Dislocation Length Research Articles

Theory of kink migration in dilute BCC alloys

Plastic deformation in elemental BCC metals and dilute alloys is controlled by the slower of the kink pair nucleation and kink migration processes along screw dislocations. In alloys nucleation is facilitated and migration inhibited, leading to a concentration- and temperature-dependent transition from nucleation dominance to migration dominance. Here, an analytical statistical model for the stress-dependent kink migration barrier in dilute BCC alloys is developed and validated. The barrier depends only on a clearly-defined solute/screw dislocation interaction parameter, the kink width, and dislocation length between jogs. The analytic model is extensively validated via fully atomistic nudged-elastic band calculations and stochastic simulations in a model Fe-Si alloy. Combined with a recent validated double-kink nucleation theory, a fully-analytic model for the temperature- and concentration-dependent flow stress is obtained that includes the transition from nucleation to migration control. The overall model is applied to Fe-Si and W-Re using independently-determined material properties and good agreement is obtained with experiments over a range of concentrations and temperatures. Overall, the two theories represent a unified, fully-statistical, parameter-free understanding of screw dislocation strength in dilute BCC alloys.

A comparative of a single novice surgeon's direct anterior approach and posterior approach learning curves in total hip arthroplasty: a retrospective cohort study.

The purpose of the present study was to compare a novice surgeon's learning curves with the direct anterior approach and posterior approach in total hip arthroplasty. A consecutive series of 376 total hip arthroplasties performed from November 2014 to September 2019 in a level-one healthcare center by a single surgeon (V.B) were retrospectively studied. Demographic data, functional outcomes, and complications were collected and compared. Within the ranks of the patients studied, we found differences between groups with respect to dislocation rate and length of stay; these were lower in the direct anterior approach (DAA) group. The approach was not associated with an increase in complications, but rather with a decrease in the rate of dislocations and better functional outcomes at 1-year follow-up. Operative time was initially higher with this approach, but equalized during the learning curve. The DAA can be safe even in the early stages of a novice surgeon's learning curve. It does not present a higher complication rate than the posterior approach, either in infection rate or in periprosthetic fractures. However, the DAA may provide greater functionality, lower dislocation rate and a shorter hospital stay. It can also be concluded that after having performed a certain number of interventions, operative time for the DAA can be comparable to that of other approaches.

Recrystallization and size distribution of dislocated segments in cellulose microfibrils\u2014a molecular dynamics perspective

The arrangement of cellulose molecules in natural environment on the nanoscale is still not fully resolved, with longitudinal disorder in cellulose microfibrils (CMF) being one relevant question. Particularly the length of the dislocated cellulose segments in CMFs is still under debate. Using molecular dynamics simulations, we are first investigating the phenomenon of pseudo-recrystallization of dislocated cellulose regions after cleavage of CMFs. Based on our simulations we propose that 3–4 glucose residues bordering to each side of a cellulose nanocrystal are actually reorganizing to a quasi-crystalline state, which are corroborating recent analytical investigations reporting an increase in crystallinity after acid vapor hydrolysis of CMFs. Combining our molecular dynamics simulation results with these analytical data we can estimate the length of the dislocated cellulose segments in CMFs. We propose that, for the investigated sources of biomass (cotton and ramie), the dislocation lengths are between 3.1–5.8 nm equaling to 6–11 glucose residues in the cellulose crystallites.Graphic abstract

23 Reaudit and Completing the Audit Cycle of Quality of Informed Consent for Surgery on Neck of Femur Fracture in Royal Stoke University Hospital

Abstract Using guidelines highlighted by the British Orthopaedic Association an reaudit was performed within our department to assess the adequacy of informed consent for NOF fractures to complete the audit cycle. 50 patients were included in the Audit and reaudit. Risk was classified as common, less common, rare and ‘other’. The adequacy of informed consent was evaluated by assessing the quality and accuracy of documentation. Infection, bleeding risks, clots and anaesthetic risks were documented in all patients (100%). Areas of improvement were seen in the documentation of neurovascular injuries (98%), pain (90%) and altered wound healing (87%). There was no significant change in the documentation of failure of surgery (83%) and neurovascular injuries (98%). The Poorly documented risk factors from the initial audit were seen to improve which included mortality (70%), prosthetic dislocation (90%) and limb length discrepancy (50%). There has been a significant improvement in the quality of Informed consent in the department and this could be attributed to the installation of ward posters and verbal dissemination of information to junior doctors. Recommendation for interventions would be to present in the next clinical governance meeting and presenting at the new junior doctors’ induction at August.

