The growth velocity during solidification of an undercooled melt of a Co-Cu alloy
processed by electromagnetic levitation was measured using a high speed video camera.
Applying a model of local non-equilibrium solidification, theoretical predictions of
dendrite growth velocity and dendritic growth radii are compared with high-accuracy
measurements of the growth kinetics. As the undercooling ΔT reaches a critical value
consistent with the dendrite growth velocity being equal to the atomic diffusion speed
VD in bulk liquid,
ΔT =
ΔT(VD),
the velocity-undercooling relationship exhibits a break-point. A distinct change in the
dendritic growth mechanism exists with the onset of complete solute trapping and
chemically partitionless solidification of the core of the main stems of the dendrites
occurs. A complete transition to the thermally controlled growth of dendrites occurs at
ΔT =
ΔT(VD)
that leads to essential changes in the microstructure of dendritic patterns The phenomenon
of dendritic fragmentation in Co-Cu melts, solidifying at ΔT <
ΔT(VD),
is discussed.