Constrained Cavitation and Fast Fracture at Metal-ceramic Interfaces at Elevated Temperatures
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Constrained cavitation and fast fracture at metal-ceramic interfaces at elevated temperatures C. M. Kenneficka) U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (Received 30 December 1996; accepted 14 September 1998)
Processes that constrain or promote cavity growth and fast fracture at elevated temperatures are examined. Solutions are given for the stress caused by inhomogeneous deposition of matter in metal-ceramic and alumina grain boundaries and for the tensile stress near the top of a hemispherical pore during pore growth. Velocities of dislocation climb that could promote fast fracture are calculated for elastic stresses acting upon dislocations arising from both a crack tip and interface repulsion. The rates for the atomic diffusive processes and the magnitudes of stresses resulting from them are found to agree well with experimental rate of pore growth, and new data on pore growth and fracture at an aluminum-sapphire interface are presented.
I. INTRODUCTION
With weak bonding providing crack deflection in a composite, or with strong bonding promoting a stable material boundary, bimaterial interfaces have been shown to provide toughening in materials at room temperature. These same interfaces, however, must often provide toughening at elevated temperatures, where failure by pore nucleation and growth may predominate. Models for pore growth at grain boundaries1,2 have analyzed the diffusion of atoms into a boundary driven by a tensile stress acting upon it. Constraints to cavity growth in polycrystalline metals3,4 have been attributed to a compressive backstress produced when an isolated cavitated grain extends by pore growth and is then constrained by neighboring grains. Under geometrical constraints, the growth of pores is predicted to be controlled by the creep rate of the surrounding material. There has also been some evidence of slow pore growth at bimaterial interfaces. These interfaces can be either planar,5 or curved,6 as in the case of silica particles in copper. Constrained cavity growth in the case of second phase particles has been attributed to the slow movement of vacancies around precipitates,7 and hence again to the creep rate of the surrounding material,8 or to the pinning of grain boundary dislocations.9 The analysis presented here quantitatively assesses processes for slow pore growth and fast fracture that are atomically self induced at the interface, and not largely dependent upon the remote applied stress or upon the exact value of the interfacial diffusion coefficient. Specifically examined are the elastic stress produced
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Present address: Air Force Research Laboratory, AFRL/MLBC, 2941 P Street, Ste. 1, Bldg. 654, Wright Patterson Air Force Base, Ohio 45433-7750.
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http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 4, Apr 1999
Downloaded: 18 Mar 2015
by the nonhomogeneous deposition of matter at the boundary during pore formation and growth, the elastic stress gradient due to a pore itself in promot
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