Sessile Configuration Research Articles

Sonographic Characteristics of ≥7 mm Gallbladder Polyps: A Retrospective Analysis.

To describe the sonographic characteristics of gallbladder polyps measuring ≥7 mm focusing on echogenicity, correlate with surgical pathology when available, and assess stability in size over time. This retrospective study used a natural language processing application to screen ultrasound (US) reports between January 1, 2012, and December 31, 2020, that contained the words "gallbladder polyp" or "polyps." Reports were reviewed to identify polyps ≥7 mm. The most hyperechoic components of the polyps were compared to the adjacent inner wall of the gallbladder and categorized as more echogenic, isoechoic, and less echogenic. Other sonographic characteristics such as heterogeneity, sessile configuration, vascularity, multiplicity, presence of gallstones, and wall thickening were recorded. Surgical pathology reports were reviewed when available. Polyps in nonsurgical patients with ≥48-month US follow-up and ≤1 mm/year growth rate were characterized as benign. Clinical outcomes were followed until December 31, 2023. Review of 4897 reports yielded 550 reports in 450 patients with polyps ≥7 mm. Surgical pathology reports were available in 22.0% (99/450) of patients; 96 (97%) had non-neoplastic etiologies and 3 (3.0%) neoplastic. There were no malignancies. All of the neoplastic polyps and 56.1% (87/155) of non-neoplastic polyps had components more echogenic than the adjacent inner wall. There were no deaths related to the polyps. The vast majority of gallbladder polyps are benign. In our large series of gallbladder polyps, we found that increased echogenicity is a nonspecific feature, found in slightly more than half of benign but also in all neoplastic polyps. Our findings support current Society of Radiologists in Ultrasound consensus guidelines.

Coalescence of vertically aligned drops over a superhydrophobic surface

The coalescence process of two liquid droplets where one is placed initially over the other is investigated. The lower drop is placed over a horizontal surface in a sessile configuration. The liquids of interest selected are water, glycerin, and Cs-alloy. The two liquid drops merge under atmospheric conditions. The substrate is superhydrophobic with respect to the three liquids, the equilibrium contact angle being 150°. For the combined drop, the Bond number is ∼0.2. Numerical simulations have been performed in an axisymmetric coordinate system along with supporting experiments. A variety of contact line models reported in the literature have been adopted and compared. Experiments are carried out for validation against simulation with water as the liquid medium. The coalescence phenomenon is recorded by a high-speed camera. The two drops coalesce spontaneously and generate interfacial shapes, velocity fields, footprint, and wall shear stress in time. In water, the combined drop recoils from the surface before spreading over the surface and approaching equilibrium. This trend, including the instant and height of recoil, is correctly realized in the contact line models. Additionally, two distinct timescales originate during the coalescence process. These are associated with inertia and surface tension at small times and inertia–viscosity for longer durations. The instantaneous footprint radius and the average wall shear stress fall to zero during recoil, increase then to a maximum, and diminish to zero with damped oscillations over the longer timescale. Recoil is seen in water as well as Cs-alloy, but not in glycerin. Despite differences in the instantaneous data, these predictions are broadly reproduced by each of the contact line models.

Experimental Observations of the Mechanisms Associated with the High Hardening and Low Strain to Failure of Magnesium

Magnesium holds considerable promise as a lightweight structural material, but structural applications of Mg alloys are generally limited to cast components because of its limited formability. Mg exhibits high hardening and low strain to failure, the attendant mechanisms of which remain elusive. In this work, we performed detailed TEM investigations on the dislocation structures in the c-axis compressed Mg single crystals. Systematic tilting experiments in TEM revealed the majority of dislocations to be dissociated and basal bound, suggesting that glissile pyramidal dislocations may transition and dissociate into a sessile configuration. Other dislocations were observed to decompose into 19 individual and dislocations. Since and dislocations cannot accommodate c20 axis compression, this process also manifests a glissile-to-sessile transition. Moreover, numerous sessile nanoscale dislocation loops were observed to form during plastic deformation, which can obstruct the movement 22 of the mobile dislocations. These three mechanisms all impede the motion of glissile dislocations. Comparing these experimental with simulations that have been published in the literature is used to glean insight on the characteristics of dislocations and to explain the high hardening and low strain to failure that are widely observed in Mg.

