Dislocation Nucleation Sites Research Articles

Orientation effects on shock-induced plastic deformation in FeNiCoCu high entropy alloy

FeNiCoCu high-entropy alloys (HEAs) demonstrate promising potential for widespread use in structural and functional applications. However, a thorough understanding of dynamic deformation processes in FeNiCoCu HEA is limited due to technological constraints in detecting real-time microstructural developments at the atomic level. This study examines the shock-induced plastic deformations in the equiatomic FeNiCoCu HEA, focusing on crystallographic orientation and particle velocity, using nonequilibrium molecular dynamics simulations. We obtained the P−V/V0, P−T, P−Up, and Us−Up Hugoniot relations and evaluated their anisotropy. The shock velocity, stress, and shear stress exhibit orientation dependence due to the differences in the plastic deformation mechanism. For shock loading along [100] orientations, dislocation dominates at lower shock intensities. However, a phase transition from face-centered-cubic (FCC) to body-centered-cubic becomes the primary plastic deformation at high shock intensity. For shock loading along [112¯] and [111] orientations, the generation of disordered structures and dislocation activities is revealed to play an important role in the development of localized plastic deformation. Moreover, the competition of disordered and hexagonal-close-packed (HCP) atoms is observed. The transition from FCC to disordered atoms provides nucleation sites for dislocations, and the slip of dislocations around disordered atoms leads to the formation of HCP structures. These findings are very helpful for learning the dynamic deformation behavior of FeNCoCu HEA.

Equiaxed microstructure design enables strength-ductility synergy in the eutectic high-entropy alloy

Eutectic high-entropy alloys (EHEAs) represent attractive candidate materials for overcoming the strength-ductility trade-off, which can be enhanced through the directional alignment of the lamellar structure along the loading direction. Here, we put forward a new route to optimize the strength-ductility synergy without orientation dependence. Through a combination of severe plastic deformation and annealing, we convert the initially lamellar structure into a dual-phase structure comprised of ultrafine equiaxed grains. The significant grain refinement improves the yield strength from 703 MPa to 1199 MPa without sacrificing any ductility. During deformation, the localized softening resistance of the achieved dual-phase microstructure avoids necking, and the intrinsic microcrack-arresting mechanism effectively improves the fracture resistance. Grain boundaries and phase boundaries provide nucleation sites for dislocations and restrict dislocation transfer while the strain incompatibility is accommodated by geometrically necessary dislocations. This work demonstrates that dual-phase alloys comprised of ultrafine equiaxed grains provide a pathway for strengthening without loss of ductility.

Transitioning of deformation mode in bicrystalline Al-Mg alloy with Σ3 [1 1 1] 60° {11 8 5} faceted grain boundary

In this letter, the authors have elucidated the effect of alloying biocompatible and lighter magnesium (Mg) solute atoms in a bicrystalline aluminium (Al) solvent incorporated with a faceted coherent-incoherent grain boundary (GB) to analyze the tensile deformation mechanisms. Cryogenic temperature and high strain rate conditions were imposed through molecular dynamics simulations to report the detrimental effect of Mg addition on the tensile strength of Al-Mg alloys. Evidence of transitioning in deformation modes for pure Al and Al-Mg alloys was seen. On elaborating, for pure Al, GBs acted as the dislocation nucleation site; whereas for Al-Mg alloy, it was both the GBs and grains. It was observed that stacking faults (SFs) originated from GBs and propagated toward grains in Al. In contrast, in Al-Mg alloys, SFs were generated simultaneously from both GBs and grains. This simultaneous nucleation of dislocations and SFs from both GBs and grains renders bicrystalline alloys lower the elastic limit than pure metal. Conversely, beyond this elastic limit, the alloys exhibit higher flow stress than the pure metal due to the Mg-dislocation interactions. These findings offer valuable insights for tailoring the mechanical properties of Al-Mg alloys to meet aerospace, automotive and marine engineering-related application requirements.

Experimental Study on Liquid Metal Embrittlement of Al-Zn-Mg Aluminum Alloy (7075): From Macromechanical Property Experiment to Microscopic Characterization.

