= Constitutional superheating
A solid can locally exceed its <solidus> because a thin solute-diffusion layer changes composition faster than <thermal conduction> changes <temperature>. Near a melting <phase boundary>, a depleted solid can join the equilibrium solidus at the interface while the adjacent, more concentrated solid remains above its own lower solidus. The assumed single sharp interface is then thermodynamically unstable to additional melting and may be replaced by a <mushy layer>.
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