Ductile-phase toughening and Fatigue-Crack Growth in Nb-Reinforced Molybdenum Disilicide Intermetallic Composites
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INTRODUCTION
THE concept of toughening brittle solids through the incorporation of a ductile phase has attracted considerable attention recently in an attempt to enhance the ductility and fracture toughness of intermetallic and ceramic materials, t~-~2]The mechanism is based on the notion that, if the crack can be made to intercept the reinforcement phase, catastrophic fracture can be impeded through the formation of unbroken ductile-phase ligaments in the crack wake. The resulting crack bridging and plastic deformation of the particles, together with possible additional effects from crack deflection and interfacial debonding, provide the main contributions to composite toughness. The extent of toughening depends upon the length of the bridging zone in the crack wake; at steady state, where the zone is at a maximum length governed by ductileligament rupture, the increase in fracture energy, AGc, has been estimated in terms of the area fractionfof ductile ligaments intersecting the crack path, their individual yield strength, O'y, and a representative cross-sectional radius, r, as 141 AGe = f ~ryrX
[ 1]
where X is a dimensionless function representing the work of rupture which can vary between - 0 . 5 and - 8 , depending upon the degree of interface debonding and constitutive properties of the reinforcement phase.t4-8] This approach has been used with success in several ceramic/ K.T. VENKATESWARA RAO, Research Engineer, and R.O. RITCHIE, Professor, are with the Department of Materials Science and Mineral Engineering, University of California at Berkeley, Berkeley, CA 94720. W.O. SOBOYEJO, formerly with McDonnell Douglas Research Laboratories, St. Louis, MO, is with the Edison Welding Institute, Columbus, OH 43212. Manuscript submitted August 26, 1991. METALLURGICAL TRANSACTIONS A
metal and intermetallic/metal systems, including glass/Al, glass/Ni, AI203/AI, WC/Co, TiA1/Nb, TiAI/TiNb, and NbsSi3/Nb, under monotonic loading conditions, tj-~2] There is still concern, however, over the effectiveness of such ductile-phase toughening mechanisms under cyclic loading conditions, t~3] Many of the toughened intermetallic and ceramic composites under current active study are being developed as potential high-temperature structural materials for the next generation of aerospace propulsion systems. Molybdenum disilicide (MoSi2) is a promising candidate for such applications due to its high melting point (2050 ~ moderate density (6.31 g/cm3), and excellent oxidation resistance, resulting from the formation of a protective glassy SiO2 film at temperatures above 600 ~ In fact, for this reason, silicides such as MoSi2 *Between 300 ~ and 600 ~ MoSi2 is prone to severe oxidation in air or oxygen-beating atmospheres, referred to as "pest failures.-fJ4j
are widely used as oxidation-resistant coatings on refractory-metal components. [jS] However, like most intermetallics, the structural use of MoSi2 is severely limited by problems of low ductility, toughness, and impact strength at room temperature (the ductile-to-brittle transi
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