On the oxidation resistance of a powder metallurgy nickel-based superalloy

To evaluate the influence of reactive minor elements on oxidation resistance of a nickel-based superalloy, 0.5w/o Si, and/or 0.1w/o Y were added in various combinations to a quaternary powder metallurgy alloy that consisted of a ternary master alloy, Ni-12Cr-9Fez6w/o Al (w/o). The processing included production of transverse rupture strength (TRS) bars by uniaxial pressing followed by sintering to 1300 degrees C and Gleeble thermo-mechanical deformation to reduce porosity. Sectioned TRS bars were then oxidised (static) at 900 degrees C in air for times up to 1000 h and the influence of the Si/Y additions on oxidation resistance was determined via a combination of weight gain data and microstructural examination. It was found that the addition of 0.5w/o Si to the quaternary Ni-Cr-Fe-Al powder metallurgy system provided a measureable improvement in oxidation resistance both in terms of thickness of oxide layer and in overall weight gain. Conversely, 0.1w/o Y provided little benefit when added with the Si and was shown to be detrimental in the absence of Si. Interestingly, little difference was noted in final microstructures. Specifically, variation in %gamma' between samples was minimal (58.3-61.7 v/o) and a distinct precipitate free zone was always present between the oxide and the gamma+gamma' matrix.