The role of shear deformation in pure tungsten: Mechanical properties and micro-mechanism

The effects of shear deformation at 1173 K on the mechanical properties and deformation mechanism of pure tungsten are investigated by molecular dynamics (MD). The results show that the shear deformation of pure tungsten is dominated by dislocation multiplication and slip band deformation. The shear angle has a significant effect on the mechanical properties of pure tungsten. The yield strength is 4.21 Gpa at a shear angle of 11[Formula: see text], and it increases significantly to 11.84 Gpa while the shear angle increasing to 27[Formula: see text]. In the plastic deformation stage, the stress–strain curve shows obvious oscillation due to the interaction of dislocations in the single-crystal tungsten and the effect of strain strengthening. In addition, the evolution of dislocation and twining in the compression system against shear angle indicates the variation of deformation behavior. When the shear angle is 11[Formula: see text], the lengths of dislocation 1/2[Formula: see text] and [Formula: see text] increase to a peak rapidly, which illustrates dislocation strengthening. However, when the shear angle is more than 11[Formula: see text], the decrease of dislocation length and the appearance of twins along [Formula: see text] direction demonstrate the twining accompanied with dislocation tangling, resulting in the additional increase of strength.

Understanding the linear relation between pop-in excursion length and critical force for spherical nanoindentation

ABSTRACT Pop-in is a widely observed phenomenon in nanoindentation. In this paper, dislocation evolution in pop-in processes is analysed in detail through molecular dynamics (MD) simulations. We found that a large number of dislocations nucleate homogeneously at the initiation of pop-in, followed by extensive dislocation propagation, which is the dominant mode of plastic deformation during pop-in. Moreover, we noted that establishing the correct dislocation evolution mechanisms of pop-in can serve to explain the overshoot phenomenon observed in nanoindentation experiments. Through our MD analysis on the obtained dislocation structures, therefore, we were able to propose a model that can predict the total length of dislocations associated with the plastic processes underneath a spherical indenter. In addition, the Taylor model was used to verify that our proposed dislocation length model sits well with the MD simulated force-displacement curves of nanoindentation. In fact, the MD simulated linear relation between critical force and indentation depth during pop-in is consistent with the Hertzian and Taylor models. Our MD simulations, therefore, can provide significant insight into the experimentally observed pop-in phenomena.

Creep behaviour of two-phase lamellar TiAl: Crystal plasticity modelling and analysis

Two-phase (α2Ti3Al+γTiAl) fully lamellar TiAl alloys show highly anisotropic creep behaviour. Depending on lamellar orientations, locally in the TiAl phases hard and easy deformation modes are active, which significantly influence the creep behaviour at a global scale. To obtain comprehensive understanding how the locally activated crystallographic deformation modes influence the macroscopic creep behaviour, a crystal plasticity (CP) model with modified hardening behaviour is proposed in this work. For this purpose, a softening parameter is introduced in the model that phenomenologically describes the threshold stresses for material softening depending on pile-up dislocation lengths during creep deformation. The model allows us to interpret the cause of creep anisotropy at macro-scale deformation based on the orientation dependent activation and interactions of slip systems at the TiAl phases. Comprehensive study was performed to validate the model and to predict creep parameters under different temperature and stress loading conditions. Comparing the experimental data with simulation results, we found that the proposed CP model captures creep behaviour in primary, secondary, and tertiary regime for all the lamellar orientations with good accuracy. This model can be further used for investigating microstructure-sensitive creep behaviour of a wide range of TiAl alloys.

Mathematical modeling of elastic wave radiation during infinite screw dislocation bending vibrations

In this paper, we consider elastic waves radiation (acoustic emission) by infinite screw dislocation performing small bending vibrations under action of external influences. Braking of the dislocation vibrations by medium was not taken into account. To solve this problem, inverse generalized susceptibility of an infinite screw dislocation is written down. It is found that condition for the emission of elastic waves by a screw dislocation is satisfied for two ranges of wave vector component values along the dislocation line. For average value of energy emitted per unit time per unit length of the screw dislocation, corresponding expressions are obtained. Internal friction caused by radiation losses during bending vibrations of the screw dislocation is found.

