IN-SITU AND EX-SITU PULSED LASER MELTING AND RAPID LATERAL SOLIDIFICATION OF Al THIN FILMS

Under extreme conditions (e.g. changes in temperatures and pressures) phase transformations can proceed along various different transformation routes that are not accessible under equilibrium conditions, involving non-equilibrium processes, and can therefore result in various unusual and novel microstructures. The investigation of the transformation path for specific thermodynamic and kinetic conditions and understanding of the resulting microstructural evolution is both scientifically intriguing and technologically important, as many materials properties are strongly dependent on the scale and morphology of the microstructure of a material. Pulsed-laser-irradiation of thin films is a relatively novel processing route that induces melting and rapid re-solidification under extreme conditions of rapid heating-cooling cycles in thin films on amorphous substrates. To date most studies on laser processing of thin films have focused on rapid solidification of Si and Cu thin films. This thesis reports on the application of pulsed-laser processing and rapid solidification of technologically important Al thin films. It is demonstrated that pulsed-laser melting and rapid re-solidification is feasible for Al thin films in various configurations. Rapid solidification after pulsed-laser melting has been accomplished for Al films on bulk Si supported amorphous underlayers, with and without a thin synthetic amorphous capping layer, and also on free-standing and electron-transparent (no bulk Si support) Al films on thin amorphous underlayers, with and without thin synthetic amorphous capping layers. This dissertation reports only on a comparison of the microstructures resulting after laser processing of free-standing and electron-transparent (no bulk Si support) Al films on thin amorphous underlayer without thin synthetic amorphous capping layers. Excimer-laser-induced melting and rapid solidification of such Al thin films have been carried out in air. The resulting microstructure has been investigated post-mortem using transmission electron microscopy (TEM). The thin film microstructure resulting from re-solidification after pulsed-laser melting consisted predominantly of directionally solidified columnar grains, approximately 80nm thick (full film thickness), 0.5 micrometer wide and up to 5 micrometer long. Additionally, laser induced melting and re-solidification experiments were carried out using the dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory, which enabled in-situ observations of the transient phenomena associated with the liquid-solid transformation with its unprecedented 15ns temporal and nano-meter (nm) spatial resolution. The in-situ DTEM experiments were used to study details of the morphology and dynamics at the liquid-solid interface, revealing a planar liquid-solid interface that moves at an average velocity of ~ 3m/s. Ex-situ post-mortem TEM investigations of samples that were laser-processed during the in-situ DTEM experiments showed that microstructures were similar in morphology for the in-situ and the ex-situ rapid solidification experiments

Wear behavior of in-situ oxide dispersion strengthened Fe-8Ni alloy with Zr additions

MakaleWOS:000959908100001In this study, in-situ oxide dispersion strengthened (ODS) Fe91Ni8Zr1 and Fe88Ni8Zr4 alloys were produced by combination of high energy mechanical alloying (HEMA) and high temperature equal channel angular extrusion (HT-ECAE). The wear behaviors of the consolidated samples were investigated under different loads from 1 N to 4 N by reciprocating wear tests at room temperature. The Scanning electron microscopy (SEM) was used to examine the wear tracks to analyze the wear characteristics as a function of applied loads. The relative comparison of the wear results showed that under the lower loads of 1 N and 2 N, Fe88Ni8Zr4 alloy has lower wear rate than Fe91Ni8Zr1 alloy whereas under the higher loads of 3 N and 4 N, it is vice versa. Additionally, the friction coefficient of Fe91Ni8Zr1 alloy was found to be lower than that of Fe88Ni8Zr4 alloy under all the applied loads. The results were comparatively discussed with respect to microstructural features of 1 at% Zr and 4 at% Zr containing ODS alloys produced by HEMA followed by ECAE. The obtained results of ODS alloys with different grain size, precipitate size, and number density of the precipitates, may disclose a new sight for using such alloys in wear applications just as cutting tools, turbine blades, and discs

A case of bilateral spontaneous pneumothorax in a patient with covid-19 pneumonia

