Intercrystalline fracture in hydrogen-charged nickel

Abstract

The results of this investigation and the previous work of Boniszewski and Smith (1) have shown that the embrittlement of nickel by hydrogen is sensitive to strain rate, grain size, test temperature and hydrogen concentration. The embrittlement is most pronounced under test conditions which promote serrated yielding (the Portevin-Le Chatelier effect). It has been found that all fractures of hydrogen-charged nickel tested at − 80°C are completely intercrystalline. The grain-boundary cracks form after 7 to 9% elongation and propagate slowly to fracture (probably in a stepwise fashion) which generally occurs after about 15 to 25% elongation. It is suggested that two conditions are necessary for crack nucleation: 1. (1) a large stress concentration, which is provided by slip bands held up at grain boundaries, or more commonly, at grain boundary triple junctions and 2. (2) a high concentration of hydrogen at the boundary in the vicinity of the stress concentration. This is provided by dislocations running into the boundary (in slip bands) and carrying their associated hydrogen atmospheres to points of stress concentration. It is suggested that the excess hydrogen lowers the surface energy thereby facilitating grain boundary crack nuoleation. Evidence is presented which indicates that possibly a hydrogen pressure builds up in propagating microcracks and aids the applied stress in the fracturing process, and also that the rate of crack propagation may be a hydrogen diffusion controlled process.