Date of Award
Doctor of Philosophy
Materials Science and Engineering
Easo P. George
George M. Pharr, Claudia J. Rawn, Madhu S. Madhukar
The thermal stability of multiphase intermetallics at temperatures to 1400°C was investigated by studying two model eutectic systems: Cr-Cr3Si having a lamellar microstructure and NiAl-Mo having a fibrous microstructure. In drop cast Cr-Cr3Si, coarsening was found to be interface controlled. The coarsening rate could be significantly reduced by microalloying with Ce and Re, two elements which were chosen because they were expected to segregate to the Cr-Cr3Si interfaces and decrease their energies. Similarly, directional solidification, which is also expected to lower the Cr- Cr3Si interfacial energy, was found to dramatically decrease the coarsening rate. In the case of NiAl-Mo, coarsening was found to occur by fault migration and annihilation. Microalloying with B was found to significantly decrease the coarsening rate. The fiber density in the B-doped alloy was smaller than in the undoped alloy, suggesting that B affects the coarsening rate by lowering the fault density. X-ray energy dispersive spectroscopy (XEDS) analysis of Ce doped drop cast Cr-Cr3Si showed that Ce is present at the Cr/Cr3Si interface. XEDS analysis of Re doped Cr-Cr3Si and EELS analysis of B doped NiAl-Mo were inconclusive.
Gali, Aravind, "Thermal Stability of Binary Cr-Cr3Si and Ternary NiAl-Mo Eutectic Alloys. " PhD diss., University of Tennessee, 2006.