Dec 202013
 

As T = 0 K, the heat capacity approaches zero, thus, it is assumed that the atoms do not oscillate at their lattice points.

By means of supplying heat, the atoms behave as oscillator and start to vibrate with certain frequencies around their lattice sites. With increasing temperature and therefore increasing oscillation of the atoms the distance or to be more precisely, the distance of the center points of the atoms becomes larger; the lattice will be widened.
the lattice

Thus, the heat supplied will be converted into vibration energy leading to both, increasing the temperature as well as increasing the volume. When the melting temperature is reached, the temperature will not increase further since the energy supplied is used to detach the atoms from the lattice bond and to transfer them into the more disordered state of a melt.

The vibrations are replaced by an undefined motion. The melting heat is called a latent heat since it does not lead to an increase of temperature.

Cooling
On the other hand, the heat supplied before will be delivered again during cooling. Crystallization of the melt starts with the formation of germs that is carried out in the atomic magnitude first. The first atoms arrange randomly to form clusters or sub germs that already have an atomic arrangement typical for the solidifying metal. Such clusters are formed as a consequence of the thermal movement of the atoms but will decompose again. Crystallization is only stable when the sub germs have obtained the capability to grow further. This accumulation of atoms which is able to increase is called a germ.
In the ideal case the germs become thermodynamically stable when the solidification temperature has been reached and so the melt can solidify.
At that point the respective cooling curves show a point of arrest here due to the latent melting enthalpy being released. Further growing of the germs will be determined by the crystallization rate.
The germs that collide in the final state are also called grains, which are separated from each other by the respective grain boundaries.
It is distinguished between a homoge-nous and a heterogeneous formation of germs.

There is a homogoneous formation of germs when everywhere in the melt there are the same thermomechanical conditions for the formation of generic germs. The germs generated are distributed irregularly.
In the heterogenous formation of germs there are thermomechanically favoured parts in the melt, e.g. by the mould walls, impurities or intentionally added germ formers (inoculants, grain finer).
It is obvious that by means of germ formers the solidification behavior of a melt and therefore the structure formation of a material can considerably be influenced.

In practice, the cooling curves are different from the theoretical ones, since due to a delay in crystallization the melt will be supercooled. The higher the supercooling is, the larger will be the force for formation and growing of the germs. However, when supercooling proceeds, the moving ability of the atoms will be decreased so that both, the number of germs and the growth of the germs will decrease again.
The ratio of germ number and rate of germ growth determines the resulting grain size as well as the distribution of the grains in the structure. If the number of germs is small and the crystallization rate is high, a course grained structure will be generated.
On the other hand, a fine grained structure will be generated, if the number of germs due to e.g. supercooling or an addition of grain finer is high and the germ growth is low.
Theoretical course of the cooling curve

Practical course of the cooling curve

Foreign germs in the melt may neutralize the delay in crystallization. At a defined number of foreign germs supercooling is proportional to the cooling rate.
The knowledge gained during the solidification of melts can principally be transferred to crystallization processes in the solid state. Here, the same correlation between germ number, crystallization rate and supercooling is valid.

An amorphous material has no defined melting or solidification point. It becomes more viscous the lower the temperature is.

 

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