Effects of Heat Treatment and Reinforcement Size on Reinforcement Fracture during Tension Testing of a SiC_p Discontinuously Reinforced Aluminum Alloy

Abstract

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 μm 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 thepercentage of cracked SiC_p at each global strain typically was equal or somewhat lower in the material reinforced with 5 μm average size SiC_p, theabsolute number of cracked SiC_p was always higher at each global stress and strain in the material containing 5 μm 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.