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

Five typical dendrite morphologies of the metastable bcc phase from underco oled Fe-30 at % Co melt have been observed by TEM technique. The morphologi es of the metastable phase exhibited well-developed dendrite with the prima ry trunk and second arms, well-developed second arms, radiated structure, l ath structure, and bifurcated structure. The crystal growth mode and the fo rmation of different dendrite morphologies were discussed on the basis of t he morphological patterns from undercooled melts. In the mean while, the br eakage mode for the primary dendrite was suggested according to the observa tion of microstructures of the alloy solidified at various undercoolings. T he EDS (Energy Dispersive Spectrum) analysis has confirmed enrichment of th e solute Co in metastable dendrite cores in comparison to that predicted fr om the view of equilibrium solidification. Further investigation after anne aling showed that the solute diffusion controlled the stability of the meta stable phase; the disappearance of dendrite morphologies was mainly attribu ted to the constituent homogenization within dendrite cores and the decreas e 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.