What is Alloy Segregation?

When an alloy solidifies from liquid to solid, the constituent elements are unevenly distributed in different parts. This phenomenon is called segregation.

1. Gravity segregation. This segregation is caused by the difference in density between the constituent phase and the molten liquid phase.

For example, when a hypereutectic or hypoeutectic alloy crystallizes, if the density of the hypoeutectic or hypereutectic phase that crystallizes first is greatly different from that of the liquid phase, these crystal phases will float or sink if they are slowly cooled. This will lead to inconsistent chemical compositions in the upper and lower regions due to the order of crystallization, forming gravity segregation.

The floating of graphite in cast iron is a kind of gravity segregation.

Obviously, gravity segregation is related to the density difference of each component of the alloy, the interval of crystallization components on the phase diagram, and the temperature interval.

The greater the difference in density of the alloy components, the greater the interval of crystallization components on the phase diagram, and the greater the density difference between the first eutectic phase and the liquid phase. The larger the crystallization temperature interval of the phase diagram, the slower the cooling rate, the more time the crystal phase has to float or sink in the liquid phase, and the more serious the gravity segregation. The phase diagram of iron-carbon alloy is shown in Figure 1.

How to prevent gravity segregation? First, fast cooling. The pro-eutectic phase does not have enough time to float or sink. Second, add a third element. When the liquid phase solidifies, a phase with a density close to that of the liquid phase is first formed to prevent the pro-eutectic phase from floating or sinking. Third, heat convection and stirring are used to prevent the segregation of the pro-eutectic phase.

2. Regional segregation. When the solid solution alloy is in unbalanced crystallization, the solute content of the alloy that solidifies first is lower, and the solute content of the alloy that solidifies later is higher. This will cause macroscopic segregation or regional segregation. The banded structure caused by segregation is shown in Figure 2.

The melting point of purer crystals is higher, and the melting point of crystals with more impurities is relatively low. When the road is blocked by heavy snow in winter, salt can be sprinkled on the road to melt the snow. This is the reason. The melting point of salt water is below zero degrees. Sprinkling salt on the road surface does not require spraying hot water, and the snow will slowly melt by itself.

The above is the case of unbalanced crystallization. If it is balanced crystallization, the crystallization process is slow, and the components of the solid phase and the liquid phase have enough time and temperature to diffuse and mix evenly, segregation will not occur.

3. Dendrite segregation. When the solid solution crystallizes, for a single grain, the first crystallized part contains more high-melting point components, and the later crystallized part contains more low-melting point components. That is to say, within a grain, the composition will also be uneven due to different solidification times, which is called intracrystalline segregation.

The crystals of the solid solution are often dendritic, and the composition of the branches and branches is different, so intracrystalline segregation is sometimes also called dendritic segregation.

Intracrystalline segregation will reduce the mechanical properties of the material and the corrosion resistance will also decrease. The way to eliminate intracrystalline segregation is diffusion annealing or homogenization annealing.