Effect of Specimen Surface Layer on Fatigue Strength of an Al-1.2%Si Alloy

Abstract

The dependence of precipitation hardening on the distance from specimen surface and the effect of the surface layer on fatigue strength of an Al-1.2mass%Si alloy was studied by microhardness test, transmission electron microscopy and repeated tension fatigue test The rate of age-hardening was slower in the vicinity of the surface than in the interior of the specimen aged at 423K after quenching from 853K. Electron microscopy results showed that the size of Si precipitates formed in the vicinity of surface was smaller than that in the interior of specimen aged for 6ks at 423K. This difference was considered to be caused by the effect of the surface as vacancy sinks which slow down the growth of Si precipitates in the vicinity of the specimen surface. Therefore, regions with a slightly lower hardness than the interior of the specimen are formed in the vicinity of the specimen surface, when the specimen is aged at a relatively low temperature such as 423K. These regions are called the Soft Surface Layer. It was found that the existence of the soft surface layer improved the fatigue strength in repeated tensile test.