Micro segregation

This is the most common appearance of segregation in real castings and is also referred to as crystal segregation or grain boundary segregation. It is caused by a disruption of the diffusion compensation between mixed crystals and the residual melt and is a concentration difference arising in the mixed crystals of an alloy after complete solidification

These segregation effects cannot be completely avoided, the aim is rather to mitigate them. For example, it is certainly admissible to assume the following context for iron alloys:

Micro segregation = f (Alloy elements, Balance of nuclei, Cooling conditions)

It is a known fact that Mg-Al alloys are prone to segregation due to the slow diffusion process of the Al dissolved in the Mg. Therefore, if the solidification rate is high, it cannot be expected that the entire solidified Mg mixed crystal has a homogenous aluminum concentration of 55.5% at 550°C. The reason for this is, that the core contains the mixed crystal diffused first at liquidus temperature which had only 3% at 598°C, which can be seen on the tie-line through the liquidus point in Fig. 1. Consequently, between 598°C and 550°C, it would have been necessary for the concentration up to inside the core to rise to 5.5% Al by means of diffusion in the growing crystal. If the diffusion is too slow a concentration gradient remains in the Mg mixed crystal resulting in an average composition below 5.5% Al because the external crystal layer in the local immediate equilibrium with the residual melt must have precisely 5.5% Al according to C. Kammer.

E. In the 1940ies, Scheil developed a model based on the following prerequisites:

  • completely prevented diffusion of the elements in the solidified solid body
  • local equilibrium at the liquid-solid phase boundary
  • macroscopic mass transport within the speciment
  • negligible influence of the boundary surface shape on the melting point

This model has proven increasingly useful as a tool for calculation of micro segregation. The principle is represented schematically by isoconcentration lines in a metallographic cross section of a primary dendrite after solidification (Fig. 2) and by the associated binary phase diagram (Fig. 3).

Additional references:
Ingot segregation
Reverse ingot segregation
Segregation factor (secondary aluminum cast alloys)
Segregation factor (cast iron)

 

  • Fig. 1: Phase diagram up to 50% Al by mass according to P. Liang et. al.
  • Fig. 2: Isoconcentration lines in a metallographic cross section of a primary dendrite after solidification (schematic) according to E. Scheil
  • Fig. 3: Binary phase diagram with solidification path (schematic) according to E. Scheil
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