Experimental study and modelling of the role of solutes, precipitates and temperature on the work-hardening of AA6xxx aluminium alloys

Attempts have been registered scientifically mostly for strained hardened Al alloys towards improvement of formability at warm temperatures. The varieties of possible precipitate populations add complexity in determining the mechanisms of work-hardening behaviour of the precipitation hardened alloys. Efforts were made, in the current work, to investigate the work-hardening response for EN AW-6016 and EN AW-6061 sheets of different temper (T4 and T6) at room and warm temperatures (150 and 250 °C) and at two different strain rates (0.1 and 0.01 s −1 ) through tensile experiments. Among these variants, the influence of temperature has been detected the most on yield strength and work-hardening rate. Lowering the efficiency of storing dislocations as well as activating dynamic recovery, both are responsible for the reduction in work hardening rate proportionally with temperature. The work-hardening model originally developed by Prof. E. Nes (Nes model) has been chosen to simulate the experimental stress-strain response of both alloys at room and warm temperature as the phenomenology of the model promised a better constitutive description of work hardening. Because Nes model was primarily designed for strained hardened Al alloys, the bottleneck, to apply this model for precipitation hardened alloys, was removed by adding contribution from the precipitates in terms of mean slip length of dislocations in the current version. The validity of the model was examined for 130 °C, 150 °C and 180 °C and at a strain rate of 0.1s −1 , once satisfied fitting was achieved for 6016 and 6061 alloys at RT and 250 °C at a strain rate of 0.01s −1 . The decrease of dynamic recovery during the change of temper state (T4 to T6) during RT deformation can be explained as a competition between solute depletion and precipitate growth. With minute quantitative differences, the influence of strain rate and temperature was reproduced well by the model.

Influences of grain size, alloy composition, and temperature on mechanical characteristics of Si100-xGex alloys during indentation process

We perform molecular dynamics simulations to explore the effects of grain size, alloy composition, and temperature on the mechanical characteristics of Si100-xGex alloys during the indentation process. The structural evolution, von Mises shear strain, von Mises stress, and dislocation in workpiece are evaluated to explain the transformation of the loading force. Moreover, hardness and Young's modulus are also computed to evaluate the mechanical characteristics of different specimens. The results present that the loading force, hardness, Young's modulus of single crystalline are higher than polycrystalline. Besides, the loading force and hardness reduce as the grain size reduces; this result reveals a reverse Hall-Petch relation. In the indentation process of small grain size workpiece, the dislocation freely migrates to the grain boundary and absorbed, leading the amount of dislocation movement significantly reduces, which will instead promote the migration and sliding at the grain boundaries. It is confirmed that the grain boundaries play an essential role in the deformation behavior of the material. For the purpose of study the effect of various alloy compositions on the mechanical characteristics of the Si100-xGex substrate, a series of various alloy compositions are conducted: Si20Ge80, Si40Ge60, Si50Ge50, Si60Ge40, and Si80Ge20. The results reveal that the hardness, and Young's modulus of specimens increase as the Si content increases. In addition, the influence of temperature is also investigated in this work. It is found that when the temperature increases, leading to the atomic displacement and shear strain increase, the total dislocation length and the number of defect atoms increase, which leads to the hardness and Young's modulus reduction.

The Sensitivity of Dislocation Rosettes to the Shape of a Berkovich Indenter on LiF and MgO Crystals

The peculiar effect observed under the indentation of the (001) plane of LiF and MgO single crystals, namely, the sensitivity of the form of dislocation rosettes to the shape of the indenter, is considered. It is shown that the dislocation rosettes being formed around Vickers indentations are symmetrical, while the rosettes around Berkovich indentations are asymmetrical, the edge arms of the dislocation rosettes are different with respect to their length, and the screw arms do not demonstrate such asymmetry. Also, an orientational effect is found under the penetration of a Berkovich indenter, thus, the form of the dislocation rosettes and length of the edge arms change at varying the orientation of the indenter relative to the crystallographic directions of the sample. It is also shown that the asymmetry of the dislocation rosettes is more pronounced on harder crystals and increases with the growth in the load on the indenter. The factors responsible for the anomaly in the development of the dislocation rosettes under nano- and microindentation of the (001) plane of cubic crystals by a Berkovich indenter are found.

