Several nickel based solid solution alloys are under consideration for application in heat exchangers for very high temperature gas cooled reactors. The principal candidates being considered for this application by the Next Generation Nuclear Plant (NGNP) project are Inconel 617 and Haynes 230. While both of these alloys have an attractive combination of creep strength, fabricability, and oxidation resistance a good deal remains to be determined about their environmental resistance in the expected NGNP helium chemistry and their long term response to thermal aging. A series of experiments has been carried out in a He loop with controlled impurity chemistries within the range expected for the NGNP. The influence of oxygen partial pressure and carbon activity on the microstructure and mechanical properties of Alloys 617 and 230 has been characterized. A relatively simple phenomenological model of the environmental interaction for these alloys has been developed.

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