Environmentally assisted cracking: Overview of evidence for an adsorption-induced localised-slip process

Abstract

Metallographic and fractographic studies of crack growth in aluminium alloys, nickel, iron-silicon, high-strength steels, magnesium, and titanium alloys in liquid-metal, aqueous, hydrogen, and inert environments are reviewed. Remarkable similarities between hydrogen-assisted cracking, stresscorrosion cracking, and adsorption-induced liquid-metal embrittlement were observed for all the above materials, suggesting that hydrogen-assisted cracking and stress-corrosion cracking were also due to adsorption (of hydrogen) at crack tips. Embrittlement of aluminium alloys, nickel, titanium alloys, and magnesium in aqueous or hydrogen environments was also observed at such high crack velocities that there was insufficient time for diffusion of hydrogen ahead of cracks, and the characteristics of fracture at high and low velocities were similar, further supporting a mechanism based on adsorption at crack tips. The observations also showed that environmentally assisted cracking occurred by a more localised plastic-flow/microvoid-coalescence process than that which occurred in inert environments. Dislocation processes occurring during ductile and brittle fracture are discussed, and it is concluded that environmentally assisted cracking occurs because adsorption facilitates the injection of dislocations from crack tips and thereby promotes the coalescence of cracks with voids ahead of cracks. High-voltage transmission-electron microscopy studies, surface-science observations, and theoretical work, which support an adsorption mechanism or a localised-slip mechanism (or both) for environmentally assisted cracking, are also reviewed.