QCAA Engineering External Practice Test

The key idea is that a phase diagram shows which phases exist at equilibrium for different temperatures and compositions. The mushy middle is the two-phase region where solid and liquid coexist, which occurs between the solidus and liquidus lines. When a solid phase (like alpha) is present together with liquid, that region is labeled alpha + liquid, so it correctly identifies the mushy zone. The liquidus marks the boundary above where everything is liquid, the solidus marks the boundary below where everything is solid, and the eutectic point is a specific condition where liquid turns into two solids at once, not a broad two-phase region.