Non-Equilibrium Solute Partitioning and Microstructure Formation in Rapidly Solidified Al-Sm Alloys

A critical issue, central to microstructural selection in highly driven systems, is the role of nonequilibrium solute partitioning and the formation of metastable phases. When local chemical equilibrium breaks down at very high undercoolings, solidification dynamics are influenced by limitations to atomic attachment kinetics, decreasing solute diffusivities, changing thermodynamic driving forces, and the energetics of ordering or clustering in the liquid phase. The scope of the current study is to employ free-jet melt spinning along with advanced e-beam, x-ray diffraction techniques to investigate the degree of solute partitioning and metastable phase formation during rapid solidification of Al-Sm alloys with compositions at or near the Al-rich eutectic (15wt.% or 3 at.%). At all melt spinning rates employed (10-40m/s), solidification morphologies consist of the Al (fcc) and All i Sm3 (tetragonal) phases. Certain morphological transitions occur through the thickness of the ribbon, indicating a change in the prevailing cooling conditions as solidification proceeds. In addition, x-ray diffraction analysis indicates that the Sm content in the fcc Al phase is well above that given by local two-phase (fcc+liquid) equilibrium.