The fracture behaviour of A356 alloys with different iron contents under resonant vibration

The fracture behaviour of A356 alloys with different iron contents under resonant vibration has been investigated by examining the morphology of eutectic silicon and Fe-rich intermetallic phases. The test materials were prepared by varying the iron content within the range of 0.14 to 0.97 wt%. The experimental results shows that while the iron content is below 0.57 wt%, the Fe-rich intermetallic phases are mainly π-Al₈Mg₃FeSi₆ and α-Al₁₅Fe₃Si₂ and there is no obvious difference in the fracture resistance under resonant vibration. However, when the specimen has higher iron content (0.97 wt% Fe), the fracture resistance drops due to the formation of β-Al₅FeSi. Hence, the β-Al₅FeSi morphology is more detrimental to the fracture resistance than that of π-Al₈Mg₃FeSi₆ or α-Al₁₅Fe₃Si₂. The results indicate that the vibration fatigue cracks initiate at the surface and the fracture paths go predominantly through the silicon particles, and occasionally through the Fe-rich intermetallic phase particles. The detrimental β-Al₅FeSi becomes influential in the specimen with higher iron content. The results also show that the line intercepted density (LID) and projected crack intercepted density (PCID) values of Fe-rich intermetallic phases increased with increasing area fraction of Fe-rich intermetallic phases (or iron content). This shows that an increase in area fraction of Fe-rich intermetallic phases (or iron content) may promote the crack propagation and growth under resonant vibration.