(Thermal and mechanical integrity of alumina scales on high temperature alloys)
The aim of the present work was the investigation of the thermal and mechanical integrity of alumina scales on the high temperature alloys MA 956 and Kanthal APM. The majority of the experiments were conducted at 1100 °C whereas for Kanthal APM partly tests at lower temperatures were carried out. The behaviour of the oxide scale was investigated with and without superimposed tensile stress.
Oxide growth kinetics on both alloys followed a subparabolic time dependence. The oxide scale on MA 956 contained numerous chemical as well as physical defects. Chemical defects consisted of Ti-Al-oxides and of Y-Al-oxides. Physical defects were characterised by pores of different sizes, pore-chains and intergranular microcracks.
For Kanthal APM the oxide scale exhibited much less chemical and physical defects. In this case chemical defects consisted mainly of Zr-rich oxides. In contrast to MA 956 a deformation of the relatively soft Kanthal APM substrate was observed which was caused by oxide growth stresses.
In the case of MA 956 the thermal integrity of the Al2O3-scales has been studied systematically for exposure times up to 3500 h with partial extension up to 5000 h. Especially rough oxide surfaces were prone to spalling. Oxide buckling was identified as the predominant spallation mechanism whereas spalling through a wedging mechanism could not be observed.
For the mechanical integrity of the oxide scale on MA 956 physical defects played the decisive role. Intergranular pore-chains were identified as the sources for crack initiation. Analysis showed a mean fracture toughness of KIc = 0.8 MNm-3/2. From deformation mechanism maps, Coble-creep was identified as the possible deformation mechanism of the oxide scale.
In the case of Kanthal APM, chemical defects again were not responsible for crack initiation. Isothermal experiments at 1100 °C did not lead to oxide fracture for strains up to 7.5 %. Additional experiments were conducted at 700 °C, 900 °C and 1100 °C for samples which had been preoxidised at 1100 °C. For deformation at 1100 °C, Coble-creep was identified as the predominant deformation mechanism. Elastic deformation at 700 °C revealed an oxide scale fracture toughness of 1.0 MNm-3/2. In this case cracks were initiated by the micro-roughness of the oxide surface.
The results of the present work reveal a contribution to the clarification of the fundamental thermal and mechanical behaviour of oxide scales on high temperature alloys. Furthermore they give a possiblity to predict the behaviour under service conditions. Although a higher defect density is present in the oxide scale on MA 956 this alloy has the advantage over Kanthal APM for technical use under mechanical load, because MA 956 exhibits a much higher strength with respect to Kanthal APM.