One of the rare complications of coronavirus disease-2019 (COVID-19), which can present with different clinical pictures, is pneumothorax. In our case, a patient who did not have predisposing risk factors for pneumothorax such as a history of trauma, smoking, past intubations, asthma, chronic obstructive pulmonary disease, and who developed bilateral spontaneous pneumothorax while under treatment with the diagnosis of COVID-19 is presented. Acute worsening in COVID-19 patients may be due to primary disease or pulmonary embolism. With this case, we emphasize the importance of considering spontaneous pneumothorax in patients with acute clinical deterioration

Global overview of the management of acute cholecystitis during the COVID-19 pandemic (CHOLECOVID study)

Background: This study provides a global overview of the management of patients with acute cholecystitis during the initial phase of the COVID-19 pandemic. Methods: CHOLECOVID is an international, multicentre, observational comparative study of patients admitted to hospital with acute cholecystitis during the COVID-19 pandemic. Data on management were collected for a 2-month study interval coincident with the WHO declaration of the SARS-CoV-2 pandemic and compared with an equivalent pre-pandemic time interval. Mediation analysis examined the influence of SARS-COV-2 infection on 30-day mortality. Results: This study collected data on 9783 patients with acute cholecystitis admitted to 247 hospitals across the world. The pandemic was associated with reduced availability of surgical workforce and operating facilities globally, a significant shift to worse severity of disease, and increased use of conservative management. There was a reduction (both absolute and proportionate) in the number of patients undergoing cholecystectomy from 3095 patients (56.2 per cent) pre-pandemic to 1998 patients (46.2 per cent) during the pandemic but there was no difference in 30-day all-cause mortality after cholecystectomy comparing the pre-pandemic interval with the pandemic (13 patients (0.4 per cent) pre-pandemic to 13 patients (0.6 per cent) pandemic; P = 0.355). In mediation analysis, an admission with acute cholecystitis during the pandemic was associated with a non-significant increased risk of death (OR 1.29, 95 per cent c.i. 0.93 to 1.79, P = 0.121). Conclusion: CHOLECOVID provides a unique overview of the treatment of patients with cholecystitis across the globe during the first months of the SARS-CoV-2 pandemic. The study highlights the need for system resilience in retention of elective surgical activity. Cholecystectomy was associated with a low risk of mortality and deferral of treatment results in an increase in avoidable morbidity that represents the non-COVID cost of this pandemic

Consolidation of thermally stabilized Fe based ferritic steels via hot pressing and hot extrusion (ECAE)

Bilyeli değirmenler ile üretilen yüksek mukavemetli tozların konsolidasyonunda temel hedef mekanik özelliklerinde önemli bir kayıp olmadan bu tozların birleştirilerek porozite içermeyen ürüne dönüştürülebilmesidir. Son yıllarda yapılan araştırmalar sıcak ekstrüzyon ile yüksek kayma ve basınca maruz bırakılan tozların porozite içermeden, teorik yoğunluklarında birleştirilebileceğini göstermiştir. Bu çalışmada sıcak presleme ve yüksek sıcaklıklarda çok geçişli eş kanallı açısal ekstrüzyon (ECAE - equal channel angular extrusion) teknikleri kullanılarak nano-kristalin Fe-Ni-Zr tozlarından konsolide edilmiş ferritik alaşımların üretimi araştırılmıştır. Elde edilen sonuçlar sıcak presleme metodu ile teorik yoğunluğa ulaşılamadığını, ancak sıcak ekstrüzyon yöntemi ile yapısında porozite içermeyen ve teorik yoğunlukta malzeme üretilebileceğini göstermiştir. Konsolidasyon sonucunda mikroyapıda mikrometre seviyelerine ulaşan tane büyümesi gözlenmesine rağmen üretilen bu çeliklerin sertliği 4-6 GPa civarındadır.The key characteristic of ball-milled powders consolidation process is to achieve densification and particle bonding without degradation in mechanical properties. Recent demonstrations of novel processing methods involving temperature, high shear and high pressure have shown promise for bonding high strength particulate materials. In this study, we report the ability of multi-pass high temperature equal channel angular extrusion (ECAE) and hot pressing to produce fully dense and well-bonded bulk Ferritic alloys from nanocrystalline FeNi-Zr powders. Subsequent microstructural characterizations indicate full consolidation via hot extrusion with grain growth to micron sizes. The consolidated alloys demonstrate an extremely high strength (4-6 GPa) at room temperature

