Interdiffusion and diffusion structure development in selected refractory metal silicides

Solid–solid diffusion couples set up with disks of Mo, W, Re, Nb, or Ta in contact with disks of a single crystal of MoSi2 were annealed at selected temperatures between 1300° and 1700°C for diffusion structure, determination of interdiffusion coefficients and energies of activation for interdiffusion in various silicides developed in the couples. The couples were analyzed and characterized by SEM and optical microscopy, microprobe analysis, X-ray diffraction and orientation imaging microscopy. From the interdiffusion fluxes determined directly from the concentration profiles, integrated and average effective interdiffusion coefficients were calculated for the components in the various binary and ternary silicide layers. The Mo vs. MoSi2 couples developed Mo5Si3 and Mo3Si layers with non-planar interface morphologies; the Mo5Si3 layer exhibited oriented growth and microcracking. The W vs. MoSi2 couples developed W5Si3 and (W,Mo)5Si3 layers with little microcracking. The diffusion structure of Re vs. MoSi2 diffusion couples consisted of layers of Re2Si, (Re,Mo)Si and (Re,Mo)5Si3 phases and cracks were blunted in the (Re,Mo)5Si3 layer. The Nb vs. MoSi2 couples developed the Nb5Si3 and (Nb,Mo)5Si3 phase layers with porosity in the diffusion zone. Layers of Ta2Si, Ta5Si3 and (Ta,Mo)5Si3 phases were observed in the Ta vs. MoSi2 couples. The activation energies (Q) for the interdiffusion of both Si and W are calculated to be about 360 and 450 kJ mol−1 in the W5Si3 and (W,Mo)5Si3 layers, respectively. Values of Q for the interdiffusion of both Re and Si are about 190 kJ mol−1 in the Re2Si phase, 325 kJ mol−1 in the (Re,Mo)Si phase, and 270 kJ mol−1 in the (Re,Mo)5Si3 phase. For the interdiffusion of Si and Nb in the (Nb,Mo)5Si3 phase, Q values are about 300 and 240 kJ mol−1, respectively; Q is about 265 kJ mol−1 in the binary Nb5Si3 phase. Zero-flux planes (ZFP) and uphill diffusion are observed for Mo in the (Re,Mo)Si and (Me,Mo)5Si3 layers where Me=W, Re, Nb or Ta; in these layers, the ternary cross coefficient D̃MoMeSi is about as large as the main ternary interdiffusion coefficient D̃MoMoSi and the interdiffusion of Mo is enhanced down the Me concentration gradient.