Temperature Dependence of Laves Phase Composition in Nb, W and Si-Alloyed High Chromium Ferritic Steels for SOFC Interconnect Applications

The Laves phase strengthened ferritic steel Crofer 22 H has been proposed as construction material for interconnects in solid oxide fuel cells. The background of the present study relates to the further qualification of this steel, especially with respect to a possible optimization of amount and composition of the strengthening Laves phase precipitates. For this purpose the chemical composition of the Laves phase in a number of high purity model alloys as well as in Crofer 22 H equilibrated at temperatures between 700 and 1100 °C was measured by EDX/WDX and atom probe tomography (APT). The obtained chemical compositions were used for a qualitative estimation of the site occupancy for Fe, Cr, Nb, W and Si in the Laves phase unit cell. Additionally, the results from APT measurements indicate the important role of impurities such as e.g. titanium in the Laves phase formation. Finally, the experimental results were compared with Thermocalc calculations using the database TCFE 7. This revealed that within the temperature range 800-900 °C a qualitative description of phases is possible, however, substantial differences existed particularly for the steel Crofer 22 H at and below 700 °C and above 950 °C