Dislocation Processes Research Articles

Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Hot deformation behavior, microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach. The true stress–strain curves exhibited typical work hardening and flow softening, in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon (Z) parameter, and the activation energy was calculated to be 446.9 kJ/mol. The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation. The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy. The coarse recrystallized γ and several massive γ phase were observed at grain boundaries. While in the case of Nb segregation, β/B2 phase harmonized bending of lamellae, combined with the growth of recrystallized γ grains and α + β + γ → α + γ transition under conditions of temperature and stress, leading to the breakdown of α2/γ lamellar colony. During the hot compression process, gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism, leading to extensive γ twins and cross twins in α/γ lamellae and at grain boundaries. In general, homogeneous microstructure during the hot deformation process can be obtained in TiAl alloy with high Nb addition and low Al segregation. The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.

Probing elementary dislocation mechanisms of local plastic deformation by the advanced acoustic emission technique

To gain a deeper insight into fundamental processes of localised plastic deformation, we propose a new strategy for micro-mechanical testing combining the micro-scale scratching and analysis of low amplitude acoustic emissions (AE) accompanying dynamic dislocation processes in solids. The analysis of the AE spectral density reveals its strong dependence on crystallographic orientations of the grains along the indenter path, thus reflecting the finest features of the local dislocation activity. Adding a temporal dimension with a sub-microsecond resolution, the proposed methodology substantially enhances the existing capacity of micro-mechanical testing and can be used in a wide range of testing schemes.

Identifying strain localization and dislocation processes in fatigued Inconel 718 manufactured from selective laser melting

The microstructure and fatigue behavior of additive manufactured Inconel 718 (IN718) were studied using digital image correlation (DIC), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The samples were produced with selective laser melting and underwent two different heat treatments (stress relieved and fully post-processed) that produced different microstructures. The solidification process led to the formation of dendrites where misorientation between each branch was accommodated with dislocations, producing subgranular cell structures. EDS observations revealed that Ti and Nb segregate to the cell and grain boundaries, forming carbides. The stress relieved sample retained the cell boundaries, which acted as barriers for dislocation propagation, thereby distributing strain evenly throughout the microstructure. In contrast, the post-processed sample contained no cell boundaries, and strain was localized at the grain boundaries. Mechanical tests showed that the precipitates and the annealing twins in the post-processed sample were much more effective at improving the strength than small grain size and cell boundaries found in the stress relieved sample.

Discrete shear band plasticity through dislocation activities in body-centered cubic tungsten nanowires

Shear band in metallic crystals is localized deformation with high dislocation density, which is often observed in nanopillar deformation experiments. The shear band dynamics coupled with dislocation activities, however, remains unclear. Here, we investigate the dynamic processes of dislocation and shear band in body-centered cubic (BCC) tungsten nanowires via an integrated approach of in situ nanomechanical testing and atomistic simulation. We find a strong effect of surface orientation on dislocation nucleation in tungsten nanowires, in which {111} surfaces act as favorite sites under high strain. While dislocation activities in a localized region give rise to an initially thin shear band, self-catalyzed stress concentration and dislocation nucleation at shear band interfaces cause a discrete thickening of shear band. Our findings not only advance the current understanding of defect activities and deformation morphology of BCC nanowires, but also shed light on the deformation dynamics in other microscopic crystals where jerky motion of deformation band is observed.

Temperature-dependent nanoindentation response of materials

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Abstract In situ straining experiments on α-Al2O3/Nb/α-Al2O3 single crystal sandwiches have been performed in a high-voltage electron microscope in order to observe the dislocation motion connected with the fracture of these samples. The tricrystals had two different orientation relationships at the interfaces, both possessing high fracture energies. The in situ studies showed that the fracture occurs along the interfaces but more frequently inside the Nb sheet. The fracture is connected with strong plastic deformation. The crack opening and the dislocation processes are analysed.

Morphology of single Shockley-type stacking faults generated by recombination enhanced dislocation glide in 4H–SiC

Morphology of single Shockley-type stacking faults (SFs) generated by recombination enhanced dislocation glide (REDG) in 4H–SiC are discussed and analysed. A complete set of the 12 different dissociated states of basal-plane dislocation loops is obtained using the crystallographic space group operations. From this set, six different double rhombic-shaped SFs are derived. These tables indicate the rules that connect shapes of SFs with the locations of partial dislocations having different core structures, the positions of slip planes in a unit cell, and the Burgers vectors of partial dislocations. We applied these tables for the analysis of SFs generated by the REDG effect reported in the past articles. Shapes, growing process of SFs and perfect dislocations for origins of SFs were well analysed systematically.

