How to use the lever law of phase diagram

The lever law only applies to the two-phase region in the phase diagram and can only be used in the equilibrium state.

Lever law is widely used in phase equilibrium, which can be simply described as "the amount of one phase multiplied by the length of its own side line segment, which is equal to the amount of another phase multiplied by the length of the other side line segment". In the phase diagram of multi-component system, the system point falls in the two-phase coexistence area of temperature-composition diagram (or pressure composition diagram), and the system is in a two-phase equilibrium state.

The intersection of the horizontal line passing through the object point and the two-phase line is two phase points, and the connecting line of these two phase points is called the tie line. The object point divides the connecting line into two segments, which can prove the lever law. Lever law is a mathematical formula for calculating the relative quantity of two equilibrium phases by using phase diagram when the alloy with known composition is in two-phase region.

Because the form is very similar to the lever theorem in mechanics, it is called the lever law. When binary alloy is in two-phase equilibrium at a certain temperature t 1, the composition of two equilibrium phases can be known by means of binary equilibrium phase diagram. The parallel arb passes through the temperature t 1 as the component axis, and the two-phase region where its two ends intersect is the two phases contained in the alloy of component C at the temperature t 1.

The composition of the two intersections indicates the composition of the relevant phase. Lever law is the core formula for calculating iron-carbon phase diagram, and the essence of lever law should be deeply understood. As shown in the figure, it is a simple binary system phase diagram. When B melts into A, the abscissa represents the solubility of B in A, and the ordinate represents the temperature, then the two components A and B are infinitely fused with each other to form a solid solution α.