DIFFUSION IN MoSi2 AND MoSi2-BASED COMPOSITES
Prof. M.A. Dayananda
School of Materials Engineering
Supported by ONR; Grant N00014-95-1-0466; 1995-1999
|Columnar grains of MoSi2 grown in the Mo vs.
Si diffusion couple annealed at 1350oC
for 10 hours.
Multiphase diffusion studies were carried out at selected temperatures between 900o
-C to investigate the development of diffusion structures in the Mo vs. Si and W vs. Si binary systems and Me vs. MoSi2
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 MoSi2
layers developed with a columnar microstructure exhibiting a  orientation in the direction of diffusion. In the Me vs. MoSi2
diffusion couples, both planar and non-planar morphologies were encountered in the development of multiple diffusion layers, including a (Me,Mo)Si3
silicide layer. For the Mo5
layers formed in the ternary couples, OIM techniques revealed the development of 001 textures in the diffusion zone. In the (Me,Mo)5
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 were also reported for Mo in the Me vs. MoSi2
Mo vs. single crystal MoSi2 diffusion couple
annealed at 1500o C for 6 hours.
Backscattered electron micrograph of the Re vs.
MoSi2 diffusion couple annealed at 1700oC for
- P.C. Tortorici and M.A. Dayananda: " Diffusion Structures in Mo vs. Si Solid-Solid Diffusion Couples", Scripta Materialia, Vol.38, (1998) pp. 1863-69.
- P.C. Tortorici and M.A. Dayananda: " Growth of Silicides and Interdiffusion in the Mo-Si System", Metallurgical Transaction A. vol.30A, (1999) pp. 545-550.
- P.C. Tortorici and M.A. Dayananda: "Interdiffusion and Diffusion Structure Development in Selected Refractory Metal Silicides", Materials Science & Engineering, vol. A261, (1999) pp. 64-77.
- M. A. Dayananda: "Diffusion Structures and Diffusion Paths in Ternary Systems-Selected Unusual Observations", 2nd International Conference on Diffusion and Reactions, Krakow, Poland, Sept. 1999, Solid State Phenomena, 72, (2000) pp. 123-134.
- P.C. Tortorici and M.A. Dayananda: " Development of Zero-Flux Planes in Silicides of Mo containing W, Re, Nb or Ta", (in preparation).