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12.7: The Lever Rule

  • Page ID
    36301
  • If an alloy consists of more than one phase, the amount of each phase present can be found by applying the lever rule to the phase diagram.

    The lever rule can be explained by considering a simple balance. The composition of the alloy is represented by the fulcrum, and the compositions of the two phases by the ends of a bar. The proportions of the phases present are determined by the weights needed to balance the system.

    Diagram illustrating the lever rule

    So,

    fraction of phase 1 = (C2 - C) / (C2 - C1)

    and,

    fraction of phase 2 = (C - C1) / (C2 - C1).

    Lever rule applied to a binary system

    Schematic phase diagram for a binary system

    Point 1

    At point 1 the alloy is completely liquid, with a composition C. Let

    C = 65 weight% B.

    Point 2

    At point 2 the alloy has cooled as far as the liquidus, and solid phase β starts to form. Phase β first forms with a composition of 96 weight% B. The green dashed line below is an example of a tie-line. A tie-line is a horizontal (i.e., constant-temperature) line through the chosen point, which intersects the phase boundary lines on either side.

    Part of a phase diagram

    Point 3

    A tie-line is drawn through the point, and the lever rule is applied to identify the proportions of phases present.

    Part of a phase diagram

    Intersection of the lines gives compositions C1 and C2 as shown.

    Let

    C1 = 58 weight% B

    and

    C2 = 92 weight% B

    So,

    fraction of solid β = (65 - 58) / (92 - 58) = 20 weight%

    and

    fraction of liquid = (92 - 65) / (92 - 58) = 80 weight%

    Point 4

    Part of a phase diagram

    Let

    C3 = 48 weight% B

    and

    C4 = 87 weight% B

    So

    fraction of solid β = (65 - 48) / (87 - 48) = 44 weight%.

    As the alloy is cooled, more solid β phase forms.

    At point 4, the remainder of the liquid becomes a eutectic phase of α+β and

    fraction of eutectic = 56 weight%

    Point 5

    Part of a phase diagram

    Let

    C5 = 9 weight% B

    and

    C6 = 91 weight% B

    So

    fraction of solid β = (65 - 9) / (91 - 9) = 68 weight%

    and

    fraction of solid α = (91 - 65) / (91 - 9) = 32 weight%.

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