Temperature dependence of phase composition in W and Si-alloyed high chromium ferritic steels for SOFC interconnect applications

Tungsten alloyed high-chromium ferritic steels such as the commercially available alloy Crofer 22 H have been proposed as construction materials for interconnects in Solid Oxide Fuel Cells. The background of the present study relates to the qualification of such alloys, especially with respect to a possible optimization of the tungsten concentration, aiming at formation of the strengthening intermetallic phases. For this purpose the chemical composition of intermetallic phases in a number of Fe-Cr-W-base model alloys after exposure at temperatures between 600 °C and 900 °C was measured by SEM/EDX and TEM/EDX. The obtained chemical and phase compositions were used for estimation of the iron-rich corner of the Fe-Cr-W system in the temperature range 600-900 °C. Finally, the experimental results were compared with values calculated using the Thermocalc software and the database TCFE 7. This comparison showed that in the temperature range 800 °C-900 °C the calculations gave a qualitatively correct description of phases present in the microstructure, however, substantial differences between calculations and experiments existed in the temperature range 600 °C-700 °C. Moreover, it was found that the solubility of silicon in the C14 Laves phase of the type Fe2W is substantially smaller than that in the Fe2Nb based laves phase present in Crofer 22 H.