Soldering terminology

The temperature occurring as a function of the composition of an alloy at which the cooling melt of this alloy completely solidifies is known as its solidus line (analogous to the liquidus line) . Between these two lines, a mixture of the metals is present in liquid and solid phases.

A‘ A‘‘ E – primary crystal A (solid phase) and eutectic (liquid phase)
B‘ B‘‘ E – primary crystal B (solid phase) and eutectic (liquid phase)

When a eutectic alloy reaches the solidus line, i.e. the eutectic temperature, the solidification process is completed. The graph refers to a eutectic system composed of A and B where the shaded areas, namely the triangles A 'A ''E and B 'B'' E of the area enclosed by the liquidus and solidus lines correspond to the phase mixtures.