Abstract
Metallographic and fractographic studies of crack growth in nickel polycrystals and single crystals in a number of environments are described. “Brittle” intercrystalline and transcrystalline cleavage-like fractures were observed for specimens tested in liquid mercury, liquid lithium, liquid sodium, gaseous hydrogen, and for hydrogen-charged specimens tested in air. “Brittle” fractures were associated with considerable slip, and dimples/tear ridges were observed on fracture surfaces, suggesting that crack growth occurred by localized plastic flow. There were remarkable similarities between adsorption-induced liquid-metal embrittlement and hydrogen-assisted cracking which, along with other observations, suggested that adsorbed hydrogen at crack tips was responsible for hydrogen-assisted cracking. It is concluded that adsorbed atoms weaken interatomic bonds at crack tips thereby facilitating the nucleation of dislocations and promoting crack growth by localized plastic flow.
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