Generation of crystals in liquid (Part 4).

To a large extent, the process of generation is affected by the inclusion surface’ topography. If there are depressions on the surface, the critical dimension nucleus’ size and the work required for its formation are smaller than those for the flat surface. The influence that the nonisomorphic impurities exert on the process of crystal formation can be boosted if they have previously been in contact with the crystals of the given substance. At the temperatures that are not too low, under the influence of the crystal’s atoms, the structure of the impurity inclusion’ surface abnormality changes, accommodating the crystal structure. Such process is called “impurity activation”. The atoms of the liquid are adsorbed easier on the activated impurity surface; such surface becomes thesubstrate. Thus, the impurities play a role of the crystallization process catalysts.

In case of overheating above the melting point, the adsorbed layer is destroyed. The surface inclusion layer can be significantly altered too. The impurity can partially dissolve and become fused. This deactivates the impurity; and, when cooled, the overheated liquid behaves similarly to the purified one. If after the melting the metal was not overheated, its crystallization is accompanied by the formation of the same fine-grained structure that existed prior to the melting. In this connection, we may speak about the heredity that is eliminated by overheating.

Thus, the active and activated inclusions and the mold walls, being instrumental in forming the flat nuclei on the substrate, facilitate the appearance of the melt’s crystallization centers. But the fundamental crystallization pattern remains the same. The speed performance curve of the nuclei generation for the liquids with impurities is offset towards the less intensive overcoolings. The offset magnitude depends on the nature of the liquid and of the inclusions, as well as on their surfaces’ condition. For example, the overcooling capacity of thoroughly purified bismuth is 45 – 46°C, and that of the oxidated one is 12°C.

The speed of the crystallization centers’ formation is also affected by the dissolved impurities that are present in a liquid metal in the form of individual atoms (ions). They exert their influence on the interphase surface energy, as a result of which the critical nucleus size is altered.

The impurities that facilitate the process of crystal generation are added to liquid metals on a large technological scale. An impurity that catalyzes the formation of crystallization centers is called a modifier, and the process of its introduction into the melt and of influencing the crystallization – the modification. The aluminum or vanadium oxide particles facilitate the generation of iron crystals. Aluminum, copper and their alloys are modified by titanium and iron, respectively.

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