Environmental fatigue of an Al-Li-Cu alloy; Part II: Microscopic hydrogen cracking processes
- NASA, Hampton, VA (United States). Langley Research Center
- Univ. of Virginia, Charlottesville, VA (United States). Dept. of Materials Science
Microscopic fatigue crack propagation (FCP) paths in peak-aged unrecrystallized alloy 2090 are identified as functions of intrinsic da/dN-{Delta}K 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 1. Subgrain boundary cracking (SGC) dominates for {Delta}K values where the cyclic plastic zone is sufficient to envelop subgrains. At low {Delta}K, 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 O{sub 2} with crack surface oxidation, FCP produces large facets along [111] oriented slip bands. This mode does not change with {Delta}K, and T{sub 1} decorated subgrain boundaries do not effect an expected da/dN-{Delta}K transition for the inert environments. Rather, the complex dependence of da/dN on {Delta}K 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 ({Delta}K), and the benignancy of O{sub 2}. The SGC may be due to hydrogen production and trapping at T{sub 1} bearing sub-boundaries after process zone dislocation transport, while crystallographic cracking may be due to lattice decohesion or hydride cracking.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 142324
- Journal Information:
- Metallurgical Transactions, A, Vol. 24A, Issue 12; Other Information: PBD: Dec 1993
- Country of Publication:
- United States
- Language:
- English
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