Effects of alloying on the brittle fracture of zinc in liquid mercury

Abstract

The susceptibility of polycrystalline zinc to embrittlement by liquid mercury is markedly increased by alloying with as little as 0.2 at.% of copper or gold in solid solution. To determine the cause of this phenomenon, a study has been made of the effects of 0.05 or 0.2 at.% copper on the flow and fracture behavior of amalgamated zinc monocrystals and asymmetric bicrystals. Fracture stress data from these experiments were used in conjunction with a criterion for crack initiation to determine the influence of alloying on cleavage surface energy, γ. It was found that alloying increased the critical resolved shear stress (c.r.s.s.) ( τ c ) of amalgamated bicrystals by a factor of ten, and their fracture stresses by factors of 2–4, but that γ was increased only from 45 ± 5 ergs/cm 2 (pure zinc) to 60 ± 7 ergs/cm 2. The significance of this and other observations are discussed, and it is concluded that the increased susceptibility to embrittlement by liquid mercury of polycrystalline zinc on alloying is not related to solute-induced changes in macroscopic flow stress, stacking fault energy, slip mode, or state of bonding, but in τ C . Increasing τ C inhibits the relaxation by plastic flow of stress concentrations at grain boundaries, and, in the presence of mercury, facilitates crack initiation.