On Improving The Oxidation Resistance Of A Nickel-Based Superalloy Produced By Powder Metallurgy

Abstract

Nickel-based Superalloys are widely used in the steam turbine power generation and aerospace industries. They possess the desirable qualities of high-temperature strength, oxidation and corrosion resistance and can operate in some of the highest temperature ranges of the structural metals. The oxidation resistance of a Superalloy is achieved primarily through the formation of a dense alumina and/or chromia oxide layer(s) including spinels. This resistance has been further improved in wrought and cast alloys through the addition of reactive elements such as silicon, yttrium and lanthanum, although the exact effects of these elements have not been well defined. This project concentrated on a powder metallurgy ternary master alloy consisting of Ni-12Cr-9Fe (w/o) with additions of 6w/o aluminum, 0.5w/o Si, and 0.1w/o Y, in various combinations. Specifically, the primary goal was to produce and characterize a PM manufactured nickel-based Superalloy with minor additions of reactive elements and to assess the effectiveness of the Si and/or Y in improving the oxidation resistance. JMatPro modeling software was first used to help determine temperatures at which various events would occur in the alloys such as solutionizing and liquation temperatures. Subsequently green compacts were produced by a press (uni-axially) and sinter route to create transverse rupture strength bars (TRS bars). These bars were then thermomechanically deformed using a Gleeble tester to reduce porosity followed by a heat treatment to restore a microstructure better suited for high temperature oxidation. Sectioned TRS bars were then oxidized (static) 900?C in air for times up to 1000h and the influence of the Si/Y additions on oxidation resistance was determined via a combination of weight gain data and microstructural examination. Whereas JMatPro predicted solutionizing temperature of the compositions studied (1010°C quaternary; 1020°C quaternary + Si, respectively) these values were slightly lower than the results observed through DSC experiments (1045°C quaternary; 1065°C quaternary + Si, respectively). A w/o ?’ of approximately 25% was predicted by the modeling tool, but values of 58.3% to 61.7% were determined using a point count method. Finally, the addition of 0.5w/o Si to the quaternary Ni-Cr-Fe-Al PM system provided a measureable improvement in the oxidation resistance both in terms of thickness of oxide layer and in overall weight gain. Conversely, 0.1w/o Y provided little benefit, and was shown to be detrimental to alloys not containing Si.