THE NATURE OF METASTABLE PHASE FORMATION IN THE MICROSECOND PULSED LASER-MELTED POOL OF AL-GE EUTECTIC ALLOY

The solidification behaviour of a microsecond pulsed neodymium-doped glass laser-melted AlGe eutectic alloy was studied. The results indicate the nucleation and columnar growth of a metastable monoclinic phase from the melt-substrate interface at a high power density. An equiaxed microstructure containing the same monoclinic phase is obtained at a low power level. It is shown that the requirement of solute partition acts as a barrier to eutectic regrowth from the substrate. The resultant interface undercooling favours nucleation of the monoclinic phase. Further, it is shown that this phase grows with very little solute partitioning at this composition, which imparts a growth advantage over other competitive phases. © 1990.