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Homework Help: Sn-Pb phase diagram

  1. Dec 28, 2008 #1
    Hi everyone,

    1. The problem statement, all variables and given/known data

    A lead–tin alloy of composition 30 wt% Sn–70 wt% Pb is slowly heated from a temperature
    of 150 C (300F).

    (a) At what temperature does the first liquid phase form?

    (b) What is the composition of this liquid phase?

    (c) At what temperature does complete melting of the alloy occur?

    (d) What is the composition of the last solid remaining prior to complete melting?

    2. Relevant equations


    3. The attempt at a solution

    It is probably an easy question, but I thought it is a good idea to consult first.
    a) At 183 C, the first liquid phase forms.

    b) We can draw a tie line and the point intersects with the liquidus line, gives us the composition of liquid. It is 61.9 wt % Sn.

    c) It is around 250 C. Because phase diagram is on liquidus line.

    d) Composition is 15wt % Sn. We can find it by drawing a tie line.

    I am not sure about the question b and d. Because as far as I know composition of this liquid means that the composition of the components in this liquid. For instance, in some specific temperature liquid is composed of 30wt % Sn - 70 wt & Pb. However, in some sources I saw that they interpreted this statement as the composition of liquid phase in the total alloy. Like, again in some temperature, 30 % liquid- 70 % solid. Which one is correct. I am a bit confused. Any help will be greatly appreciated.

    Thank you.

    Attached Files:

    Last edited: Dec 28, 2008
  2. jcsd
  3. Dec 28, 2008 #2


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    Staff: Mentor

    Hard to help without having phase diagram at hand.
  4. Dec 28, 2008 #3
    I have put it below the heading of "relevant equations", but I guess there is a problem about seeing it so I attached it to the problem. Thanks for your warning.
  5. Dec 28, 2008 #4


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    Staff: Mentor

    I can see it now, could be that was my internet playing its dirty tricks, I have serious problems with web access lately.

    I think you are right, but I have not analyzed phase diagrams for eons, so second opinion won't hurt.
  6. Dec 29, 2008 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Your answers look good to me.

    I think you interpreted it fine, since the question asked for the composition of the liquid phase, not the amount of liquid in the total system.
  7. Jun 5, 2010 #6
    ternary Zr-Nb-Y

    Please help me do you have ternary phase diagram of Zr-Nb-Y
    thanks in advance
  8. Oct 1, 2010 #7
    what is the structural difference between eutectic & proeutectic alpha???
  9. Oct 1, 2010 #8
    your answers of question b&d are absolutely correct..if you want to know the composition in liq. then draw tie line at given temperature , where the tie line cuts the liqidus line drop a vertical line where it cut the x axis is the correct composition (61.9 wt% sn)......
    but if it is asked that what is the amount of liquid at a given temp. & for an particular alloy then u have to use the lever rule......
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