Role of heat treatment and cathodic charging conditions on the hydrogen embrittlement of HP 7075 aluminum alloy
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I.
INTRODUCTION
H Y D R O G E N studies in aluminum alloys have mainly concentrated on tensile properties and crack growth behavior as a function of microstructure.[1.2] In particular, such results clearly show a significant sensitivity to hydrogen environment, particularly in the 7000 series tl-41 and to a lesser extent in 2000 series tSl alloys. The anticipated low hydrogen permeation and diffusivity at room t e m p e r a t u r e (RT) I6'71 are parameters incompatible with the observed specimen depth dependence of embrittlement either under severe cathodic charging taJ or in moist air during concurrent slow straining. [9] Additionally, these hydrogen effects, which are often manifested by a significant decrease in reduction of area, usually accompanied by intergranular fracture in the hydrogen-rich surface brittle zone, are strongly dependent on microstructure. Attempts have previously been made to explain this intergranular zone by invoking the existence of a stress-induced hydride and correlating the postulated phase with observed patterns on the intergranular fracture surface, t1~ However, significant discrepancies have been observed between the expected thermal stability of a magnesium hydride and observed behavior in aluminum alloys; tH] less information is available on the formation and stability of aluminum hydridet~2~and especially on the roles of alloying elements, microstructure, and charging conditions on hydride nucleation and growth. Thus, the existence and importance of a hydride appears to remain an open question. It is the goal of this study to characterize the roles of the alloy microstructure and cathodic charging conditions on hydrogen uptake and, specifically, to investigate the possible occurrence of a hydride and its consequences for subsequent mechanical behavior.
J. CHENE is with the Laboratoire de M6tallurgie Structurale, Universit6 de Paris-Sud, Orsay 91405, France. I.M. BERNSTEIN, formerly with Carnegie Mellon University, is Provost, Illinois Institute of Technology, Chicago, IL 60616. A.W. THOMPSON, Professor and Department Head, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted May 14, 1987. METALLURGICALTRANSACTIONSA
II.
EXPERIMENTAL TECHNIQUES
The material used was a "high-purity" version of the commercial AI-Zn-Mg-Cu 7075 high-strength aluminum alloy, prepared nominally free of chromium and with reduced levels of Fe and Si; its chemical analysis is given in Table I. Because of the expected very low hydrogen diffusivity in aluminum, tTl small flat tensile specimens (thickness, 1 ram; width, 2 mm; gage length, 16 mm) were prepared to increase the hydrogen-containing volume of material. The as-received specimens were solutionized at 465 ~ for 0.5 hours and ice-water quenched, producing an average equiaxed grain diameter of 100/zm. After aging for 48 hours at RT, the specimens were then given one of the following heat treatments. (1) 24 hours at 100 ~ "underaged" condition; (2) 24 hour
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