Interdiffusion Kinetics in the Mo<SUB>5</SUB>SiB<SUB>2</SUB> (T<SUB>2</SUB>) Phase

Abstract

Diffusion couples based on Mo2B and Mo5Si3 were used to determine the diffusion kinetics of T2 phase development and the relative diffusivities controlling the kinetics. Annealing the Mo2B/Mo5Si3 diffusion couple above 1600 °C yielded an initial diffusion path sequence of Mo2B/T2/Mo3Si/Mo5Si3, which was subsequently transformed to Mo2B/T2/(Mo3Si+Mo5Si3)/Mo5Si3 by shrinkage of the Mo3Si phase upon long-term annealing. The T2 phase developed from Mo2B and during the growth of the T2 phase, Si and B atom movements were driven by the Si concentration gradient. The activation energy for Si interdiffusion in the T2 phase was evaluated to be 355 kJ/mole compared with about 300 kJ/mole for Mo5Si3 in both Mo/Si and MoSi2/T2 diffusion couples. The larger activation energy is mainly responsible for the 103 lower diffusivity for Si in the T2 phase compared with Mo5Si3. The relatively slow diffusion in the T2 phase is consistent with the enhanced creep resistance exhibited by T2 phase microstructures.