Abstract
Dependence of precipitation hardening and aged structures of an Al–1.2 mass%Si alloy on the distance from specimen surface was studied by microhardness test and transmission electron microscopy. The specimen was water-quenched from 833 K or 853 K to 273 K, held for 60 s and subsequently aged at 373 K, 423 K or 473 K. Rate of age-hardening was slower in the surface layer than in the interior when the specimen was aged at relatively low temperatures (373 K and 423 K) .However the value of maximum hardness in the surface layer became the same as that in the interior. According to the results of electron microscopy, the size of Si phase precipitates formed in the surface layer was smaller than that in the center of specimen aged at 423 K. This difference was considered to be caused by the effect of the surface as a vacancy sink which slowed down the growth of Si phase precipitates near the surface.
