Novel High-Temperature Austenitic Alloys for Energy Conversion Applications

The critical factor limiting the operation of power generation plants at higher temperatures is the lack of suitable materials that are strong, corrosion resistant and economically viable. Operation at high temperature can lead to energy conversion efficiencies of >50%, which will not only reduce running costs, but also extend the lifetime of fossil fuels and/or reduce the carbon footprint of the plants. Currently-used ferritic steels cannot satisfy the requirements for future operating temperatures of ≥700°C. Iron-based austenitic steels strengthened with Laves phase precipitates, and alloyed with aluminum for improved oxidation resistance, e.g. Fe-20Cr-20Ni-2Nb-5Al (at.%), are potential candidate materials. Whilst promising, currently these materials suffer from two drawbacks, i.e. they are not strong enough at high temperatures, and the growth of precipitates on the grain boundaries limits the creep life.

We are attempting to generate finer, higher volume fractions of Laves precipitates in the matrix of Fe-20Cr-20Ni-2Nb-5Al by using enhanced nucleation on dislocations (introduced through cold work) through carefully controlled ageing. These precipitates will increase the strength and also minimize the formation of grain boundary precipitates, thus retarding the growth of the latter and extending the creep life of the alloy. We are also looking at the effects of Si, which appears to accelerate precipitation of the Laves phase. The precipitates are being characterized using TEM and APT after various ageing treatments. TEM hot in-situ straining experiments are being used to examine the deformation mechanisms.

Funded by the National Science Foundation, Division of Materials Research