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I.

INTRODUCTION

ALUMINUM-LITHIUM alloy is a new kind of structural material. Its density is less than that o f conventional aluminum alloy, and its strength and modulus are higher than those o f the latter. It has great promise f o r potential applications in the aerospace and weapon industries.l 1-7] Nevertheless, the effects of hydrogen on the mechanical properties and fracture behavior o f A1-Li alloy have become so important that they need to be studied urgently. Although in recent years a few articles have been published on hydrogen embrittlement of 8090 and 2090 A1-Li alloys, I8-~l~ many problems remain to be solved. Thus, in this article, the effects o f hydrogen and strain rate on the mechanical properties and fracture mechanism o f 8090 A1-Li alloy are studied.

II.

EXPERIMENTAL P R O C E D U R E S

The chemical composition o f 8090 A1-Li alloy is listed in Table I. The alloy was vacuum melted in an induction furnace and cast in a high-purity argon atmosphere. The ingot was annealed at 783 K for 12 hours, and then the surface layer was machined a w a y . A f t e r being preheated at 783 K for 2 hours, the ingot was hot rolled to plates of 2-mm thickness. The plate was solution treated at 803 K for 1 hour and water quenched at room temperature. All specimens were aged at 463 K f o r 16 hours in o r d e r to obtain a microstructure corresponding to the peak-aged condition. The grain structure o f the plate was partially recrystallized LIAN CHEN, Professor, WENXIU CHEN, Professor, ZHONGHAO LIU, Assistant Professor, YUXIA SHAO, Assistant Professor, and ZHUANGQI HU, Professor, are with the Institute of Metal Research, Academia Sinica, Shenyang 110015, People's Republic of China. Manuscript submitted April 2 3 , 1992. METALLURGICAL TRANSACTIONS A

with larger pancake-shaped grains. Owing to the presence o f pancake-shaped grains, tensile specimens were taken along the long transverse directions and perpendicular to the rolling direction. The tensile specimens had a gauge length o f 26 mm and width o f 6 mm. All o f the specimens were carefully polished to r e move the surface damage. After polishing, some o f these specimens were cathodically hydrogen charged in a 0.1 N NaOH solution containing 250 mg A s 2 0 3 p e r liter. The charging current density was 100 A / m e, and the hydrogen charging time was 6 hours. At room temperature (293 K), the tensile test was carried out on a Shimadzu Autograph Ag-6000A testing machine to fracture with three kinds of strain rates (~): 3,4 x 10-3/s, 3.4 x 10 4/ s, and 3.4 x 10 5/s. The fracture morphology was observed by a Shimadzu EPM-810 electron microscope. Commercial secondary ion mass spectroscopy (SIMS) equipment o f "second generation," KYKY LT-1A, was used to detect the change o f binding energy o f aluminum atom before and a f t e r hydrogen charging. A schematic d i a g r a m o f LT-1A type o f SIMS is shown in Figure 1, in which an energy filter grid was inserted between the collection slit and transfer electrode. Changing the grid potential, beginning with the lo