Ductile-Phase Toughening and Fatigue-Crack Growth in Nb-Reinforced Molybdenum Disilicide Intermetallic Composites K.T. VENKATESWARA RAO, W.O. SOBOYEJO, and R.O. RITCHIE A study has been made of the role of ductile-phase toughening on the ambient temperature fracture toughness and fatigue-crack propagation behavior of a molybdenum disilicide intermetallic- matrix composite reinforced with 20 vol pct niobium spheres. Using disk-shaped compact DC(T) samples, only moderate improvements (-24 pct) in fracture toughness Ktc values were found for the composite compared to the unreinforced MoSi2 matrix material. Moreover, (cyclic) fatigue- crack propagation was seen at stress intensities as low as 75 to 90 pct of Ktc, with growth rates displaying a high dependency (- 14) on the applied stress-intensity range. The lack of significant toughening due to the incorporation of ductile Nb particles is associated with an absence of crack/particle interactions. This is attributed to the formation of a weak reaction-layer interface and elastic mismatch stresses at the crack tip between the Nb and MoSi2, both factors which favor interfacial debonding; moreover, the spherical morphology of Nb phase stabilizes cracking around the particle. Results suggest that increasing the aspect ratio of the distributed Nb rein- forcement phase with attendant interfacial debonding and eliminating possible Nb-phase em- brittlement due to interstitial impurity contamination are critical factors for the successful development of tougher Nb/MoSiz structural composites. I. INTRODUCTION THE concept of toughening brittle solids through the incorporation of a ductile phase has attracted consider- able attention recently in an attempt to enhance the duc- tility 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 defor- mation 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 ductile- ligament rupture, the increase in fracture energy, AGc, has been estimated in terms of the area fractionfof duc- tile ligaments intersecting the crack path, their individual yield strength, O'y, and a representative cross-sectional radius, r, a s 141 AGe = f ~ryrX [ 1] where X is a dimensionless function representing the work of rupture which can vary between -0.5 and -8, de- pending upon the degree of interface debonding and con- stitutive 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. 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 devel- oped 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 im- pact strength at room temperature (the ductile-to-brittle transition temperature is between 900 ~ and 1000 ~ in addition, its high-temperature strength is relatively l o w . 114-17| Accordingly, it is a model material system for reinforcement with second-phase particles to achieve op- timal strength-toughness combinations at ambient and el- evated temperatures. Limited studies to date on MoSi2-based intermetallic- matrix composites reveal only modest improvements in toughness with the addition of second-phase reinforce- ments, t9"18-25] For example, TiC particulate reinforce- ments show little effect, I22]whereas crack deflection and crack bridging promoted by the inclusion of 20 vol pct METALLURGICAL TRANSACTIONS A VOLUME 23A, AUGUST 1992--2249