The mechanism of hydrogen induced cleavage in Fe-3 pct Si alloy
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I.
INTRODUCTION
THE mechanism
of hydrogen induced cracking has been widely studied and discussed by numerous investigators, but is still unclear and a number of different theories have been proposed, m For example, the planar pressure theory proposes that hydrogen precipitates in pre-existing voids and exerts a pressure that reduces the load-bearing ability of the material. 121The adsorption theory proposes that absorbed hydrogen lowers the surface energy of the lattice. 131 McMahon et al. propose that a small decrease of the surface energy can induce a large decrease of the plastic deformation work. t41 The decohesive theory proposes that hydrogen dissolved in the lattice reduces the interatomic cohesive forces and lowers the fracture stress] sl Hydrogen promotes the local plastic deformation to facilitate the nucleation of the crack. [6.7] The planar pressure theory is applicable only in some particular conditions, e.g., severely electrolytic charging, supersaturated hydrogen precipitation during cooling. It is generally agreed that if hydrogen reduces the cohesive forces it must be able to decrease the surface energy and the elastic modulus. Recent work, however, showed that although the apparent Young's modulus after charging with hydrogen was decreased the Young's modulus associated with the interatomic cohesive force did not evidently change during aging with escaped hydrogen of 7 to 8 wppm at room temperature; i.e., hydrogen of 7 to 8 wppm did not decrease the cohesive force of a-Fe. I81The result of the study of in situ high-voltage electron microscope environmental cell shows that hydrogen can promote the multiplication and motion of dislocations and that hydrogen induced plastic deformation will stop if argon is substituted for hydrogen. [9] A series of tests on low-alloy steels, aluminum alloys, and austenitic stainless steels indicate that hydrogen can induce a time-dependent, i.e., delayed, plastic deformation and a hydrogen-induced delayed cracking occurs when the hydrogen induced delayed plastic deformation develops to a critical extent. Ix~ Further experiment s h o w s [131 that although hydrogen does not evidently decrease the yield YAN-BIN WANG, Instructor, WU-YANG CHU, Professor, and CHIMEI HSIAO, Professor and Head, are with the Department of Metal Physics, Beijing University of Iron and Steel Technology, Beijing, China. Manuscript submitted July II, 1986.
METALLURGICAL TRANSACTIONS A
strength %, the hydrogen atmosphere enriched at a crack tip can produce an additional stress ~'H which will help an external stress to move the dislocations against the resistance. As a result, the apparent yield stress L , which is an exterior stress necessary for local macroscopic plastic deformation, will be decreased markedly. This is the reason why hydrogen induced delayed plastic deformation and then hydrogen induced cracking can occur under the exterior stress ~'c less than ~'s or a stress intensity factor KZHless than K~c. One purpose of the present work is to clarify further why hydrogen can promote t
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