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The automotive Stirling engine now under development by DOE/NASA as an alternative to the internal combustion engine imposes severe materials requirements for the hot portion of the engine. Materials selected must have low cost and contain a minimum of strategic elements so that availability is not a problem. Heater head tubes contain high pressure hydrogen on the inside and are exposed to hot combustion gases on the outside surface. The cylinders and regenerator housings must be readily castable into complex shapes having varying wall thicknesses and be amenable to brazing and welding operations. Also, high strength, oxidation resistance, resistance to hydrogen permeation, cyclic operation, and long-life are required. A research program conducted by NASA Lewis focused on alloy chemistry and microstructural control to achieve the desired properties over the life of the engine. Results of alloy selection, characterization, evaluation, and actual engine testing of selected materials will be presented. The purpose of this paper is to describe the effects of alloy chemistry and microstructure on the oxidation/corrosion behavior, strength (endurance), and resistance to hydrogen permeability of candidate alloys.
automotive Stirling engine, alloy chemistry, microstructure, creep resistance, oxidation, hydrogen permeation, iron-base alloys
Chief, National Aeronautics and Space Administration, Lewis Research Center, Cleveland, OH