Uniaxial deformation of face-centered-cubic(Ni)-ordered B2(NiAl) bicrystals: atomistic mechanisms near a Kurdjumov–Sachs interface

Creating tailored interfaces between soft and hard materials is a promising route to simultaneously enhance ductility and strength of multicomponent materials. Here, we study deformation mechanisms in a model bicrystal, with a Kurdjumov–Sachs (KS) interface, between face-centered-cubic Ni and ordered-B2 NiAl slabs using molecular dynamics simulations. The bicrystals were uniaxially deformed by strain rates of $$10^7$$ and $$10^9\,\hbox {s}^{-1}$$ by holding temperatures constant at 300, 500, 700, and 900 K for each strain rate. Our simulations reveal atomistic processes that create sessile and glissile dislocations, and their reactions during high-strain rate deformation. At $$10^9\,\hbox {s}^{-1}$$ strain rates, dislocation processes enhance ductility and cause large-scale atomic rearrangements in the KS interfacial region. This subsequently causes nucleation, growth, and coalescence of nano-voids into cracks inside the harder B2-ordered phase bordering the interface. Our results suggest that interfaces between “soft”–“hard” materials likely withstand high-strain rates better.

Temperature-based plastic deformation mechanism of Cu/Ag nanocomposites: A molecular dynamics study

Recently, copper-silver nanocomposites (NCs) have been introduced as ideal candidates for use in medical implants owing to the antibacterial features of their constituents. Since evaluation of the mechanical properties of the implants is crucial, identification of the mechanism responsible for their plastic deformation has been a challenging issue. With respect to the fact that service operation and sterilization of these NCs are performed in high temperatures, the governing mechanism should include the role of temperature on their deformation characteristics. Accordingly, in this article, we report a series of MD-based tensile tests performed at different temperatures ranging from 1 to 600 K to assess the issue. It is demonstrated that increasing the temperature can substantially reduce the yield strength and Young’s modulus of the NC sample. To reveal the temperature-based underlying mechanism, dislocation extraction analysis is implemented to identify the type and extent of dislocations in each case. Results show that due to the lack of adhesion to the copper matrix, presence of silver nanoparticles has a significant effect on the nucleation of dislocations that weakens the mechanical properties of the NC sample. It is also observed that at low temperatures, plastic deformation is followed by twining. As the temperature rises, as a result of the supply of thermal energy required for diffusion, the process of dislocation climb is accelerated. Consequently, the amount of partial dislocations as well as the extent of stacking faults is reduced providing more chances for creation and gliding of perfect dislocations.

Microstructural evolution of adiabatic shear bands in pure copper during impact at high strain rates

A systematic study was conducted on copper of commercial purity (99.9% purity) to elucidate the sequence of events that leads to the evolution of adiabatic shear bands and the structure of the evolved shear bands after strain localization during deformation at high strain rates and large strains. A direct impact Hopkinson Pressure Bar was used to deform different specimens at increasing impact momentum and strain rate followed by microstructural characterization using metallographic techniques and transmission electron microscopy. It was observed that sequential occurrence of emergence of dislocations from grain and twin boundaries, formation of dislocation cell structures and substructures with varying cell sizes and cell walls, dislocation-nucleation controlled softening and extensive micro twinning characterized the structure of the evolved adiabatic shear bands as a function of impact momentum and strain rate. The dislocation cell structures and substructures were typically made up of high-density dislocation walls surrounding low-density dislocation cell interiors. The microhardness distribution within the evolved shear bands increased up to a peak value at a critical impact momentum and strain rate (≥ 45kgm/s and ≥6827s−1). Above this threshold, microhardness of the regions within the evolved shear bands decreased because of the occurrence of softening. However, the first specimen that exhibited softening had high-density dislocation cell walls surrounding dislocation-free cell interiors with no observed recrystallized grains. Despite the onset of softening both within and outside the shear bands, the regions within the shear bands were always harder than the regions outside the shear bands. The shear bands that exhibited significant softening were clad with vast distribution of microtwins in addition to evolved refined grains and sub-grains. It is discussed that thermally activated dislocation processes as a result of the rise in temperature during impact, dynamic recovery and dynamic recrystallization do not necessarily result in strain localization in the impacted copper specimens because their effects are observed in the structure of the evolved shear bands at a latter stage when strain localization had already occurred.

