On the mechanism of anomalous slip in bcc metals Hsiung, Luke L.
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
11/2010, Letnik:
528, Številka:
1
Journal Article
Recenzirano
Odprti dostop
▶ Understanding the anomalous slip behavior of bcc metals. ▶ TEM observation of coplanar screw-dislocation arrays. ▶ TEM analysis for the formation of 〈0
1
0〉-type junction dislocations. ▶ A novel ...mechanism proposed for the anomalous slip in bcc metals.
The anomalous-slip behavior of bcc metals has been studied by TEM analyses of dislocation substructures developed in a
2
¯
9
20
-oriented
Mo single crystal uniaxially compressed at room temperature to a total-strain of 0.4%. It is found that the initial dislocation lines in association with “grown-in” super-jogs can act as effective sources for the formation of both
a
0/21
1
1 (Schmid factor
=
0.5) and
a
0
/
2
1
1
¯
1
(Schmid factor
=
0.167) coplanar screw dislocation arrays in the
(
1
¯
0
1
)
primary slip plane. The interaction between the multiplied
a
0/21
1
1 dislocations and pre-existing
a
0
/
2
1
1
¯
1
dislocation segments, which block the motion of the
a
0/21
1
1 dislocations, renders the multiplication of
a
0
/
2
1
1
¯
1
dislocations and leads to the formation of
a
0/21
1
1 and
a
0
/
2
1
1
¯
1
dislocation arrays on the
(
1
¯
0
1
)
primary slip plane. The occurrence of
{
0
1
¯
1
}
anomalous slip is accordingly proposed to be resulting from the mutual trapping of
a
0/21
1
1 and
a
0
/
2
1
1
¯
1
coplanar dislocation arrays on the
(
1
¯
0
1
)
primary slip plane, which renders a cross-slip propagation of both
a
0/21
1
1 and
a
0
/
2
1
1
¯
1
screw dislocations from the
(
1
¯
0
1
)
plane onto the
{
0
1
¯
1
}
planes and thus activates the
{
0
1
¯
1
}
〈1
1
1〉 slip systems.
This paper presents for the first time an analytical solution to the boundary-value problem in the theory of elasticity for a circular prismatic dislocation loop (PDL) coaxial to a hollow cylindrical ...channel in an elastically isotropic infinite matrix. The stress fields and energy of the PDL are calculated and analyzed in detail. Based on the solution, a theoretical model for the misfit stress relaxation through the formation of a misfit PDL around a misfitting nanotube embedded in an infinite matrix is suggested. The critical radii of the embedded nanotube are found and discussed. It is shown that, for thin nanotubes prepared by nanolayer growth on the initial channel surface, there are two critical inner radii of the nanotube, between which the formation of a misfit PDL is energetically favorable.
•Circular prismatic dislocation loop coaxial to a hollow channel in a matrix.•Stress fields and energy of the loop are calculated and analyzed in detail.•Modeling the misfit stress relaxation in nanotube embedded to a matrix.•Critical radii of the embedded nanotube are found and discussed.
X‐Ray diffraction microstrain characterization is a technique which enables the quantification of threading dislocations by measuring the radial microstrain field surrounding these defects. This work ...demonstrates how to accurately perform these measurements on a broad range of GaN samples. In particular, for GaN grown on Si substrates with large stress gradient through the layer, the measurements need to be performed on the (20–25) reflection to avoid being affected by a macrostrain influence. A comparison of this characterization with cathodoluminescence and X‐ray diffraction lattice misorientations measurements shows differences in the measurement depth, so the results are affected by the various evolutions of dislocations densities through the layers of the different samples. Independently of the measurement depth, the analysis also highlights a dependence of the X‐ray measurements results on the type of sample, in particular the starting substrate. Microstrain measurements appear therefore as being well suited for the comparison of threading dislocations densities between similar GaN layers. It can be used to replace lattice misorientations measurements, or be associated with them to obtain an edge versus screw dislocations ratio.
This work presents an X‐ray diffraction measurement of threading dislocations densities, based on the surrounding microstrain field analysis. It demonstrates how to accurately characterize different gallium nitride samples, including specimens grown on silicon substrates with large stress gradient through the layer. A comparison between microstrain, lattice misorientations, and cathodoluminescence measurements highlights the potential and the issues of this technique.
