Dendrite morphologies of the metastable phase from undercooled Fe-30at % Co melts and its stabilities

Abstract

Five typical dendrite morphologies of the metastable bcc phase fromundercooled Fe-30 at % Co melt have been observed by TEMtechnique. The morphologies of the metastable phase exhibitedwell-developed dendrite with the primary trunk and second arms,well-developed second arms, radiated structure, lath structure,and bifurcated structure. The crystal growth mode and theformation of different dendrite morphologies were discussed onthe basis of the morphological patterns from undercooled melts.In the mean while, the breakage mode for the primary dendritewas suggested according to the observation of microstructures ofthe alloy solidified at various undercoolings. The EDS (EnergyDispersive Spectrum) analysis has confirmed enrichment of thesolute Co in metastable dendrite cores in comparison to thatpredicted from the view of equilibrium solidification. Furtherinvestigation after annealing showed that the solute diffusioncontrolled the stability of the metastable phase; thedisappearance of dendrite morphologies was mainly attributed tothe constituent homogenization within dendrite cores and thedecrease in the number of dendrite cores was chiefly owing tothe solute diffusion between dendrite cores and the subsequentlysolidified equilibrium phase.