Formation of σ phase in highly purified Fe-Cr alloys

The formation behavior of the σ phase in Fe-Cr alloys was investigated using highly purified Fe-Cr alloys containing between 40 and 50 mass% chromium. The amount of σ phase was determined by optical microscopy. In annealed highly purified Fe-40 and 45 mass% Cr alloys, the σ phase nucleates at the surface of the specimen and grows inward, and the formation of this phase is finished completely within 360 ks. In annealed highly purified Fe-50 mass% Cr alloy the nucleation rate and the growth rate of the σ phase were extremely small. The formation of the σ phase was suppressed by purification, by the reduction of nitrogen, and by the presence of a small amount of carbon. (orig.)