Environmental fatigue of an Al-Li-Cu alloy: part I. Intrinsic crack propagation kinetics in hydrogenous environments
- PDF / 1,695,289 Bytes
- 14 Pages / 630 x 792 pts Page_size
- 99 Downloads / 163 Views
I.
INTRODUCTION
PRECIPITATION-hardened A1-Li-Cu-based alloys are of considerable interest for aerospace applications because of low density, high stiffness, superior fatigue crack growth resistance, and high cryogenic toughness. I~-5[ Research has concentrated on improving those properties controlled by high-angle grain boundaries, including low short-transverse fracture toughness, [6-91 localized aqueous corrosion, [~~ and intergranular stress corrosion cracking. [14,15,161In contrast, studies are lacking regarding environmental effects on fatigue crack growth and associated damage mechanisms. The outstanding fatigue crack propagation (FCP) resistance of A1-Li alloys in moist air is controlled by two factors: increased extrinsic shielding by crack closure and reduced environmental embrittlement compared to other high-strength aluminum alloys. I~7,~81 Increased elastic modulus may provide some improvement in cracking resistance. The strong effect of closure on FCP is traceable to the highly localized planar and reversible character of slip in under- to peak-aged AI-Li microstructures hardened by ordered coherent precipitates. ]~9,2~ Such deformation exacerbates roughness-induced closure by producing microscopic crack deflections, highly faceted fatigue crack surfaces, and mode II displacements. The mechanical and microstructural factors which affect crack closure in AI-Li alloys are reviewed elsewhere. ~181 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 23665. RICHARD P. GANGLOFF, Professor, is with the Department of Materials Science, University of Virginia, Charlottesville, VA 22903. Manuscript submitted August 1, 1990. METALLURGICAL TRANSACTIONS A
Aluminum alloys are prone to cracking in water vapor or halogen-bearing electrolytes and under monotonic or cyclic loading. [22,231 Extensive environmental FCP rate (da/dN) studies have been conducted on conventional 2000 and 7000 series aluminum alloys. [231 This work identified the effects of important variables including: stress intensity range (AK = Km~x - gmin), [241 s t r e s s ratio (R gmin/Kmax),[25"261water vapor p r e s s u r e , [27'28'291 o x y g e n partial pressure, 13~ electrode potential, [31,32,331loading frequency,[26-28.32.34] alloy composition,[351 and deformation mode. [361 The preeminent mechanisms for environmental cracking of aluminum alloys include: hydrogen environment embrittlement, t27,32,37-4~ passive film formation/rupture/anodic dissolution, [41] and surface film effects on plastic deformation, r31,421 Uncertainties exist regarding environmental fatigue in aluminum alloys. While hydrogen embrittlement is established for 2000 and 7000 series alloys in water vapor, cracking mechanisms for aqueous solutions are controversial because of the simultaneous action of hydrogen production, metal dissolution, and surface film formation. [27,371 Quantitative models for da/dN-AK were developed for specific c
Data Loading...
