The influence of hydrogen on atomic binding energy, critical slip shear stress, and fatigue crack propagation rate of al
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1.
INTRODUCTION
S O M E high strength A1-Zn-Mg alloy polycrystalstt-Sl and aluminum single crystals t6t showed enhanced crack propagation in aqueous environments. To interpret these phenomena, a number of possible mechanisms associated with hydrogen have been suggested. The research described in this paper was aimed at investigating the change of atomic binding energy, critical slip shear stress, and fatigue crack propagation rate of pure aluminum single crystals after hydrogen-charging. These results may provide some fundamental and valuable information for clarifying the mechanism of hydrogen induced cracking of aluminum.
II.
EXPERIMENTAL PROCEDURES
High purity (99.999 pct) aluminum single-crystal specimens used in this study were obtained by recrystallization after deformation. To investigate the effect of hydrogen, some specimens were hydrogen-charged by immersing them into 1 N HC1 + 0.25 g/1 A s 2 0 3 solution for several days. After immersing, they were washed by blowing with cool air. Hydrogen content was measured either by means of a LECO Hydrogen Determinator or by collecting the gaseous hydrogen escaped from the specimens while heated by silicon oil bath. The specimens used in the content determination tests were 4 • 4 x 25 mm in size and oriented in several different directions other than [100] for their surface normal, the surface of which was electrolytically polished; the specimens used in gas-collecting tests were 4 • 5 x 10 mm in size and oriented in the [1001 direction for surface normal, the surface of which was polished only by grit paper. The tensile tests were performed in laboratory air (RH = 30 to 60 pet) at room temperature, and the strain rate was kept at 2.08 x 10-6/second. The specimens used in tensile tests were 4 x 4 x 80 mm in size with a gage length more than 30 mm and oriented in the single slip direction. Before hydrogen-charging and tensile testing,
ZHAO-XIONG TONG, Instructor, and SHI LIN and CHI-MEI HSIAO, Professors, are with the Department of Metal Physics, Beijing University of Iron and Steel Technology, Beijing, China. Manuscript submitted September 23, 1987. METALLURGICAL TRANSACTIONS A
all of them were annealed to eliminate residual stress and hydrogen which may exist in crystals. The fatigue tests were performed in dry air (RH = 20 pct) at room temperature with an Amsler fatigue test machine. The loading ratio was R Pmin/Pmax = --1 and the frequency was 50 Hz. The Mode It specimen was used in the fatigue test and oriented in such a way that its prenotched plane was nearly parallel to a (111) plane (Figure 1). After electrolytically polishing, the specimens were annealed at 300 ~ for two hours to eliminate residual stress and hydrogen. Then, half of them were hydrogen-charged by immersing them partially to keep the region ahead of the prenotch from touching the acid solution in case the polished surface was pitted. The experiments for measuring the change in atomic binding energy of aluminum were performed with a LT1A Ion Microprobe Mass Analyzer. The primary beam of
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