MICROSTRUCTURAL INFLUENCES ON FATIGUE CRACK INITIATION IN A MODEL PARTICULATE-REINFORCED ALUMINUM-ALLOY MMC

Preliminary results are presented for fatigue crack initiation within a particulate-reinforced aluminium alloy metal-matrix composite. The study examines the response of a model system consisting of Si particles in an Al-Mg-Si matrix age hardened to the peak strength T6 temper. Experimental observations indicate two sites for crack initiation for the high stress amplitudes (sigma(max) greater-than-or-equal-to 0.95 sigma(ys)) used in this study. In specimens containing smaller Si particles (almost-equal-to 4 mum), crack initiation occurs at almost-equal-to 0.4 N(f) within thin ligaments of the matrix which separate uncracked subsurface particles from the free specimen surface. In contrast, specimens containing larger Si particles (almost-equal-to 8 mum) are susceptible to crack initiation from particles which crack within 0.05 N(f). The potential contribution of thermally induced residual stresses is discussed qualitatively for the case of particles located at or near a free surface.