PIT MORPHOLOGY OF ALUMINUM-ALLOY AND SILICON-CARBIDE ALUMINUM-ALLOY METAL MATRIX COMPOSITES

Pit morphology of Al 5456 (UNS A95456), Al 6061 (UNS A96061), SiCw/Al 5456, and SiCw/Al 6061 was studied in order to compare pitting processes of SiCw/Al metal matrix composites with that of corresponding unreinforced alloys. Work on anodized samples of Al 6061 and SiCw/Al 6061 is also included. Pits on the composites more numerous, shallow, and widespread than on the monolithic materials. Studies of pit structure suggest there are two stages in pit development. The first involves the initial dissolution of metal atoms and opening of the pit, and the second involves pit enlargement or growth. For both materials, pits initiate at secondary particles within the metal matrix. In the case of Al 5456 and SiCw/Al 5456, these particles are intermetallic phases composed of alloying elements Mg, Cr, Mn, and Al, as well as Fe, an impurity of the metal. Under equivalent conditions of preparation and processing, a larger number of intermetallic phases form in the composite than the alloy hence the composite has more pit initiation sites. Examination of larger pits indicates normal pit growth processes are altered by the presence of the SiC reinforcement.