• The new dislocation density based continuum model is dislocation evolution mechanism based. • Cross slip is taken into consideration in dislocation evolution mechanism. • The defects density is ...incorporated to consider the effect of irradiation. • This model is a breakthrough by bridging the discrete dislocation dynamics and crystal plasticity.
Continuum dislocation dynamics (CDD) with a novel constitutive law based on dislocation density evolution mechanisms was developed to investigate the deformation behaviors of single crystals. The dislocation density evolution law in this model is mechanism-based, with parameters predicted by lower-length scale models or measured from experiments, not an empirical law with parameters back-fitted from the flow curves. Applied on iron single crystal, this model was validated by experimental data and compared with traditional single crystal constitutive models using a Hutchinson-type hardening law or a dislocation-based hardening law. The CDD model demonstrated higher fidelity than other constitutive models when anisotropic single crystal deformation behaviors were investigated. The traditional Hutchinson type hardening laws and other constitutive laws based on a Kocks formulated dislocation density evolution law will only succeed in a limited number of loading directions. The main advantage of CDD is the novel physics-based dislocation density evolution laws in describing the meso-scale microstructure evolution. Another advantage of CDD is on cross-slip, which is very important when loading conditions activate only one primary slip system. In addition to the dislocation hardening, CDD also takes into consideration dislocation defect interactions. Irradiation hardening of iron single crystal was simulated with validation from experimental results.
The density of geometrically necessary dislocations (GND) obtained from the lattice curvature was studied in commercially pure copper up to extreme large strains (von Mises strain of 63). Its ...evolution shows an increase to a maximum at a strain of about 2, then decreases until reaching the stationary limiting stage of grain refinement at a von Mises strain of about 14. At the same time, the total dislocation density is also decreasing. It is shown that the variation in the GND density correlates with the difference between the correlated (first neighbor grains) and the non-correlated (random neighbor) misorientation angle distributions. The low quantity of GND at extreme large strains is a consequence of the near Taylor-type homogeneous behavior of the polycrystalline ultrafine-grained structure.
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This work presents a regularized eigenstrain formulation around the slip plane of dislocations and the resultant non-singular solutions for various dislocation configurations. Moreover, we derive the ...generalized Eshelby stress tensor of the configurational force theory in the context of the proposed dislocation model. Based on the non-singular finite element solutions and the generalized configurational force formulation, we calculate the driving force on dislocations of various configurations, including single edge/screw dislocation, dislocation loop, interaction between a vacancy dislocation loop and an edge dislocation, as well as a dislocation cluster. The non-singular solutions and the driving force results are well benchmarked for different cases. The proposed formulation and the numerical scheme can be applied to any general dislocation configuration with complex geometry and loading conditions.
Solutions to the differential equations of linear elasticity in the continuum limit in arbitrary crystal symmetry are known only for steady-state dislocations of arbitrary character, i.e. line ...defects moving at constant velocity. Troubled by singularities at certain ‘critical’ velocities (typically close to certain sound speeds), these dislocation fields are thought to be too idealized, and divergences are usually attributed to neglecting the finite size of the core and to the restriction to constant velocity. In the isotropic limit, accelerating pure screw and edge dislocations were studied some time ago. A generalization to anisotropic crystals has been attempted for pure screw and edge dislocations only for some special cases. This work aims to fill the gap of deriving a general anisotropic solution for pure screw dislocations applicable to slip systems featuring a reflection symmetry, a prerequisite to studying pure screw dislocations without mixing with edge dislocations. Further generalizations to arbitrary mixed dislocations as well as regularizations of the dislocation core are beyond the scope of this paper and are left for future work.
We aimed to compare the intraoperative and early postoperative clinical outcomes of using an acromioclavicular joint hook plate (AJHP) versus a locking plate (LP) in the treatment of anterior ...sternoclavicular joint dislocation.
Seventeen patients with anterior sternoclavicular joint dislocation were retrospectively analyzed from May 2014 to September 2019. Six patients were surgically treated with an AJHP, and 11 were surgically treated with an LP. Five male and one female patients composed the AJHP group, and nine male and two female patients composed the LP group. The mean age of all patients was 49.5 years.
Reduction and fixation were performed with AJHP or LP in all 17 patients. The mean operative blood loss, operative time, and length of incision in the AJHP group were significantly better than those in the LP group. Shoulder girdle movement of the AJHP group was significantly better than that of the LP group.
