Interdiffusion and diffusion structure development in selected refractory metal silicides

Abstract

Abstract Solid–solid diffusion couples set up with disks of Mo, W, Re, Nb, or Ta in contact with disks of a single crystal of MoSi 2 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. MoSi 2 couples developed Mo 5 Si 3 and Mo 3 Si layers with non-planar interface morphologies; the Mo 5 Si 3 layer exhibited oriented growth and microcracking. The W vs. MoSi 2 couples developed W 5 Si 3 and (W,Mo) 5 Si 3 layers with little microcracking. The diffusion structure of Re vs. MoSi 2 diffusion couples consisted of layers of Re 2 Si, (Re,Mo)Si and (Re,Mo) 5 Si 3 phases and cracks were blunted in the (Re,Mo) 5 Si 3 layer. The Nb vs. MoSi 2 couples developed the Nb 5 Si 3 and (Nb,Mo) 5 Si 3 phase layers with porosity in the diffusion zone. Layers of Ta 2 Si, Ta 5 Si 3 and (Ta,Mo) 5 Si 3 phases were observed in the Ta vs. MoSi 2 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 W 5 Si 3 and (W,Mo) 5 Si 3 layers, respectively. Values of Q for the interdiffusion of both Re and Si are about 190 kJ mol −1 in the Re 2 Si phase, 325 kJ mol −1 in the (Re,Mo)Si phase, and 270 kJ mol −1 in the (Re,Mo) 5 Si 3 phase. For the interdiffusion of Si and Nb in the (Nb,Mo) 5 Si 3 phase, Q values are about 300 and 240 kJ mol −1 , respectively; Q is about 265 kJ mol −1 in the binary Nb 5 Si 3 phase. Zero-flux planes (ZFP) and uphill diffusion are observed for Mo in the (Re,Mo)Si and (Me,Mo) 5 Si 3 layers where Me=W, Re, Nb or Ta; in these layers, the ternary cross coefficient D MoMe Si is about as large as the main ternary interdiffusion coefficient D MoMo Si and the interdiffusion of Mo is enhanced down the Me concentration gradient.