Bridge toughening enhancement in double-notched MoSi 2 /Nb model composites
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INTRODUCTION
MOLYBDENUM disilicide (MoSi2) is believed to be a promising intermetallic matrix for high-temperature applications, m It offers oxidation resistance which is superior to most other intermetallics at elevated operating temperature above 1000 ~ (melting temperature of MoSi2 is 2030 ~ MoSi2 also retains high strength and stiffness and significantly lower density (6.24 g/cm3) in comparison to nickelbase superalloys (8.2 to 8.4 g/cm3). Unfortunately, the brittle-to-ductile transition for MoSi 2 occurs in the region of 1000 ~ which poses a major problem with respect to toughness of this material at lower temperature. The room-temperature toughness of MoSi 2 is about 3 MPa'~/-m.m Various reinforcement strategies have been used including both brittld z] and ductile reinforcement~3,4s]of the intermetallic matrix to impart low-temperature toughness and damage tolerance. Various reinforcements based on refractory metal, e.g., Mo or Nb wire, provide energy dissipation on crack opening which increases fracture toughness of the intermetallic. Ductile phase toughening, using refractory metals and alloys, has the advantage of increasing toughness at low temperatures without losing creep resistance at high temperatures,t61Additionally, Nb (and to some extent Mo) refractory metal provides a close match of the thermal expansion coefficient (CTE Nb = 7 • 10 -6 K -1, CTE Mo = 6 • 10-6 K -I) with MoSi2 (CTE = 8 • 10-6 K-~), which alleviates problems associated with thermal cycling fatigue commonly associated with ceramic reinforcement such as SiC in MoSi23~]
S.M. PICKARD, Research Fellow, and A.K. GHOSH, Professor, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136. This article ~s based on a presentation made at the "High Temperature Fracture Mechanisms in Advanced Materials" symposium as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee. METALLURGICALAND MATERIALSTRANSACTIONSA
The evolution of damage in the process of ductile phase toughening in MoSi2/Nb composites is investigated in this study. Particular attention is paid to interface deformation and fracture aided by a weak debond coating and an adherent reaction layer with subsequent deformation of the Nb metal. Prior studies on ductile phase toughening in brittle solids concentrated on crack bridging across the ductile phase for symmetric cracks and determination of constrained stress effects for the ductile phase, tSJ Asymmetric crack trajectories have been studied in model laminates of glass and lead,t9] where increased toughening potential is found in asymmetric geometries subject to debonding of the ductile phase. When a weak debond path is unavailable for asymmetric crack geometry, increased shear stress at large offset angles and increased shear failure strain causes an increased energy to failure. However, the toughening derived for a well-bonded system with asymmetric cracks is substantially less than
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