Deformation twinning and dislocation processes in nanotwinned copper by molecular dynamics simulations

Nanotwinned materials exhibit a combination of high strength and good ductility which is attributed to the interactions between dislocations and twin boundaries. But no attempt has been made to explore the possibility for deformation twinning in nanotwinned face-centered cubic materials. Here we use large scale molecular dynamics simulations to elucidate the mechanical behaviour of nanotwinned Cu. We demonstrate that deformation twinning plays an important role in the deformation of nanotwinned Cu with specific twin orientations, in addition to conventional dislocation slip. Deformation twins are formed through the glide of Shockley partials on adjacent {1 1 1} slip planes and two twinning mechanisms are identified based on the arrangement of Shockley partials. The first mechanism involves the successive motion of Shockley partials of different types, named as double-Shockley partials, which forms unstable thin twin plates. The second process involves the successive passage of the same twinning dislocations on neighbouring slip planes, which forms stable deformation twins along one primary twinning system or symmetric twinning systems. The dislocation processes involved in the dislocation-twin reactions are analysed at atomic level. The orientation dependence of deformation twinning is discussed and compared with available experimental results.

Antisymmetric vortex interactions in the wake behind a step cylinder

Flow around a step cylinder at the Reynolds number 150 was simulated by directly solving the full Navier-Stokes equations. The configuration was adopted from the work of Morton and Yarusevych [“Vortex shedding in the wake of a step cylinder,” Phys. Fluids 22, 083602 (2010)], in which the wake dynamics were systematically described. A more detailed investigation of the vortex dislocation process has now been performed. Two kinds of new loop vortex structures were identified. Additionally, antisymmetric vortex interactions in two adjacent vortex dislocation processes were observed and explained. The results in this letter serve as a supplement for a more thorough understanding of the vortex dynamics in the step cylinder wake.

Плитотектоническая металлогения Урало-Тимано-Палеоазиатского сегмента Евразии на основе мобилистских реконструкций

General positions on plate-tectonic metallogeny are presented and the grounds for singling out the metallogenic factors of orogenic systems are given. Mobilistic paradigma corresponds to the genotype of orogens and belts which being formed at the expense of oceanic basin transformations. Their alternative character for the factors of fixistic constructions is stressed. The conclusion is made that as a theoretical basis for the analysis of orogen system metallogeny - that is the metallogeny of considering in the paper segment - are the metallogenic factors connected with processes of movement and dislocation of lithospheric plates, geodynamical regimes of riftogenic extension and spreading of plates, geodynamical environments of subduction, accretion and collision. A decisive role of the factor of movement and dislocation of lithospheric plates makes it possible to introduce for this type of metallogeny the denomination of plate-tectonic metallogeny . A comparative analysis of metallogeny of modern oceanic basins and of epioceanic orogens showed that metallogenetic factors of geodynamical environments characterize their ore specialization, while the accretions and collisions define the location of ore concentrations, determining, in the long run, combination in orogenic systems of pre-accretion, accretion and post-accretion metallogenic elements. On the given basis taking into account regional particularities the metallogeny analysis of Ural-Timan-Paleoasian segment of Eurasia has been made. Diagrams of metallogeny and the segment metallogenic zoning were suggested.

Theoretical derivation of flow laws for quartz dislocation creep: Comparisons with experimental creep data and extrapolation to natural conditions using water fugacity corrections

