Coring microstructure resulting from non-equilibrium cooling

Phase diagrams are valid under the condition of thermodynamic equilibrium. The challenge is that sometimes it takes a long time to reach thermodynamic equilibrium. For example, if you are cooling through a liquid + solid region then you might require a large change in composition in the solid phase. Diffusion in the solid phase might be too slow so that you end up with a cored microstructure. The microstructure ends up with a gradient in composition and the solidus line shifts downward in departure from what you'd expect on the equilibrium phase diagram.