Dislocation Nucleation Process Research Articles

Determination of the controlling parameters for dislocation nucleation in SrTiO3: An investigation by nanoindentation

AbstractWe conduct nanoindentation to investigate dislocation nucleation in SrTiO3 (STO) single crystals with surface orientations of (0 0 1), (0 1 1), and (1 1 1) with loading/unloading rates of 25, 250, and 2500 μN/s. Results reveal that the critical loads (Pc) at which “pop‐in” event occurs depend strongly on surface orientations, but slightly related to loading rate. Based on Pc, the critical shear stress that triggers dislocation nucleation was determined by extracting the maximum resolved shear stress (τmax) along the slip systems of STO using the Hertzian solution. The dislocation activation shear stress (τa) was determined by averaging τmax. The determined τa is 9.0–12.0 GPa, close to the shear strength (∼G/2π) of STO, indicating that homogeneous dislocation nucleation dominates the pop‐in events. The consistency of the determined τa demonstrates that the frameworks for nanoindentation pop‐in analysis established for metals can be extended to ceramics, whereas the influence of the limited slip systems should be taken into consideration. Additionally, we estimated the activation volume and the activation energy via the statistical model proposed by Schuh et al. The small values of the determined activation volume (0.6–9.8 Å3) and the activation energy (0.13–0.70 eV) indicate that the dislocation nucleation possibly begins from a single‐atom migration and local point defects may participate in the dislocation nucleation process. That is, heterogeneous nucleation may exist initially but the homogeneous dislocation nucleation dominates the pop‐in events.

Role of temperature and preexisting dislocation network on the shock compression of copper crystals

Shock wave experiments show that many annealed face-centered cubic metals exhibit an anomalously increasing yield strength with temperature at high strain rates. Such an increase is usually associated with the multiplication of dislocations in the phonon friction mode, which slows with temperature. However, the effect of temperature on the structure of the shock wave and the yield strength of metal crystals, including those with microstructure, remains elusive. In this paper, we perform molecular dynamic simulations of shock-wave loading for [110] and [111] copper crystals of 0.45 and 0.80μm length in a wide range of temperatures between 100 and 1100K to understand the role of temperature and preexisting dislocations on the Hugoniot Elastic Limit (HEL). We show that, in ideal copper crystals, the elastic precursor exhibits a form of plateau, and the HEL almost does not change with shock propagation distance. However, at higher impacts, the perturbations of an elastic precursor are observed leading to fluctuations in the HEL value. We show that temperature dependencies of the HEL are strongly anisotropic. The HEL values tend to decrease with temperature for [110] perfect copper crystals, and to increase with temperature for [111] copper crystals. This surprising result is explained by the presence of dislocation substructures in a plastic wave in [110] crystals, which reduces the mobility of dislocations and makes the process of dislocation nucleation more dominant than in [111] crystals. Preexisting dislocations in copper crystals allow the HEL to decay much faster than in ideal crystals. In contrast to ideal [110] crystals, [110] crystals with dislocations exhibit increasing HEL values with temperature, as do [111] crystals. We find that the HEL dependence could be well approximated by a power law, but the decay power changes as the wave propagates. The decay power values lie between 0.5 to 0.7 in the second stage for both [110] and [111] crystals, which is consistent with experimental results with polycrystalline annealed copper. Our findings extend our understanding of temperature-dependent mechanical properties of materials under high strain rate and crystal plasticity.

Atomistic modeling of surface and grain boundary dislocation nucleation in FCC metals

Dislocation nucleation plays a critical role in the plastic deformation of crystalline materials. However, it is challenging to predict the active mode and associated rate of dislocation nucleation under typical experimental loading conditions through molecular dynamics simulation due to timescale limitations. Here we use the free-end nudged elastic band method to determine the activation energies and activation volumes of dislocation nucleation in four typical face-centered cubic metals of Au, Al, Cu and Ni. We focus on the representative processes of surface and grain boundary dislocation nucleation. The atomistically determined activation volumes of these dislocation nucleation processes are larger than 10b3 (with b being the Burgers vector length) under typical experimental loading conditions. These results are compared with experimentally measured activation volumes in ultrafine-grained and nanocrystalline metals, thereby providing mechanistic insight into their rate-controlling deformation mechanisms.