Investigation on mechanical behaviors of Cu-Ni binary alloy nanopillars: a molecular dynamics study.

Cu-Ni binary alloy has become the attention of scientific world for its potentials in nanodevices. It is indispensable to investigate on the mechanical properties of this material due to lack of previous work done regarding this binary alloy. Molecular dynamics (MD) studies were performed on nanopillar (NP) structures comprised of Cu-Ni binary alloy having an FCC unit cell with Cu atoms selectively replaced by Ni atoms. This selective replacement resulted in a better stress behavior than the randomly replaced alloy structure when both tension and compression load were applied. The effect of crystal orientation, NP dimensions, temperature, and strain rate on the stress-strain curve of Cu-Ni binary alloy NPs was thoroughly investigated under tensile loading. This investigation reveals significant influence of crystal orientation on ultimate strength and flow stress region. Among four different crystal orientations, <111> orientation shows maximum strength behavior under tensile loading, while <110> shows highest strength under compressive load. However, in both cases, i.e. tension and compression, the poorest stress behavior was observed for <001> orientation. Under tensile load, <111>-oriented binary alloy fails due to the formation of Shockley partials followed by formation of complex dislocation network. On the other hand, <110>-oriented binary alloy fails due to the formation of Lomer-Cottrell (LC) lock from the Shockley partials. Total dislocation length is calculated, and its effect on the stress-strain behavior of the Cu-Ni binary alloy is discussed. Highest Young's modulus and yield stress are observed on <111>-oriented binary alloy among other orientations, and these values for <111>-oriented NP was found to decrease with the increment of temperature. If the temperature is increased, yield stress and Young's modulus decrease. The effect of cross section width was also investigated in this study, and it was found that yield stress decreases with the increment of cross section width due to the effect of surface atom fraction. Increasing the strain rate causes the initiation of amorphous structure, resulting in superplastic behavior of the <111>-oriented Cu-Ni binary alloy NP.

Reduction of dislocation, mean free path, and migration barriers using high entropy alloy: insights from the atomistic study of irradiation damage of CoNiCrFeMn

High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradiation resistance, partially due to sluggish diffusion. The mechanism from a defect-generation perspective, however, has received much less attention. In this paper, the formation of dislocation loops, and migration of interstitials and vacancies in CoNiCrFeMn high entropy alloy under consecutive bombardments were studied by molecular dynamics simulations. Compared to pure Ni, less defects were produced in the CoNiCrFeMn. Only a few small dislocation loops were observed, and the length of dislocation was small. The dislocation loops in Ni matrix were obviously longer and so was the length of dislocation. The interstitial clusters had much smaller mean free path during migration in the CoNiCrFeMn. The mean free path of 10-interstitial clusters in CoNiCrFeMn was reduced over 40 times compared to that in pure Ni. In addition, CoNiCrFeMn had a smaller difference of migration energy between interstitial and vacancy, which increased the opportunity of recombination of defects, therefore, led to less defects and much fewer dislocation loops. Our results provide insights into the mechanism of irradiation resistance in the high entropy alloy and could be useful in material design for irradiation tolerance and accident tolerance materials in nuclear energy.

Atomistic deformation behavior of single and twin crystalline Cu nanopillars with preexisting dislocations

Molecular dynamics simulations are performed to investigate the impact of a coherent Σ3 (111) twin boundary on the plastic deformation behavior of Cu nanopillars. Our work reveals that the mechanical response of pillars with and without the twin boundary is decisively driven by the characteristics of initial dislocation sources. In the condition of comparably large pillar size and abundant initial mobile dislocations, overall yield and flow stresses are controlled by the longest, available mobile dislocation. An inverse correlation of the yield and flow stresses with the length of the longest dislocation is established and compared to experimental data. The experimentally reported subtle differences in yield and flow stresses between pillars with and without the twin boundary are likely related to the maximum lengths of the mobile dislocations. In the condition of comparably small pillar size, for which a reduction of mobile dislocations during heat treatment and mechanical loading occurs, the mechanical response of pillars with and without the twin boundary can be clearly distinguished. Dislocation starvation during deformation is more pronounced in pillars without the twin boundary than in pillars with the twin boundary because the twin boundary acts as a pinning surface for the dislocation network.