This study is a multiscale experimental investigation into the embrittlement of Al-Zn-Mg aluminum alloy (7075-T6) caused by liquid metal gallium. The results of the experiment demonstrate that the tensile strength of the 7075-T6 aluminum alloy significantly weakens with an increase in the embrittlement temperature and a prolonged embrittlement time, whereas it improves with an increase in the strain rate. On the basis of the analysis of the experimental data, the sensitivity of the embrittlement of 7075-T6 aluminum alloy by liquid gallium to the loading strain rate is significantly higher compared to other environmental factors. In addition, this study also includes several experiments for microscopic observation, such as Scanning Electron Microscope (SEM) observation, Energy-Dispersive Spectrometer (EDS) spectroscopy, and Electron Back Scatter Diffraction (EBSD) analysis. The experimental observations confirmed the following: (1) gallium is enriched in the intergranular space of aluminum; (2) the fracture mode of 7075-T6 aluminum alloy changes from ductile to brittle fracture; and (3) the infiltration of liquid gallium into aluminum alloys and its enrichment in the intergranular space result in the formation of new dislocation nucleation sites, in addition to the original dislocations cutting and entanglement. This reduces the material's ability to undergo plastic deformation, intensifies stress concentration at the dislocation nucleation point, and, ultimately, leads to the evolution of dislocations into cracks.

Crack tip dislocation activity in refractory high-entropy alloys

Dislocation activities play an important role in the deformation and failure of refractory high-entropy alloys (RHEAs). However, the impact of chemical short-range ordering (SRO) on dislocation activities at a crack tip in RHEAs remains unclear. Here, we investigate the effect of SRO on the dislocation nucleation, propagation, and reaction at a crack tip in a body-centered-cubic (BCC) MoTaTiWZr RHEA, using a combination of molecular dynamic simulations and Monte Carlo methods. Our results indicate that this RHEA is energetically favorable to form SRO, developing a pseudo-composite microstructure with low-energy clusters (LECs), medium-energy clusters (MECs), and high-energy clusters (HECs). The HECs at the crack tip are favorable sites for dislocation nucleation whereas the MECs surrounding the HECs function as a strong matrix to stabilize the weak HECs. At elevated temperatures, the HECs near the crack tip transform to severely distorted BCC and disordered structures, which can cause the breakup, absorption, and annihilation of emitted dislocations and nucleation of new dislocations. Our work reveals the interesting role of SRO in altering the dislocation activities at the crack tip of RHEAs and suggests alternative routes for designing superior RHEAs at both room and elevated temperatures.

Atomistic insights into the synergistic effect of nanotwins and nano-precipitates on the mechanical behavior of superalloys

As one of the typical nanostructures, nanotwins (NTs) have been utilized to evade the trade-off between strength and ductility, which is also applicable to improve the mechanical properties of superalloys. However, the optimization capability of mechanical properties by tailoring NTs has encountered bottleneck for the reasons such as inverse Hall-Petch effect and anisotropic effect. Based on the experimental observation of a special distribution pattern of NTs and nano-precipitates (NPs) in superalloys, that is, NPs are located at the twin boundaries (TBs), our work aims to study the distinctive synergistic deformation mechanism between NTs and NPs, which could become a new microstructure design strategy to optimize the mechanical properties of superalloys. Specifically, molecular dynamic (MD) method was employed to unveil the synergistic deformation mechanism of NTs and NPs in a model Ni-based superalloy. As expected, the introduction of NPs onto TBs can inhibit dislocation motion along or parallel to TBs, despite the softening deformation mode of NTs is activated. Moreover, the existence of NPs onto TBs can greatly improve the strengthening efficiency of NTs by impeding twin thickening/thinning. On the other hand, both phase boundaries and NPs interior have obstructive effect on dislocation motion, which could also be beneficial for material strengthening. More interestingly, anti-phase boundary (APB) structure will form along with the dislocations cutting through NPs. Subsequently, the APB structure can act as channels for dislocation slip and dislocation nucleation site induced by a kind of TB confinement effect, which may effectively accommodate plastic deformation. Our research outcomes could be utilized to facilitate the development of high-performance superalloys via rational collaborative design of NTs and NPs.

Synergetic Enhancement of Strength-Ductility and Thermoelectric Properties of Ag2 Te by Domain Boundaries.

