Environmental fatigue of an Al-Li-Cu alloy: Part II. Microscopic hydrogen cracking processes

Abstract

Microscopic fatigue crack propagation (FCP) paths in peak-aged unrecrystallized alloy 2090 are identified as functions of intrinsicda/dN- δK kinetics and environment. The FCP rates in longitudinal-transverse (LT)-oriented 2090 are accelerated by hydrogen-producing environments (pure water vapor, moist air, and aqueous NaCl), as defined in Part I. Subgrain boundary cracking (SGC) dominates for δK values where the cyclic plastic zone is sufficient to envelop subgrains. At low δK, when this crack tip process zone is smaller than the subgrain size, 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 oriented slip bands. This mode does not change with δK, and T1 decorated subgrain boundaries do not affect an expectedda/dN- δK transition for the inert environments. Rather, the complex dependence ofda/dN on δK is controlled by the environmental contribution to process zone microstructure-plastic strain interactions. A hydrogen embrittlement 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 plastic zone volume (δK), and the benignancy of O2. The SGC may be due to hydrogen production and trapping at T1 bearing sub-boundaries after process zone dislocation transport, while crystallographic cracking may be due to lattice decohesion or hydride cracking.