Chemical diffusion in the iridium-rich A1 and L1 ₂ phases in the Ir-Nb system

The diffusion in iridium-rich Ir-Nh alloys has been studied by single-phase interdiffusion experiments. The chemical diffusion coefficient has been measured for the primary fee solid-solution and the L1 ₂ ordered compound Ir ₃Nb in the temperature range between 1650 and 1950°C, using Ir/Ir-8Nb and Ir-26Nb/Ir-28Nb diffusion couples, respectively (numbers indicate mol%). While the chemical diffusion coefficient in the solid-solution phase is close to the tracer self-diffusion coefficient of pure iridium, the diffusion in the compound phase is extremely slow: the chemical diffusion coefficient is 1/40 to 1/50 of that in the solid solution. The low diffusion rate in the compound must be beneficial for high-temperature performance of refractory superalloys based on the Ir-Nb system.