Effect of dispersoids on fatigue crack propagation in aluminium alloys

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Abstract
The fatigue crack propagation characteristics of three peak aged Al–Mg–Si alloys have been determined. One was of essentially ternary composition, the others contained increasing volume fractions of 0·1 μm dia. dispersoid particles. The stress intensity thresholds for fatigue crack propagation ∆K TH increased with increasing volume fraction of dispersoid. At low and intermediate values of stress intensity ∆K, the crack growth rate decreased as the volume fraction of dispersoid increased. However, at high values of ∆K, void coalescence around coarse particles could outweigh the beneficial effect of the dispersoids. The degree of intergranular fracture occurring in the alloys was shown to be dependent on the maximum stress intensity in the fatigue cycle K max. The micromechanism of intergranular fracture was identified as microvoid coalescence along the grain boundary precipitate free zone. The concept of a semicohesive zone ahead of the fatigue crack tip is shown to be useful in modelling the micromechanisms of crack propagation. In particular, the proposed model predicts that the crack growth rate at intermediate values of ∆K should be controlled by K max, in good agreement with the observed behaviour. The decrease in fatigue crack growth rate with increasing volume fraction of dispersoid is shown to be a result of a decrease in the degree of intergranular fracture due to the homogenization of slip by the dispersoid particles.