Thermal Stability of Binary Cr-Cr₃Si and Ternary NiAl-Mo Eutectic Alloys

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.