There are three classes of nucleation: (1) homogeneous nucleation or spontaneous nucleation; (2) self-nucleation; and (3) heterogeneous nucleation , Homogeneous nucleation occurs in pure polymers due to the influence of supercooling also known as undercooling
, which is the difference between the melt temperature (Tm) and the onset crystallization temperature ([T.sub.0]) .
The presence of any undercooling
will reflect in the relatively large grain size of the sample.
 report that, during solidification, the eutectic undercooling
increases up to 15 K at 1 pct MM addition and up to 25 K at 2 pct addition.
The nucleation process highly depends on the temperature and movement of the molecular chains, and a higher undercooling
is needed to form nuclei.
* the instability of the eutectic front occurs because of constitutional undercooling
caused by accumulation of solute ahead of the eutectic growth front.
It should be emphasized here that the addition of the RE elements changes the nucleation process of the molten alloy and, hence, the undercooling
of the melt will be influenced.
Droplets in the metastable area grow through nucleation-growth (NG) mechanism, and a lower degree of undercooling
can result in a cellular structure , The cellular structure usually is closed and it decreases the flux of the membrane.
Goal: Canada Centre for Mineral and Energy Technology (CANMET) has quantified the effects of undercooling
versus holding time for base iron with Gray Iron Research Committee (5-H) sponsorship.
This could be attributed to the fact that the undercooling
shows an increasing trend with enlarging the supercell sizes.
G values (solid blue line) measured in nonisothermal condition (cooled at 1[degrees]C/min), are low at high temperatures, then gradually increase with the increase of the undercooling
to reach a maximum, as predicted by the theory .
The white eutectic, which occurs at the edges of thin-wall ductile iron plate castings, is the plate-like eutectic, which probably forms at much higher undercooling
than ledeburite (the usual structure available in white iron).
During the twin-roll continuous casting, the undercooling
of the liquid phase is generally far less than the undercooling
required for uniform nucleation and liquid-phase surface oxidation, broken dendrites, and heterogeneous nucleation exist in the liquid metal.