Diffusion and phase formation in selected refractory metal silicides

Peter Christopher Tortorici, Purdue University

Abstract

Multiphase diffusion studies were carried out at selected temperatures between 900$\sp\circ$-1700$\sp\circ$C to investigate the development of diffusion structures in the Mo vs. Si and W vs. Si binary systems and Me vs. MoSi$\sb2$ systems, where Me = Mo, W, Re, Nb, and Ta. Several series of diffusion couples were examined for the formation of binary and ternary silicides and were analyzed and characterized by microprobe analyses, SEM and optical microscopy, x-ray diffraction, and orientation imaging microscopy (OIM). Concepts of integrated and average effective interdiffusion coefficients were employed for the description of interdiffusion, and the calculation of energies of activation for interdiffusion in the silicide layers. A new relationship was derived to describe the growth rate constant of a diffusion layer in terms of integrated interdiffusion coefficients. In the Mo vs. Si and W vs. Si couples, tetragonal MoSi$\sb2$ and WSi$\sb2$ layers developed with a columnar microstructure exhibiting a hk0 orientation in the direction of diffusion. In the Me vs. MoSi$\sb2$ diffusion couples, both planar and non-planar morphologies were encountered in the development of multiple diffusion layers, including a (Me,Mo)$\rm\sb5Si\sb3$ silicide layer. For the $\rm Mo\sb5Si\sb3$ and $\rm W\sb5Si\sb3$ layers formed in the ternary couples, OIM techniques revealed the development of 001 textures in the diffusion zone. In the (Me,Mo)$\rm\sb5Si\sb3$ layers, Mo exhibited "up-hill" diffusion against its own concentration gradient in a direction opposite the flow of the other refractory element W, Re, Nb, or Ta. Such diffusion of Mo was characterized by negative effective interdiffusion coefficients. The interactions on Mo interdiffusion exhibited by Ta, Nb, W and Re increased in the order of the elements cited. New observations of zero-flux planes without the formation of a relative extremum in concentration profiles are also reported for Mo in the Me vs. MoSi$\sb2$ couples.

Degree

Ph.D.

Advisors

Dayananda, Purdue University.

Subject Area

Materials science|Metallurgy

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