AbstractWe theoretically derived flow laws for quartz dislocation creep using climb‐controlled dislocation creep models and compared them with available laboratory data for quartz plastic deformation. We assumed volume diffusion of oxygen‐bearing species along different crystallographic axes (//c, ⊥R, and ⊥c) of α‐quartz and β‐quartz, and pipe diffusion of H2O, to be the elementary processes of dislocation climb. The relationships between differential stress (σ) and strain rate ( ) are written as and for cases controlled by volume and pipe diffusion, respectively, where Dv and Dp are coefficients of diffusion for volume and pipe diffusion. In previous experimental work, there were up to ~1.5 orders of magnitude difference in the water fugacity values in experiments that used either gas‐pressure‐medium or solid‐pressure‐medium deformation apparatus. Therefore, in both the theories and flow laws, we included water fugacity effects as modified preexponential factors and water fugacity terms. Previous experimental data were obtained mainly in the β‐quartz field and are highly consistent with the volume‐diffusion‐controlled dislocation creep models of β‐quartz involving the water fugacity term. The theory also predicts significant effects for the transition of α‐β quartz under crustal conditions. Under experimental pressure and temperature conditions, the flow stress of pipe‐diffusion‐controlled dislocation creep is higher than that for volume‐diffusion‐controlled creep. Extrapolation of the flow laws to natural conditions indicates that the contributions of pipe diffusion may dominate over volume diffusion under low‐temperature conditions of the middle crust around the brittle‐plastic transition zone.

Grain‐ to multiple‐grain‐scale deformation processes during diffusion creep of forsterite + diopside aggregate: 1. Direct observations

AbstractWe uniaxially deformed fine‐grained (~ 1 μm) forsterite + diopside (5 and 20 vol %) aggregates in the diffusion creep regime. Prior to deformation, line markers were milled on a lateral surface of a cylindrical sample to detect single‐ to multiple‐grain‐scale deformation. We performed deformation experiments and observations of the marker‐etched surface after sample cooling multiple times on the same specimens. The strain measured at the scale of several tens of grains from macroscopic shortening of the markers parallel to the compression axis is consistent with the total strain of the sample. However, microscopically, the markers are intensely segmented and rotated at the grain scale increasing with the sample strain. Meanwhile, essentially, no deformation is observed within the grains in most of the samples. The surface microstructures, including the deformation of the markers, reveal the serial operations of grain boundary migration, grain boundary sliding, rigid body grain rotation, and grain neighbor switching, which correspond well to processes expected in diffusion‐controlled superplasticity. This sequence is commonly observed in both samples consisting of forsterite grains of tabular and equiaxed grain shapes, which have been shown to develop notable crystallographic preferred orientation (CPO) and random (or weak) CPO, respectively, during diffusion creep. Intragranular regions of relatively larger forsterite grains in the specimens deformed at stresses near the transition between deformation mechanisms from diffusion creep to dislocation creep reveal marker deformation and formation of surface creases and subgrain boundaries, which indicate intragranular dislocation processes. Overall, the surface microstructures reflect the deformation state of the materials well.

A wake for urban theory

AbstractIn a critical essay on new books by Matthew Desmond and Mitchell Duneier, I interrogate the demise of social theory in urban sociology following what I call the consolidation of a post‐Wilsonian consensus: a turn from properly urban sociology to a field of descriptive poverty studies. This is what Duneier's intellectual history of the idea of the ghetto illustrates most unambiguously, viz. that sociologists today rarely study cities as such, but instead treat urban poverty that happens to occur in cities. Divorced from the theories of racialization, urban political economy, and space that once defined the subfield, writers have shifted toward a localist descriptivism, embodying what Castells once called phenomena in the city, but not of the city. This reluctance to theorize characterizes Desmond's book on evictions, his prior pleas for a relational ethnography notwithstanding. Far from laying out the field of evictions, I argue, he instead treats immediate landlord‐tenant relations, never exploring how the state, investors, and other key actors are involved in the process. As Duneier unwittingly demonstrates in Ghetto, urban theory withers away in the subfield at precisely the moment when it is needed to make sense of new processes of dispossession and dislocation that have characterized urban change in the United States since the implosion of the ghetto in the 1970s. Hopefully Evicted will serve as a clarion call to reverse this tendency with its rich descriptions of such processes. But this sort of reversal would require us to engage in a project of theoretical reconstruction, piecing together the insightful theoretical fragments scattered throughout Desmond's book, as he remains hesitant to promulgate an actually relational sociology.

