Hydrogen diffusion and its relevance to intergranular cracking in nickel
- PDF / 2,142,605 Bytes
- 8 Pages / 597 x 774 pts Page_size
- 10 Downloads / 205 Views
I.
INTRODUCTION
IN two recent publications, t~,2j grain boundary diffusion of hydrogen has been dealt with both theoretically and experimentally. Based upon the test results of permeation rate measurements and the silver decoration technique, it was concluded that grain boundaries are not fast paths for hydrogen diffusion in nickel. [2j The calculations of permeation transients using a new grain boundary diffusion model have revealed the reasons behind some contradictory findings. Indeed, it was found that distinct early breakthrough t3] is doubtful. Kimura and Birnbaum [41 precharged tensile specimens of pure nickel with hydrogen at different temperatures for various periods and then fractured these specimens at 77 K. Based on the measurements of intergranular (IG) fracture depths, they concluded that hydrogen transport was enhanced by grain boundaries. However, as pointed out by Yao and Cahoon, t51 an inadequate grain boundary diffusion model was used in their analysis of results. If Whipple's trl model were used to analyze the data of high purity specimens tested in Reference 4, a large diffusivity ratio of grain boundary to lattice diffusion (K = De/ Dr > 1000) would have been derived. Furthermore, the activation energy for diffusion along grain boundaries would have been found even higher than that for diffusion through the lattice. This higher activation energy, in itself, is contradictory to the fast-diffusion-path theory. An alternative attempt to rationalize the results of Reference 4, assuming the lattice diffusion to be dominant, has resulted in a large binding energy, [SJ 20.5 kJ/mol. This value is consistent with that proposed by Lee and Lee 17] but much greater than the value obtained by Lassila and Birnbaum, tSJ 11.6 kJ/mol. Furthermore, Lassila and Birnbaum found that a grain boundary J. YAO, Research Associate, and S.A. MEGUID, Professor, are with the Engineering Mechanics and Design Laboratory, Department of Mechanical Engineering, University of Toronto, Toronto, ON, Canada M5S 1A4. J.R. CAHOON, Professor, is with the Metallurgical Sciences Laboratory, Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 2N2. Manuscript submitted June 14, 1991. METALLURGICAL TRANSACTIONS A
concentration of 10 at. pet hydrogen is needed to change the fracture mode from transgranular (TG) to IG. t8"9]In view of the importance of all these parameters to hydrogen embrittlement in metals, it was decided that further work is needed to clarify the situation. Accordingly, three aspects were examined: (1) the grain boundary diffusion coefficient of hydrogen, (2) the critical grain boundary concentration of hydrogen for the TG/IG transition, and (3) the binding energy of grain boundaries with hydrogen. Tensile tests were performed at 77 K on thin nickel specimens precharged with hydrogen. Since hydrogen atoms are "frozen" in the host lattice and thus become unable to move along with the carrying dislocations, the effect of presegregated hydrogen at grain boundaries upon th
Data Loading...