Characterization of Microstructure and Stress Corrosion Cracking Susceptibility in a Multi-pass Austenitic Stainless Steel Weld Joint by Narrow-Gap TIG

In the present study, correlation of the microstructure and stress corrosion cracking (SCC) susceptibility in the top, middle and root regions of a multi-pass austenitic stainless steel weld joint fabricated by narrow-gap tungsten inert gas welding (NG-TIG) was investigated by slow strain rate tests (SSRT) following a microstructure characterization. The results show that from the top to root regions in both the heat-affected zone (HAZ) and weld metal (WM), the dislocation density and length fraction of the Σ3 boundaries present a decreased trend, while the grain size, length fraction of LAB and residual strain all increase. By comparing the microstructure of WM with HAZ, δ-ferrite in WM is excessive and continuous while that in HAZ is island-shaped and discontinuous. In addition, larger grain size, lower dislocation density, higher residual strain, lower length fraction of Σ3 boundaries and higher length fraction of RGB are observed in WM. As a result, the SCC susceptibility in various regions of the weld joint follows the order of root regions > middle regions > top regions and WM > HAZ.

Deformation Induced Soft and Hard Lath Packets Enhance Ductility in Martensitic Steels

Martensitic steels are widely used structural materials with outstanding mechanical properties. Their high strength is provided by the non-diffusional phase transformation of fcc γ into thin lamellar bcc plates during fast cooling. Coherency strains between the fcc and bcc lamellae induce large dislocation densities in the range of 1016 m−2, well above the densities attainable by conventional plastic deformation. Using high resolution X-ray line profile analysis, scanning electron microscopy, and hardness tests we show that during tensile deformation when the active Burgers vectors are within the lath plane the lath-packets work soften. On the contrary, when the active Burgers vectors are oblique to the lath-plane the lath-packets work harden. The softening and hardening processes in the differently oriented lath-packets produce a composite of hard and soft components on the length scale of lath-packet size. The stress–strain response of the alloy is discussed in terms of the different mean free paths and the different annihilation lengths of dislocations in the softened and hardened lath-packets. The relatively good ductility is shown to be produced by the composite microstructure induced by plastic strain.

Length of dislocations during band formation of irregular plastic flow

Average dislocation length during the fast regime (formation and development phase) of deformation bands in aluminum alloy AA 3204 undergoing irregular plastic flow was estimated. It has already been demonstrated that the electrons embedded in dislocation core domains (bound states) in slip planes during the regime expose out to specimen surface and create an extra potential. Hence, the potential at a fixed location was monitored in an electrochemical cell containing a dilute electrolyte. From the measured potential the number of the bound states was calculated and an estimation for total number of dislocated atoms considering probable single and double charge states was carried out. Then, using the data in the literature on number of dislocation events per band taken by acoustic emission technique, the average dislocation length during the fast regime was calculated. For probable charge states such as doublets and singlets, the dislocation lengths were respectively found to be 190 nm and 380 nm for the fast regime. Dislocations of such length were thought to be critical for band formation events of an irregular plastic deformation.

Informed consent for surgery on neck of femur fractures: A multi-loop clinical audit.

BackgroundThe Montgomery case in 2015 resulted in a pivotal change in practice, leading to a patient-centric approach for informed consent. Neck of femur (NOF) fractures are associated with a high rates perioperative morbidity and mortality. Using guidelines highlighted by the British Orthopaedic Association we performed a multi-loop audit within our department to assess the adequacy of informed consent for NOF fractures.MethodsTwo prior cycles had been performed utilising a similar framework. Prior interventions included ward posters, verbal dissemination of information at Junior Doctor's (JD) induction and amendments to the JD handbook. For the latest audit loop, a retrospective analysis of 100 patients was performed. Risk were classified as common, less common, rare and ‘other’ non-classifiable risks. The adequacy of informed consent was evaluated by assessing the quality and accuracy of documentation in the signed Consent Form-1s for compos mentis patients.ResultsInfection, bleeding risks, clots and anaesthetic risks were documented in all patients (100%). Areas of improvement included documentation of neurovascular injuries (98%), pain (75%) and altered wound healing (69%). There was no significant change in the documentation of failure of surgery (83%) and neurovascular injuries (98%). Poorly documented risk factors included mortality (21%), prosthetic dislocation (14%) and limb length discrepancy (6%).ConclusionFollowing the latest cycle, the trust has now approved the use of 2 consent-specific stickers (for arthroplasty or fixation), amendable on a patient-to-patient basis. As part of the multi-loop process, the cycle will be repeated every year, in line with Junior Doctor rotations. Medical professionals have an ethical, moral and legal obligation to ensure they provide all information regarding surgical interventions to aid patients in making an informed decision.

Recovery kinetics in high temperature annealed AlN heteroepitaxial films

Based on the experimental dislocation annihilation rates, vacancy core diffusion-controlled dislocation climb was found as a dominant recovery mechanism in high temperature annealing of AlN heteroepitaxial films. Dislocation annihilation mechanisms via dislocation glide (with or without kinks) and vacancy bulk diffusion were found to be less significant. Cross-slip was also ruled out as a possible mechanism as a majority of dislocations in heteroepitaxial AlN films are threading edge dislocations. While dislocation climb through both vacancy bulk and core diffusion could offer a plausible explanation of the recovery process, the activation energy for the vacancy core diffusion-controlled dislocation climb was relatively low (4.3 ± 0.1 eV), as estimated from an Arrhenius plot. The validity of the vacancy core diffusion mechanism was also supported by a large vacancy mean free path (∼240 nm), which was comparable to the sample thickness and thus the average dislocation length. Finally, the experimentally observed dislocation density reduction as a function of the annealing temperature and time was in good agreement with the vacancy core diffusion mechanism.

Atomistic Study of Interactions between Intrinsic Kink Defects and Dislocations in Twin Boundaries of Nanotwinned Copper during Nanoindentation.

In order to study the effects of kink-like defects in twin boundaries on deformation mechanisms and interaction between dislocations and defects in twin boundaries under localized load, nanotwinned Cu with two defective twin (TDT) boundaries is compared with the nanotwinned Cu with two perfect twin (TPT) boundaries, and nanotwinned Cu with single defective twin (SDT) boundary and single perfect twin boundary by simulating spherical nanoindentations using molecular mechanics. The indenter force-depth and hardness-contact strain responses were analyzed. Results show that the existence of intrinsic defects in twin boundary could reduce the critical load and critical hardness of nanotwinned material. A quantitative parameter was first proposed to evaluate the degree of surface atom accumulation around the indenter during nanoindentation, and it can be inferred that the surface morphology in TDT changes more frequently than the surface morphologies in TPT and SDT. The atomistic configurations of incipient plastic structures of three different models were also analyzed. We found that the intrinsic defects in twin boundary will affect the incipient plastic structures. The formation of twinning partial slip on the defective twin boundary happens before the contact of the dislocation and twin boundary. The kink-like defects could introduce Frank partial dislocation to the twin boundary during interaction between dislocation and twin boundary, which was not detected on the perfect twin boundary. In addition, the area of twinning partial slips on the upper twin boundary in the incipient plastic structures in SDT and TDT are larger than the twinning partial slip area in TPT, which results in the reduction of the critical hardness in SDT and TDT. The kink-like defects could also block the expansion of twinning partial slip on the twin boundary. Furthermore, we investigated the dislocation transmission processes in three different models. It is found that the dislocation transmission event could be delayed in model containing single defective twin boundary, while the transmission process could be advanced in model containing two consecutive defective twin boundaries. The quantitative analysis of dislocation length was also implemented. Result shows that the main emitted dislocation during nanoindentation is Shockley partial, and the dislocation nucleation in SDT and TDT is earlier than the dislocation nucleation in TPT due to the existence of defects. It is inferred that the intrinsic defects on twin boundaries could enhance the interaction between dislocations and twin boundaries, and could strongly change the structure evolution and promote the dislocation nucleation and emission. These findings about kink-like defects in twin boundaries show that the inherent kink-like defects play a crucial role in the deformation mechanisms and it should be taken into consideration in future investigations. Single defective twin boundary structure is recommended to delay the transmission and block the expansion of twin boundary migration. Some of the results are in good agreement with experiments.