Closure-affected fatigue crack propagation behaviors of powder metallurgy-processed Al-Li alloys in various environments

The environment-affected fatigue crack propagation (FCP) behavior of rapid solidification-processed (RSP) Al-2.6Li-1.0Cu-0.5Mg-0.5Zr (RSP 644-B) and mechanically alloyed (MA) Al-4.0Mg-1.5Li-1.1C-0.8O₂ (MA 905-XL) were examined in air, in vacuum, and in an aqueous 3.5 pct NaCl solution at R=0.1 and a sinusoidal frequency of 20 Hz. The emphasis was placed on the effect of environment-sensitive crack closure on the FCP behavior of fine-grain-sized powder metallurgy (P/M)-processed Al-Li alloys. The present study suggests that closure is extremely sensitive to environmental factors and significantly alters the environment-affected da/dN-ΔK relationships for both alloys. In the submicron grain-sized MA 905-XL, for example, increased corrosion product-induced closure in aqueous NaCl appeared to overwhelm the detrimental environmental effects in low- and intermediate-ΔK regimes. The environment-sensitive closure contribution alone, however, cannot completely explain the FCP behavior of P/M-processed Al-Li alloys. The intrinsic environmental effects also need to be considered for further understanding of this behavior.