High temperature internal oxidation behaviour of dilute Ni-Al alloys

Abstract

The internal oxidation of dilute Ni-Al alloys, either pure or containing small additions of tantalum or hafnium, was investigated at 1200° C in air. The advance of the precipitation zone followed a parabolic relationship at a slightly decreasing rate with increasing aluminium content. The presence of active elements had no appreciable effect on the growth rate. The precipitate shape, size and spacing depended upon the aluminium concentration and more significantly on the active element additions. The morphology varied from polyhedral crystallites and epitaxial platelets in the Ni-0.5 wt% Al alloy, to well defined cylindrical rods extending across the precipitation zone approximately normal to the reaction interface, in the alloys of higher aluminium content. An also continuous and similarly oriented plate-like morphology was observed in the active element-containing alloys. X-ray microanalysis indicated that the continuous precipitates consisted of NiAl₂O₄ and Al₂O₃. The former phase comprised approximately 65±5% of the outer region of the precipitation zone. Aluminium depletion in the alloy ahead of the precipitation front and a consequent enrichment in the form of oxide within the reaction zone inferred that growth was controlled by simultaneous outward aluminium and inward oxygen diffusion. No correlation was found between either the growth rate or oxygen permeability and the distribution of the precipitates. It was, therefore, concluded that the interfacial boundaries were ineffective in accelerating oxygen transport at this elevated temperature.