solidus line
/ SOL-ih-dus /
The solidus is the floor of melting. Below it the alloy is fully solid; heat up to touch it and the very first drop of liquid appears. Read the other way, on cooling it is the temperature at which the last remaining liquid finally freezes solid.
More precisely, the solidus is the line joining the temperatures below which the alloy is 100 percent solid. Between the solidus and the liquidus, solid and liquid coexist. For Cu-Ni at 35% Ni the solidus is about 1200 degrees C: cooling from the melt, solidification begins at the liquidus near 1280 C and finishes at the solidus near 1200 C. That roughly 80-degree gap is the freezing range.
A wide freezing range promotes coring and segregation, because the first solid to form is richer in the higher-melting component and the composition only evens out if diffusion can keep up. It also affects castability and hot tearing. Welders and foundry engineers treat the solidus as the safe fully-solid temperature.
Cu-Ni at 35% Ni: solidification runs from the liquidus (~1280 C) down to the solidus (~1200 C).
Below the solidus is all solid; the gap up to the liquidus is the freezing range.
At true equilibrium the final solid has uniform composition, but real (fast) cooling leaves each grain cored and layered because solid-state diffusion cannot keep up.