MICROMECHANISMS OF CREEP-FATIGUE CRACK-GROWTH IN A SILICIDE-MATRIX COMPOSITE WITH SIC PARTICLES

An experimental study has been conducted to examine the cyclic fatigue crack growth characteristics in 1200-degrees-C air of a MoSi2-50 mol% WSi2 alloy in the unreinforced condition and with 30 vol% SiC particl es. For comparison purposes, crack growth experiments under sustained loads were also carried out in the silicide-matrix composite. Particul ar attention is devoted to developing an understanding of the micromec hanisms of subcritical crack growth by recourse to optical and electro n microscopy, including transmission electron microscopy of crack-tip damage. The results indicate that enhanced viscous flow of glass films along interfaces and grain boundaries imparts pronounced levels of su bcritical crack growth in the composite material; the composite exhibi ts a higher fatigue fracture threshold and a more extended range of st able fracture than the unreinforced alloy. The effects of glass phase in influencing fatigue crack growth in the silicide-based material are compared to the influence of in situ-formed and preexisting glass fil ms on high-temperature cyclic fatigue crack growth in ceramics and cer amic composites. The paper concludes with a comparison of present resu lts with the high-temperature damage tolerance or a variety of interme tallic alloys and ceramic materials.