Corrosion fatigue of a precipitation-hardened Al-Li-Zr alloy in a 0.5 M sodium chloride solution
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INTRODUCTION
S T R E S S corrosion cracking (SCC) and corrosion fatigue (CF) of conventional aluminum alloys have been extensively investigated for many years. Exposure to environments containing liquid or vapor water is detrimental to the cracking resistance of these alloys. Anodic dissolution ~1"2jand hydrogen embrittlement [3,4,5] are two major mechanistic theories generally proposed to explain these phenomena, although the latter has been supported by an increase in experimental evidence, such as reversible embrittling phenomena, [6,7] stress-state ( i . e . , mode I v s mode III) dependent behavior, [4,8] observations of hydrides or hydrogen bubbles at grain b o u n d a r i e s . [9,1~ e t c .
In recent years, research also has been conducted to investigate environmental cracking behavior of newly developed lithium-containing aluminum alloys. It was found that, similar to conventional alloys, A1-Li alloys are susceptible to S f C [11'12'13] and C F . H4'151 However, since relevant experimental evidence is incomplete, knowledge regarding the fracture mechanism for A1-Li alloys subjected to a combination of aggressive environments and applied stresses is still limited. On the one hand, it has been observed that lithium-containing aluminum alloys are susceptible to hydrogen embrittlement. For example, Holroyd et al. [11] have demonstrated that the SCC resistance of an AI-Li-Cu-Mg-Zr (8090) alloy tested in laboratory air dramatically decreased if the sample had been embrittled by pre-exposure to 3.5 pct NaC1 solutions, while vacuum outgassing for 28 days after the pre-exposure treatment eliminated the damaging effect of absorbed hydrogen. Also, Kim e t al. I16]have reported that the tensile fracture strain of an AI-Li-Cu (2090) alloy decreased with an increase in relative hydrogen content by cathodic precharging of the specimens for an increased period of time and that the fracture surfaces of G.S. CHEN, Research Assistant, and D.J. DUQUETTE, Professor, are with the Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Manuscript submitted April 22, 1991. METALLURGICAL TRANSACTIONS A
the charged samples revealed relatively brittle features as compared to uncharged samples. However, anodic dissolution has been thought to play a major role in environment-assisted fracture of A1-Li alloys since lithium is one of the most reactive elements. For example, it has been suggested that TI (A12CuLi) and 6 (AILi) precipitates, which commonly form at subgrain and grain boundaries of A1-Li alloys, are very anodic phases with respect to the alloy matrix. [~7,18] Thus, preferential dissolution of these particles at grain or subgrain boundaries has been thought to be responsible for the difference in SCC resistance for A1-Li alloys with various aging conditions.t~2.13] A program has been recently conducted to study the hydrogen embrittlement phenomenon of a high-purity A12.5Li-0.12Zr alloy under cyclic stresses, t19] Hydrogen precharged samples and uncharged samples were fatigued in dry ai
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