Simultaneously improving the mechanical and thermoelectric (TE) properties is significant for the engineering applications of inorganic TE materials. In this work, a novel nanodomain strategy is developed for Ag2 Te compounds to yield 40% and 200% improved compressive strength (160MPa) and fracture strain (16%) when compared to domain-free samples (115MPa and 5.5%, respectively). The domained samples also achieve a 45% improvement in average ZT value. The domain boundaries (DBs) provide extra sites for dislocation nucleation while pinning the dislocation movement, resulting in superior strength and ductility. In addition, phonon scattering induced by DBs suppresses the lattice thermal conductivity of Ag2 Te and also reduces the weighted mobility. These findings provide new insights into grain and DB engineering for high-performance inorganic semiconductors with robust mechanical properties.

Hardening-softening of Al0.3CoCrFeNi high-entropy alloy under nanoindentation

The competition and balance mechanism between work hardening resulting from the surge in dislocations and material softening caused by plastic deformation during contact loading in Al0.3CoCrFeNi high-entropy alloy is unclear. The structural transformation and strain localization of Al0.3CoCrFeNi high-entropy alloy during nanoindentation are investigated using molecular dynamics, and the hardening-softening mechanism is discussed. The simulations demonstrate that the prismatic dislocation loop with independent nucleation is discovered for the first time in the [111] orientation. Dislocation multiplication and cross-slip lead to an increase in indentation resistance, resulting in work hardening. Free dislocation slip and dislocation annihilation accommodate plastic strain will reduce indentation resistance, resulting in plastic softening. Twin boundaries can effectively block dislocation propagation, which contributes to hardening. However, twin boundaries cause a softening effect in the later stage of plastic deformation owing to two reasons: (1) the formation of steps and the partial slip where the slip plane and Burgers vector are parallel to the twin boundary, (2) steps and local damage zones in twin boundaries become new nucleation sites for dislocations. The phase-field-crystal method confirms that Al0.3CoCrFeNi high-entropy alloy has the highest hardness when the twin spacing is 2.467 nm.

Irradiation hardening and deuterium retention behaviour of tungsten under synergistic irradiations of 3.5 MeV Fe13+ ions and deuterium plasma

This work investigates the irradiation hardening and deuterium (D) retention behaviour of tungsten (W) under synergistic irradiations of heavy ions and D plasma. 3.5 MeV iron (Fe13+) ion irradiation was performed on recrystallized W samples (RW) to produce displacement damage of 1 dpa. Then, low-energy (38 eV) D plasma exposure was conducted at 500 K. It is found that Fe ion irradiation creates substantial vacancy-type defects and dislocation loops/networks in RW. These irradiation-induced defects not only function as nucleation sites for dislocations that increase the activation probability of new dislocations and suppress the pop-in events, but also act as barriers for dislocations that result in irradiation hardening. Nano-indentation results show that the average hardness of RW-D, RW-Fe and RW-Fe-D increases from 5.26 GPa for RW to 5.28, 6.23 and 6.56 GPa, respectively. The strong interaction between dislocations and high density of damage-induced defects is suggested to be the chief source for the obvious irradiation hardening observed in the synergistic irradiation case. Besides, compared with RW-D, the total D retention in RW-Fe-D is increased by a factor of 2.65. NRA and TDS results suggest that Fe pre-irradiation not only increases D retention within the damage layer (within the first 1.3 μm), but also enhances that beyond the damage layer (> 1.3 μm) before surface blisters are formed. This work further improves the fundamental understanding on the microstructure evolution, D retention and nano-mechanical behaviour of W under synergistic irradiation effect of heavy ions and D plasma.

Superior low cycle fatigue property from cell structures in additively manufactured 316L stainless steel

We have investigated the low cycle fatigue (LCF) properties and the extent of strengthening in a dense additively manufactured stainless steel containing different volume fractions of cell structures but having all other microstructure characteristics the same. The samples were produced by laser powder bed fusion (L-PBF), and the concentration of cell structures was varied systematically by varying the annealing treatments. Load-controlled fatigue experiments performed on samples with a high fraction of cell structures reveal an up to 23 times increase in fatigue life compared to an essentially cell-free sample of the same grain configuration. Multiscale electron microscopy characterizations reveal that the cell structures serve as the soft barriers to the dislocation propagation and the partials are the main carrier for cyclic loading. The cell structures, stabilized by the segregated atoms and misorientation between the adjacent cells, are retained during the entire plastic deformation, hence, can continuously interact with dislocations, promote the formation of nanotwins, and provide massive 3D network obstacles to the dislocation motion. The compositional micro-segregation caused by the cellular solidification features serves as another non-negligible strengthening mechanism to dislocation motion. Specifically, the cell structures with a high density of dislocation debris also appear to act as dislocation nucleation sites, very much like coherent twin boundaries. This work indicates the potential of additive manufacturing to design energy absorbent alloys with high performance by tailoring the microstructure through the printing process.

