Hydrogen-enhanced localized plasticity--a mechanism for hydrogen- related fracture

The mechanisms of hydrogen-related fracture are briefly reviewed and a few evaluative statements are made about the stress-induced hydride formation, decohesion, and hydrogen-enhanced localized plasticity mechanisms. A more complete discussion of the failure mechanism based on hydrogen-enhanced dislocation mobility is presented, and these observations are related to measurements of the macroscopic flow stress. The effects of hydrogen-induced slip localization on the measured flow stress is discussed. A theory of hydrogen shielding of the interaction of dislocations with elastic stress centres is outlined. It is shown that this shielding effect can account for the observed hydrogen-enhanced dislocation mobility. 1. Review of pertinent observations Despite extensive study, the mechanism(s) of hydrogen embrittlement have remained unclear. Several candidate mechanisms have evolved, each of which is supported by sets of experimental observations and strong personal views. One reasonably certain aspect of this controversy is that there are several viable mechanisms of hydrogen-related failure and that the search for a single mechanism to explain all observations is doomed to failure. Of the many suggestions, three mechanisms appear to be viable: stress-induced hydride formation and cleavage [1-4], hydrogenenhanced localized plasticity [5-9], and hydrogeninduced decohesion [10, 11]. The first of these has been definitively established to be operative in systems in which hydrides are either stable, or can be stabilized by the application of a stress field, e.g. Group Vb metals [3, 12-14], Ti [4, 15], and Zr [16]. This "second phase" mechanism is supported by microscopic observations [11, 17] and thermodynamic calculations [18]. In these hydride-forming systems it has been shown [4] that under conditions in which the hydride cannot form, hydrogen "embrittlement" will occur by the second mechanism named above, hydrogenenhanced localized plasticity. The hydrogen-enhanced localized plasticity mechanism is based on observations that in a range of temperatures and strain rates, the presence of hydrogen in solid solution decreases the barriers to dislocation motion, thereby increasing the amount of deformation that occurs in a localized region adjacent to the fracture surface [19-25]. The fracture process is a highly localized plastic failure process rather than an embrittlement. This counter-intuitive process says that the macroscopic ductility is limited by the onset of extensive localized plasticity and is supported by microscopic observations. The third viable mechanism is the hydrogen-related decohesion mechanism, in which the atomic bonding at the crack tip is weakened by the presence of hydrogen in solid solution [10, l 1]. This mechanism is supported primarily by the observations that in some non-hydride forming systems, hydrogen embrittlement appears to occur in the absence of significant local deformation, by theoretical calculations of the effect of hydrogen on the atomic potentials [26] and by a thermodynamic argument [27, 28]. Direct evidence for this mechanism has not been obtained and measurements which have been made on the effects of hydrogen on small strain aspects of the lattice potential suggest no softening of the lattice potential [7]. In the present paper we shall review the observations supporting the hydrogen-enhanced localized plasticity (HELP) mechanism of hydrogen embrittlement and then discuss the mechanism by which hydrogen enhances the mobility of dislocations. Experiments on which the HELP mechanism is based are founded on the premise that detailed understanding of fracture mechanisms requires observations at sufficiently high resolution to allow the mechanistic details to be revealed. High-resolution fractography of hydrogen embrittled metals, such as Ni and Fe, show extensive plastic deformation localized along the fracture surfaces [25, 29]. Particularly revealing results have been obtained with the technique of in situ TEM environmental cell deformation and fracture. These methods 11921-5093/94/$7. (10 © 1994 Elsevier Sequoia. All rights reserved ,%1)1 (1921-5/)93( 93 )02554-G 192 11. K. Birnbaum, P. Sofronis / Hydrogen-enhanced localized plasticity allow observation of the fracture process in real time, at high spatial resolution, and in vacuum or in H 2 atmospheres. These studies have been carried out for b.c.c. , f.c. c, and h.c. p, metals having various solute contents and for solid solutions, precipitation strengthened alloys, and intermetallics [19-24, 30]. In systems which exhibit hydrogen embrittlement, the nature of hydrogen effects, while differing in details, were the same in fundamental character. The most dramatic observation was that hydrogen increased the dislocation mobility under conditions of constant stress. In stressed specimens, dislocation mobility could be increased by the addition of H 2 to the environmental cell and decreased