Homogenization



Nomenclature
cconstant
Ssecondary dendrite arm spacing
Dsrate of diffusion of the solute in the matrix


The interdendritic segregation and problems with hot shortness can be reduced by means of a homogenization treatment. Consider an alloy such as Cu-40% Ni. If a casting is heated to a temperature below the nonequilibrium solidus, the nickel atoms in the centers of the dendrites diffuse to the interdendritic regions; copper atoms diffuse in the opposite direction as demonstrated in Fig1. Since the diffusion distances are relatively short, only a few hours are required to eliminate most of the composition differences. The homogenization time is related to:

(Eq1)    
t  =  
cS 2
Ds

A small S reduces the diffusion distance and permits short homogenization times.

Microsegregation between dendrites can be reduced by a homogenization heat treatment. Counterdiffusion of nickel and copper atoms may eventually eliminate the composition gradients and produce a homogeneous composition.


Fig1

Fig2
InitialFinal