High temperature dislocation processes in precipitation hardened crystals investigated by a 3D discrete dislocation dynamics

3D discrete dislocation dynamics is employed to investigate motion of general mixed dislocation segments subjected to high temperature loadings in microstructures with impenetrable particles. The implementations of the model first address several benchmark processes including shrinkage of a glissile dislocation loop driven by self-stresses and an annihilation of mutually interacting co-axial prismatic dislocation loops. In particular, we show that models of the microstructure with planar and/or translational symmetry improve efficiency, speed and stability of the calculations. Our simulations then focus on migration of low angle dislocation boundaries in an array of particles while taking into account all mutual dislocation-dislocation interactions and the action of an externally applied stress. The results show for the first time that the migration of tilt dislocation boundaries in crystals with particles can be associated with threshold stresses. The calculated thresholds are in a good agreement with experimental threshold stresses that characterize creep behaviour of precipitation hardened alloys.

The core structure and recombination energy of a copper screw dislocation: a Peierls study

The recombination process of dislocations is central to cross-slip, and transmission through 3 grain boundaries among other fundamental plastic deformation processes. Despite its importance, a detailed mechanistic understanding remains lacking. We apply a continuous dislocation model, inspired by Peierls and Nabarro, complete with an ab-initio computed -surface and continuous units of infinitesimal dislocation slip, towards computing the stress-dependent recombination path of both an isotropic and anisotropic Cu screw dislocation. Under no applied stress, our model reproduces the stacking fault width between Shockley partial dislocations as predicted by discrete linear elasticity. Upon application of a compressive Escaig stress, the two partial dislocations coalesce to a separation of . Upon increased loading the edge components of each partial dislocation recede, leaving behind a spread Peierls screw dislocation, indicating the recombined state. We demonstrate that the critical stress required to achieve the recombined state is independent of the shear modulus. Rather the critical recombination stress depends on an energy difference between an unstable fault energy () and the intrinsic stacking fault energy (-). We report recombination energies of W = 0.168 eV/Å and W = 0.084 eV/Å, respectively, for the Cu screw dislocation within isotropic and anisotropic media. We develop an analytic model which provides insight into our simulation results which compare favourably with other (similar) models.

Multiscale Crystal Plasticity Modeling Considering Nucleation of Dislocations Based on Thermal Activation Process on Ultrafine-grained Aluminum

In this study, a crystal plasticity model expressing the behavior of the dislocation source and the mobile dislocations is proposed by considering a thermal activation process of dislocations. In order to predict the variation of critical resolved shear stress due to grain boundaries, mobile dislocations, or dislocation sources, information on these crystal defects is introduced into a hardening law of crystal plasticity. The crystal orientation and shape of ultrafine-grained (UFG) aluminum produced by accumulative roll bonding processes are measured by electron backscatter diffraction (EBSD). Mechanical properties of the UFG aluminum are estimated using tensile test and indentation test. Results obtained by EBSD are introduced into a computational model. Finite element simulation for polycrystal of aluminum investigates the effect of microstructure on mechanical properties of UFG aluminum.

Size effects on the mechanical properties of nanocrystalline NbMoTaW refractory high entropy alloy thin films

The newly-surged high-entropy alloys (HEAs), in particular the refractory HEAs, manifest excellent properties for practice applications. In this work, we systematically investigated the plastic deformation characteristics (including the hardness, strain rate sensitivity (SRS), and activation volume) of nanocrystalline (NC) equiatomic NbMoTaW body-centered cubic (BCC) high-entropy alloy films (HEAFs) with thicknesses spanning from 100 to 2000 nm at room temperature. The BCC NbMoTaW HEAFs exhibited the strongest grain size ∼10 nm with the hardness of ∼16.0 GPa, above which the hardness decreases with increasing the grain size or film thickness, while below this critical size the material softening occurs. Given the friction stress and the source activation stress cannot be additive because they don't appear under the same circumstances, the size-dependent ultra-high strength in BCC HEAFs was discussed in terms of the solid solution hardening, the boundary dislocation density and the source strength. It was unveiled that above a critical grain size of ∼40 nm, the solid solution hardening becomes the predominant strengthening mechanism, while below this critical size the source strengthening becomes the predominant one in BCC NbMoTaW HEAs. Also, the strong size-dependence of SRS is emerged, which monotonically increased with decreasing thicknesses (just opposite to the activation volume). A thermally activated model based on the depinning process of bowed-out dislocations was employed to quantitatively account for the size-dependent SRS of the NbMoTaW HEAFs. Our findings not only provide deep insight into understanding the mechanical behavior of the NC BCC NbMoTaW HEAFs, at least in the studied size-range, also offer some clues for the metastability engineering strategy to design high performance HEAFs at small-scales.