Experimental observations of the mechanisms associated with the high hardening and low strain to failure of magnesium

Magnesium holds considerable promise as a lightweight structural material, but structural applications of Mg alloys are generally limited to cast components because of its limited formability. Mg exhibits high hardening and low strain to failure, the attendant mechanisms of which remain elusive. In this work, we performed detailed TEM investigations on the dislocation structures in the c-axis compressed Mg single crystals. Systematic tilting experiments in TEM revealed the majority of 〈c + +a〉 dislocations to be dissociated and basal bound, suggesting that glissile pyramidal 〈c + +a〉 dislocations may transition and dissociate into a sessile configuration. Other 〈c + +a〉 dislocations were observed to decompose into individual 〈a〉 and 〈c〉 dislocations. Since 〈a〉 and 〈c〉 dislocations cannot accommodate c-axis compression, this process also manifests a glissile-to-sessile transition. Moreover, numerous sessile nanoscale dislocation loops were observed to form during plastic deformation, which can obstruct the movement of the mobile 〈c + +a〉 dislocations. These three mechanisms all impede the motion of glissile 〈c + +a〉 dislocations. Comparing these experimental with simulations that have been published in the literature is used to glean insight on the characteristics of 〈c + +a〉 dislocations and to explain the high hardening and low strain to failure that are widely observed in Mg.

P670Clincal and echocardiographic characteristics in patients with pathology proven cardiac papillary fibroelastomas

Abstract Background Papillary fibroelastoma (PFE) is a rare benign cardiac tumor with embolic potential. It is most commonly found on cardiac valves but can also be present on non-valvular endocardial surfaces. We sought to better understand the average PFE growth and recurrence rates, and characteristics that might be associated with embolization. Purpose To examine growth and recurrence rates and factors associated with embolization in patients with pathology-proven PFE. Methods Pathology-proven PFEs from 279 patients were identified at a single center between January 1995 and December 2018,and those with at least two transesophageal echocardiograms (TEE) more than 30 days apart were analyzed (n=62). Medical records were retrospectively reviewed for clinical characteristics and outcomes. In addition, intra-operative TEEs were manually reviewed and compared to previous TEEs at our institution to estimate overall size, location, and average PFE growth rates. Results The TEEs from 62 patients with pathology-proven PFE (mean age 65±12 years, female 63%) were reviewed. PFE was discovered incidentally during cardiac surgery in 18% of patients. Most PFEs were located on the aortic valve (AV) (73%) followed by the mitral valve (MV) (16%). The majority of PFEs (71%) presented with a stalk versus a sessile configuration. Average maximal length for PFE (including stalk if present) on the AV was 8.5±3.3 mm, and on the MV 6.8±2.0 mm. The number of PFEs located in other locations was too small to be analyzed for growth. PFE growth varied depending on location and configuration. The PFE growth on the AV was an average increase of 0.47 mm/year, those on the MV was 0.115 mm/year. Valvular PFE was associated with significant functional valve abnormality in 15% of patients, and the abnormality was attributed to the PFE in 9% of patients. There were 25 patients (40%) who had a documented stroke/transient ischemic attack prior to PFE removal (46% located on AV, 44% on MV, 10% other locations). In addition, some patients had other documented embolic complications, such as myocardial infarction, amaurosis fugax, and peripheral embolization to the spleen and kidney. PFE size and location (up-/ downstream of the valve) were not predictive of cardiovascular thromboembolic events. At a median follow up of 2 years, PFEs recurred in the same location in 7.5% of the patients. Conclusion PFE is highly associated with thromboembolic events and seem to grow slowly, with an average growth rate of 0.47mm/year on AV and 0.115mm/year on MV. Growth rates for PFE have not been previously described. PFE can be safely excised with preservation of the native valve and recurs in 7.5% of patients, suggesting that close follow-up maybe warranted.

