coatings exhibited pore-filling down to ∼55 μm and 1.5 mm in the porous CCCs and graphite foam, respectively. Furthermore, x-ray diffraction and high resolution transmission electron microscopy determined that the {0002}-textured nanocrystalline ZnO (wurtzite), amorphous Al2O3, and {101}-textured tetragonal ZrO2 nanolaminates exhibited excellent coating conformality and uniformity inside the pores. A significant reduction in the sliding wear factor (2.3 × 10−5 to 4.8 × 10−6 mm3/Nm) and friction coefficient (0.22 to 0.15) was observed with the ALD ZnO/Al2O3/ZrO2 nanolaminate in comparison to uncoated CCCs. The mechanism for improved tribological properties was attributed to intrafilm shear (slip) of partial dislocations along the ZnO {0002} basal stacking faults by a dislocation glide process.

Signature of surface energy dependence of partial dislocation slip in a gold nanometer-sized protrusion

► Here we show strong surface-energy dependence of crystal plasticity of a nanostructure . ► In situ HRTEM of a gold nanoprotrusion at room temperature at the atomic scale. ► The slip of Shockley partial dislocations is determined to reduce surface energy. The mechanical behavior of metallic nanowires has been intensively studied, demonstrating the strong sample-size dependence of crystal plasticity. However, the effect of surface energy has not yet been fully resolved. Here we have used in situ high-resolution transmission electron microscopy of gold nanometer-sized protrusions at room temperature to address this problem. Our experimental results reveal that the slip of Shockley partial dislocations reduces the energy cost of creating the extra surface area resulting from the dislocation slip (i.e. surface steps).

Simulations of stress-induced twinning and de-twinning: A phase field model

Twinning in certain metals or under certain conditions is a major plastic deformation mode. Here we present a phase field model to describe twin formation and evolution in a polycrystalline fcc metal under loading and unloading. The model assumes that twin nucleation, growth and de-twinning is a process of partial dislocation nucleation and slip on successive habit planes. Stacking fault energies, energy pathways (γ surfaces), critical shear stresses for the formation of stacking faults and dislocation core energies are used to construct the thermodynamic model. The simulation results demonstrate that the model is able to predict the nucleation of twins and partial dislocations, as well as the morphology of the twin nuclei, and to reasonably describe twin growth and interaction. The twin microstructures at grain boundaries are in agreement with experimental observation. It was found that de-twinning occurs during unloading in the simulations, however, a strong dependence of twin structure evolution on loading history was observed.

Double-inverse grain size dependence of deformation twinning in nanocrystalline Cu

While a Hall-Petch-type dependence is known for deformation twinning (DT) in Cu and other metals of conventional grain sizes $(Dg1\text{ }\ensuremath{\mu}\text{m})$, with $D$ decreasing into the nanocrystalline regime, the propensity for DT turns around to increase, exhibiting an inverse grain size dependence. This trend is inversed again at still smaller grain sizes, returning to the behavior of increased difficulty in DT with $D$ going further down. This double-inverse behavior with respect to the normal Hall-Petch $D$ dependence is demonstrated here for nanocrystalline Cu films, deformed in tension at room temperature and slow strain rates. The nonmonotonic $D$ dependence of DT is explained by modeling the competing grain size effects on the emission of the first partial dislocation and the plane-to-plane promotion of partial dislocation slip afterwards.

Plastic deformation of nanocrystalline nickel

A high-resolution electron microscopy study has uncovered the plastic behavior of accommodating large strains in nanocrystalline (NC) Ni subject to cold rolling at liquid nitrogen temperature. The activation of grain-boundary-mediated-plasticity is evidenced in NC-Ni, including twinning and formation of stacking fault via partial dislocation slips from the grain boundary. The formation and storage of 60° full dislocations are observed inside NC-grains. The grain/twin boundaries act as the barriers of dislocation slips, leading to dislocation pile-up, severe lattice distortion, and formation of sub-grain boundary. The vicinity of grain/twin boundary is where defects preferentially accumulate and likely the favorable place for onset of plastic deformation. The present results indicate the heterogeneous and multiple natures of accommodating plastic strains in NC-grains.

High-temperature strength and deformation of γ/γ′ two-phase Co–Al–W-base alloys

The high-temperature strength and deformation behavior of γ/γ′ two-phase Co–Al–W-base alloys have been studied with polycrystalline and single-crystal materials. The ternary, quaternary and higher-order alloys containing Ta, Cr and/or Re exhibit flow stress anomalies above 873 K due to slip of pairs of 1/2〈1 1 0〉 superpartial dislocations on {0 0 1} planes, in addition to {1 1 1} planes, in the γ′ precipitates. Compression tests on the single-crystal specimens reveal a true anomalous peak temperature of 1073 K for both ternary and Ta-containing quaternary alloys. Above the peak, the ternary alloy exhibits a rapid decrease in strength with temperature, as 1/2〈1 1 0〉 dislocations bypass the γ′ precipitates without significant shearing. Conversely, the Ta-containing quaternary alloy sustains strength to higher temperatures due to the activation of 1/3〈1 1 2〉 partial dislocation slip that introduces a high density of stacking faults in the γ′ precipitates.

Atomistic simulation studies on deformation mechanism of nanocrystalline cobalt

An approach is developed to investigate the deformation behavior of hexagonal close-packed (hcp) nanocrystalline (nc) cobalt by computer simulations. The microstructures are modeled by a grain growth theory, and the mechanical deformation behavior is investigated using molecular dynamics simulation in nc-cobalt samples with an average grain size of 10 nm. The deformation mechanisms are found to involve both full and partial dislocation activities. Despite the small stacking fault energy of nc-cobalt, surprisingly the deformation twinning is not prevalent in the model cobalt sample. The simulation suggests that unlike the easy twinning events in coarse-grained hcp metals, deformation of nanocrystalline cobalt is primarily controlled by partial dislocation slips and stacking faults. The continuous accumulation of deformation faults eventually leads to hcp to face-centred cubic allotropic phase transformation during tension and compression of nc-cobalt.

Electron-beam-induced surface reactions in Pd/PdO films

Profile imaging observations of clean palladium surfaces suggested that surface diffusion occurred by the slip of surface Shockley partial dislocations resulting in the depletion of material from the surface. The formation of PdO patches under strong electron irradiation and the epitaxial orientation relationships which developed between the oxide and the Pd matrix were studied at the atomic scale. The newly formed PdO crystals were not usually stable against further intense irradiation. Their disintegration as well as depletion has been considered in terms of the mechanical behavior of PdO crystals.