The fracture characteristics of Al-9Ti/SiC p metal matrix composites
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INTRODUCTION
T H E aerospace industry is developing aluminum matrix composites for high-temperature applications. One system developed for this purpose is dispersionstrengthened AI-9Ti reinforced with cobalt sol-gel-coated SiC particles. Sol-gel coatings are applied to ceramic reinforcement to provide diffusion bonding of the reinforcement to the matrix at lower than normal fabrication temperatures, as well as to maintain high interfacial shear strength at high use temperatures, m High shear strength interfaces are developed through intermetallic phase formation during consolidation. Tensile strengths of up to 120 MPa at 500 ~ have been measured in A I - 9 T i / C o / SiC particle-reinforced composites.t2] Processing defects from the sol-gel coating step consisting of SiC particle clumps bonded by cobalt have been observed in the microstructure of these materials. An A1-Co intermetallic reaction zone forms around these defects, and exposure to elevated temperature causes the intermetallic region to grow. pj The objective of this research was to apply a linear elastic fracture mechanics (LEFM) approach to evaluate the effect of annealing A1-9Ti/Co/ SiC particle-reinforced composites on the fracture toughness of the material. The C o / S i C particle defects which initiate tensile failure were studied at room temperature and 200 ~ for as-received composites, as well as for two annealing times at 500 ~
A. Linear Elastic Fracture Mechanics Considerations The fracture toughness (Kc) for a material is defined as the critical value of an applied stress intensity at which unstable propagation of an existing crack occurs. Irwin [4] showed that the local stresses near a c r a c k are dependent T.D. BAYHA, Semor Materials and Processes Engineer, is with Lockheed Aeronautical Systems Company, Marietta, GA 30063. R.J. KILMER, Graduate Rcsearch Assistant, and F.E. WAWNER, Research Professor, are with the Department of Materials Science, University of Virginia, Charlottesville, VA 22903. Manuscript submitted August 26, 1991. METALLURGICALTRANSACTIONSA
on the nominal stress (or0), the square root of the crack length, and a constant ( F ) dependent on specimen and crack geometry: K, = F0-o(Tra) t/2 [1] In Eq. [1], Kc is the stress intensity factor. For a discontinuously reinforced composite which is linear elastic to the fracture stress, Eq. [1] holds, and the fracture criterion is based on LEFM. In this case, Kc equals K~c, the plane strain fracture toughness; K~c is a material constant and can be determined from a tensile test on a sample having a known initial crack size. [~] The material is assumed to be homogeneous and isotropic, and the crack that initiates failure is assumed to propagate normal to the applied stress. The width of the composite sample must be much greater than the defect size, as well as at least 50 times the radius of the plastic zone. Im A large plastic zone size in the material would imply significant plasticity, thereby weakening the validity of the analysis. The plastic zone radius can be calculat
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