At the large overcooling, the mechanism of the crystals growth by means of dislocations and steps is replaced normal, is not connected with the origin of new atomic layers in usual sense. In these conditions the prize of thermodynamic potential when hardening is so huge that the crystal can become covered by rough and step sides with big excess of energy. On these sides, there are many places convenient for fixing of atoms. Therefore with such crystals growth there is no need for formation of new steps. The crystals of all metals can have the rough surface. For overcoming potential barriers upon transition from liquid to a crystal atoms have to be activated.
On fig. 1 the curve growth rates of crystals when hardening metals (a) and some nonmetals (b) are shown.
Increasing the degree of supercooling (ΔT) increases the rate of crystal growth (Up). The reason is that with the growth of hypothermia increases the difference between the chemical potentials Δμ.
In some nonmetallic liquids at the large overcooling, the viscosity so increases that transitions of atoms through phase border are at a loss and the crystals growth rate decreases. In this case the curve of dependence of growth rate on overcooling consists of the ascending and falling branches. For the metals which are characterized by small viscosity in liquid state it isn’t possible to reach reduction of growth rate usually.
The great influence on the metals hardening is exerted by withdrawal of crystallization warmth. It is carried out or through a crystal (Fig. 2, a), or through a crystal and liquid (Fig. 2, b).
In the first case, the heat stream through the hardening part of metal goes to walls of a form and before the front of crystallization there is an overcooling ΔT, sufficient for the crystal growth. In the process of heat removal temperature of liquid becomes below Тб and the crystal grows. In the second case, the crystal grows in the overcooled liquid, and its temperature can be higher, than liquids.