MODULE 3
Solidification
Solidification is the process where liquid metal transforms into solid upon cooling.
The properties of the solidified metal depends on its microstructure and the microstructure in turn
depends on the ‘Solidification Mechanism’ employed to freeze the metal.
Transformation from liquid metal to solid metal is accompanied by a shrinkage in the volume. This
volume shrinkage takes place in three stages:
(i) Liquid-Liquid
(ii) Liquid-Solid
(iii) Solid –Solid
Mechanism of Solidification in pure metals and alloys
Solidification of all liquid metals consist of two stages
A) Nucleation of minute Crystals
B) Growth of these crystals into grains
A) Nucleation:
This is the starting point of the solidification process of any metal or alloy. In this stage a number of
minute crystals nucleate throughout the melt.
Metal in molten condition possesses high energy and the atoms have high mobility.
As the liquid metal cools, the atoms gradually lose their energy and their movements become less
vigorous. Then when atoms collide with each other, attractive forces are set up and atomic bonds are
reestablished between them.
Therefore, small cluster of atoms, usually consisting of 10 –15 atoms are formed at several places in the
molten metal and are known as nuclei.
It is the number of nuclei formed during the stage of nucleation which decides the final grain structure
of the solidified metal and therefore some of its properties.
Nucleation in metals and alloys may occur by two mechanisms namely
(i) Homogenous Nucleation (ii) Heterogeneous nucleation
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, B) Crystal Growth:
This is the stage where molten metal continues to solidify around the nuclei which are already formed.
The nuclei and the metal solidifying around them continuously release latent heat which it had acquired
during melting. This heat is absorbed by the surrounding molten metal and therefore for solidification to
continue more and more heat has to be extracted I.e., cooling of metals should continue.
As the temperature decreases due to cooling the nuclei grow rapidly with atoms attaching themselves in
identical layers around the nuclei grow rapidly with atoms attaching themselves in identical layers
around the nuclei and a dendritic formation takes place.
When more and more atoms attach themselves in a particular direction, they form a trunk like that of a
tree. Out of this trunk branches grow out and from these branches some more smaller branches grow
out perpendicularly and altogether forms a tree like structure. This type of growth is called the dendritic
growth (fig 2.51 a and b)
The dendrites are like skeleton in the microstructure of the solidified metal. When one big dendrite tree
meets another, a grain boundary is formed along their border while the dendrites themselves become
grains or crystals (fig 2.51c). The liquid metal along the grain boundaries will be the last to solidify and
this marks the end of solidification.
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Solidification
Solidification is the process where liquid metal transforms into solid upon cooling.
The properties of the solidified metal depends on its microstructure and the microstructure in turn
depends on the ‘Solidification Mechanism’ employed to freeze the metal.
Transformation from liquid metal to solid metal is accompanied by a shrinkage in the volume. This
volume shrinkage takes place in three stages:
(i) Liquid-Liquid
(ii) Liquid-Solid
(iii) Solid –Solid
Mechanism of Solidification in pure metals and alloys
Solidification of all liquid metals consist of two stages
A) Nucleation of minute Crystals
B) Growth of these crystals into grains
A) Nucleation:
This is the starting point of the solidification process of any metal or alloy. In this stage a number of
minute crystals nucleate throughout the melt.
Metal in molten condition possesses high energy and the atoms have high mobility.
As the liquid metal cools, the atoms gradually lose their energy and their movements become less
vigorous. Then when atoms collide with each other, attractive forces are set up and atomic bonds are
reestablished between them.
Therefore, small cluster of atoms, usually consisting of 10 –15 atoms are formed at several places in the
molten metal and are known as nuclei.
It is the number of nuclei formed during the stage of nucleation which decides the final grain structure
of the solidified metal and therefore some of its properties.
Nucleation in metals and alloys may occur by two mechanisms namely
(i) Homogenous Nucleation (ii) Heterogeneous nucleation
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, B) Crystal Growth:
This is the stage where molten metal continues to solidify around the nuclei which are already formed.
The nuclei and the metal solidifying around them continuously release latent heat which it had acquired
during melting. This heat is absorbed by the surrounding molten metal and therefore for solidification to
continue more and more heat has to be extracted I.e., cooling of metals should continue.
As the temperature decreases due to cooling the nuclei grow rapidly with atoms attaching themselves in
identical layers around the nuclei grow rapidly with atoms attaching themselves in identical layers
around the nuclei and a dendritic formation takes place.
When more and more atoms attach themselves in a particular direction, they form a trunk like that of a
tree. Out of this trunk branches grow out and from these branches some more smaller branches grow
out perpendicularly and altogether forms a tree like structure. This type of growth is called the dendritic
growth (fig 2.51 a and b)
The dendrites are like skeleton in the microstructure of the solidified metal. When one big dendrite tree
meets another, a grain boundary is formed along their border while the dendrites themselves become
grains or crystals (fig 2.51c). The liquid metal along the grain boundaries will be the last to solidify and
this marks the end of solidification.
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