Microstructure influence on the oxidation behaviour at 750°C of astroloy

Abstract
γ-γ' nickel base alloy, as Astroloy is sensitive to environment for fatigue crack propagation at 750°C. Two main oxides are obtained, especially enriched with Ni, Al (+Cr) in the outer zone and with Al, Cr in the inner one. The oxide scale growth is mainly controled by oxygen bulk difision, expect in the case of rolled sample where a linear diffusion of oxygen can also intervene for the growth of inner oxide layer. The inner oxide scale grows 10³ times faster than the outer one. On account of the different diffusivities of metal and oxygen together with the dissolution of oxygen and formation of suboxides, (containing Ni, Cr in γ phase and Al, Ti in γ' islands) at the oxide scale-alloy matrix interface, an accumulation of mechanical constraints occurs in the alloy during oxidation. With a model based on "a solutal elastic effect when the concentration of solute varies" [l] a depth corresponding to a maximum stress was estimated between 3.4 x 10^-8 cm to 1154 x 10^-8 cm in front of the oxide scale-alloy matrix interface. Local enrichment in oxygen on induced defects (vacancies, holes) or precipitates then disturb the ¹⁸O diffusion profile versus alloy's depth.