Solidification variables 

 
Solidification behavior depends on parameters such as growth rate, temperature gradient, under cooling and alloy constitution. The temperature gradient (G) and growth rate (R) influence the solidification morphology and solidification substructure respectively.


Growth rate or solidification rate is the rate of advance of the solid/liquid interface into the liquid. The rate of movement of solidification front determines solute redistribution during solidification, scale of solidification substructure and the growth undercooling. Rapid solidification (faster movement of solid/liquid interface) minimizes the tendency of segregation of elements


Temperature gradients both in solid and liquid are important. Thermal gradient in liquid is more critical as compared with solid and is strongly affected by mixing in liquid. Temperature gradient in solid is diffusion dependent.


Undercooling is the difference between the liquid temperature of the alloy of nominal composition and the actual temperature.


The total undercooling    


.      
    
 Constitutional undercooling

 Kinetic undercooling,

    Undercooling due to curvature at solid/liquid interface.

 = Thermal undercooling


  and   are usually negligible in casting. Thermal under cooling is required in solidification of pure metal where there is a significant nucleation barrier for liquid to solid transformation or when directional solidification is carried out at a lower rate where cellular structure forms. In the solidification of ingot and continuous casting where alloys are solidified, thermal undercooling is not of interest.

In solidification of alloy, the liquidus temperature of the melt is composition dependent. If the liquid composition deviates from the bulk composition, then the melt temperature may differ from the liquidus temperature of the overall alloy. The change in transformation temperature due to compositional effect is called constitution undercooling .