Interaction of the solute niobium or molybdenum with grain boundaries in α-iron

Three-dimensional atom probe (3DAP) was used to estimate solute–grain boundary interactions for Nb or Mo in α-Fe. By using the Langmuir–McLean isotherm, effective Gibbs free energies of segregation in α-Fe, ΔGb, at 800 °C were estimated to be −38±2 kJ mol−1 for Nb and −28±2 kJ mol−1 for Mo. The apparent width of solute segregation was shown to be broader when the direction of analysis is not perpendicular to the boundary plane, due to limits of the lateral resolution of the 3DAP. However, even taking this effect into account, we measured a width of solute segregation of 1.0–2.0 nm, i.e. extending a few atom layers on both side of grain boundary. In order to represent the actual solute segregation profile, a wedge-shaped interaction potential across grain boundaries was used for the estimation of the interaction from atom probe data. The difference in the solute–grain boundary interaction between Nb and Mo, and the solute distribution within grain boundary is also discussed.