The planned evolutionary advancements in aircraft gas turbine technology in the past three decades has been nothing less than phenomenal. The demands for increased engine performance with the resultant higher turbine inlet temperatures, closer tolerances in the rotating elements, and high strength material requirements have resulted in an essentially new alloy technology. Formerly, these demands had been met by the development of a class of nickel- and cobalt-base superalloys. This was possible because the stronger alloys were still operating at temperature levels and in environments where oxidation and sulfidation resistance were not limiting factors. These first generation superalloys were limited, however, by strength restrictions at elevated temperatures. To circumvent the slow progress in materials development air cooling of hot section components was introduced and provided a modicum of growth potential for turbine performance. However, the averaged and localized temperatures of the blades and vanes have continued to increase. To achieve the strength and stability at the superalloys in these increasing temperature environments, the trend has been to decrease the average chromium level, with a resultant undesirable decrease in corrosion resistance. Also, to effect an increase in strength the chromium has been partially replaced with elements such as tantalum, columbium, molybdenum, or tungsten. These elements usually have an adverse effect on oxidation or sulfidation resistance,1 figure 1.
