1991

85Al2O3-15ZrO2 (wt%) powders were synthesized by gel precipitation starting from AlCl3 · 6H2O and ZrCl4 solutions and dried by two different methods: (i) by spray-drying and (ii) via a sol-gel route by n-octanol in a pilot plant. The particles by process (i) were spherical granules of diameter 15 μm, and those by process (ii) were microspheres of 25 μm. The powders were characterized in terms of morphology, particle size distribution, surface area, weight loss and crystallization behaviour at different temperatures.

Carbon free Fe-Ni alloys (12 and 20 wt.% Ni) have been analysed by X-ray diffraction and surface Mössbauer spectroscopy (CXMS and CEMS) after boriding treatment at 1273 K for 20 hours. Some (FexNil-x)2B and FexNil-xB samples, with different values of x, obtained by heating at 1073 K mixtures of elements in powder form, were used as reference. Besides (FexNil-x)2B and FexNil-xB, a third boride phase rich in boron has been detected in the outer borided layers of the alloy specimens. A third phase appears also in the corresponding X-ray patterns.

The aging response at room temperature of aluminium alloy 6061 was compared with that of its composites containing SiC whiskers or particles. Materials were solutionized for 2 h at two different temperatures (529 and 557°C), quenched in cold water and naturally aged for up to 90 days. The improvement in mechanical characteristics was checked by means of hardness measurements. The microstructural features of aged samples were studied by differential scanning calorimetry. Both aluminium alloy and its composites showed the same aging sequence.

Ageing behaviour at 180 °C of 6061 aluminium alloy-SiCw composites, drawn from bars obtained in various extruded ratios, and 6061 aluminium alloy used as matrix, have been compared. These materials were dissolved in a salt bath at 529 and 557 °C for 2 h, quenched in ice-water, and aged at 180 °C in an oil bath for increasing periods. Ageing kinetics were studied with Brinell hardness measurements and differential scanning calorimetry (DSC).