An in Situ Experimental Study of Grain Growth in a Nanocrystalline Fe91Ni8Zr1 Alloy

Grain growth and microstructural evolution of thermally stabilized Fe91Ni8Zr1 were investigated by in situ and ex situ studies. Our investigations suggest that the microstructural evolution is fairly slow and the microstructure shows stabilization up to about 700 °C. Above this temperature, a certain fraction of grains grow abnormally into the nanocrystalline matrix, resulting in a bimodal microstructure and causing the complete loss of thermal stability. The reason for abnormal grain growth and the loss of thermal stability is identified as the appearance of the fcc γ-phase and consequent reduction in the total area of grain boundaries and the overall stored energy

Grain size stabilization of oxide dispersion strengthened CoCrFeNi-Y(2)O3 high entropy alloys synthesized by mechanical alloying

Nanocrystalline CoCrFeNi high entropy alloys (HEAs) with 1 and 4 wt% nanosized Y2O3 were synthesized by high energy mechanical alloying and subjected to annealing treatments at different temperatures up to 1100 degrees C. X-ray diffraction (XRD), focused ion beam microscopy (FIB), and transmission electron microscopy (TEM) were used to investigate the microstructures of as-milled and annealed HEAs as a function of annealing temperature and Y2O3 content. The results have shown that the as-milled HEAs were solid solutions with face-centered cubic (fcc) crystal structure, which remained stable even after annealing at 1100 degrees C. The as-milled nanocrystalline CoCrFeNi HEA revealed grain growth upon annealing, reaching 293 nm and 1.45 mu m after annealing at 900 and 1100 degrees C, respectively. This suggests that the nanocrystalline microstructure of CoCrFeNi is not thermally stable at high temperatures. The grain size stability was found to reach around 72 nm with nanosized Y2O3 particles after annealing at 1100 degrees C. Accordingly, 477 +/- 20 HV asmilled hardness of CoCrFeNi was dramatically reduced to 220 +/- 14 HV after annealing at 1100 degrees C due to severe grain coarsening but retained around 450 +/- 23 HV with 4 wt% Y2O3 addition. The correlation between microstructure and hardness was utilized to evaluate the mechanical properties. (c) 2021 Elsevier B.V. All rights reserved

Effect of Y addition on the structural transformation and thermal stability of Ti-22Al-25Nb alloy produced by mechanical alloying

In this study, a Ti-22Al-25Nb alloy with nanocrystalline structure was produced by high energy mechanical alloying (HEMA) and 1 at.-% yttrium was added as a thermal stabilizer. The as-milled samples were annealed at various temperatures up to 900 °C in a protective gas atmosphere, and the samples were allowed to cool to room temperature in the furnace. The phase transformations and microstructural changes as a function of the annealing temperatures and alloy compositions were studied using room- and high-temperature X-ray diffraction (XRD), focused ion beam microscopy (FIB), and scanning electron microscopy (SEM). The mechanical properties of the samples were interpreted based on the hardness results and their correlation with the microstructures. The results showed that the as-milled nanocrystalline structure of Ti-22Al-25Nb alloy increased from 3.4 nm to 350 nm after annealing at 800 °C due to the high driving force induced by the large grain boundary area. Consequently, the as-milled hardness of the Ti-22Al-25Nb alloy dropped from 7.63 ± 0.18 GPa to 5.37 ± 0.28 GPa. The grain size stability of the Ti-22Al-25Nb alloy after annealing at elevated temperature was ensured through the addition of yttrium. Thus, the grain size remained at the level of 125 nm, and the hardness value was maintained at around 6.98 ± 0.43 GPa after annealing at 800 °C