Effect of the Cobalt Content and Chromium Alloying on the Structure of the Scale Formed on Structural β-NiAl + γ'-Ni3Al + γ-Ni Alloys (Ni–Al–Co System) at 1300°C

Abstract

The structures of the scale and the subscale layers that form during the air oxidation of the β + γ' + γ alloys of the Ni–Al–Co system containing (at %) ~27 Al (alloys 1–3), 10 Co (base alloy 1), 16 Co (alloy 2), and 10 Co and 4.25 Cr (alloy 3) at 1300°C for 100 h are studied. The formation of scale (Al2O3) in all alloys leads to the aluminum depletion of the subscale layers and to the γ' → γ + γ' transition due to the interdiffusion of aluminum and oxygen. An increase in the cobalt content from 5 to 10 at % increases the heat resistance. The degradation of the heat resistance induced by an increase in the cobalt content to 16 at % is related to an increase in the volume fraction of the γ phase in an alloy as compared to the base alloy with 10 at %. The introduction of small amounts of chromium degrades the heat resistance because of the development of internal oxidation, i.e., the penetration of alumina into an alloy along interdendritic γ-phase precipitates. The heat resistance of the chromium-free β + γ' + γ alloy with 10 at % Co at 1300°C, which was estimated from the sample mass increment per unit surface area in 100 h, is higher than the heat resistance of the well-known γ' + γ VKNA and VIN alloys at 1100–1200°C.