A transition to diffusionless solidification
Modern techniques of experiments and advanced technologies of melting and solidification of metallic systems allow us to reach deep undercoolings, large temperature and concentration gradients, and high velocities of the phase transformations. E.g., in modern experiments [ Metastable solids from undercooled melts ] the interface velocity reaches 10-100 m/s and the liquid phase can be undercooled in a wide range of 10-400 K. For such large driving forces and with increasing of velocity of solidification, a deviation from local thermodynamic equilibrium can occur at the advancing solid-liquid interface and in bulk phases. Application of the LNSM to description of rapidly solidified patterns leads to the conclusion that the transition to diffusionless solidification occurs at a finite solidification velocity V equals the diffusion speed VD in bulk liquid. Figures (*) and (**) show the transition from the chemically partition, Fig. (*), to chemically partitionless (diffusionless) solidification of dendrite, Fig. (**). As the undercooling in bulk liquid provides the higher growth velocity for the dendritic tip, V>VD, a core of the main stem of dendrite forms with the initial chemical composition marked in white. Numeric modelling of a transition to diffusionless (chemically partitionless) growth was performed for a dilute Fe-C binary alloy.
Figure (*). The concentration (left) and temperature (right) fields in chemically
partition growth with the initial (base)
undercooling of ΔT=140 K.
Figure (**). The concentration (left) and temperature (right) fields in diffusionless
(chemically partitionless) growth with
the initial (base) undercooling of ΔT=390 K. The white color corresponds to the initial (nominal) composition of the system.