In this paper, we investigated effects of aging at 473K on the relationship between
microstructure in the vicinity of the grain boundaries and fatigue strength for Al-1.2%Si alloy.
Results obtained show the following features. (1) As aging time, tA increase, the tensile strength (σB)
and 0.2% proof stress (σ0.2) increase slowly, but gradually decrease after reaching a maximum at
around 18 ks. On the other hand, fracture elongation shows an opposite trend, suggesting that at aging
times above 18ks, over aging occurs. (2) The fatigue strength lowers with increasing aging time,
however, when the aging time is more than 18 ks at 473K, the fatigue strength remains almost the
same. (3) When the aging time is more than 6 ks, grain boundary precipitates with a size greater than
several 10s of nm are observed. (4) When the aging time is 18 ks, an accumulation of dislocations are
observed at the grain boundaries and in the vicinity of grain boundary precipitates, and dislocations
increase with the number of stress cycles. (5) When the aging time is more than 6 ks, the fatigue
fracture surface is mainly intergranular. These results suggest that reduction of fatigue strength
results from propagation of micro-cracks which are initiated at the large precipitates on the grain
boundaries.