Fatigue crack initiation and propagation in A356 alloy reinforced with in situ TiB2 particles

Fatigue crack initiation in the in situ TiB2/A356 composite has been investigated by high cycle fatigue testing using smooth specimens. Fatigue crack propagation in the composite was also studied through fatigue crack growth testing using pre-cracked three point bending specimens. The results showed that TiB2/A356 composite exhibits a higher high cycle fatigue life but a faster crack growth rate compared with the unreinforced alloy, suggesting that the composite has a higher resistance to crack initiation but a lower resistance to crack propagation. Fatigue fractography indicated that pores and inclusions near the specimen surface are the preferred crack initiation sites. Cracks also prefer to initiate at fractured eutectic silicon particles. After initiation, cracks first tend to propagate within the matrix and avoid TiB2 particles, and there is no cracking in the in situ TiB2 particles. With increasing stress intensity, more eutectic silicon particles fracture or debond, and then the separated TiB2 appears. As stress intensity increases, cracks propagate along TiB2/matrix interfaces and link the fractured eutectic silicon particles, and then causes composite to fracture.

► TiB2/A356 composite exhibits higher fatigue life but faster crack growth rate. ► Pores and inclusions near specimen surface are the preferred crack initiation sites. ► Cracks also prefer to initiate at fractured eutectic silicon particles. ► Cracks propagate within matrix and avoid TiB2 particles at intermediate ΔK level. ► Cracks propagate along TiB2 interface and link cracked Si with increasing ΔK.