Influence of Aluminum and Rhenium on the Isothermal Oxidation Behavior of CoNiCrAlY Alloys

Abstract

The oxidation behavior of a Co32Ni21Cr8Al0.6Y (wt%) alloy with and without the addition of 3.5 wt% rhenium, 2 wt% aluminum or a combination of the two was investigated at 1000°C. Results showed that increasing the Al content from 8 to 10 wt% led to an increase of the alloy b-phase, but did not affect the oxidation behavior. Re addition induced (Cr,Re,Y)-rich phase to precipitate in the alloy, accelerated the h-to a-alumina transformation, reduced the oxidation rate and enhanced the rate of alloy Al diffusion. Adding both Al and Re further improved the oxidation behavior by promoting the development of the external alumina scale and suppressing the formation of Ni, Co containing spinel. This alloy also showed the largest reduction of oxidation rate and emerged to be the most beneficial. A continuous Cr-Re rich layer was observed at the oxide/alloy interface of the Re, Al containing alloy after longer oxidation times, but this layer is not expected to affect the continued growth of the alumina scale.

Phase evolution in an MCrAlY coating during high temperature exposure

“…For this temperature it is easier to identify the Cr-rich layer beneath the oxide [14]: it is made up of a string of carbide with a blocky morphology. Electron Microscopy allowed the characterization of phases in the CoNiCrAlY coating.…”

“…After longer oxidation time, a Cr-rich layer, identified as α-Cr, accumulate at the alloy / thermally grown oxide (TGO) interface; together with the Y precipitates, these phases are considered to play a role in improving the oxidation performance of the coating [14].…”