ABSTRACT
The liquidus and the solidus, which are represented by curved surfaces, enclose a space constituting the two-phase region of liquid + solid solution. This two-phase region may be considered as consisting of a bundle of tie-lines of varying direction; each tie-line links the composition points of liquid and solid co-existing at a particular temperature, and no two-tie-lines at the same temperature can cross one another. During the solidification of alloys whose compositions lie in the section, the liquid and solid compositions change out of the plane of the section. If it is required to know the actual compositions of liquid and solid co-existing at any temperature, experimental work or thermodynamic calculations are necessary. For a given alloy, the tie-lines at various temperatures do not generally lie in the same vertical plane and the paths of the extremities of the tie-lines on the liquidus and solidus are generally curved.
