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The fracture behavior of Al,O, containing 5 vol% 0.15 pm SIC particles was investigated using indentation techniques. A significant increase in strength was achieved by the addition of SIC particles to the base AI,O,. Specifically, the strength increased from 560 MPa for A1,0, to 760 MPa for the composite samples (average values for unindented hotpressed bars tested in four-point bending). After annealing for 2 h at 1300°C, the average strength of the composite samples increased to about 1000 MPa. Toughness was estimated using indentation-strength data. While there was a slight increase in toughness, the increase was not sufficient to account for the increase in the unindented strength on Sic particle addition. It is suggested that the observed strengthening and apparent toughening were due to a machining-induced compressive surface stress.

The 1980 encounter by the Voyager 1 spacecraft with Titan, Saturn's largest moon, revealed the presence of a thick atmosphere containing nitrogen and methane (1.4 and approximately 0.05 bar, respectively). Methane was found to be nearly saturated at Titan's tropopause, which, with other considerations, led to the hypothesis that Titan might experience a methane analogue of Earth's vigorous hydrological cycle, with clouds, rain and seas. Yet recent analyses of Voyager data indicate large areas of super-saturated methane, more indicative of dry and stagnant conditions. A resolution to this apparent contradiction requires observations of Titan's lower atmosphere, which was hidden from the Voyager cameras by the photochemical haze (or smog) in Titan's stratosphere. Here we report near-infrared spectroscopic observations of Titan within four narrow spectral windows where the moon's atmosphere is ostensibly transparent. We detect pronounced flux enhancements that indicate the presence of reflective methane condensation clouds in the troposphere. These clouds occur at a relatively low altitude (15+/-10 km), at low latitudes, and appear to cover approximately 9 per cent of Titan's disk.

The tensile creep behavior of two ceramic composite systems exhibiting duplex microstructures was studied relative to their single-phase constituents in the temperature and stress ranges of 1100-1350°C and 35-75 MPa. The equivolumetric compositions in the Al,O,:c-ZrO, (8 mol% Y,O,) and AI,O,:Y,AI,O,, systems both exhibit lower creep rates than either of their single-phase constituents. This effect is attributed to Y3+ (and possibly Zr4+) present in the AI,O, as a segregant which lowers the creep rate by -2 orders of magnitude. It is believed that the segregation of Y3+ to the AI,O, grain boundaries hinders the interface reaction believed to control the creep. If one of the singlephase constituents is taken to be the Y3+-doped A1,0,, the creep of the duplex microstructures can be modeled using standard composite theory applied to flow.

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