Stress corrosion cracking of an Al-Li alloy
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I.
INTRODUCTION
ALUMINUM-lithium alloys are attractive materials for aircraft component and aerospace applications due to their reduced density and increased stiffness compared to conventional aluminum alloys, tq Although many behaviors of A1-Li alloys have been and are being studied, it is necessary to understand their stress corrosion cracking (SCC) characteristics and mechanism because of the high SCC susceptibility of other high-strength aluminum alloys. The purpose of the investigation of AI-Li alloys is to look for the best combination of strength, ductility, and SCC resistance and to understand the mechanism of SCC. It is known that the SCC behavior of A1-Li alloys is affected strongly by mechanics, [2] chemistry, [3m and heat treatment, [1'5-71 but the results of these factors were different due to the variation of materials and testing conditions. Up to now, there were two important points of view on the SCC mechanism of AI-Li alloy. One was local anodic dissolution (LAD), tE'Sjwhich showed a driving force for preferential dissolution of the precipitates in grain boundaries or subgrain boundaries due to the electrochemical potential difference between the grain boundaries or the subgrain boundaries and the interior of grains; the other was hydrogen embrittlement (HE) 14,6,8] of materials at the crack tip, which was testified by banded texture in some grains and discontinuous propagation of the crack. [5] The present work studied the behavior of the SCC and discussed a possible SCC mechanism of an AI-Li alloy.
II.
EXPERIMENTAL PROCEDURE
The material used in this investigation was an alloy of weight percent (wt pet) 2.7 pet Li, 1.33 pct Cu, 0.99 pct Mg, 0.15 pct Zr, 0.007 pct Fe, and 0.054 pct Si. It was melted and cast in a vacuum induction furnace with argon Z.F. WANG, Research Associate, Z.Y. ZHU, Associate Professor, Y. ZHANG, Associate Professor, and W. KE, Professor and Director, are with the Corrosion Science Laboratory, Institute of Corrosion and Protection of Metals, Academia Sinica, Shenyang l l0015, People's Republic of China. Manuscript submitted October 8, 1991. METALLURGICAL TRANSACTIONS A
protective atmosphere. The ingots (150N) were homogenized in two stages, at 460 ~ for 10 hours and 510 ~ for 10 hours, then heated at 450 ~ for 2 hours followed by extruding and cogging. The aging conditions were natural aged (NA), peak aged (190 ~ hours) (PA), and overaged (210 ~ hours) (OA) after solution heat treatment at 530 ~ for 1 hour and water quenched. The main mechanical properties of NA, PA, and OA specimens with different directions were shown in Table I. The stress corrosion crack propagation resistance of the tested alloy was evaluated using single edge notched plate specimens shown in Figure l(a) (30-mm width, 150-mm length, and 2-mm thickness) machined from plate with the longitudinal direction (LT) and transversal direction (TL). Before testing, the specimens were fatigue precracked to 4.5 mm in air, and the maximum stress intensity factor during precracking did not exceed 80 pct of the
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