Sigma-phase formation in high chromium ferritic steels at 650°C

•Formation of σ-FeCr phase at 650°C in alloys Fe–30%Cr–2%(Mn,Mo,W) was investigated.•Formation of σ-FeCr phase was accelerated by interdiffusion with Ni-coating.•Mechanism of heterogeneous nucleation of σ-FeCr at BCC/FCC interface was discussed.•Mechanisms of homogenous and heterogeneous nucleation were discussed.•Improvement of isothermal section of Fe–Cr–Ni phase diagram at 650°C was proposed. A binary Fe–30wt.%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 3000h 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.