Kinetic enrichment of hydrogen at interfaces and voids by dislocation sweep-in of hydrogen
- PDF / 881,390 Bytes
- 9 Pages / 603.28 x 788 pts Page_size
- 47 Downloads / 186 Views
I.
INTRODUCTION
THATmoving dislocations can act as rapid and efficient carders or sweepers of hydrogen was experimentally verified in the seventies by the tritium release rate and penetration experiments of Donovan, Louthan, and co-workers,l'2'3 and others, and more recently, by the permeation experiments of Kurkela and Latanision.4 In addition, the stripping model for hydrogen transport has predicted the wellestablished strain rate and temperature dependence of hydrogen embrittlement. 5'6'7 Although there is no longer doubt that dislocation sweepin can result in deep hydrogen penetration, deeper than by lattice diffusion, the question still remains whether dislocation sweeping of hydrogen can result in significant local enrichment, or supersaturation, of hydrogen at critical failure sites in the material, such as at the interfaces of internal traps. Typically, the traps can be such mechanical weak links as carbides or other precipitate particle interfaces, grain boundaries, and inclusions or internal voids in the material whose interfaces are often preferential sites for microvoid or microcrack initiation. 8-1~ As has been proposed earlier, 29'3~microvoids or microcracks can be initiated when a critical hydrogen concentration is exceeded in local regions. Subsequent enhancement of void growth and failure can also occur if excess hydrogen atoms at the interface are trapped as hydrogen gas in these microvoids resulting in pressurization induced growth. 11 The feasibility of such a pressurization model will depend on whether significant enrichment of hydrogen atoms can take place at trap-matrix interfaces. The theoretical basis for quantitatively evaluating any possible internal supersaturation or enrichment of hydrogen at specific sites in the material as a result of dislocation sweep-in of hydrogen was originally laid by two kinetic models, both published in Metallurgical Transactions and based on two potentially independent mechanisms of hydrogen enrichment. In one, by Tien et al,5 internal traps are
S.V. NAIR, Research Associate, R.R. JENSEN, Graduate Student, and J. K. TIEN, Professor, are all with Henry Krumb School of Mines, Columbia University, New York, NY 10027. Manuscript submitted September 11, 1981.
METALLURGICALTRANSACTIONS A
visualized to strip, through satisfying appropriate binding energy considerations, hydrogen off moving dislocations along their encounter lengths, resulting in kinetic supersaturations of hydrogen at the trap-matrix interfaces. Enrichment would occur if the arrival rate of hydrogen to the traps by dislocation sweeping, Ia, were much greater than the departure rate, IL, of the hydrogen by diffusion leakage into the surrounding matrix. The model predicted that significant hydrogen build-up can occur in the interface for kinetic reasons both for low hydrogen diffusivity materials, such as nickel and austenitic steels, and also for the case of high hydrogen diffusivity materials, such as ferritic steels at ambient temperature. The other model, by Johnson and Hirth, j2 evaluated th
Data Loading...
