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to Metalsto Fundamentals of metals

Crystallization

Dr. Dmitri Kopeliovich

Some metallurgical processes involve phase transition.

The typical example of phase transition is crystallization.

Crystallization is transformation of liquid phase to solid crystalline phase.

There are two general stages of phase transformation (crystallization)process nucleation and growth :

1. Nucleation

Nucleation is a process of formation of stable crystallization centers of anew phase.

Nucleation may occur by either homogeneous or heterogeneousmechanism, depending on the value of undercooling of the liquid phase(cooling below the equilibrium freezing point ).

Presence of foreign particles or other foreign substance in the liquid alloy (walls of the casting mold) allows to initiate crystallization at minor value of undercooling (few degrees below the freezing point). This is heterogeneousnucleation .

If there is no solid substance present, undercooling of a hundred degrees isrequired in order to form stable nuclei or seeds crystals, providingfollowing crystal growth ( homogeneous nucleation )

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Undercooling value determines quantity of nuclei, forming in thecrystallizing alloy. When a liquid comes into a contact with cold andmassive mold wall (chill zone), it cools fast below the freezing point,resulting in formation of a large quantity of stable nuclei crystals.

In order to promote the nucleation process, surface-active additives are used.They decrease interfacial energy of the nuclei crystals, causing formation of many more new stable nuclei.

2. Crystal growth

Number of stable nuclei per unit volume of crystallizing alloy determines thegrain size.

When a large number of stable nuclei are present in chill zone of mold, fineequiaxed grains form. Latent crystallization heat, liberating from thecrystallizing metal, decreases the undercooling of the melt and depresses thefast grains growth.

At this stage some of small grains, having favorable growth axis, start togrow in the direction opposite to the direction of heat flow. As a resultcolumnar crystals (columnar grains) form.

Contrary to the pure metals, in alloys different type of undercooling takesplace. It is called constitutional undercooling .

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Constitutional undercooling

Since solubility of an alloying element in solid is lower, than in liquid at thesame temperature, this element (solute) is rejected by the solidifying metalto the liquid phase, enriching the region of liquid adjacent to thecrystallization front.

For the most of the alloys: the higher the concentration of alloying elementin the alloy, the lower its liquidus temperature (temperature at whichcrystallization of the alloy starts).

Thus crystallization temperature of the liquid, adjacent to the crystallizationfront, rises with increasing the distance from the front surface. Thereforethere is a layer of the liquid, where its temperature is lower, than its

crystallization temperature. This is the region of constitutionalundercooling (see the figure below).

Dendrites

If a protruding finger forms on the solidifying surface, its tip may reach theregion of constitutional undercooling . In this case the protuberance startsaccelerated growth, forming the main dendrite arms. Under certainconditions the same process may occur on the surface of the main dendritearms, causing branching off the secondary arms and then arms of higher

orders.

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Process of solidification is considered in the article Solidification .

Discuss the article and ask questions in our Materials Forum

Related internal links

Metals crystal structure Solid solutions

Diffusion in alloys Grain structure Imperfections of crystal structure Solidification Microsegregation

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