Carbide-facilitated nanocrystallization of martensitic laths and carbide deformation in AISI 420 stainless steel during laser shock peening

Severe plastic deformation (SPD) techniques such as laser shock peening (LSP) have been widely adopted to modify microstructure of materials. Understanding the plastic deformation mechanisms during SPD will enable the optimization of SPD process to obtain desired microstructures. Using LSP technique, we modified surface microstructure of AISI 420 martensitic stainless steel containing both martensitic lath and carbide, and investigated deformation mechanisms based on microstructural evolution. High-resolution transmission electron microscopy and X-ray diffraction results revealed that the laser shock wave induced severe deformation in the original sub-micron carbide, resulting in carbide fragmentation in AISI 420 martensitic stainless steel. These LSP-induced carbide fragments and original carbides promote nanocrystallization of martensitic laths, because the carbide/matrix interface acts as an effective dislocation nucleation sites and an obstacle to the dislocation movement. The LSP-generated martensitic lath nanocrystallization is rationalized by two simultaneous subdivision modes: the intersection between longitudinal and transverse sub-boundaries developed from dense dislocation walls and transverse sub-boundaries transformed from dislocation tangles. In addition, the carbide decomposition happened simultaneously with severe plastic deformation, which was reflected by the decreasing continuous Cr concentration and the increment of Fe concentration with increasing deformation degree in the deformed carbide. Finally, fine grain strengthening, precipitation strengthening and dislocation strengthening induced by LSP contribute to an increase in the strength and ductility of AISI 420 martensitic stainless steel.

Stress-dependent incipient plasticity of a face-centered-cubic-based Al0.3CoCrFeNi multi-principal element alloy with nano-scaled phase separation

The stress-dependent incipient plasticity was probed by measuring the first pop-in behavior in a face-centered-cubic-based Al0.3CoCrFeNi multi-principal element alloy with nano-scaled phase separation. A large number of indentation measurements revealed that the operation of a bimodal distribution for the maximum shear stress was associated with different dislocation nucleation sites. This bimodal distribution phenomenon can be used to characterize the structural heterogeneity of the alloy. The activation volume and the dislocation nucleation rate were calculated to make a further investigation to the different dislocation nucleation mechanisms.

Plasticity induced by nanoindentation in a CrCoNi medium-entropy alloy studied by accurate electron channeling contrast imaging revealing dislocation-low angle grain boundary interactions

In the present work, interactions of nanoindentation-induced dislocations (NIDs) with a low-angle grain boundary (LAGB) are investigated in a single-crystalline CrCoNi medium-entropy alloy (MEA). Microstructural evolutions before and after nanoindentation were examined using accurate electron channeling contrast imaging (A-ECCI). In the as-grown state, the alloy microstructure consists of subgrains separated by LAGBs. After nanoindentation on the (001) plane far away from LAGBs, the load-displacement curves exhibit the typical behavior of metals and alloys with a pop-in marking the elastic-plastic transition. This pop-in is related to the nucleation of NIDs that are observed to form pile-ups on {111} planes. In contrast, when indents are performed in the vicinity of a LAGB with a low misorientation angle of 0.24° and consisting of dislocations spaced ~60 nm apart, different micromechanical responses and deformation mechanisms are observed depending on the distance between the LAGB and the nanoindenter tip. When the distance between the LAGB and the nanoindenter tip is larger than four times the size of the indent (corresponding ratio: R > 4), the LAGB does not affect the micromechanical response nor interact with NIDs. In contrast, when the indenter comes in direct or indirect contact with the LAGB (R < 1), the load-displacement curve deviates at low loads from the elastic stage, and pop-ins are not observed. In this case, the continuous deformation is accommodated by the movement of the pre-existing LAGB dislocations. For intermediate cases with 1 < R < 4, the load of the initial pop-in is dependent on the local defect density. In this latter case, the pile-ups of NIDs directly impinge on the LAGB. Microstructural analyses reveal that the LAGB accommodates plasticity by blocking the NIDs, activating a dislocation nucleation site in the adjacent subgrain/emission of dislocation from the LAGB, and inducing slight motions of its constituent dislocations.

