Four key assumptions in Scheil analysis enable determination of phases present in a cast part.  These assumptions are:
# No diffusion occurs in solid phases once they are formed ()
# Infinitely fast diffusion occurs in the liquid at all temperatures by virtue of a high diffusion coefficient, thermal convection, Marangoni convection, etc. ()
# Equilibrium exists at the solid-liquid interface, and so compositions from the phase diagram are valid
# Solidus and liquidus are straight segments
The fourth condition (straight solidus/liquidus segments) may be relaxed when numerical techniques are used, such as those used in CALPHAD software packages, though these calculations rely on calculated equilibrium phase diagrams.  Calculated diagrams may include odd artifacts (i.e. retrograde solubility) that influence Scheil calculations.