M. Aljarrah

Microstructural characterization of Mg-Al-Sr alloys

Abstract The microstructural details of fourteen Mg–Al–Sr alloys were investigated in the as-cast form by a combination of scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) analysis and quantitative electron probe microanalysis (EPMA). The heat transfer method coupled with the DSC measurement has been utilized to determine the solidification curves of the alloys. The morphology and the chemical composition of the phases were characterized. The microstructure of the alloys is primarily dominated by (Mg) and (Al4Sr). In the present investigation, ternary solid solubility of three binary compounds extended into the ternary system has been reported and denoted as: (Al4Sr), (Mg17Sr2) and (Mg38Sr9). The (Al4Sr) phase is a substitutional solid solution represented by MgxAl4–xSr and has a plate-like structure. The maximum solubility of Al in Mg17Sr2 was found to be 21.3 at%. It was also observed that Mg38Sr9 dissolved 12.5 at% Al.

The significance of circulating endothelin-1 as a predictor of coronary artery disease status and clinical outcomes following coronary artery catheterization

BACKGROUND/OBJECTIVES Coronary artery disease (CAD) is responsible for significant morbidity and mortality. Inflammatory, pro-thrombotic and structural factors contribute to the etiology of CAD. This study sought to determine the relationship of plasma endothelin-1 (pET-1), a potent vasoconstrictor, mitogen and modulator of cardiac inflammation, to clinical characteristics and outcomes of CAD patients. METHODS Blood samples were collected from 336 patients with underlying chest pain or recent myocardial infarction (MI), prior to coronary catheterization. pET-1 was correlated with clinical characteristics and outcomes following catheterization and at 30-day follow-up. RESULTS pET-1 was higher in recent MI patients than in patients with CAD (coronary occlusion≥50%) or without CAD (<50%) (Mean±sem (pg/ml): 2.12±0.13, 1.51±0.10, 1.21±0.06; 95% confidence interval (1.85-2.38, 1.31-1.72, 1.07-1.32; respectively, P<.0001). Patients with ST elevation MI (STEMI) had higher pET-1 than non-STEMI (P=.008). pET-1 was associated with heart failure (HF) and low left ventricular ejection fraction (LVEF) and was highest in MI patients presented with acute HF. At 30-day follow up, pET-1 was not associated with the change in LVEF. In multivariate analysis, pET-1 was positively associated with age, smoking, HF, CAD status, and need for revascularization by coronary artery bypass surgery (CABG). pET-1 was negatively correlated with LVEF and preoperative statin use. CONCLUSIONS pET-1 is associated with recent MI, HF, age, smoking, CABG, and low LVEF. Preoperative statin use was associated with lower pET-1. pET-1 may serve as a risk marker and a potential therapeutic target in CAD patients.

Solidification Microstructure and Mechanical Properties of Hot Rolled and Annealed Mg Sheet Produced through Twin Roll Casting Route

The use of wrought magnesium for automobile structural components is an important component of the mass reduction strategy for automobiles to improve their fuel efficiency. Compared to Direct chill casting, Twin Roll Casting (TRC) allows major reduction of hot rolling steps in the production of Mg sheet due to the thin thickness of the as-cast strip. This TRC route can substantially reduce the time and cost to produce Mg alloy sheet product. In this work, AZ31 magnesium alloy was casted to 5 and 6 mm thick strips under different process conditions. Microstructure of these strips was analyzed using optical microscopy, SEM and EPMA. TRC strip was annealed under two different conditions: 2 hours at 330 and 1 hour at 400°C. It has been found that heat treatment at 400°C for 1 hour reduces centerline segregation significantly. TRC strips were rolled down to 2 mm and annealed at 450°C for 2 minutes. The average grain size was 4-6 µm and mechanical properties were comparable with commercial AZ31 sheet.

Influence of cooling rate on microsegregation behavior of magnesium alloys

The effect of cooling rate on microstructure and microsegregation of three commercially important magnesium alloys was investigated using Wedge (V-shaped) castings of AZ91D, AM60B, and AE44 alloys. Thermocouples were distributed to measure the cooling rate at six different locations of the wedge casts. Solute redistribution profiles were drawn based on the chemical composition analysis obtained by EDS/WDS analysis. Microstructural and morphological features such as dendrite arm spacing and secondary phase particle size were analyzed using both optical and scanning electron microscopes. Dendritic arm spacing and secondary phase particle size showed an increasing trend with decreasing cooling rate for the three alloys. Area percentage of secondary phase particles decreased with decreasing cooling rate for AE44 alloy. The trend was different for AZ91D and AM60B alloys, for both alloys, area percentage of β-Mg17Al12 increased with decreasing cooling rate up to location 4 and then decreased slightly. The tendency for microsegregation was more severe at slower cooling rates, possibly due to prolonged back diffusion. At slower cooling rate, the minimum concentration of aluminum at the dendritic core was lower compared to faster cooled locations. The segregation deviation parameter and the partition coefficient were calculated from the experimentally obtained data.

Modeling the elastic modulus of exfoliated graphite platelets filled vinyl ester: analytical predictions with consideration of filler percolation

Mechanical response of nano-based composites is generally influenced by interaction of filler and matrix at interface. Increasing filler-loading within the composite may cause spatial limitation toward best dispersion of filler, and since synthesizing a totally agglomerated-free nanocomposite is difficult, filler and matrix interaction needs to be perfectly modeled. A micromechanical model is developed in this study based on the common Halpin–Tsai theory to predict the elastic stiffness of vinyl ester/exfoliated graphite platelet nanocomposites. The model considers near-rational ideal (uniformly dispersed) mixed with clustered filler-network to simulate filler-distribution conditions. A filler-dispersion level based on the filler concentration has been proposed mathematically in this study. Predictions of the proposed model considering filler morphology were compared with the predictions of the Halpin–Tsai model and the experimentally obtained results as well. The proposed model shows better accuracy in terms of stiffness over predictions of the Halpin–Tsai model and appears in a very good agreement with the experimental results obtained for vinyl ester nanocomposites.

Experimental Study of the Al-Mg-Sr Phase Diagram at 400°C

The Al-Mg-Sr system is experimentally studied at 400°C using EPMA and XRD techniques. It was determined that the intermetallic phases in the Al-Mg-Sr system have a tendency to form extended substitutional solid solutions. Two ternary phases were found in this system. Solubility limits of binary and ternary phases were determined and the phase equilibria among phases were established. The isothermal section of the Al-Mg-Sr system at 400°C has been constructed using results of the phase analysis and experimental literature data.

The Effect of Annealing Conditions and Alloying Elements on the Microstructure Stability and Mechanical Properties of Mg-Zn-Cesheets

In this work, four Mg-Zn-Ce sheets were processed on an industrial scale by “Helholtz-ZentrumGreesthacht” company. To study the effect of alloying elements and annealing conditions on the rolled sheets, these sheets were annealed at different conditions. Mechanical properties of these rolled and annealed sheets were determined in rolling and transverse directions. Optical microscopy was used to investigate microstructure evolution and stability as well as grain size after annealing. Annealing at 450°C, 1 hr for A and D sheets provides microstructure stability of these sheets. Whereas, annealing B and C sheets for one hour at 400 and 350°C, respectively, provide the optimum annealing conditions. In the studied alloys, the highest zinc (Zn) presence alloyed with magnesium significantly increases the grain size and the amount of precipitates. Whereas, the highest cerium (Ce) presence alloyed with magnesium, refines the grain size during the recrystallization process.