This study revealed that AJHP facilitated glenohumeral joint motion, reduced the risk of rupture of mediastinal structures, required a shorter incision, and had lesser blood loss and a shorter duration of operation compared with LP. However, some deficiencies require further improvement.
High-entropy and medium-entropy alloys are presumed to have a configurational entropy as high as that of an ideally mixed solid solution (SS) of multiple elements in near-equal proportions. However, ...enthalpic interactions inevitably render such chemically disordered SSs rare and metastable, except at very high temperatures. Here we highlight the wide variety of local chemical ordering (LCO) that sets these concentrated SSs apart from traditional solvent-solute ones. Using atomistic simulations, we reveal that the LCO of the multi-principal-element NiCoCr SS changes with alloy processing conditions, producing a wide range of generalized planar fault energies. We show that the LCO heightens the ruggedness of the energy landscape and raises activation barriers governing dislocation activities. This influences the selection of dislocation pathways in slip, faulting, and twinning, and increases the lattice friction to dislocation motion via a nanoscale segment detrapping mechanism. In contrast, severe plastic deformation reduces the LCO towards random SS.
•Relationship between dislocation annihilation and dislocation type is revealed.•The cause of almost no annihilation of edge dislocations is explained.•Reason for the failure of AlGaN and AlN layers ...to block dislocations is given.
In this work, we have found that there is a clear difference between the annihilation of screw dislocation and edge dislocation, and the density of edge dislocations remains almost unchanged during the growth from AlN to GaN. Through the transmission electron microscope (TEM), the internal mechanism of this phenomenon has been explored in this paper, which can be attributed to the failure of AlGaN buffer layer and AlN insertion layer to block the edge dislocation. The explanation proposed in this paper may provide new ideas to further reduce the dislocations in GaN-on-Si, improving the crystal quality.
Purpose
The WARPS/STAID classification employs clinical assessment of presenting features and anatomic characteristics to identify two distinct subsets of patients within the patellofemoral ...instability population. The purpose of this study was to further define the specific demographics and the prevalence of risky pathoanatomies in patients classified as either WARPS or STAID presenting with recurrent patellofemoral instability. A secondary purpose was to further validate the WARPS/STAID classification with the Banff Patella Instability Instrument (BPII), the Marx activity scale and the Patellar Instability Severity Score (ISS).
Methods
A convenience sample of 50 patients with recurrent patellofemoral instability, including 25 WARPS and 25 STAID subtype patients, were assessed. Clinical data were collected including assessment of demographic risk factors (sex, BMI, bilaterality of symptoms, affected limb side and age at first dislocation) and pathoanatomic risk factors (TT-TG distance, patella height, patellar tilt, grade of trochlear dysplasia, Beighton score and rotational abnormalities of the tibia or femur). Patients completed the BPII and the Marx activity scale. The ISS was calculated from the clinical assessment data. Patients were stratified into the WARPS or STAID subtypes for comparative analysis. An independent
t
test was used to compare demographics, the pathoanatomic risk factors and subjective measures between the groups. Convergent validity was tested with a Pearson
r
correlation coefficient between the WARPS/STAID and ISS scores.
Results
Demographic risk factors statistically associated with a WARPS subtype included female sex, age at first dislocation and bilaterality. Pathoanatomic risk factors statistically associated with a WARPS subtype included trochlear dysplasia, TT-TG distance, generalized ligamentous laxity, patellar tilt and rotational abnormalities. The independent t test revealed a significant difference between the ISS scores: WARPS subtype (
M
= 4.4, SD = 1.1) and STAID subtype (
M
= 2.5, SD = 1.5);
t
(48) = 5.2,
p
< 0.001. The relationship between the WARPS/STAID and the ISS scores, measured using a Pearson r correlation coefficient, demonstrated a strong relationship:
r
= −0.61,
n
= 50,
p
< 0.001.
Conclusions
This study has demonstrated statistically significant evidence that certain demographics and pathoanatomies are more prevalent in each of the WARPS and STAID patellofemoral instability subtypes. There was no difference in quality-of-life or activity level between the subtypes. The WARPS/STAID score demonstrated convergent validity to the ISS and divergent validity to the BPII score and the Marx activity scale. This study has further validated both the WARPS/STAID classification and the ISS of patients that present with recurrent patellofemoral instability.
Level of evidence
III.
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