Effects of beryllium on fatigue crack propagation of A357 alloys containing iron

An investigation of iron-bearing compounds, silicon particles and Mg₂Si precipitates on fatigue crack growth behaviour in A357 alloys has been performed. The effect of Be on increasing ΔK_th is more apparent in alloys with higher Fe content than in alloys with medium and lower Fe content. The fatigue life of the higher and medium Fe-content Be-containing alloys will be improved much as well as that of in the lower Fe content of alloys. Major fatigue cracks induced by cracking of iron-bearing compounds cracking were found in the medium and higher Fe-content alloys. In the lower Fe-content alloys, irregular silicon particles appear, to be the main source of fatigue crack nucleation. In region I of low fatigue crack growth rates, the higher threshold stress intensity is attributed to the greater mechanical strength and to enhanced roughness-induced crack closure and deflection from more tortuous crack paths. The superior fatigue crack propagation resistance at higher growth rates of regions II and III is attributed to greater fracture toughness and to reduced crack growth rates in the Be-containing alloys regardless of Fe levels. The fracture surface morphologies of near-threshold crack growth contain stepped patterns and cleavage-like facets. With increasing crack growth rate, the fracture surface roughness increases and striations are formed in region II. At higher fatigue crack growth rates, the formation of a dimple surface and cleavage plane replaces striation formation.