An Alternative View of the Effect of Active Elements on the Mechanical Properties of Scales

Abstract

Much work has been published over recent years on the role active elements play in the growth and adhesion of oxide scales. In these studies it is widely acknowledged that the presence of active elements improves the adhesion of both Cr₂O₃ and Al₂O₃ scales as is clearly evident from the improved cyclic oxidation resistance of these alloys. Many mechanisms have been suggested to account for these improvements but few of them have addressed the role active elements play in modifying the mechanical properties of oxide scales.

This paper considers the role of active elements on scale mechanical properties by assessing the condition necessary to cause scale fracture. Examples to illustrate these effects are taken from the published literature for cerium and yttrium additions to alloys forming chromia scales. Fracture mechanics techniques are used to assess the effect of active elements in modifying the fracture behaviour of the scales.

It is shown, that the distribution of defects (cracks, voids, open boundaries) is modified by the presence of the active element and that this effect is a major contribution to improved resistance to scale fracture. Reactive element additions do not modify the fracture toughness (K_1c) of chromia scales, nor alter scale plasticity. They do modify the scales growth mechanism and hence the growth stresses generated within these scales. A similar behaviour should be expected for alumina scales, but the difficulty in measuring defect distributions in these thin scales makes it more difficult to quantify this effect.