Differential Homeostasis of Sessile and Pendant Epithelium Reconstituted in a 3D-Printed "GeminiChip".

Local mechanical cues can affect crucial fate decisions of living cells. Transepithelial stress has been discussed in the context of epithelial monolayers, but the lack of appropriate experimental systems leads current studies to approximate it simply as an in-plane stress. To evaluate possible contribution of force vectors acting in other directions, double epithelium in a 3D-printed "GeminiChip" containing a sessile and a pendant channel is reconstituted. Intriguingly, the sessile epithelia is prone to apoptotic cell extrusion upon crowding, whereas the pendant counterpart favors live cell delamination. Transcriptome analyses show upregulation of RhoA, BMP2, and hypoxia-signaling genes in the pendant epithelium, consistent with the onset of an epithelial-mesenchymal transition program. HepG2 microtumor spheroids also display differential spreading patterns in the sessile and pendant configuration. Using this multilayered GeminiChip, these results uncover a progressive yet critical role of perpendicular force vectors in collective cell behaviors and point at fundamental importance of these forces in the biology of cancer.

In situ transmission electron microscopy investigation on 〈c+a〉 slip in Mg

Abstract

Role of second transurethral resection in determining residual tumor in nonmuscle-invasive bladder cancer

Introduction: Presence of residual tumors is not an uncommon event after transurethral resection of bladder tumor, and no studies from Nepal so far has addressed this issue. We conducted this study to determine the rate of residual tumors after first transurethral resection of nonmuscle-invasive bladder cancer, and to determine the factors associated with the presence of residual tumors and upstaging of nonmuscle-invasive bladder cancer.
 Methods: This was a prospective observational study of 43 patients of bladder cancer who had a diagnosis of nonmuscle-invasive bladder cancer following an initial transurethral resection. Demographic data and data on tumor characteristics were obtained. Patients underwent a second transurethral resection within 2 to 8 weeks. Histopathological findings at first and second resection were compared.
 Results: There were 20 patients with Ta tumor and 23 patiens with T1 tumor at initial resection. Residual tumor was detected in 18 (41.86%) patients overall, 2 in patients with Ta tumor (10%) and 16 in patients with T1 tumor (69.5%). Tumors with T1 stage, high grade, size more than 3 centimeters and sessile growth pattern were seen to have significant association with the presence of residual tumors. Six patients with T1 disease upstaged to T2 disease after second resection (26%), while there were no upstaging with Ta tumors. Tumors with T1 stage, sessile configuration and size more than 3 centimeters were found to be significantly associated with upstaging.
 Conclusion: A second transurethral resection for nonmuscle-invasive bladder cancer should be considered if the initial tumor is T1 stage, high grade, more than 3 centimeters in size and has sessile growth pattern.

Sessile and Pendant Micro-Liter Drops Evaporate at Different Rates: An Experimental Approach

Evaporation of micro-liter drops from solid surfaces at room condition is mainly governed by diffusion. Therefore, there should be no difference between evaporation rate of sessile and pendant drops. However, some studies indicate a difference and explain the difference using buoyancy. The objective here is to reconcile the inconsistency in the literature. For that, first, by comparing two identical suspended drops, one with a plate on top and the other underneath with a space between drop and plate, we showed the contribution of buoyancy in evaporation is at most less than 8%. When a plate was placed on top, water (its vapor is lighter than air) evaporated slower and hydrocarbons (their vapors are heavier than air) evaporated faster. Interestingly, it was observed when drops touch the plates (i.e. sessile and pendant drops), both water and hydrocarbon drops evaporated faster in sessile configuration. The observation for hydrocarbons is in contradiction with what buoyancy explains. To describe the difference, different scenarios were studied. It was found that sessile drops stay longer in the “constant wetted area” (CWA) mode, before switching to the CCA (constant contact angle) mode, e.g. a 4 µl sessile water drop on a Poly(methyl methacrylate) coated silicon stays in the CWA mode for 318 s whereas for a similar pendant drop this time is 274 s. Considering the fact that evaporation rate in the CWA mode is 30–40% higher compared to the CCA mode, the faster evaporation rate of sessile drops may be explained.

