Effects of aging treatments on fracture characteristics of 6061 aluminum alloy reinforced with SiC whisker

The effect of matrix microstructure on fracture characteristics of 6061 aluminum alloy reinforced with SiC whisker was investigated. The underaged composite exhibits stable slow crack growth behaviour and possesses a high fracture toughness value of 20.0 MPa √m. On the other hand, with increasing aging time, the fracture characteristics changes drastically to low stress type brittle fracture without stable crack growth, and the fracture toughness is degraded to the minimum level of 13.6 MPa √m in the peakaged composite. Detailed SEM observations of the fracture surfaces and cross-section of a specimen show that the microscopic fracture mechanisms of the underaged composite has the tendency of void nucleation and growth at the sharp corners of whiskers, and it is considered due to severe stress concentration. Meanwhile whisker breakage is mainly observed in the peakaged and overaged composites, and it is considered to be attributed to immediate rapid and unstable fracture. From the microstructural observations carried out using TEM, narrow PFZ is observed around whiskers and it is accompanied by coarse precipitates on their surfaces in the peakaged composite. Further, the density and size of the precipitates, according to direct and high-magnification observations on the surfaces of whiskers, increase with increasing aging time. The change of the microscopic fracture mechanism with increasing aging time in the mechanical property tests is attributed to both lowering of whisker fracture strength due to existence of coarse precipitates on its surface and depression of hydrostatic stress around sharp edges of whiskers due to existence of ductile PFZ layers. Therefore the premature fracture of whisker is considered to occur at a lower load level than nucleation of voids at the corners of whiskers in the peakaged and overaged composites.