Fatigue crack growth behavior in powder-metallurgy 6061 aluminum alloy reinforced with submicron Al2O3 particulates

Fatigue crack growth (FCG) behavior of aluminum alloy 6061 particulate composites reinforced with submicron Al2O3 has been investigated in two volume fractions of 10% and 20%. The composites were fabricated by powder processing and consolidated by hot extrusion into 17 mm round bars. FCG rates were measured at a fixed load ratio of R = 0.1 using three-point bending specimens. Both composites showed higher FCG resistance than that reported for a monolithic AA6061 over the threshold and Paris regimes. Increasing the reinforcement content from 10% to 20% resulted in slight improvement in the near-threshold fatigue response but caused marked reduction in FCG rates over the intermediate-ΔK Paris regime. FCG rates in the 10% composite were adequately described by Paris law with a single slope for the entire range of ΔK tested. The 20% composite, however, exhibited an abrupt transition in Paris slope at Kmax ≈ 13 MPa m. Examination of the fatigue crack-surfaces revealed that this transition was accompanied with a change in the growth mechanism from FCG via matrix cracking to accelerated FCG due to concurrent damage introduced by micro-void nucleation and growth at the crack-tip process zone. Variations in FCG rates through the matrix with reinforcement content correlated proportionally with 1/(Eσy2).