EFFECTS OF HEAT-TREATMENT AND REINFORCEMENT SIZE ON REINFORCEMENT FRACTURE DURING TENSION TESTING OF A SIC(P) DISCONTINUOUSLY REINFORCED ALUMINUM-ALLOY

The effects of heat-treatment, matrix microstructure, and reinforcement size on the evolution of damage, in the form of SiC(p) cracking, during uniaxial tension testing of an aluminum-alloy based composite have been determined. A powder metallurgy Al-Zn-Mg-Cu alloy reinforced with 15 vol pct of either 5 or 13 mum average size SiC(p) was heat treated to solution annealed (SA), underaged (UA), and overaged (OA) conditions. The SA treatment exhibited lower yield strength and higher ductility for both particulate sizes compared to the UA and OA conditions. The evolution of damage, in the form of SiC(p) fracture, was monitored quantitatively using metallography and changes in modulus on sequentially strained specimens. It is shown that the evolution of SiC(p) fracture is very dependent on particulate size, matrix aging condition, and the details of the matrix-reinforcement interfacial regions. SiC(p) fracture was exhibited by the UA and OA treatment over a range of strains, while a preference for failure near the SiC(p)/matrix interfaces and in the matrix was exhibited in the OA material. While the percentage of cracked SiC(p) at each global strain typically was equal or somewhat lower in the material reinforced with 5 mum average size SiC(p), the absolute number of cracked SiC(p) was always higher at each global stress and strain in the material containing 5 mum average size SiC(p), for each heat treatment. Damage (e.g., voids) in the matrix and near the SiC(p)/matrix interfaces was additionally observed, although its extent was highly matrix and particle-size dependent. It was always observed that increases in stress (and strain) produced a larger amount of fractured SiC(p). However, neither a global stress-based nor a global strain-based model was sufficient in converging the amount of SiC(p) fractured for all heat treatments and particle sizes tested.