ENVIRONMENTAL FATIGUE OF AN AL-LI-CU ALLOY .2. MICROSCOPIC HYDROGEN CRACKING PROCESSES

Microscopic fatigue crack propagation (FCP) paths in peak-aged unrecry stallized alloy 2090 are identified as functions of intrinsic da/dN-DE LTAK kinetics and environment. The FCP rates in longitudinal-transvers e (LT)-oriented 2090 are accelerated by hydrogen-producing environment s (pure water vapor, moist air, and aqueous NaCl), as defined in Part I. Subgrain boundary cracking (SGC) dominates for DELTAK values where the cyclic plastic zone is sufficient to envelop subgrains. At low DEL TAK, when this crack tip process zone is smaller than the subgrain siz e, environmental FCP progresses on or near {100} or {110} planes based on etch-pit shape. For inert environments (vacuum and He) and pure O2 with crack surface oxidation, FCP produces large facets along {111} o riented slip bands. This mode does not change with DELTAK, and T1 deco rated subgrain boundaries do not affect an expected da/dN-DELTAK trans ition for the inert environments. Rather, the complex dependence of da /dN on DELTAK is controlled by the environmental contribution to proce ss zone microstructure-plastic strain interactions. A hydrogen embritt lement mechanism for FCP in 2090 is supported by similar brittle crack paths for low pressure water vapor and the electrolyte, the SGC and { 100}/{110} crystallographic cracking modes, the influence of cyclic pl astic zone volume (DELTAK), and the benignancy of O2. The SGC may be d ue to hydrogen production and trapping at T1 bearing sub-boundaries af ter process zone dislocation transport, while crystallographic crackin g may be due to lattice decohesion or hydride cracking.