Materials Considerations for Long Life Jet Engines 660057

The design of high-performance jet engines for long and reliable life has pose d a series of important challenges to materials technology. Reliable predictions of the behavior of the metals used is a formidable assignment because of the long time interaction of steady and cyclic stress in a corrosive environment at high temperature. The most immediate problems are the loss of strength and the reduction of ductility resulting from long-time, elevated temperature stressed exposure of the nickel alloys used for “hot” parts. The problems of agglomeration of strengthening precipitates, the formation of secondary phases, and grain boundary carbides are prominent structural changes. The formation of the acicular sigma phase is especially sensitive to the chromium content of nickel base alloys. Low chromium content inhibits the formation of this phase but invites hot corrosion. Improved strain cycling criteria are essential to designing for cyclic life. The Coffin criterion is adequate at low temperatures but non conservative at turbine operating temperatures.