Development of elevated temperature structural alloys & intermetallics

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Several classes of alloys are designed for elevated temperature applications. This research focuses on two classes of these alloys: single crystal (SX) nickel base superalloys and titanium aluminide (TiAl) intermetallics. The former are virtually the only materials used for blades in the turbine stages of gas turbine engines. The latter are emerging as useful high temperature materials. The proposed SX research represents the third phase of a long term program. The first two phases examined microsegregation and the impact of refractory elements on the solidified structure, and developed innovative heat treatments to control chemical homogenization, solutioning and onset of incipient melting. The proposed third phase will study microstructural stability as a function of refractory content by determining the response to thermal and creep exposure in terms of elemental phase partitioning, gamma prime morphology, and formation of harmful topologically close packed phases. This research will provide an understanding of the metallurgical mechanisms controlling solution heat treatments and the influence on subsequent properties and, thereby, offer the opportunity to substantially shorten solution heat treatments and improve subsequent property control, reducing the processing cost of these alloys and improving properties during extended service in gas turbine engines. The research on TiAl will utilize the massive transformation possible in this material to develop a straightforward quench and temper process that produces a microstructure with properties appropriate for application as reciprocating engine exhaust valves. This research will investigate the influence of various process parameters to determine the kinetics of the massive transformation and tempering reactions, and finally characterize the hot strength, creep and fatigue properties. The research will provide a better fundamental understanding of the massive transformation in TiAl, as well as develop a simple cast plus easy-to-apply heat treatment process to reduce the manufacturing cost of TiAl valves helping them move from the laboratory to road vehicles.