The biomechanical relationship between the lumbar spine and the hip is well-documented. It follows that fusing the lumbar spine would have implications on the outcomes of total hip arthroplasty ...(THA). This study aimed to determine the effect of preexisting lumbar spinal fusion surgery on the outcomes of THA by synthesizing the available evidence via systematic review and meta-analysis.
A systematic review with meta-analysis was performed in accordance to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Electronic searches were performed in 6 different databases for studies comparing outcomes in patients after THA with or without a history of lumbar fusion. Studies were required to report at least one outcome out of dislocation, revision due to hip instability or patient-reported outcomes.
Patients with a history of lumbar spinal fusion are at a significantly increased risk of dislocation (relative risk 2.03, P < .00001) and revision (relative risk 3.36, P = .006) after THA. Patient-reported outcomes were also poorer in patients with prior lumbar fusion compared with those without, although meta-analysis could not be performed due to heterogeneity in the outcome measure used between studies.
Previous lumbar spinal fusion increases risk of dislocation and revision, and may negatively impacts patient-reported outcomes after THA. Orthopaedic surgeons should pay particular attention to these patients and could use patient-specific planning, instrumentation, and targeted counselling to optimize clinical and subjective outcomes. Future studies could clarify the impact of prior fusion on patient-reported outcomes after THA.
Abstract Background context Occipitocervical dislocations involve translations of the craniocervical joints. The relative contributions of each ligament to overall stability and the effects of the ...occipitoatlantal joint capsules on the pathologic translation are unknown. Although incidences of occipitocervical dislocations are rare after blunt trauma, they are usually fatal. When patients do survive these dislocations, the proper diagnosis is difficult, which in turn may increase the fatality rate. A biomechanical model may provide a greater pathologic understanding of craniocervical subluxation. Purpose The purpose of the study is to build an in vitro biomechanical model to determine which stabilizing ligament(s) of the craniocervical junction are most important in restraining rotation and translations during these rotations. This may guide clinical diagnosis, which could assist in treatment options. Study design/Setting The study design includes a biomechanical cadaveric test. Methods Seven cadaveric specimens were tested using a 6-degree-of-freedom spine simulator under the following conditions: intact, clivus/alar removal (CR), transverse ligament destruction (TLD), occipitoatlantal (OA) joint capsulotomyoccipitoatlantal (OA) joint capsulotomy (C0–C1 JC), and C1–C2 joint capsulotomy (C1–C2 JC). Flexion-extension (FE), lateral bending (LB), and axial rotation (AR) were applied (2.5 Nm) to a C0–C2 segment, whereas anterior-posterior (AP) and cranial-caudal (CC) translations were recorded. Average motions were normalized to intact (100%) for each joint. Results Increases in C0–C1 angular and translational motions occurred after TLD and C0–C1 JC. At the atlantoaxial joint, there were significant (p<.05) increases from intact in FE (TLD=154%, C0–C1 JC=174%) and in AR (TLD=178%, C0–C1 JC=224%). Anterior-posterior translation during applied LB increased significantly after TLD (248% intact). Cranial-caudal translation during applied FE increased significantly after TLD (188%) and C0–C1 JC (361%). Increases in C1–C2 angular motion occurred after TLD and C1–C2 JC and in translation after CR and TLD. At the C1–C2 joint, there were significant increases from intact in FE (TLD=172%, C1–C2 JC=160%) and in LB (TLD=286%, C1–C2 JC=332%); in AR, there were no statistical differences. Anterior-posterior translation increased significantly after CR (280%). Cranial-caudal translation also increased significantly after CR (205%) and TLD (298%) during LB. Conclusions Transverse and alar ligaments appear to be the main stabilizers of the craniocervical junction. The vertical structures attached to the clivus and OA joint capsules function as secondary stabilizers. Craniocervical dislocations seem to affect FE and lateral bending the most, whereas increased translation seems to occur primarily in the AP and CC directions. Models of craniocervical trauma should section all three restraining structures for the future studies.
The objectives of this study were to compare syndesmosis dislocation and ankle ligament stress after the fixation of the posterior malleolus fracture (PMF) with four different techniques by Finite ...Element Analysis (FEM).