Atomistic modeling of Σ3 twin grain boundary in alloy 800H

Grain boundary engineering (GBE) has become an important thermomechanical processing strategy to enhance the physical and mechanical properties of polycrystalline metals in microstructure design. Solute segregation at grain boundaries can alter the electronic and strain energy states of the lattice and can impact the mechanical behavior in materials such as dislocation activity and yield strength. The twin boundary is one special type of grain boundary and has been of particular interest because of its role in deformation processes. The present work investigates twin grain boundary segregation induced strengthening in alloy 800H at temperatures of 300 K and 1000 K using Molecular Dynamics and Monte Carlo simulations. The simulations predicted no segregation preference of solute atoms (Ni and Cr) at coherent grain boundaries, but a significant segregation tendency to incoherent grain boundaries at a higher temperature during the simulated time. Shockley partial dislocations were formed and associated with Cr clusters. Deformation twins were found to only nucleate at the incoherent grain boundary. This solute segregation at the incoherent grain boundary increases the stress required to activate such a dislocation nucleation process. These findings extend the current understanding of the yielding behavior in alloy 800H, and more importantly, shed light on using GBE strategies to produce high-performance polycrystalline metallic materials.

Phase boundary segregation-induced strengthening and discontinuous yielding in ultrafine-grained duplex medium-Mn steels

The combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted discontinuous yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus discontinuous yielding. These findings extend the current understanding of the yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials.

First stages of plasticity in three-point bent Au nanowires detected by in situ Laue microdiffraction

The first stages of plasticity in three-point bent Au nanowires are investigated by in situ three-point bending tests in combination with Laue micro-diffraction. To separate the elastic and plastic deformation, loading–unloading cycles were performed with increasing load in each consecutive cycle. The storage of the first four geometrically necessary dislocations of [011¯](111) slip system is observed in the vicinity of both clamping points, which might be attributed to the local rotations induced by the rigid Si support. At later stages of the deformation, additional slip systems are activated either by the torsion of the nanowire or by unintentional indentation from the AFM tip. The cyclic loading–unloading approach combined with Laue microdiffraction thus allows to study the onset of plasticity in defect-scarce nanostructures deformed by bending, offering additional possibilities in studying the dislocation nucleation process in bent nano-objects, which are essential for future applications, e.g., in flexible electronics and nano-electromechanical systems.

Lattice distortion effect on incipient behavior of Ti-based multi-principal element alloys

In this work, we studied the influence of lattice distortion on the initiation of plasticity in Ti-based (multi-principal element alloy) MPEAs at a loading rate of 500 μN/s. The surface dependence was observed in Ti20Al20Cr20V20Nb20 (Ti20) MPEA, where the lowest first burst load was determined in (110) oriented grain, which is the same as that in Ti60Al10Cr10V10Nb10 (Ti60). This implies this kind of crystallographic dependence is the intrinsic feature of present studied alloys and it is not covered by the lattice distortion effect. In spite of similar orientation dependence, a much higher load at first burst was identified in Ti20 compared with Ti60, which is attributed to the fact that the more severe lattice distortion causes a larger critical resolved shear stress (CRSS) to nucleate the dislocation and a greater lattice friction to overcome during dislocation multiplication and emission process. In addition, based on the activation volume value, the mechanism in Ti20 is regarded as the same as that in Ti60, inferring that altering the lattice distortion does not change the pop-in mechanism in MPEAs. Lastly, the lattice distortion influence on the homogeneous dislocation nucleation process was investigated and compared with the heterogeneous dislocation nucleation event.

Effects of Re, W and Co on dislocation nucleation at the crack tip in the γ-phase of Ni-based single-crystal superalloys by atomistic simulation.

The effects of Re, W and Co on dislocation nucleation at the crack tip in Ni have been studied by the molecular dynamics method. The results show that the activation energy of dislocation nucleation is lowered by the addition of Re, W and Co; moreover, the activation energy decreases when the alloying element increases from 1 at.% to 2 at.%. The energy landscapes of the atoms are studied to elucidate these effects. Quantification analyses of the bonding strength between Ni and X (X = Re, W or Co) reveal that strong bonding between Ni and X (X = Re, W or Co) in the dislocation nucleation process can suppress the cleavage process and enhance the ability of dislocation nucleation. The surface energy and unstable stacking fault energy are also calculated to understand the alloying effects on the dislocation nucleation process. The results imply that interaction between alloying elements and Ni atoms plays a role in promoting the dislocation nucleation process at the crack tip. The ability of Re, W and Co in improving the ductility of the Ni crack system is in the order W > Re > Co. The results could provide useful information in the design of Ni-based superalloys.

