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The first grains to form are small, due to the large undercooling from the mould wall, providing a greater driving force for nucleation.
Mould wall
Chill zone
Larger grains then grow towards the centre of the casting along the maximum temperature gradient, and in this example dendrites are formed.
This occurs as the conditions ahead of the solid/liquid interface are more favourable for solidification i.e. at a lower temperature.
Mould wall
Chill zone
Columnar zone