DISLOCATION INDUCED MECHANISMS OF HYDROGENE EMBRITTLEMENT OF METALS AND ALLOYES

Abstract

The paper discusses some models of hydrogen-stress cracking of metals and alloys. These models are based on hydrogen-dislocation interaction. It is shown that the critical role of dislocation emissions in AIDE mechanism is, in its turn, similar to HELP except for a higher localization of deformations compared with microvoids coalescence that is related with HELP, because that stresses needed for the dislocation propagation are high enough to boost general dislocation activity in deformation zones in front of cracks. This results in the formation of small voids on intersecting deformation bands. It has been observed that a crack is essentially growing due to the emission of dislocations. However the emission of dislocation towards the tip of a crack and the formation of voids in front of a crack contribute a lot to the process. Furthermore, the formation of voids in front of a crack makes for a short radius of the crack tip and low angles of the crack tip opening displacement The paper considers crack growing in inert media in plastic materials. Crack plastic growth takes place mainly due to dislocations that originate from the sources in the deformation zone in front of the crack tip and are propagating backwards along the crack tip plane with a small or zero emission of the dislocations that start from the crack tip. Small number of the dislocations that originate in the sources lying closest to the crack tip will intersect the tip of the crack precisely thus promoting the crack development while the majority of the dislocation will have either blunting effect or contribute to the deformation in front of the crack. Thus to cause a crack growth due to microvoid coalescence and deep cavities with shallow depressions therein on fracture surfaces there must be a large deformation in front of the crack. It is demonstrated that the cracking mechanism resulting from the AIDE mechanism will be either intergranular or transcrystalline depending on the location where the propagation of dislocations and formation of voids run mostly easily. In case of transcrystalline cracking alternative sliding motion along the planes on either side of the crack will tend to minimize the reverse stress caused by previously emitted dislocations. Then the macroscopic transcrystalline cracking plane will divide the angle between the slide planes and the crack front will be located on the intersection line of the crack planes and the slide planes. However, if there is a difference in the number of slides that occur on either crack side because of big differences in shear stresses on different slide planes, there will be deviations from the planes and directions with low refraction index. If the plane index is not low, there still can be deviations in the failure planes depending on the location of nucleus voids in front of the crack. A detailed description of the relationship between hydrogen effect on the behavior of dislocations and voids, sliding motion localization and hydrogen embrittlement is still lacking, moreover, it presents a serious problem that can be solved by describing the kinetics of hydrogen embrittlement process. Thanks to their sophisticated nature HELP and AIDE mechanisms can be embrittlement contributors both in cracking and in the formation of cavities due to ductile fracture.