= Solution
\b[At equilibrium at room <temperature> the phases are pure Ag and pure Cu, with <mass fractions> $0.80$ and $0.20$.] A representative <solidification> structure contains primary Ag-rich grains or dendrites surrounded by a finer two-phase eutectic aggregate. Subsequent equilibrium solute rejection modifies both constituents as they cool.
\Image[/past-exam-of-the-materials-science-course-of-the-university-of-cambridge/2016/paper-1-ag-cu-microstructure.png]
{title=Original schematic primary-silver and lamellar eutectic arrangement; geometrical drawing areas do not assert mass or volume fractions}
The $42.1\%$ primary-$\alpha$ and $57.9\%$ eutectic-constituent fractions calculated at the eutectic arrest describe how the liquid froze. They are \b[not] the final pure-Ag and pure-Cu phase fractions. For example, just below the arrest the total copper-rich phase fraction is $(20-9)/(92-9)=11/83=0.1325$; it rises to $0.20$ as solubility is lost on further equilibrium cooling. The distinction separates a microstructural constituent from a <thermodynamic phase>.
Faster <solidification> usually refines the dendrite and eutectic spacings. If solid-state <diffusion> cannot keep up, composition gradients, supersaturated solids and delayed precipitation can persist. Their exact fractions then depend on the thermal history; faster cooling does not change the alloy's overall $20\%$ <copper> inventory. Complete long-term equilibration would restore the same equilibrium phase compositions and <mass fractions>.
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