undercool

There are three classes of nucleation: (1) homogeneous nucleation or spontaneous nucleation; (2) self-nucleation; and (3) heterogeneous nucleation [9], 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]) [10].

The presence of any undercooling will reflect in the relatively large grain size of the sample.

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.

Droplets in the metastable area grow through nucleation-growth (NG) mechanism, and a lower degree of undercooling can result in a cellular structure [2], 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.

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 [10].

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).