Role of Carbon and Nitrogen on the Transformation of the σ Phase in Highly Purified Fe-50 mass%Cr Alloys

The role of carbon and nitrogen on the transformation of the σ phase was investigated using highly purified Fe–Cr alloys containing 50 mass% chromium. In highly purified Fe-50 mass%Cr alloy the σ phase transforms at grain boundaries across the surface of the specimen and then grows inward along grain boundaries. Extremely small amounts of the σ phase are transformed in Fe-50 mass%Cr alloys containing carbon. On the other hand, in Fe-50 mass%Cr alloys containing nitrogen aged for long periods, 3.6 Ms and more, the σ phase transforms on grain boundaries at the inside of the specimen and grows along grain boundaries. In these alloys it was observed that plate-like nitride Cr2N precipitated along grain boundaries and resulted in a depleted zone of chromium around it. The σ phase preferentially transforms on the α grain boundaries. In Fe-50 mass%Cr alloy containing carbon and nitrogen aged for 18 Ms, the α phase consists of 49.1 mass% chromium and 50.9 mass% iron. On the other hand, the σ phase consists of 47.9 mass% chromium and 52.1 mass% iron. It was confirmed that chromium enrichment occurred in the α phase at the interface between the α and σ phases. Consequently, in annealed Fe-50 mass%Cr alloys, the transformation of the σ phase is suppressed by the presence of a small amount of carbon and by purification, especially the reduction of the nitrogen concentration and the absence of nitride.