%0 Journal Article
%A Niewolak, Leszek
%A Garcia-Fresnillo, L.
%A Meier, G. H.
%A Quadakkers, Willem J.
%T Sigma-Phase Formation in High Chromium Ferritic Steels at 650°C
%J Journal of alloys and compounds
%V 638
%@ 0925-8388
%C Lausanne
%I Elsevier
%M FZJ-2015-07475
%P 405-418
%D 2015
%X A binary Fe–30 wt.%Cr alloy and corresponding ternary alloys containing manganese, molybdenum or tungsten were studied with respect to σ-phase formation at 650 °C. Although even after 3000 h exposure complete equilibration was not attained, the presence of tungsten and especially molybdenum was found to promote σ-phase formation. More extensive σ-phase formation was observed in the tungsten and especially in the molybdenum-containing alloys than in the binary and manganese-containing alloy. Apparently the bulk free energy decrease driving the nucleation of σ-phase is substantially larger when tungsten or molybdenum are present in the alloy.The presence of a nickel layer, to simulate the contact between ferritic steel interconnects and nickel mesh in a Solid Oxide Fuel Cell (SOFC) results in the formation of an austenitic zone and in accelerated formation of a σ-phase rich layer at the ferrite/austenite interface, due to interdiffusion processes. This interface acts as a highly efficient heterogeneity for the nucleation of σ-phase. The nucleation is enhanced by an increased Cr/Fe-ratio at that interface. Several possible modes for the growth of the σ layer were identified but the available experimental data were not sufficient to distinguish among these. The σ-rich layer, which appears to act as an interdiffusion barrier, is thicker in the case of the binary Fe–Cr and the Fe–Cr–Mn alloy than for the molybdenum- or tungsten-rich alloys.The results show that the stability range of σ-phase is larger than indicated by the presently used thermodynamic data bases.
%F PUB:(DE-HGF)16
%9 Journal Article
%U <Go to ISI:>//WOS:000353377500061
%R 10.1016/j.jallcom.2015.03.076
%U https://juser.fz-juelich.de/record/279591