Effect of silicon particles on the fatigue crack growth characteristics of Al-12 Wt Pct Si-0.35 Wt Pct Mg-(0 to 0.02) Wt Pct Sr casting alloys

Abstract

Fatigue crack growth (FCG) characteristics and mechanisms in Al-Si-Mg eutectic casting alloys containing 0.35 wt pct Mg and 0 to 0.02 wt pct Sr were investigated as a function of stress ratio,R, stress-intensity-factor range, ΔK, and silicon (Si) particle size. The fatigue crack propagation behavior was compared with that observed in commercial casting alloy A356. At the same applied ΔK level, the crack growth rate was found to increase with increasing stress ratio and Si particle size. Modified (fine Si morphology) and A356 alloys showed better FCG resistance than the unmodified (coarse Si morphology) ones, for a constant applied ΔK, due to increased closure. The effects of roughness-induced and plasticity-induced crack closures, crack branching, and crack meandering on the fatigue crack propagation observed in these alloys have been discussed. The fatigue crack propagation path is found to be dependent on the Si particle characteristics. The mechanisms of silicon particle decohesion and cracking are also discussed.