Site dependence of surface dislocation nucleation in ceramic nanoparticles

The extremely elevated strength of nanoceramics under compression arises from the necessity to nucleate highly energetic dislocations from the surface, in samples that are too small to contain pre-existing defects. Here, we investigate the site dependence of surface dislocation nucleation in MgO nanocubes using a combination of molecular dynamics simulations, nudged-elastic-band method calculations and rate theory predictions. Using an original simulation setup, we obtain a complete mapping of the potential dislocation nucleation sites on the surface of the nanoparticle and find that, already at intermediate temperature, not only nanoparticle corners are favorable nucleation sites, but also the edges and even regions on the side surfaces, while other locations are intrinsically unfavorable. Results are discussed in the context of recent in situ TEM experiments, sheding new lights on the deformation mechanisms happening during ceramic nanopowder compaction and sintering processes.

Effect of irradiation damage and indenter radius on pop-in and indentation stress-strain relations: Crystal plasticity finite element simulation

In this work, a crystal plasticity finite element model is developed to simulate the spherical nano-indentation of ion-irradiated metallic materials. Two critical features are addressed by the proposed theoretical framework, which include the pop-in event observed in the loading force-indentation depth (P−h) relations and irradiation hardening characterized by the transformed indentation stress-strain (ISS) curves. For the former, the pop-in event is dominated by the dislocation nucleation mechanisms, which are affected by both the density of irradiation-induced dislocation nucleation sites and indenter radii. For the latter, irradiation hardening is closely related to the heterogeneously distributed irradiation-induced defects within the irradiated layer with a limited depth. By applying the developed model to the spherical nano-indentation of helium-irradiated single crystal tungsten, it is informed that the simulated results can match well with corresponding experimental data, which include the unirradiated and irradiated P−h and ISS relations at different indenter radii. Moreover, the evolution of different hardening mechanisms during the indentation process is systematically analyzed, which can help comprehend the fundamental deformation mechanisms of ion-irradiated materials under spherical nano-indentation.

A unified statistical model for indentation pop-in: Effects of indenter radius and microstructure density on the transition from homogeneous nucleation to heterogeneous nucleation to bulk plasticity

Indentation pop-in, i.e. a sudden displacement burst on the measured load-penetration depth curves, has been known as the onset of elastic-plastic deformation for crystalline materials. Depending on the indenter radius or density of pre-existing dislocation nucleation sources, the evolution of pop-in shear stress can generally be categorized into three deformation stages, i.e. the homogeneous dislocation nucleation stage, heterogeneous dislocation nucleation stage and bulk plasticity stage. For the former, the fluctuation of pop-in stress is dominated by the thermally activated nucleation process. For the middle, a size-dependent pop-in behavior with stress fluctuation is informed over a wide range of indenter radius and microstructure density. For the later, the materials strength is determined by the critical resolved shear stress for the motion of existing dislocations. Here, we find two additional transition stages between the adjacent deformation stages that can be effectively addressed by a novel competition mechanism, and the transition points are affected by the density of dislocation nucleation sites. All these critical features are well characterized by a unified statistical model proposed in this work. Moreover, a mechanistic model with closed-form formulas is developed for the size-dependent pop-in behavior during the heterogeneous dislocation nucleation stage. By comparing both the calibrated and predicted theoretical results with different sets of experimental data, good agreements are achieved that can rationally verify the proposed model. This work presents a theoretical framework to characterize the fundamental pop-in mechanisms, and provides an avenue towards the prediction of transition points distinguishing different plasticity deformation regimes.

Plastic Deformation and Hardening Mechanisms of a Nano-twinned Cubic Boron Nitride Ceramic.

