Initiation and Growth Behavior of Small Fatigue Cracks in Ultra Fine-Grained P/M Aluminum Alloys

Small fatigue crack initiation and growth behavior were investigated for two kinds of ultra fine-grained P/M (Powder Metallurgy) aluminum alloys of which grain size was from 200 to 500nm. Those two materials were denoted as Types M and S according to the chemical composition and manufacturing process. Type M without Si addition was extruded, while Type S with Si was forged. The shape of powders changed through manufacturing processes, where the aspect ratio of elongated powders in Type M was larger than that in Type S. Reversed plane bending fatigue tests were conducted and small fatigue cracks were observed by means of plastic replicas technique. The effect of specimen orientation on fatigue properties in those two materials was also investigated. The specimen orientations evaluated were C-C and L-C for Type M, and C-C and C-L for Type S. It was found that small fatigue cracks initiated at the inclusions for C-C orientation and at the boundary between powders for C-L orientation in Type M, while they initiated at the boundary regardless of specimen orientation in Type S. S-N curves were dependent on the crack initiation mode, and the crack initiation life was well estimated by Kmax calculated with stress amplitude and the size of inclusions or cluster of powders. The fatigue crack growth behavior was almost irrelevant to the materials and specimen orientations.