Influence of composition and phase distribution on the oxidation behaviour of NiCoCrAlY alloys

Phase composition and distribution can strongly affect the transient oxide stages as well as the internal oxide morphology of Ni(Co)CrAlY alloys and coatings. In the present paper the influence of Co-, Cr- and Al-additions on phase composition and morphology is illustrated. Co, as well as Cr, destabilises the gamma ' -phase and promotes the formation of the sigma -phase. Dilatometry is shown as an useful technique to determine phase transformations in Ni(Co)CrAlY alloys upon temperature changes. The phase distribution determines the morphology of the oxide formed during early stages of oxidation and the NiY-phase distribution. The distribution of the NiY-rich phase determines the final morphology of the internal oxidation. A coarse distribution of the NiY-phase on Ni(Co)CrAlY grain boundaries leads to a finger type internal oxidation which was found to have a detrimental effect on oxide scale adherence.