The Volume and Grain-Boundary Diffusion of Iron in 20 Cr/25 Ni/Nb Stainless Steel

Abstract

Tracer diffusion of ⁵⁹Fe in 20 Cr/25 Ni/Nb stainless steel has been measured for two different grain sizes: 2–4 mm and ∼ 50 μm. Volume diffusion dominates in the coarse-grained material in the range 750–1300°C. Above 900°C it is entirely lattice diffusion, described by D_L = 1.74 exp (− 67.9 × 10³/RT) cm², sec⁻¹. At lower temperatures there is evidence of “short-circuiting” by dislocation pipes and the results are consistent with a pipe-diffusivity Dp = 20 exp (− 37 × 10³/RT) cm², sec⁻¹. In fine-grained material, grain-boundary diffusion dominates between 600 and 900°C and is described by the relation δD_B = 8.3 × 10⁻⁷ exp (− 43 × 10³/RT) cm³.sec⁻¹.