Stress Corrosion Cracking and Liquid Metal Embrittlement in Pure Magnesium Available to Purchase

Metallographic and fractographic observations of crack growth in pure magnesium in dry air, aqueous, and liquid alkali metal environments are described. Crack growth in dry air at ambient temperatures was macroscopically brittle and occurred parallel to ${101¯ X}$ planes or along grain boundaries, but fracture surfaces were microscopically fluted or dimpled. Fluted fracture surfaces parallel to ${101¯ X}$ planes and dimpled intercrystalline facets were also produced by stress corrosion cracking (SCC) and liquid metal embrittlement (LME), but flutes and dimples were smaller and shallower than those produced by overload fracture. Cleavage-like {0001} fracture surfaces were also observed after SCC and LME. The close similarities between SCC and adsorption-induced LME, and observations that embrittlement in aqueous environments could occur at crack velocities as high as 5 cm/s, suggested that adsorbed hydrogen (rather than solute hydrogen, hydrides, or localized dissolution) was responsible for SCC. It is concluded that adsorbed hydrogen and metal atoms weaken interatomic bonds at crack tips and thereby facilitate the nucleation of dislocations from crack tips.