Four internal fixation techniques used for fixation of PMF were assessed by FEM: posterior one-third tubular 3.5 mm buttress plate (PP) with one screw (PP 1 screw), PP with two screws (PP 2 screws), two cannulated 3.5 mm lag screws in the anteroposterior (AP) direction (AP lag screws), and two posteroanterior (PA) cannulated 3.5 mm lag screws (PA lag screws). PMF with 30% fragment size was simulated through computational processing reconstructed from computed tomography (CT). The simulated loads of 700 N and 1200 N were applied to the proximal tibial end. The FEM evaluated the syndesmosis dislocation (mm) and stress values of the posterior tibiofibular ligament (PTFL) (in Kpa) and deltoid ligament (in Kpa) in the four mentioned subgroups.
We found that with a load of 700 N, syndesmosis dislocation varied from 6.5 to 7.9 mm, being the lowest and greatest for PA lag screw and PP 1 screw, respectively. In all groups was observed a greater dislocation in the syndesmosis at 1200 N of load. We observed that the stress values on the PTFL were lower for AP lag screws and PP 2 screws with 700 N and 1200 N, respectively. For both loads, PP 1 screw presented the greatest stress. Regarding the stress in the deltoid ligament, the AP lag screws presented the lowest stress for 700 N and PP 1 screw for 1200 N. For all fixation techniques, the syndesmosis displacement and ligament stresses were higher when 1200 N were imposed.
This study demonstrated that PMF fixed with lag screws presents greater stability in the distal tibiofibular syndesmosis and higher joint loadings promoted greater displacement and ligaments stress, regardless of the fixation technique. Besides, lower stress in the syndesmosis is accompanied by a greater load on the deltoid ligament.
The use of low-temperature (LT) GaAs layers as dislocation filters in GaAs/Si heterostructures (HSs) was investigated in this study. The effects of intermediate LT-GaAs layers and of the post-growth ...and cyclic in situ annealing on the structural properties of GaAs/LT-GaAs/GaAs/Si(001) HSs were studied. It was found that the introduction of LT-GaAs layers, in combination with post-growth cyclic annealing, reduced the threading dislocation density down to 5 × 10
cm
, the root-mean-square roughness of the GaAs surface down to 1.1 nm, and the concentration of non-radiative recombination centers in the near-surface GaAs/Si regions down to the homoepitaxial GaAs level. Possible reasons for the improvement in the quality of near-surface GaAs layers are discussed. On the one hand, the presence of elastic deformations in the GaAs/LT-GaAs system led to dislocation line bending. On the other hand, gallium vacancies, formed in the LT-GaAs layers, diffused into the overlying GaAs layers and led to an increase in the dislocation glide rate. It was demonstrated that the GaAs/Si HSs obtained with these techniques are suitable for growing high-quality light-emitting HSs with self-assembled quantum dots.
Cervical pedicle screw fixation is an effective procedure for stabilising an unstable motion segment; however, it has generally been considered too risky due to the potential for injury to ...neurovascular structures, such as the spinal cord, nerve roots or vertebral arteries. Since 1995, we have treated 144 unstable cervical injury patients with pedicle screws using a fluoroscopy-assisted pedicle axis view technique. The purpose of this study was to investigate the efficacy of this technique in accurately placing pedicle screws to treat unstable cervical injuries, and the ensuing clinical outcomes and complications. The accuracy of pedicle screw placement was postoperatively examined by axial computed tomography scans and oblique radiographs. Solid posterior bony fusion without secondary dislodgement was accomplished in 96% of all cases. Of the 620 cervical pedicle screws inserted, 57 (9.2%) demonstrated screw exposure (<50% of the screw outside the pedicle) and 24 (3.9%) demonstrated pedicle perforation (>50% of the screw outside the pedicle). There was one case in which a probe penetrated a vertebral artery without further complication and one case with transient radiculopathy. Pre- and postoperative tracheotomy was required in 20 (13.9%) of the 144 patients. However, the tracheotomies were easily performed, because those patients underwent posterior surgery alone without postoperative external fixation. The placement of cervical pedicle screws using a fluoroscopy-assisted pedicle axis view technique provided good clinical results and a few complications for unstable cervical injuries, but a careful surgical procedure was needed to safely insert the screws and more improvement in imaging and navigation system is expected.
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