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

Five typical dendrite morphologies of the metastable bcc phase from undercooled Fe-30 at % Co melt have been observed by TEM technique. The morphologies of the metastable phase exhibited well-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 the formation of different dendrite morphologies were discussed on the basis of the morphological patterns from undercooled melts. In the mean while, the breakage mode for the primary dendrite was suggested according to the observation of microstructures of the alloy solidified at various undercoolings. The EDS (Energy Dispersive Spectrum) analysis has confirmed enrichment of the solute Co in metastable dendrite cores in comparison to that predicted from the view of equilibrium solidification. Further investigation after annealing showed that the solute diffusion controlled the stability of the metastable phase; the disappearance of dendrite morphologies was mainly attributed to the constituent homogenization within dendrite cores and the decrease in the number of dendrite cores was chiefly owing to the solute diffusion between dendrite cores and the subsequently solidified equilibrium phase. (C) 2000 Kluwer Academic Publishers.