Materials Science Forum Vol. 560

Abstract: The effect of the chemical treatment of zirconia/alumina composites followed by a biomimetic treatment has been studied. The composites are prepared from a powder mixture of Mg- PSZ and Al2O3. The powders are ball-milled in acetone and uniaxially pressed after drying. The specimens are sintered at 1550 °C in air. After sintering, chemical treatment is performed by immersing the samples in a 5M aqueous solution of phosphoric acid at 95 °C for 4 days. The biomimetic method consists of immersing the chemically-treated samples in simulated body fluid at 36.5 °C. In some cases a wollastonite bed is used as a supplier of calcium ions, resulting in the formation of a bonelike apatite layer. The presence of this bioactive system during the biomimetic process has a positive significant effect on the bioactivation of the composites for either short or long times of immersion of the composites in simulated body fluids. The chemical treatment increases also the rate of apatite formation at short immersion periods.

Abstract: The behavior of two different types of ultra-high-performance polyamide (PA) 66 fibers under fatigue loading up to failure, and the correlation between the fibers (nano)structures and their structural heterogeneity with fatigue lifetimes, have been studied using scanning electron microscopy, differential scanning calorimetry, wide angle x-ray diffraction and micro-Raman spectroscopy. The role of the microstructure of the fibers in determining fatigue life is presented and the possibility of improving their resistance to fatigue or eliminating the fatigue process will be discussed.

Abstract: The degradation in ambient atmosphere of Al/SiCp composites prepared by the reactive infiltration of SiCp preforms containing fly ash has been investigated. SiCp/fly-ash preforms in the form of plates (3 cm x 4 cm x 0.5 cm) with 50 % porosity are infiltrated by an Al- 8 Si-15 Mg (wt. %) alloy under argon atmosphere at 1050, 1100 and 1150 °C, for 50, 60 and 70 min. Characterization by XRD, SEM and EDX of composite specimens shortly after processing do not reveal the presence of the unwanted Al4C3 phase. However, in addition to Al, Si and SiC, MgAl2O4 and Mg2Si phases are detected. One month after the infiltration trials, white and gray powders are present on the composite specimens, accompanied by pitting corrosion and cracks which propagate with time. Although analysis by XRD of the degradation products reveals only Al4C3 in addition to the above mentioned phases, results from SEM, IR absorption and ICP also suggest the presence of Al(OH)3 and Mg(OH)2, probably from the interaction of Al4C3 and Mg2Si with water. It is considered that Mg2Si in the powders acts as an anode in a galvanic couple with atmospheric moisture as the electrolyte. The crack pathway through SiC, intermetallic AlFeMnSi and Si rich zones implies that one or more of these phases worked as the cathode. In summary, degradation of the composites is explained by the combined effect of galvanic corrosion caused by second phases and the interaction of Al4C3 with atmospheric moisture.

Abstract: In this work a fundamental Eulerian mathematical model is developed to simulate fluid flow and mixing phenomena in aluminum ladles equipped with an impeller for dehydrogenization treatment. The effect of rotating speed and type of impeller, depth of immersion, and gas flow rate, on the mixing behavior and vortex formation is analyzed with this model. The model simulates operation with and without gas injection and it is developed in the commercial PHOENICS 3.4 CFD code in order to solve all conservation equations governing the process, i.e., continuity, 3D turbulent Navier-Stokes and k-ε turbulence model for a two-phase fluid flow problem using the Inter Phase Slip Algorithm. In order to realistically represent the process, the shape of the furnace and the impellers are modeled by employing Body Fitted Coordinates. It is concluded that the mixing behavior is highly dependent on the rotation speed and impeller type. Mixing time is improved when: the impeller is located at a depth of 0.229 m into the aluminum bath, and by using high rotation speeds, ladles with a high ratio of diameter to height, and impellers with notches.