Influence of Temperature and Thermal Cycling on the Oxidation of Directionally Solidified Ni 3 Al–Ni 3 Nb Eutectic

Abstract

The isothermal and thermal cycling oxidation behaviour of a directionally solidified Ni₃Al–Ni₃Nb eutecticalloy has been studied in flowing air at temperatures from 900° to 1200°c. In particular, the extent of development and subsequent breakdown of a protective layer of Al₂O₃, which separates the relatively thick NiO scale and subjacent internal oxide zone from the bulk alloy, is considered. These healing Al₂O₃ layers form in a relatively short time at the higher temperatures, but only after long periods or not at all at the lower temperatures. A lthough they afford protection to the alloy, they are often eventually undermined following mechanical failure, enabling internal oxide to penetrate deeper into the alloy.

Thermal cycling at 1000°c, and particularly at 1100°c, results in spalling of much of the external scale during cooling. This results in more rapid oxidation of the exposed alloy, but the process of scale development is similar to that observed during isothermal oxidation.