EFFECTS OF AGING TREATMENTS ON FRACTURE C HARACTERISTICS OF 6061 ALUMINUM-ALLOY REINFORCED WITH SIC WHISKER

The effect of matrix microstructure on fracture characteristics of 606 1 aluminum alloy reinforced with SiC whisker was investigated. The und eraged composite exhibits stable slow crack growth behaviour and posse sses a high fracture toughness value of 20.0 MPa square-root 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 l evel of 13.6 MPa square-root 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 compos ite has the tendency of void nucleation and growth at the sharp comers of whiskers, and it is considered due to severe stress concentration. Meanwhile whisker breakage is mainly observed in the peakaged and ove raged composites, and it is considered to be attributed to immediate r apid and unstable fracture. From the microstructural observations carr ied out using TEM, narrow PFZ is observed around whiskers and it is ac companied by coarse precipitates on their surfaces in the peakaged com posite. Further, the density and size of the precipitates, according t o direct and high-magnification observations on the surfaces of whiske rs, increase with increasing aging time. The change of the microscopic fracture mechanism with increasing aging time in the mechanical prope rty 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 comers of whiskers in the peakaged and overaged composites.