Development of New Nitrided Nickel-Base Alloys for High Temperature Applications

Abstract

The development strategy for what we believe to be a highly promising new family of superalloys is presented. Most superalloys reach their upper limit for useful service above 900°C: age hardened alloys encounter a dramatic drop of their very high strength because precipitates are no longer thermally and/or thermodynamically stable. Solid solution alloys still provide useful strengths slightly above 1000°C. In the temperature range 800 to 1100°C other strengthening mechanisms and phases must be used: examples are ODS alloys. The development presented consists in the production of a fairly fine grain structure (-50pm) in which nitride particles (with a size of 1-5 pm) are dispersed in order to avoid grain growth at high temperatures, and in the use of solid solution additions to promote additional high temperature strength and creep resistance. The useful temperature of this new alloy is situated in a range above SOO”, up to 1100°C; below 800°C age hardened alloys are much stronger and therefore more suitable. The alloys were produced by nitriding a Ni-Cr melt under high nitrogen partial pressure. For typical content of 0.5-0.8N during alloy solidification lo-20~01.~~ nitrides were formed, and homogeneously dispersed in the matrix after thermomechanical treatment. A grain size close to 50 urn was obtained. The nitridation and themomechanical treatments have been carried out for different alloy compositions with a common Ni-Cr-N-base. The microstructure was shown to have a fairly good stability at 1lOO’C. Small grains are believed to be deleterious for creep resistance, because they can lead to accelerated creep by grain boundary sliding: for this reason many high temperature alloys do not contain grain boundaries or only very low angle ones and some others (ODS or DS) have large elongated grams. However, good creep resistance is believed to be achievable by stabilizing grain boundaries by the nitrides. In the alloys at temperatures below 1120°C the 7c nitride was observed; its precipitation was accompanied by a volume variation, which was measured by dilatometry. W, MO and Ta were considerated as possible solid solution strengtheners and their effect tested on the alloys. W was identified as best candidate. The n: nitride was found to be greatly stabilized by additions of MO and W. High temperature tensile tests revealed that the presence of nitrogen and tungsten induces a remarkable strengthening as well as the appearance of a pronounced yield phenomenon in the stress-strain behavior. Strength levels above 150 MPa at 1000°C were reached already with nitride-free Ni-Cr-N-W alloys. Superalloys 2000 Edited by T.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S. Olson, and 1.1. Schim TMS (The Minerals, Metals &Materials Society), 2C4JJ