MICROSTRUCTURAL EVOLUTION AND MECHANICAL-PROPERTIES OF AC8A/AL2O3 SHORT-FIBER COMPOSITES AFTER THERMAL EXPOSURE AT 150-DEGREES-C TO 350-DEGREES-C

The microstructural evolution and mechanical properties of an AC8A/12 vol pet Al2O3 (sf) composite fabricated by squeeze casting were characterized. Thermal treatments included the normal T6 temper and thermal exposure at 150 degrees C, 250 degrees C, 300 degrees C, and 350 degrees C for 400 hours. The predominant strengthening phase in the matrix appeared to be beta' (Mg2Si) needles. Bulk pure Si particles and dendrites were commonly seen. Large particles, termed as B-type phase, might include hexagonal Al-3(Ni, Cu, Fe, Si, Mg)(2) and orthorhombic Al-3(Ni, Cu, Fe, Si, Mg) phases. Both the Si and B dispersoids were not obviously affected by artificial aging at 150 degrees C to 350 degrees C. In certain cases, large cubic beta (Mg2Si) particles, hexagonal Q' or Q (Al4Cu2Mg8Si7) precipitates, and numerous small Al particles inside Si dispersoids were also seen. No interfacial reaction product was observed along the fiber/matrix interface even after long exposure at 350 degrees C. Amorphous SiO2 gels, which were used as a binder during fabrication, were occasionally observed. The tensile and fatigue behavior of the AC8A alloys and composites after the preceding thermal exposures were evaluated over the temperature range of 25 degrees C to 350 degrees C. The composites showed similar strength as the matrix alloy at room temperature but exhibited higher strength at temperatures above 250 degrees C, with the sacrifice of the lower ductility. The strength levels of both the alloys and composites were significantly reduced after long thermal exposure, especially for temperatures higher than 250 degrees C. The loss of strength after long-term exposure at elevated temperatures may be attributed to age-softening of the matrix.