Oxide plasticity in the oxidation mechanism of zirconium and its alloys

The question of how crack-free, protective oxide films can form on zirconium during oxidation when the Pilling-Bedworth ratio is about 1-5 has been considered by a study of the relative plasticity of various forms of zirconia. Hot hardness measurements showed that doping monoclinic zirconia with iron, nickel, or chromium resulted in softer (more plastic) structures and that yttrium additions slightly reduced the plasticity. Calcia-stabilized cubic zirconia was found to be more plastic than monoclinic zirconia when tested at temperatures above 200°C. The behaviour of anion-deficient oxides indicated that they were more plastic than stoichiometric oxides even though the hardness values were identical at 23°C. The former were free from cracks at the indentations, whereas stoichiometric oxides exhibited extensive cracking around and between indentations. The behaviour of actual, thick (72 μm) oxide films during tensile deformation of oxidized metal samples indicated that considerable plasticity occurs in the oxide at 500°C but that the films are brittle at 23°C. It was concluded that the plasticity of the oxide may be greater than that of the oxygen-contaminated substrate at elevated temperatures and may be the means by which epitaxial strains are minimized.