Influence of hydrogen environment on dislocation nucleation and fracture response of 〈1 1 0〉 grain boundaries in nickel

The segregation of solute H has a profound effect on mechanical properties of grain boundaries (GBs). In this work, systematic molecular dynamics (MD) simulations have been performed to elucidate the influence of H environment on dislocation nucleation process and fracture response of different Ni GBs. The results show that the effect of H on dislocation nucleation depends strongly on GB structures. The presence of H decreases the yield stress required to nucleate dislocations from GBs with C and D structural units (SUs), while increases the yield stress of GBs with E SUs. This difference is attributed to different deformation mechanisms related to the dislocation nucleation from various GBs with H. By analysing the decohesion results, it is found that H has stronger embrittling effect on GBs with E SUs than C and D SUs, originating from the fact that H significantly elongates the Ni-Ni bonds around the open E SUs while slightly expands the compact C and D SUs. These findings present fundamental understanding on H embrittlement mechanism and provide mechanistic insights for engineering microstructure against H embrittlement in metallic materials.

Influence of an amorphous surface layer on the mechanical properties of metallic nanoparticles under compression

This study aims to investigate the role of amorphous surface layers on the mechanical response of metallic nanoparticles under compression using molecular dynamics simulations. For this purpose, the transferability of three embedded-atom-method (EAM) potentials to model monoatomic Ni glass and amorphous-crystalline structures is examined. Particular attention is paid to the crystallisation rate of the amorphous shell surrounding the crystalline inner structure. Relying on the most appropriate model, the influence of the amorphous layer on the mechanical response of crystalline-amorphous Ni nanoparticles is further investigated. Regardless of its thickness, the amorphous surface layer significantly changes both the effective elastic modulus of the nanoparticle and the flow stress. Besides stress, dislocation nucleation processes as well as the final shape of the compressed particles are influenced by the presence of the amorphous shell. These results bring new insights on the influence of surface state on the mechanics of metallic nano-objects.

Three-point bending behavior of a Au nanowire studied by in-situ Laue micro-diffraction

The elastic and plastic deformation of a gold nanowire tested in a three-point bending configuration using the custom-built scanning force microscope SFINX was studied in situ by Laue micro-diffraction. A new data treatment method based on the integration of diffraction patterns recorded along the deformed nanostructure is introduced visualizing both the movement and shape of the diffraction peaks as a function of the measurement position. Besides bending, torsion is evidenced during the elastic deformation originating from a misalignment of the SFINX-tip of the order of 60 nm with respect to the nanowire center. As demonstrated by post-mortem Laue micro-diffraction maps, the plastic deformation is governed by the storage of geometrically necessary dislocations. Analyzing the shape of the diffraction peaks, the activation of two unexpected slip systems is found which does not coincide with the slip systems with the highest resolved shear stress. These unexpected slip systems are probably related to the dislocation nucleation process at the clamping point, which is influenced by the local curvature.

A predictive model for plastic relaxation in (0001)-oriented wurtzite thin films and heterostructures

In this work, we present an experimental and theoretical study of the process of plastic strain relaxation of (0001)-oriented wurtzite heterostructures. By means of transmission electron microscopy and atomic force microscopy, we show that plastic relaxation of tensile strained AlxGa1-xN/GaN heterostructures proceeds predominantly by nucleation of a-type misfit dislocations in the 13⟨112¯0⟩|0001 slip-system driven by a three-dimensional surface morphology, either due to island growth or due to cracking of the layer. Based on our experimental results, we derive a quantitative model for the dislocation nucleation process. With the shear stress gradients at the nucleation sites of a-type misfit dislocations obtained by the finite element method, we calculate the critical thickness for plastic relaxation of strained wurtzite films and heterostructures as dependent on the surface morphology. The crucial role of the growth mode of the film on the strain relaxation process and the resulting consequences is discussed in the paper.

