EFFECT OF CARBON ADDITION ON ELEVATED-TEMPERATURE CRACK-GROWTH RESISTANCE IN (MO,W)SI2-SICP COMPOSITE

Experimental results on subcritical crack growth behavior of hot-press ed MoSi2-50 mol% WSi2 alloy reinforced with 30 vol% SiC particles in t he temperature range 1200-degrees-1300-degrees-C are presented. The ef fect of 2 wt% C addition on the stable crack growth resistance of this composite was investigated under both static and cyclic loading condi tions. The results indicate that the addition of carbon to the composi te improves the subcritical crack growth resistance under both static and cyclic loads and increases the elevated temperature capabilities o f the (Mo, W)Si2 composite. Increasing the temperature from 1200-degre es to 1300-degrees-C is found to increase the crack growth velocities with a concomitant decrease in the crack growth initiation thresholds. Electron microscopy of the crack-tip region indicates that the stable crack growth process is influenced primarily by interfacial cavitatio n. At 1300-degrees-C, deformation processes such as twinning of the Si C particles and dislocation motion within the matrix grains appear to play an active role in determining the crack growth kinetics. The role of glassy phase in influencing the high-temperature fracture behavior and its implications for design of the microstructures of the brittle materials are discussed.