RECOIL OF DROPS DURING COALESCENCE ON SUPERHYDROPHOBIC SURFACES

A numerical and experimental study has been carried out with a pair of water drops coalescing over a superhydrophobic surface. Initially, a stationary isolated drop (of given radius R1) in the sessile configuration is just touched by another drop (of radius R2), vertically in line with it, at zero speed. The solid substrate is a chemically treated, polished superhydrophobic surface made up of copper, for which the equilibrium contact angle for water was measured to be 150 ◦ . The Bond number, considering the total volume of the combined drop, falls in the range of 0.3–0.5. For these conditions, experiments clearly reveal that for a given range of R1=R2, the merged drop bounces off once before it starts to eventually spread and attain an equilibrium configuration. The above experiment has been numerically simulated using COMSOL c ⃝ , in an axisymmetric coordinate system. The simulations have been carried out for different combinations of droplet volumes, while keeping their combined volume to be fixed. The predictions of the simulation are compared with the experiment in terms of the interface shapes attained, distinctive timescales, and recoil height. The comparison between the two sets of data is seen to be favorable. Factors leading to recoil are delineated according to the available energy budget.

Oncological outcomes after endoscopic removal of malignant colorectal polyps

malignant colorectal polyp, defined by submucosally invasive adenocarcinoma, is the earliest form of clinically relevant colorectal cancer (CRC). After endoscopic resection additional surgery may be necessary, although decision criteria remain debatable. The objective of this study was to assess oncologic outcomes in terms of locoregional disease and to identify areas of improvement that may facilitate patients' management. retrospective study of 40 patients with T1 CRC endoscopically resected between 2007 and 2012. Clinicopathological features were assessed and correlated with residual disease (RD), defined as presence of adenocarcinoma in intestinal wall and/ or lymph nodes. thirty-one patients underwent surgery while 9 were followed-up. After surgery, RD was confirmed in 15 (48.4%) patients: 8 (53.3%) wall disease, 5 (33%) nodal metastasis, and 2 (13.3%) with both. No recurrence was detected in the follow-up group. The characteristics of the lesions that were associated with DR were sessile configuration (p = 0.03), the degree of differentiation G3 (p = 0.01) and intercepted/indeterminate margins (p = 0.01). Twenty-two patients were operated because of inadequate evaluation, mainly due to piecemeal resection, and half of them were disease free. Postoperative complications were found in 9 (30%) patients, mainly anastomotic leakage that was associated with rectum anterior resection (p = 0.03). surgery should be considered in the presence of any risk factor for residual disease, while follow-up can be offered in low risk settings. Was also demonstrated a clear need for technical improvement in endoscopic resection and pathology evaluation in order to prevent unnecessary surgeries.

Influence of surface orientation on the organization of nanoparticles in drying nanofluid droplets

The influence of droplet orientation on the flow directed organization of nanoparticles in evaporating nanofluid droplets is important for the efficiency of foliar applied fertilizers and contamination adhesion to the exterior of buildings. The so called “coffee ring” deposit resulting from the evaporation of a sessile nanofluid drop on a hydrophilic surface has received much attention in the literature. Deposits forming on hydrophobic surfaces in the pendant drop position (i.e. hanging drop), which are of importance in foliar fertilizer and exterior building contamination, have received much less attention. In this study, the deposit patterns resulting from the evaporation of water droplets containing silica nanoparticles on hydrophobic surfaces orientated in the sessile or pendant configuration are compared. In the case of a sessile drop the well known coffee ring pattern surrounding a thin nanoparticle layer was formed. A deposit consisting of a thin coffee ring surrounding a bump was formed in the pendant position. A mechanism involving flow induced aggregation at the droplet waist, settling, orientation dependant accumulation within the drop and pinning of the contact line is suggested to explain the findings. Differences in the contact area and adhesion of deposits with surface orientation will affect the efficiency and rainfastness of foliar fertilizers and the cleanliness of building exteriors.

