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Fatigue crack initiation and propagation have been investigated in relation to the aged microstructure in an Al 4Ge alloy by means of a fatigue test, microstructural characterization, and fractography. Specimens were solution treated at 693 K, quenched in iced water, kept at 273 K for 60 s, aged at 473 K for times up to 72 ks and then fatigue tested. It was confirmed that as the aging time increases, the tensile strength initially increases, reaches a maximum, and then decreases, while the fatigue strength monotonously decreases. Both in the interior and at the surface of the specimen, as the aging proceeded, the growth of precipitates inside the grain as well as on the grain boundary was observed while the width of precipitate free zones (PFZs) adjacent to the grain boundary was almost constant. Fractographic observations showed that intergranular fracture became predominant with increasing aging time and decreasing stress amplitude. From this result and close observations of the fatigue cracks initiation sites, the cracking was deduced to proceed in the following steps: 1) cracks initiate in the intergranular region of the specimen surface, 2) the cracks propagate along the intergranular region towards the interior of the specimen, 3) the crack path transits to the transgranular region because of the increase in true stress, 4) ductile fracture occurs with a marked reduction in area. The crack initiation was presumed to occur as a result of intrusion and extrusion formations along the soft PFZs that play a similar role to that of the persistent slip bands in most metallic materials.

Variation of hardness with position in the grain was measured during aging and reversion of Al-2 mass%Cu alloy.Age-hardening was suppressed in the surface layer relative to the interior of the specimen aged at 273K after quenching from 823K. Rate of reversion was faster in the surface layer than in the interior when the age-hardened specimen was annealed below 403K where few secondary defects should decay. When the specimen aged after quenching from high temperature was annealed at 403K, the surface layer and the interior softened nearly at the same rate. Those softenings appeared also in the regions near grain boundary.