Plasticity of protective oxide scales

Abstract

There is still some debate on the extent of plasticity of oxide scales at high temperatures. Examination of scales after creep of the substrate often shows that at higher strain rates scale cracks are visible after only very low strains (brittle behaviour), whereas at low strain rates no scale cracks are observed after large strains, at least when simple optical methods are applied. In this paper, the possible mechanisms of scale deformation are discussed with the help of deformation mechanism maps constructed for grain sizes of the order of magnitude encountered in surface oxide scales. As well as ‘genuine’ plasticity by creep of the scale, mechanisms of ‘pseudoplasticity’ are discussed. These consist of simultaneous cracking and healing processes in the scale and simulate macroscopic plasticity of the oxide. Examples from experiments are shown for through scale cracking plus rapid healing and microcracking plus healing. In model calculations critical strain rates are assessed below which the scales are macroscopically intact.

MST/1160