Fatigue crack growth behavior in powder-metallurgy 6061 aluminum alloy reinforced with submicron Al ₂O ₃ particulates

Fatigue crack growth (FCG) behavior of aluminum alloy 6061 particulate composites reinforced with submicron Al ₂O ₃ 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 K _max ≈ 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/(Eσ _y ²).