Environmental fatigue of an Al-Li-Cu alloy: Part II. Microscopic hydrogen cracking processes
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I.
INTRODUCTION
R E S E A R C H in Part I m characterized fatigue crack propagation (FCP) in precipitation hardened A1-Li-Cu alloy 2090 for inert (vacuum and helium), gaseous oxygen, and hydrogenous (pure water vapor, moist air, and aqueous NaC1 with varying polarization) environments. Intrinsic FCP rates (da/dN) were measured for several constant stress intensity ranges (AK = Kma• - Kmi.) and at programmed high stress ratios (R = Kmin/KmaOto minimize crack closure and transient chemical effects. The moist gas and aqueous environments accelerated FCP in alloy 2090, as qualitatively explained by hydrogen environment embrittlement and similar to other 2000 series aluminum alloys, t~,2J While simple power-law da/dN-AK behavior was observed for FCP in vacuum, He, and 02, multisloped growth rate relationships were produced by the moist air, water vapor, and chloride environments. For each environment, fatigue process zone size substantially varied relative to several microstructural parameters. ~3~
A. Objective The objective of the work in Part II is to physically define the microscopic processes for environmental fatigue crack tip damage in A1-Li-Cu alloys. The approach is to correlate the intrinsic 2if( dependence of crack growth rates for 2090 in the inert and aggressive environments ROBERT S. PIASCIK, formerly Graduate Student, Department of Materials Science, University of Virginia, is Scientist, Mechanics of Materials Branch, NASA-Langley Research Center, Hampton, VA 23681-0001. RICHARD P. GANGLOFF, Professor, is with the Depam-nent of Materials Science, University of Virginia, Charlottesville, VA 22903. Manuscript submitted April 13, 1992. METALLURGICAL TRANSACTIONS A
with fractographic observations of crack path, facet crystallography, and the underlying microstructure. Such determinations are necessary to assess the hydrogen mechanism for crack tip process zone damage and the role of microstructure-environment-plasticity interactions.
B. Background Environment affects the crack path and damage mechanism for FCP in precipitation-hardened aluminum alloys. For inert environments, or at high AK and rapid da/dN where environmental effects are minimal, crack surface morphologies include mechanical striations; precipitate-free zone shear, planar slip band separation, and ductile rupture. E4-161Limited fractographic studies have been conducted on aluminum alloys stressed in inert environments free of water or 02 contaminants. Considering environments that produce hydrogen, studies of Pax'is-regime FCP identified brittle striated crack growth in the Al-Zn-Mg/sea w a t e r s y s t e m . [1~ In contrast to blunting-buckling-based striations, t6J repeating brittle striations involve "cleavage" crack growth on {100} or {110} planes, t15,16] For low AK, FCP produces crystallographic facets for a variety of aggressive environments, including moist air. [1'9A4'16'18-2~ Facets were identified as hydrogen-promoted cleavage parallel to {100} [4] o r as slip band cracking along {111} planes. [9"121 Both high-angle and subgrai
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