Micromechanism of Fatigue Crack Growth in Melt-Sintered Al-Al₂O₃ Alloys

Abstract

The melt-sintered Al-Al₂O₃ dispersion-strengthened alloy has been fatigued in plane bending at the resonant frequency for the second-order mode of oscillation to study the micromechanism of fatigue crack growth. Specimens were annealed at 373K for 1hr to obtain the structure for recovery stage heat treatment, having the average cell size of 0.7μm in diameter. Fatigue damage was examined by optical and scanning electron microscopy. It was found from the microstructural observations that the fracture surface morphology with fatigue crack growth was classified into three categories depending on ΔK values: in ΔK<5MPa√m the granular mode of fracture covered over fracture surfaces, in 5<ΔK<10MPa√m striation-like marking and microcracking were observed together with the granular mode, and in ΔK>10MPa√m the granular mode has been hardly observed. The ΔK≈5MPa√m seemed to be the critical value at which the striation-like marking and microcracking appeared on fracture surfaces. It was concluded that fatigue damage concentrated on cell boundaries. The results obtained are discussed in connection with the effect of the Al₂O₃ particles dispersed in cells on dislocation motion.