Volume and Grain‐Boundary Self‐Diffusion in a High‐Purity Fe–50 mass% Cr Alloy

Volume and grain-boundary self-diffusion coefficients of 51 Cr and 59 Fe in a high-purity Fe-49.7 mass% Cr alloy containing 13 mass ppm carbon and 6 mass ppm nitrogen have been determined in the temperature range between 923 and 1453 K by use of the sputter-microsectioning technique. Linear Arrhenius lines for volume self-diffusion coefficients of both tracers have been observed across the α-σ phase-transformation temperature 1103 K, which is consistent with the suppression of the formation of the σ-phase by purification. The Arrhenius equations can be expressed by D Fe = 1.2 x 10 -4 exp(-256 kJ mol -1 /RT) m 2 s -1 and D Cr = 1.3 × 10 -4 exp (-259 kJ mol -1 /RT) m 2 s -1 . Downward curved Arrhenius lines for the grain boundary diffusivity aδD gb (s segregation factor, δ width of grain boundary, and D gb grain boundary diffusion coefficient) of both tracers have been observed. This is attributed to the formation of very thin layer of the (Cr, Fe) 23 C 6 phase at the grain boundary near the surface of the specimens.