A microstructure sensitive grain boundary sliding and slip based constitutive model for machining of Ti-6Al-4V

A composite dual phase internal state variable constitutive model was developed for Ti-6Al-4V. The proposed model includes diffusion assisted grain boundary sliding based physics in addition to a traditional slip-based plasticity. Influence of microstructure on the flow stress is introduced via dislocation density and mean grain size internal state variables. The dislocation density evolves according to a physics based law that considers dislocation nucleation and annihilation processes. Grain refinement is driven by dynamic recrystallization, which is modeled phenomenologically. The model is calibrated with uniaxial stress–strain data that ranges between quasi-static and dynamic rates across a wide range of temperatures. Validation against machining data shows that the model predicts chip segmentation frequency, machining forces, and tool temperatures reasonably well. The newly introduced grain boundary sliding physics was found to dominate deformation following sufficient grain refinement. This deformation mode provides softening at the constitutive level without the need for invoking damage based softening mechanisms. This physical interpretation is something that has not previously been explored in the machining literature.

Effects of solutes on dislocation nucleation from grain boundaries

When grain sizes are reduced to the nanoscale, grain boundaries (GB) become the dominant sources of the dislocations that enable plastic deformation. We present the first molecular dynamics (MD) study of the effect of substitutional solutes on the dislocation nucleation process from GBs during uniaxial tensile deformation. A simple bi-crystal geometry is utilized in which the nucleation and propagation of dislocations away from a GB is the only active mechanism of plastic deformation. Solutes with atomic radii both larger and smaller than the solvent atomic radius were considered. Although the segregation sites are different for the two cases, both produce increases in the stress required to nucleate a dislocation. MD simulations at room temperature revealed that this increase in the nucleation stress is associated with changes of the GB structure at the emission site caused by dislocation emission, leading to increases in the heats of segregation of the solute atoms, which cannot diffuse to lower-energy sites on the timescale of the nucleation event. These results contribute directly to understanding the strength of nanocrystalline materials, and suggest suitable directions for nanocrystalline alloy design leading toward structural applications.

A review on atomistic simulation of grain boundary behaviors in face-centered cubic metals

Grain boundaries are the interfaces between differently oriented crystals of the same material. The underlying structures of grain boundary play a significant role in mechanical properties of polycrystalline materials. This influence becomes more significant when the grain size is reduced to ultrafine or nano size scale where the dislocation activities in the interior of grains lessen and mechanisms mediated by the grain boundary become dominant. This paper reviewed recent results in the atomistic simulation of the nanoscale behavior of grain boundary in face-centered cubic (fcc) metals. Three different simulation models were introduced to investigate the grain boundary behavior during plastic deformation, including three-dimensional (3D) nanocrystalline model, columnar nanocrystalline model and bicrystal model. The grain boundary was found to contribute to plastic deformation through the process of dislocation absorption, transmission or nucleation at boundary plane, as well as grain boundary accommodation mechanisms such as GB sliding and GB migration. These grain boundary mediated mechanisms were widely studied by the previous atomistic simulation works and were extensively reviewed here. Future challenges and directions in the computational study of grain boundary behaviors were also discussed.

Dislocation "Bubble-Like-Effect" and the Ambient Temperature Super-plastic Elongation of Body-centred Cubic Single Crystalline Molybdenum.

With our recently developed deformation device, the in situ tensile tests of single crystal molybdenum nanowires with various size and aspect ratio were conducted inside a transmission electron microscope (TEM). We report an unusual ambient temperature (close to room temperature) super-plastic elongation above 127% on single crystal body-centred cubic (bcc) molybdenum nanowires with an optimized aspect ratio and size. A novel dislocation “bubble-like-effect” was uncovered for leading to the homogeneous, large and super-plastic elongation strain in the bcc Mo nanowires. The dislocation bubble-like-effect refers to the process of dislocation nucleation and annihilation, which likes the nucleation and annihilation process of the water bubbles. A significant plastic deformation dependence on the sample’s aspect ratio and size was revealed. The atomic scale TEM observations also demonstrated that a single crystal to poly-crystal transition and a bcc to face-centred cubic phase transformation took place, which assisted the plastic deformation of Mo in small scale.

