Metallographic contributions to understanding mechanisms of environmentally assisted cracking

Abstract

Mechanisms of environmentally assisted cracking can often be deduced from a knowledge of the relationship between cracking and microstructure, the fracture planes and directions, the detailed appearance of fracture-surfaces, the slip planes that are active during cracking, and the strains associated with crack growth. Metallographic and fractographic techniques for determining these characteristics are briefly reviewed, and the advantages of studying cracking in single crystals are outlined. Transcrystalline, cleavage-like cracking in various materials is then discussed, and it is concluded that the metallographic observations strongly support the following: 1) a mechanism of hydrogen embrittlement based on repeated formation and fracture of brittle hydrides for materials such as niobium, vanadium, and zirconium; and 2) a mechanism of liquid-metal embrittlement, hydrogen embrittlement, and stress-corrosion cracking, based on an adsorption-induced localized-slip process for materials such as aluminium alloys, nickel, and high-strength steels. There is, however, less support for other proposed mechanisms of transcrystalline stress-corrosion cracking, such as those based on film-induced cleavage or localized dissolution at crack tips, and the mechanism of cleavage-like cracking in some materials such as copper alloys is still unresolved.