Watching formation of solid object by casting always fascinates us no matter how many times we saw it, so today I will tell you the scientific reason and the step by which a hot burning liquid metal transforms to a useful solid state . 

Solidification of metal is a physical phenomenon where in a metal which is in a liquid state transforms to a solid state whereas scientifically it's transformation of one phase with higher free energy to another phase with lower free energy , as every elements and objects In this universe has  tendency to remain in lower free energy state .

Before we discuss why and how it happens let me tell you what is a phase,

A phase is a chemically homogeneous physically distinct and mechanically separable part of a system.

For the solidification to happen, A driving force is required , which is provided by the difference of the free energy of the two state, i.e difference of free energy of liquid state metal and the free energy of the solid state metal.

Solidification process happens in three stages , nucleation , stable grain (nucleus) formation and grain growth respectively .

Nucleation can happen by two ways ,

Homogenous nucleation route and Heterogeneous nucleation route.

Homogenous nucleation :- Homogenous nucleation happens when an under cooling is applied to the liquid, i.e the liquid metal is exposed to a temperature below it’s freezing point.

Taking the example of pure copper metal , which has a freezing point of 1083°C , if we want to solidify a liquid copper metal by this route ,we need to give cooling of temperature below 1083°C . This temperature difference will provide the driving force for the nucleation to happen ,and hence many small sized nucleus would be created .

Heterogeneous nucleation:- heterogeneous nucleation happens when there is a solid foreign material presents in the hot metal or there is solid surface in contact with the liquid metal , which provides the ready sites for the nucleation to happen.

Taking an example of copper again , the crucible walls will provide a ready site for the nucleation to happen and hence direction of solidification will be from the walls towards the centre. 

Similarly if there is a solid foreign material like some metal oxide that has melting point greater than melting point of copper metal , then it will act as the nucleation site and  help the nucleation process.

Now let’s  see what happens after nucleus formation, after the nucleus has formed , as we keep decreasing the temperature further the nucleus will tend to coalesce with each other and form stable nucleus or grains, unstable nucleus will get destroyed .

After the formation of stable nucleus or preliminary grains ,as the temperature decreases further, grain growth begins where in the grain size increases at the cost of smaller grains or the liquid which failed to form nucleus or grain.

Speaking in simple language nucleation is more feasible at higher temperature hence more number of nucleus are formed at higher temperature

 Whereas grain growth is more feasible at lower temperature , but only upto certain degree below which the atomic movement slows down restricting the further grain growth.

This is how all the magic happens.

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