Simulating complex atomistic processes: On-the-fly kinetic Monte Carlo scheme with selective active volumes

An accelerated atomistic kinetic Monte Carlo (KMC) approach for evolving complex atomistic structures has been developed. The method incorporates on-the-fly calculations of transition states (TSs) with a scheme for defining active volumes (AVs) in an off-lattice (relaxed) system. In contrast to conventional KMC models that require all reactions to be predetermined, this approach is self-evolving and any physically relevant motion or reaction may occur. Application of this self-evolving atomistic kinetic Monte Carlo (SEAK-MC) approach is illustrated by predicting the evolution of a complex defect configuration obtained in a molecular dynamics (MD) simulation of a displacement cascade in Fe. Over much longer times, it was shown that interstitial clusters interacting with other defects may change their structure, e.g., from glissile to sessile configuration. The direct comparison with MD modeling confirms the atomistic fidelity of the approach, while the longer time simulation demonstrates the unique capability of the model.

Stability of sessile and pendant liquid drops

The solution space of axisymmetric liquid drops attached to a horizontal plane is investigated, and the stability of hydrostatic shapes is assessed by a novel numerical linear stability analysis involving discrete perturbations. For a given contact angle and Bond number, multiple interfacial shapes exist with compact, lightbulb, hourglass, and more convoluted pearly shapes. It is found that more than one solution branch can be stable, and that negative curvature at the contact line of a pendant drop is not a prerequisite for instability. Numerical simulations based on the boundary-integral method for Stokes flow illustrate the process of unstable drop detachment. Unstable drops transform into elongated threads with a spherical head whose volume is determined by a Bond number expressing the significance of surface tension. A complementary investigation of the shape and stability of two-dimensional drops attached to a horizontal or inclined plane reveals that hydrostatic shapes are least stable in the inclined configuration and most stable in the pendant or sessile configuration.

Strain hardening due to deformation twinning in α-titanium: Mechanisms

Novel experiments were conducted to elucidate the effect of deformation twinning on the mechanical response of high-purity α-titanium deformed at room temperature. Orientation-imaging microscopy (OIM), microhardness, and nanohardness evaluations were employed in conjunction with optical microscopy and quasi-static compression testing to obtain insight into the deformation mechanisms. Hardness measurements revealed that the newly formed deformation twins were harder than the matrix. This observation is perhaps the first experimental evidence for the Basinski mechanism for hardening associated with twinning, arising from the transition of glissile dislocations to a sessile configuration upon the lattice reorientation by twinning shear. This work also provided direct evidence for two competing effects of deformation twinning on the overall stress-strain response: (1) hardening via both a reduction of the effective slip length (Hall-Petch effect) and an increase in the hardness of twinned regions (Basinski mechanism) and (2) softening due to the lattice reorientation of the twinned regions.