Understanding pop-in phenomena in FeNi3 nanoindentation

In this work, the spherical nanoindentation of FeNi3 crystal was simulated by molecular dynamics (MD) for (0 1 0), (1 1 0) and (1 1 1) surfaces to study the origin of pop-in phenomena at the atomistic level. Simulated results demonstrate that the critical load, indentation modulus as well as dislocation nucleation processes are strongly dependent upon crystallographic orientations. The pop-in behaviors observed in nanoindentation are associated with dislocation reactions. The first pop-in event indicates the homogeneous nucleation of the 1/6<1 1 2>-type partial dislocations. Activated partial dislocations can transform into dislocation locks through complex dislocation reactions.

Dynamic and atomic-scale understanding of the twin thickness effect on dislocation nucleation and propagation activities by in situ bending of Ni nanowires

Although their mechanical behavior has been extensively studied, the atomic-scale deformation mechanisms of metallic nanowires (NWs) with growth twins are not completely understood. Using our own atomic-scale and dynamic mechanical testing techniques, bending experiments were conducted on single-crystalline and twin-structural Ni NWs (D=∼40nm) using a high-resolution transmission electron microscope (HRTEM). Atomic-scale and time-resolved dislocation nucleation and propagation activities were captured in situ. A large number of in situ HRTEM observations indicated strong effects from the twin thickness (TT) on dislocation type and glide system. In thick twin lamella (TT>∼12nm) and single-crystalline NWs, the plasticity was controlled by full dislocation nucleation. For NWs with twin thicknesses of ∼9nm<TT<∼12nm, full and partial dislocation nucleation occurred from the free surface, and the dislocations glided on multiple systems and interacted with each other during plastic deformation. For NWs with twin thicknesses of ∼6nm<TT<∼9nm, partial dislocation nucleation from the free surface and the gliding of those dislocations on the plane that intersected the twin boundaries (TBs) were the dominant plasticity events. For the NWs with twin thicknesses of ∼1nm<TT<∼6nm, the plasticity was accommodated by a partial dislocation nucleation process and glide parallel to the TBs. When TT<∼1nm, TB migration and detwinning processes resulting from partial dislocation nucleation and glide adjacent to the TBs were frequently observed.

Dislocation creation and void nucleation in FCC ductile metals under tensile loading: a general microscopic picture.

Numerous theoretical and experimental efforts have been paid to describe and understand the dislocation and void nucleation processes that are fundamental for dynamic fracture modeling of strained metals. To date an essential physical picture on the self-organized atomic collective motions during dislocation creation, as well as the essential mechanisms for the void nucleation obscured by the extreme diversity in structural configurations around the void nucleation core, is still severely lacking in literature. Here, we depict the origin of dislocation creation and void nucleation during uniaxial high strain rate tensile processes in face-centered-cubic (FCC) ductile metals. We find that the dislocations are created through three distinguished stages: (i) Flattened octahedral structures (FOSs) are randomly activated by thermal fluctuations; (ii) The double-layer defect clusters are formed by self-organized stacking of FOSs on the close-packed plane; (iii) The stacking faults are formed and the Shockley partial dislocations are created from the double-layer defect clusters. Whereas, the void nucleation is shown to follow a two-stage description. We demonstrate that our findings on the origin of dislocation creation and void nucleation are universal for a variety of FCC ductile metals with low stacking fault energies.

ATOMISTIC SIMULATION OF TORSIONAL VIBRATION AND PLASTIC DEFORMATION OF FIVE-FOLD TWINNED COPPER NANOWIRES

In this paper, atomistic simulations have been conducted to investigate the torsional mechanical behaviors of five-fold twinned nanowires (FTNs), including the torsional vibration properties, elasto-plastic deformation behaviors and activation process of the first partial dislocation nucleation. Simulation results show that the fundamental torsional vibration frequency is inversely proportional to the wire length and is independent of the wire radius. Provided that an effective shear modulus of FTNs is used, the classic elastic torsional theory may be applicable to nanoscale. Furthermore, it is found that the plastic deformation of FTNs is dominated by partial dislocation activities. The normalized critical torsional angle corresponding to the onset of plastic deformation increases with the decrease of the wire radius and temperature, while it is almost independent of the wire length and loading rate. In addition, the activation energy of the first partial dislocation nucleation is about several electric voltages and decreases with the increase of the wire radius and applied torsional load.