A very interesting experimental finding shows that a nano-twinned cubic boron nitride (NT-cBN) ceramic has size-dependent hardness. In order to reveal the hardening mechanism of NT-cBN, the plastic deformation mechanism of single-crystalline cubic boron nitride (SC-cBN) under nano-indentation is first studied, and then that of NT-cBN is further investigated using atomistic simulations with a parameter-modified Tersoff potential. It is found that the plastic deformation of SC-cBN under nano-indentation is mainly attributed to serial dislocation behaviors, such as the formation of dislocation embryos, shear loops, and prismatic loops. In comparison, for NT-cBN, the plastic behavior is much more complex, which is influenced by a dislocation blockage, absorption, dissociation, and re-nucleation due to the interaction between dislocations and twin boundaries (TBs). From the plastic deformation mechanism of NT-cBN, it is found that the size-dependent hardening behavior of NT-cBN is a competitive result between the hardening sources, including slip transfer, dislocation accumulation, and suppression of dislocation nucleation, and the softening sources, including TBs being destroyed, parallel slips of dislocations, and the formation of new sites for dislocation nucleation. The smaller the distance between the adjacent TBs, the more dominant the role of hardening sources is, resulting in the high size-dependent hardness of NT-cBN. The results in this paper should be helpful for the optimized design of high strength and toughness of nano-structured cBN ceramics.

Effect of Void on Yielding Behaviors in a Bicrystalline Copper

Nanovoids make a big contribution to the plasticity and failure of nanocrystalline metallic materials. To develop a further understanding of the role played by spherical voids in governing the plasticity of nanocrystalline metals, molecular dynamics simulations are performed on bicrystalline copper models with a spherical void at room temperature under 1 × 108 s−1 strain rate. The simulation results indicate that the introduction of void lead to the reduction in yield stress with increasing void diameter. The spherical void acts as either a barrier or a source for dislocations. For intergranular spherical void, the dislocations are emitted from the intersections between the void and grain boundary (GB). However, the initial dislocation nucleation site transits from GBs to spherical void surface at a critical diameter of 9.5 nm in the models with intragranular void. According to the Lubarda model, the dislocation emission critical stress at intermediate void diameters is effectively predicted, but it is not applicable for the model with extremely large and small void since the factors of GB and grain orientation are not considered. Some more suitable models need to be developed.

Influence of lattice misfit on the deformation behaviour of [formula omitted]/[formula omitted] lamellae in TiAl alloys

Interfaces play a significant role in the deformation behaviour of lamellar two-phase TiAl alloys and contribute to their increased strength. We study the deformation behaviour of α2/γ bilayers with either coherent or semicoherent interfaces, using atomistic simulations. We identify the nucleation sites for dislocations and decouple the effects of the microstructural parameters volume fraction and layer thickness on the yield stress and strain. Uniaxial tensile tests are carried out on bi-layer specimens with α2 and γ phases along directions parallel and perpendicular to the interface. Coherent α2∕γ bi-layers show residual stresses due to lattice mismatch which are linearly related to the volume fractions of the phases. These residual stresses, superimposed with tensile stresses during loading, lead to early yielding of the γ phase. In contrast, a semi-coherent interface leads to negligible residual stresses, but contains misfit dislocations which create localized stresses within the γ layer and thus contributes to dislocation nucleation. We show that along loading directions parallel to the interface, the layer thickness does not affect the deformation behaviour, irrespective of the type of interface, instead volume fraction is the governing parameter. When loading perpendicular to the interface, the absolute layer thickness does not affect the deformation behaviour of a bi-layer with a coherent interface, but determines the yield stress and strain in case of a semi coherent interface.

Time-dependent plasticity in silicon microbeams mediated by dislocation nucleation

Understanding deformation mechanisms in silicon is critical for reliable design of miniaturized devices operating at high temperatures. Bulk silicon is brittle, but it becomes ductile at about 540 °C. It creeps (deforms plastically with time) at high temperatures (∼800 °C). However, the effect of small size on ductility and creep of silicon remains elusive. Here, we report that silicon at small scales may deform plastically with time at lower temperatures (400 °C) above a threshold stress. We achieve this stress by bending single-crystal silicon microbeams using an in situ thermomechanical testing stage. Small size, together with bending, localize high stress near the surface of the beam close to the anchor. This localization offers flaw tolerance, allowing ductility to win over fracture. Our combined scanning, transmission electron microscopy, and atomic force microscopy analysis reveals that as the threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high-stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time, lowering the stress while bending the beam plastically. This process continues until the effective shear stress drops and dislocation activities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon.