Dislocation Character Research Articles

A comparison of different continuum approaches in modeling mixed-type dislocations in Al

Mixed-type dislocations are prevalent in metals and play an important role in their plastic deformation. Key characteristics of mixed-type dislocations cannot simply be extrapolated from those of dislocations with pure edge or pure screw characters. However, mixed-type dislocations traditionally received disproportionately less attention in the modeling and simulation community. In this work, we explore core structures of mixed-type dislocations in Al using three continuum approaches, namely, the phase-field dislocation dynamics (PFDD) method, the atomistic phase-field microelasticity (APFM) method, and the concurrent atomistic-continuum (CAC) method. Results are benchmarked against molecular statics. We advance the PFDD and APFM methods in several aspects such that they can better describe the dislocation core structure. In particular, in these two approaches, the gradient energy coefficients for mixed-type dislocations are determined based on those for pure-type ones using a trigonometric interpolation scheme, which is shown to provide better prediction than a linear interpolation scheme. The dependence of the in-slip-plane spatial numerical resolution in PFDD and CAC is also quantified.

A New Phenomenological Approach for Modeling Strain Hardening Behavior of Face Centered Cubic Materials

A new methodology for analyzing strain hardening behavior of face centered cubic materials based on transition from restricted glide/single slip to multiple slip has been developed. The proposed modification considers strain dependence of orientation factor spanning between lower bound iso-stress Sachs model and upper bound iso-strain Taylor model. The modifications are suitably incorporated in the classical two internal variable model to develop a new slip activity based strain hardening model. The proposed model is shown to be performing better than the existing one parameter forest strengthening model and two internal variable model in predicting strain hardening behavior in the presence of wide range of microstructural features such as solutes, semi-coherent and incoherent precipitates, grain size and twins. Experimental validation of the proposed concept of transition in slip behavior is shown in terms of evolution of dislocation density and character from X-ray diffraction and surface roughness, slip lines and micro-texture from in-situ electron back scatter diffraction tests.

The Epidemiology of Sports Related Dislocations among Collegiate and High School Athletes

Athletes are at risk of sustaining a dislocation, ranging from relatively minor with minimal time lost from play to more severe with long recovery times and costly treatments. Previous studies have examined dislocations; however, most focused on specific sports or joints. PURPOSE: To describe dislocations sustained by collegiate and high school athletes. METHODS: Athletic trainers (ATs) participating in the National Collegiate Athletic Association (NCAA) Injury Surveillance Program reported athlete-exposure (AE) and injury data for 25 sports during the 2009/10-2016/17 academic years. ATs participating in the National Athletic Treatment Injury and Outcomes Network (NATION) reported AE and injury data for 27 sports during 2011/12-2013/14. Dislocations occurred during a school-sanctioned practice or competition and required medical attention. Along with dislocation characteristics, injury rates per 10,000 AEs were reported with 95% confidence intervals (CIs). RESULTS: From 2009/10-2016/17, ATs reported 542 dislocations among NCAA athletes for an injury rate of 0.81/10,000 AEs (95% CI:0.74-0.87). Men’s football (1.74; 95% CI:1.53-1.94), men’s wrestling (1.53; 95% CI:0.91-2.16), and women’s gymnastics (1.31; 95% CI:0.54-2.09) had the highest rate of dislocations. From 2011/12-2013/14, ATs reported 149 dislocations among high school athletes for an injury rate of 0.29/10,000 AEs (95% CI:0.24-0.34). Boys’ football (0.79; 95% CI:0.62-0.95), girls’ basketball (0.52; 95% CI:0.26-0.78), and boys’ wrestling (0.50; 95% CI:0.22-0.79) had the highest rate of dislocations. For NCAA and high school athletes, the most commonly dislocated body parts were the hand/finger (40.2% and 25.8%) and shoulder/clavicle (36.7% and 40.3%). A greater proportion of dislocations resulted from player contact (55.8%) and were non-time loss (40.9%) in NCAA athletes than high school athletes (39.6% and 25.9%). A similar proportion of NCAA and high school dislocations were recurrent (20.6% and 18.8%) and required surgery (21.4% and 15.8%). CONCLUSION: Some characteristics of dislocations vary across collegiate and high school athletes including mechanism of injury and time loss. More research is needed to determine how to effectively reduce the incidence of dislocations among all collegiate and high school athletes.

TEM investigation of the influence of dose rate on radiation damage and deuterium retention in tungsten

Tungsten targets were first irradiated with high energy W6+ ions (20 MeV) with different dose rates at 800 K and then loaded with deuterium at room temperature to decorate the displacement damage created in the first step. Detailed microstructure investigations were performed and compared with calculated damage profiles and deuterium depth profiles to link defect characteristics with the ability to retain hydrogen isotopes. Results directly indicate that depending on the used dose rate dislocation density and characteristics change. However, deuterium retention does not. Thus, we conclude that overall dislocation density does not affect deuterium retention significantly. Changes in D retention were observed as function of the depth of the damaged zone. Variations of dislocation density were also depth dependent. Consequently, we link changes in D retention with the dislocation presence, number of implanted tungsten ions and other point defects. The latter seem to affects D retention the most.

Revealing the atomistic nature of dislocation-precipitate interactions in Al-Cu alloys

Despite significant gains on understanding strengthening mechanisms in precipitate strengthened materials, such as aluminum alloys, there persists a sizeable gap in the atomistic understanding of how different precipitate types and their morphology along with dislocation character affects the hardening mechanisms. Toward this, the paper examines nature of precipitation strengthening behavior observed in the Al-Cu alloys using atomistic simulations. Specifically, the critical resolved shear stress is quantified across a wide range of dislocation-precipitate interactions scenarios for both θ′ and θ phase of Al2Cu. Overall, the simulations reveal that the dislocation character (edge or screw) plays a key role in determining the predominant hardening mechanism (shearing vs. Orowan looping) employed to overcome the θ′ Al2Cu precipitate. Furthermore, the critical shear stress and mechanism to overcome the precipitate is sensitivity to the position of the glide plane with respect to the precipitate and its orientation. Interestingly in our findings, the θ Al2Cu precipitate conventionally regarded as un-shearable particle was overcome by shear cutting mechanism for small equivalent precipitate radius, which agrees with recent TEM observations. These findings provide necessary information for the development of atomistically informed precipitate hardening models for the traditional continuum scale modeling efforts.

Dislocation Substructure Analysis in γ′-Precipitated Ni-Based Alloy by X-Ray Diffraction Combined with Positron Annihilation

The systematic changes in the dislocation density and characteristics of γ′-precipitated Ni-based model alloys that develop under cold rolling are studied as simulated deformations, to examine the fundamental dislocation behavior in terms of the dislocation substructure formation. In particular, the dislocation density is quantified through X-ray line profile analysis (XLPA), which is effective for quantifying the dislocation density and characteristics, as well as positron annihilation lifetime (PAL) measurements, which are sensitive to vacancy-type lattice defects. Similar tendencies are obtained for the strain dependency of the dislocation density analyzed using XLPA and PAL. Hence, the influence of the γ/γ′ coherent interface and γ′ precipitation on the dislocation substructure and vacancies is shown by comparing with a Ni-Cr solid solution. These results help to understand the interaction of solute atoms, vacancies, and dislocation with regard to substructure formation in Ni-based alloys.

Disconnection description of triple-junction motion

Grain boundary (GB) migration in polycrystalline materials necessarily implies the concurrent motion of triple junctions (TJs), the lines along which three GBs meet. Today, we understand that GB migration occurs through the motion of disconnections in the GB plane (line defects with both step and dislocation character). We present evidence from molecular dynamics grain growth simulations and idealized microstructures that demonstrates that TJ motion and GB migration are coupled through disconnection dynamics. Based on these results, we develop a theory of coupled GB/TJ migration and use it to develop a physically based, disconnection mechanism-specific continuum model of microstructure evolution. The continuum approach provides a means of reducing the complexity of the discrete disconnection picture to extract the features of disconnection dynamics that are important for microstructure evolution. We implement this model in a numerical, continuum simulation and demonstrate that it is capable of reproducing the molecular dynamics (MD) simulation results.

Clinical and Radiological Predictors of Functional Outcome After Isolated Medial Patellofemoral Ligament Reconstruction at Midterm Follow-up

Background: Isolated medial patellofemoral ligament reconstruction (iMPFLR) is increasingly used for the surgical treatment of recurrent patellofemoral instability. Purpose: The purpose of this study was to identify the clinical and radiological predictors that can significantly influence the functional outcomes after an iMPFLR. Study Design: Case series; Level of evidence, 4. Methods: A total of 107 patients (112 ligament reconstructions) who underwent iMPFLR were evaluated with a mean ± SD follow-up of 59 ± 33 months (range, 12-123 months). Functional International Knee Documentation Committee (IKDC) and Kujala scores were assessed preoperatively and every 12 months. Radiological assessment of patellar height and tilt (Laurin angle, Merchant angle, Maldague classification) and computed tomography (CT) scan measurement of patellar tilt (contracted and relaxed quadriceps) and tibial tubercle–trochlear groove distance were performed preoperatively and at 6 months. Femoral tunnel position was assessed following the criteria formulated by Schöttle. The amount of femoral tunnel widening was measured by means of 3-dimensional CT scan at 6 months. Predictors were determined from univariate and multivariate regression analyses integrating clinical and radiological criteria pre- and postoperatively. The dependent variable was defined as the difference between pre- and postoperative scores. Results: Between pre- and postoperative measurement at last follow-up, a significant improvement for IKDC and Kujala functional scores was observed (Kujala: 57 ± 11.3 to 87 ± 12.9, P < .001; IKDC: 47.8 ± 13.1 to 79 ± 15.8, P < .001). Demographics (age, body mass index, sex), dislocation characteristics (number of dislocations, time between first dislocation and surgery, age at first dislocation, mechanism of first dislocation, knee side), clinical data (frontal limb alignment, hyperlaxity, recurvatum, pre- and postoperative range of motion), and complications (quadriceps atrophy, complex regional pain syndrome) did not influence functional scores. The predictors of lower improvement in functional scores included small correction of the patellar tilt reported on the CT scan measurement, malpositioning of the femoral tunnel, and a widening of this tunnel near the medial cortex. Malpositioning of the femoral tunnel was correlated with tunnel widening, and patients with anterior and proximal malpositioning experienced stiffness in flexion. Conclusion: Overall, iMPFLR demonstrated good outcomes. Predictors influencing the functional results were identified. Less improvement in clinical outcome was reported for patients with a high preoperative patellar tilt and only a small correction in tilt and for those who had femoral tunnel malpositioning, which was correlated with tunnel widening.

Clinical characteristics of arytenoid dislocation in patients undergoing bariatric/metabolic surgery: A STROBE-complaint retrospective study.

Tracheal intubation and the use of a large-bore calibrating orogastric (OG) tube have been reported to increase the incidence of arytenoid dislocation (AD) in patients undergoing bariatric/metabolic surgery. This study aimed at identifying the clinical characteristics of this patient subgroup.We retrospectively examined the clinical characteristics of 14 patients with AD (study group) who received tracheal intubation and OG insertion for bariatric/metabolic surgery between 2011 and 2016. For comparison, another group of 19 patients with postoperative AD collected from published literature and 3 patients from the authors’ institute served as controls in whom only tracheal intubation was performed. Information on patient characteristics, anesthetic time, symptoms, time of symptom onset, intervention, and postinterventional impact on vocalization of the 2 groups were collected and compared.Patients in the study group were younger than those in the control group (38 [25–60] vs 54.5 [19–88] years, P = .03). Compared with the control group, anesthetic time (282.5 [155–360] vs 225 [25–480] minutes, P = .041) was longer and symptom onset (1.0 [0–6] vs 1.0 [0–6] days, P = .018) was more delayed in the study group. After closed reduction, the frequency of voice recovery was comparable in both groups in a time interval of 12 weeks (84.6% vs 92.9%, P = .59).Our report demonstrates that the clinical characteristics of patients with AD who received tracheal intubation and OG insertion for bariatric/metabolic surgery were different from those with postoperative AD receiving only tracheal intubation, highlighting the importance of implementing individualized strategies for AD prevention in this patient population.

Identification of dislocation characteristics in Na-flux-grown GaN substrates using bright-field X-ray topography under multiple-diffraction conditions

Basal-plane and threading dislocations in multipoint-seed Na-flux-grown GaN single crystals are characterized in terms of Burgers vectors mainly by using bright-field X-ray topography under multiple-diffraction conditions. The technique, combined with a CMOS camera system with high spatial resolution, can provide topographic images of the dislocations with a relatively high dislocation density (up to approximately 5×105cm−2). It is possible to directly determine the Burgers vector of individual dislocations based on invisibility criteria. From the present experiment, it is found that almost all basal-plane dislocations have a-type Burgers vectors, and threading dislocations have a- and (a + c)-type Burgers vectors.

PNADIS: An automated Peierls–Nabarro analyzer for dislocation core structure and slip resistance

Dislocation is one of the most critical and fundamental crystal defects that dominate the mechanical behavior of crystalline solids, however, a quantitative determination of its character and property in experiments is quite challenging and limited so far. In this paper, a fully automated Peierls–Nabarro (P–N) analyzer named PNADIS is presented; a complete set of the character and property of dislocation can be automatically derived, including the dislocation core structure, Peierls energy and stress, pressure field around dislocation core, solute/dislocation interaction energy, as well as the energy barrier and increase in critical-resolved shear stress at 0 K for solid solution strengthening. Furthermore, both one-dimensional (1D) and two-dimensional (2D) P–N models are implemented to meet the demand to analyze the character and property of dislocation for not only simple FCC and HCP structures but also complex crystals. The implementation of this code has been critically validated by a lot of evaluations and tests including 1D P–N model for complex crystals, 2D P–N model for FCC and HCP metals, pressure field around dislocation core, and solid solution strengthening for alloys. We expect that the automated feature of this code would provide a high-efficiency solution for determining the character and property of dislocation. Program summaryProgram title: PNADISProgram Files doi:http://dx.doi.org/10.17632/whk6wdy3nn.1Licensing provisions: GNU General Public License 3Programming language: MATLABNature of problem: To determine automatically the character and property of dislocation, including dislocation core structure, Peierls stress, pressure field around dislocation core and solid solution strengthening, for not only FCC and HCP structures but also complex crystals.Solution method: The generalized stacking fault energy is firstly fitted by Fourier expansion, and meanwhile an appropriate trial function of disregistry vector is chosen. Afterwards, a least square minimization of the difference between elastic resistance and restoring force for one-dimensional Peierls–Nabarro model, or a global minimization of the total dislocation energy via particle swarm optimization or genetic algorithm for two-dimensional Peierls–Nabarro model, will be performed to determine the dislocation core structure of complex crystals, or FCC and HCP structures. Finally, the Peierls stress, pressure field around dislocation core and solid solute strengthening are derived from the calculated dislocation core structure.

Influences of solute concentration on uniform elongation and dislocation characteristics of Al–Mg and Al–Si alloys

Influences of solute concentration on uniform elongation and dislocation characteristics of Al–Mg and Al–Si alloys

Properties of Dislocation Drag from Phonon Wind at Ambient Conditions.

It is well known that, under plastic deformation, dislocations are not only created but also move through the crystal, and their mobility is impeded by their interaction with the crystal structure. At high stress and temperature, this “drag” is dominated by phonon wind, i.e., phonons scattering off dislocations. Employing the semi-isotropic approach discussed in detail in a previous paper (J. Phys. Chem. Solids 2019, 124, 24–35), we discuss here the approximate functional dependence of dislocation drag B on dislocation velocity in various regimes between a few percent of transverse sound speed and (where is the effective average transverse sound speed of the polycrystal). In doing so, we find an effective functional form for dislocation drag for different slip systems and dislocation characters at fixed (room) temperature and low pressure.

Slip transmission for dislocations across incoherent twin boundary

We performed molecular dynamic simulations to elucidate the transmission mechanisms of ½〈110〉 dislocation across a Σ3{112} incoherent twin boundary (ITB). There are the certain relations between incoming and outcoming dislocations, determined by the dislocation structure of the ITB and the character of incoming dislocation. A screw dislocation can cross-slip onto the ITB and push out the pre-existing interface dislocation into the twin. The Burgers vector of the incoming dislocation determines the emitted dislocation and corresponding glide plane. A mixed dislocation can only directly cross the ITB when its leading partial reacts with pre-existing interface dislocation to produce a full dislocation.

Insights into image contrast from dislocations in ADF-STEM

Competitive mechanisms contribute to image contrast from dislocations in annular dark-field scanning transmission electron microscopy (ADF-STEM). A clear theoretical understanding of the mechanisms underlying the ADF-STEM contrast is therefore essential for correct interpretation of dislocation images. This paper reports on a systematic study of the ADF-STEM contrast from dislocations in a GaN specimen, both experimentally and computationally. Systematic experimental ADF-STEM images of the edge-character dislocations reveal a number of characteristic contrast features that are shown to depend on both the angular detection range and specific position of the dislocation in the sample. A theoretical model based on electron channelling and Bloch-wave scattering theories, supported by numerical simulations based on Grillo's strain-channelling equation, is proposed to elucidate the physical origin of such complex contrast phenomena.