Three‐dimensional ultrasonographic findings in bladder cancer

Bladder cancer is the second most common cancer of the genitourinary tract, and its most common symptom is hematuria, either gross or microscopic. However, the differential diagnosis for hematuria is extensive1. Patients referred with hematuria in the absence of infection should have renal ultrasonography or intravenous urography, urine cytology and cystourethroscopy to distinguish between a renal or postrenal source of the bleeding1. We report the three-dimensional (3D) sonographic findings in two cases of bladder carcinoma. The first case was a 59-year-old postmenopausal woman who presented with a history of urinary frequency, nocturia and dysuria associated with gross hematuria. Transvaginal ultrasonography (TVS) revealed an ill-defined, 3.2 × 2.7-cm hypoechogenic nodule with an inner calcified ring located in the right anterior lateral aspect of the bladder, protruding into the bladder lumen. 3D scanning revealed invasion of the bladder wall (Figure 1a) and plaque-like extension of the tumor, which increased the thickness of the underlying bladder wall (Figure 1b). Doppler studies revealed increased vascularity in the periphery of the tumor mass (Figure 2). The lowest resistance index value of the tumor vessels was 0.29. Cystoscopy revealed an irregular mass protruding into the bladder cavity (Figure 3). A transurethral biopsy specimen contained malignant cells. Subsequent surgery showed a Stage IV tumor of the right anterior bladder wall. The pathological diagnosis was a poorly differentiated adenosquamous carcinoma. In Case 1, three-dimensional transvaginal ultrasound examination revealed bladder wall invasion (a) and plaque-like extension (b) of the bladder tumor. (a) Bladder wall invasion is recognized by the normal bladder wall (white arrows), which is relatively echogenic, being interrupted by hypoechogenic tumor tissue. (b) The bladder wall underlying the plaque-like extension of the tumor is 11.7 mm thick (black arrows). Doppler study in Case 1. The vascularity is increased at the periphery of the tumor mass. Cystoscopic examination in Case 1 showing an irregular mass. The second case was a 50-year-old woman who was referred to our clinic with a history of frequency, nocturia, urgency and urinary incontinence. Urinalysis revealed numerous red and white blood cells. TVS revealed a 3.2 × 3.1-cm echogenic mass located at the junction of the bladder dome and anterior bladder wall and protruding into the bladder cavity. 3D scanning revealed a tiny tumor invading the bladder wall (Figure 4). Increased vascularity was seen centrally on Doppler studies (Figure 5). Cystoscopy and transurethral biopsy revealed a malignant bladder tumor. On subsequent surgery, a Stage I adenocarcinoma was found. In Case 2, a three-dimensional transvaginal sonographic scan revealed a tiny protuberance (arrows) invading the bladder wall. In Case 2, a Doppler study revealed increased vascularity centrally within the tumor mass. Ultrasound has been reported to be useful for screening and detecting bladder tumors and is considered superior to intravenous urography for this purpose2. Dibb et al. reported varying morphological features of bladder tumors on transabdominal ultrasonography, including: a polypoid, sessile or plaque-like configuration; regular or irregular surface; calcified or uncalcified texture2. TVS with a high-resolution probe can clearly differentiate various intravesical masses such as bladder calculus3, foreign bodies, intravesical blood clots4 and hemorrhagic cystitis5. These conditions may cause hematuria and can appear simply as an intravesical mass on imaging studies, making them difficult to distinguish from bladder cancer3-5. Abnormal ultrasonographic changes of the bladder wall are one of the indications for cystoscopic examination. We suggest that TVS scanning of the lower urinary tract may serve as an initial work-up for women with either gross or microscopic hematuria. For those women who have abnormal sonographic findings, cystourethroscopy is suggested, even in the presence of pyuria. However, for those women with normal ultrasound findings, medical treatment may be attempted to avoid the discomfort of cystourethroscopy. 3D TVS, in addition to detecting a bladder tumor, may also demonstrate bladder wall invasion, and thus assist in surgical planning6. W.-C. Huang*, S.-H. Yang , J.-M. Yang , * Department of Obstetrics Gynecology, Cathay General Hospital, 92 Chung-Shan North Road, Section 2, Taipei 104, Taiwan, R.O.C., Taipei Medical University, 92 Chung-Shan North Road, Section 2, Taipei 104, Taiwan, R.O.C., Division of Urogynecology, Department of Obstetrics Gynecology, Mackay Memorial Hospital, 92 Chung-Shan North Road, Section 2, Taipei 104, Taiwan, R.O.C.

Histoclinical analysis of early colorectal cancer.

