Consider a hypothetical equilibrium phase diagram between two elements A and B $.$ Component A melts at $900C$; component B melts at $1,000C$; and there is an invariant reaction at $600C.$ The solubility of B in A is known to increase from almost nil at room temperature to a maximum of $10wt.\%.$ When an alloy containing $30wt.\%B$ is cooled under equilibrium conditions to just above $600C,$ a two$-$phase mixture of $40\%$ and $60\%$ liquid is present. When the alloy is cooled just below $600C,$ the alloy contains two solid phases, $$ and $.$ The fraction of $$ is $0\mathrm{.}70.$ After cooling under equilibrium conditions to room temperature, the amount of $$ in the $+$ mixture decreases to $66\%.$