by removal of H 2 and restoration of vacuum. This behaviour was observed for edge, screw and mixed dislocations and for isolated dislocations as well as dislocation tangles. In b.c.c, metals the enhanced dislocation velocities were observed on {112] and {110} slip planes and the enhancement was least for extremely high purity Fe and increased as interstitial solutes (C) were added to the solid solution. Hydrogen-enhanced dislocation velocity was observed for dislocations completely contained within the specimens as for Frank-Read sources and for dislocations which terminated at the surfaces. Hydrogen-enhanced operation of dislocation sources was observed within the crystals and at grain boundaries. Specimens which contained stress concentrations, such as notches, failed by ductile plastic processes at the front of the notch when stressed in Mode 1 in vacuum. When stressed under gaseous H2, similar fracture was observed, with the exception that the extent of plasticity was more confined to regions adjacent to the fracture surface in the case of fracture in H 2. In relatively pure specimens (Ni, Fe etc.) the fracture tended to be along slip planes and the deformation accompanying the fracture in H 2 was within 1 ~m of the active slip plane. In alloys such as stainless steels [23], the fracture in H 2 was much less crystallographic and tended to be along the plane of maximum normal stress. In all of the systems, the cracks which had propagated in vacuum and then stopped under a constant external load could be started and continued to propagate without any increase in external load when H 2 was added to the environmental cell. This process occurred by increasing the dislocation activity at the crack tip when the specimen was exposed t o U 2 gas. Since the fracture process in H 2 was by localized ductile failure, e.g. by the formation of very shallow, localized microvoids [31], the increase in the dislocation velocity at the crack tip is the root cause of the hydrogen embrittlement. The effect of hydrogen is greatest at the crack tip where either hydrogen entry is facilitated by slip processes or the local hydrogen concentration is increased by the effect of stress on the chemical potential of the solute H. Hydrogen locally softens the material in front of the crack, allowing ductile fracture to occur there, prior to general yielding away from the crack tip. In the cases where the concentration of hydrogen is greatest in the vicinity of grain boundaries [32-34], it is in these regions that deformation occurs at the lowest stresses and hence "intergranular" fracture is observed. In cases where high resolution studies of these "intergranular" fractures have been carried out [4, 35] it is clear that the fracture in fact occurs by plastic processes in the vicinity of the boundaries rather than along the boundaries themselves. 2. Hydrogen effects on macroscopic deformation Despite the many observations of hydrogenenhanced dislocation mobility in TEM specimens, relatively few observations of softening due to hydrogen in solid solution have been reported. In most cases, the introduction of hydrogen has been reported to result in increased flow stress and only in a small number of cases has softening been observed [36-39]. Significant decreases of the flow stress have been reported in high purity iron which was cathodically charged at very low current densities and during hydrogen charging under conditions in which the hydrogen was introduced without damage to the specimen. Two significant differences appear to divide the experiments that exhibit softening due to hydrogen from those which do not. Hydrogen decreases have been observed at very low strain rates and when hydrogen was introduced under conditions which did not cause any structural damage to the specimens. The latter condition is particularly significant, as most cathodic charging conditions used to introduce high supersaturations of hydrogen introduced high concentration gradients and correspondingly high stresses near the surface. The consequent deformation in the near surface region gives rise to hardening which may mask the softening caused by hydrogen. In the case of the cathodic charging conditions utilized by Kimura et al. [36-38], no surface deformation was observed. The effects of hydrogen on the thermal activation of dislocations over barriers has been studied using the techniques of load relaxation, temperature and strain rate changes in high purity Ni and Ni-C alloys [40]. The strain rate can be expressed as: g = g0 exp [(AH0* f bA*ro*) / kT] ( 1 ) H. K. Birnbaum, P. Sofronis / Hydrogen-enhanced localized plastici O' 193 where g, is a constant which is given by go = VDPmA exp(AS*/ k ), VD is the dislocation at tempt frequency, A is the slip plane area swept by the dislocation per activated event, AS* is the activation entropy for slip activation, AH0* is the activation enthalpy for slip activation at zero stress, A* is the activation area, b is the Burgers vector and o* is the effective