Reduction of the Lorenz Number in Copper at Room Temperature due to Strong Inelastic Electron Scattering Brought about by High-Density Dislocations.

In this work, heat and charge transport were measured in a series of deformed bulk Cu samples where dislocation density was tuned but dislocation character generally remained unchanged. We observed a notable violation of the Wiedemann-Franz law at room temperature for such a conventional metal. We show that high-density dislocations introduce strong inelastic electron scattering, which relax heat and charge currents to different extents. A reduction of Lorenz number by 15% was observed. We reveal that the contribution from elastic scattering to the incremental thermal resistivity scarcely varies with dislocation density, but the contribution due to inelastic scattering monotonically increases and becomes overwhelmingly dominant for dislocation density above 1.0 × 1015 m-2.

Effect of {10−12} twinning on the deformation behavior of AZ31 magnesium alloy

Twinning process and dislocation characteristics were determined by means of quasi in-situ electron backscatter diffraction and high-resolution transmission electron microscopy techniques in a rolled AZ31 magnesium alloy under different strain paths (compression along rolling direction and tension along normal direction). It is demonstrated that the activation of different twin variants depends on the strain path, and thus these twin variants can further induce different texture characteristics. In addition, two independent slip systems were activated during deformation of both strain paths. However, the dislocation slip modes of the two kinds of deformation are different, resulting in different stress-strain response.

Relationship among Elongation, Work Hardening Behavior and Dislocation Characteristics of Al–Mg–Si Series Alloys

Relationship among Elongation, Work Hardening Behavior and Dislocation Characteristics of Al–Mg–Si Series Alloys

An approach in the analysis of microstructure of proton irradiated T91 through XRDLPA using synchrotron and laboratory source

The changes in the microstructure of 3.5 MeV proton irradiated T91 Ferritic-Martensitic steel samples with dose have been evaluated using detailed X-ray diffraction line profile analysis of the data collected using both laboratory and the synchrotron source. Different line profile analysis techniques like Williamson-Hall, modified Rietveld method and convolutional multiple whole profile fitting have been applied to evaluate the microstructural parameters such as domain size, microstrain, dislocation density and the character of the dislocation from both the data. The coherent domain size decreases and the corresponding microstrain increases at the first dose of irradiation. With further irradiation, these parameters show a slight recovery and eventually tend towards saturation with increasing dose. The dislocation density follows similar trend as the microstrain showing saturation at higher doses of irradiation in both the XRD data. However, a systematic change in the character of the dislocation from screw-type to edge-type could be ascertained only from the data obtained using synchrotron source. The Vickers microhardness of the samples is found to increase continuously with increasing irradiation dose supporting the observed change in the dislocation character from screw-type (glissile) to edge-type (sessile). Both EBSD and TEM analysis of the unirradiated and irradiated samples have supported the results obtained from the XRD analysis.

Atomistic insight into the dislocation nucleation at crystalline/crystalline and crystalline/amorphous interfaces without full symmetry

Misfit dislocations at bimetal interfaces play a decisive role in determining various deformation behaviors by carrying the shear sliding, serving as a barrier for dislocation transmission and a source of dislocation nucleation. However, when the interface does not possess the distinct feature of misfit dislocations, the nucleation mechanism of lattice dislocations at the interfaces cannot be simply quantified by previously developed atomistic mechanisms based on characteristic misfit dislocations. Using crystalline/crystalline interfaces with a large lattice mismatch and crystalline/amorphous interfaces without local symmetry as prototypes, we show for the first time that the dislocation nucleation at such interfaces is attributable to the localized strain heterogeneities by modifying the volumetric and shear strain components at the atomic level to mechanically respond to different loadings. Using atomic strain tensor analysis, we found that in-plane localized shearing plays a critical role in the emission of lattice dislocations from interfaces, while the corresponding normal components of the volumetric strain tensor will dominate the character of the nucleated lattice dislocation by modifying the atomic excess volume at the interface to overcome the barrier to dislocation nucleation. Further exploration of various crystalline/amorphous interfaces by varying the chemical composition of the amorphous side indicates that chemical heterogeneity may substantially change the strain heterogeneity by forming a different clustered structure at the interface, resulting in the preferred choice of nucleation sites at the boundary regions that can be defined as nano shear traces (NSTs). These results provide a foundation to investigate the effects of strain and chemical heterogeneities in order to provide a realistic explanation of interface mediated deformation mechanisms and an efficient solution to tune interface dominated plasticity.