To evaluate the clinicopathologic characteristics of early colorectal cancer (ECC), histopathologic and clinical features of 90 ECC patients who underwent surgical resection (not including the endoscopic polypectomized cases) and 1704 patients with advanced colorectal cancer were analyzed. Smaller size, better histologic differentiation, less lymph node (LN) metastasis, and better clinical outcome were observed in those with ECC than in patients with more advanced lesions. LN metastasis was found in 5 patients with ECC among the 56 patients who underwent bowel resection (8.9%). Tumors with LN metastasis were more frequently associated with deep submucosal invasion, absence of an adenomatous component within the tumor, sessile configuration, and lymphovascular invasion. Tumors showing deep submucosal layer invasion were associated with a more unfavorable histologic grade, lymphovascular invasion, LN metastasis, sessile morphology, and absence of an adenomatous component within the tumor. Recurrence was observed in two patients who underwent local excision for their submucosal cancer. One of the patients was salvaged after bowel resection, but one patient died of distant metastasis. From our data analysis and literature review, extensive submucosal invasion, lymphovascular invasion, sessile configuration, and tumors with no adenomatous component should be considered high risk factors for LN metastasis and recurrence after limited therapy.

Locking of super shockley partials in the super-alloy SRR99 after creep at 1033K

After plastic deformation superalloys show dissociated superdislocations with Burgers vector [bar b] = 2 [center dot] a/2 (a = lattice constant) or super Shockley partials (SSPs) with [bar b] = a/3 . Superdislocations can change from a glissile into a sessile configuration by thermal activation. This Kear-Wilsdorf-mechanism is the reason for the increase of the critical shear stress with increasing temperature. An analogous mechanism for SSPs is the Giamei-mechanism. It was suggested already in 1971, but first proven in 1991 by Sun et al. in Ni[sub 3]Al and Ni[sub 3]Ga-single crystals after compression tests at 77 K. In this work it is shown that this mechanism is also active in a commercial superalloy under typical test conditions. The material used was SRR99, a Ni-base single crystal in [001]-orientation.

Dislocation core structures in the ordered intermetallic alloy TiAl

Evidence for the locking of both “ordinary” ( b=(a/2)<110]) and “super” ( b=a<101]) dislocations has been reported in the literature, and possible models for the locking processes have been proposed. In the present study, high resolution transmission electron microscopy (HREM) and calculated image simulations were combined to provide atomic level observations of these dislocation core structures. The results of this study suggest that the core of ordinary dislocations is compact and that the motion of these dislocations is inhibited by intrinsic lattice friction and not by dislocation core dissociations. In contrast, the core of screw superdislocations was observed to be dissociated in a non-planar configuration. The results suggest that the leading super partial dislocation cross-slipped from one {111} plane to a cube plane and then redissociated on another {111} plane. The end result of this process is a sessile configuration that contains an intrinsic stacking fault on the ( 1 1 1 ) plane, an extrinsic stacking fault on the (111) plane, and an antiphase boundary on the (010) plane in between.

Dislocation mechanisms in Ni3Al at room temperature.In situstraining experiments in TEM

Abstract Transmission electron microscopy in situ straining experiments were performed under weak beam conditions, in Ni3Al(0.25 at.% Hg) at room temperature. They revealed rectilinear screw superdislocations dissociated either in the two octahedral or in the cube cross-slip plane, and fully reversible cross-slip between these planes. Screw superdislocations have the lowest mobility. They glide in octahedral planes through series of jumps and waiting times in sessile positions for which the antiphase boundary (APB) is contained in the glide plane. The jump amplitude can either be large, or equal to the superpartial separation. In order to explain these experimental results, a new sessile configuration of screw superdislocations has been introduced, with the APB ribbon in {111}, and both superpartials cross-slipped in {100} on a short distance (about 20 A). This configuration has some common points with that already proposed by Paidar, Pope and Vitek (1984), but is different in several aspects. It is consi...