ENGR 170 -- Introduction To Materials Science

LABIV

PHASE TRANSFORMATIONS

Study Questions:

  1. A 50wt% Pb-50wt% Sn alloy is cooled from 350oC to room temperature; At what temperature does the first solid appear ?
  2. What is the composition of the first solid? How about that of the remaining liquid?
  3. What is the composition of the last liquid to freeze when the temperature decreases to 183oC?
  4. How much liquid is still left upon first reaching that temperature?
  5. What would you see if you looked at the microstructure at room temperature?
  6. Now consider a 95wt% Sn-5wt% Sb alloy at 300oC. What is the temperature at which the first solid appears? What is the composition of the first solid and of the remaining liquid?
  7. What is the composition of the last liquid to freeze at about 232oC? How much liquid is left to solidify at that temperature, if any?
  8. Now compare what you would see as microstructure for Sb-Sn and Pb-Sn at room temperature in terms of the phases present and their percent amount.
  9. A steep liquidus line leads to a high constitutional supercooling, therefore to high instability ahead of a growing solid/liquid interface. Comparing the phase diagrams of Pb-Sn and Sb-Sn, which of the two alloys is expected to produce more dendrites?

Laboratory Construction of a Eutectic Phase Diagram

Samples

Eleven molten bismuth - tin alloys are located in the furnaces. Their compositions are:

  1. 100%Bi
  2. 40 Bi - 60 Sn
  3. 90 Bi - 10 Sn
  4. 30 Bi - 70 Sn
  5. 80 Bi - 20 Sn
  6. 20 Bi - 80 Sn
  7. 70 Bi - 30 Sn
  8. 10 Bi - 90 Sn
  9. 60 Bi - 40 Sn
  10. 57 Bi - 43 Sn
  11. 100% Sn

Each lab group is required to determine the cooling curves for two alloys.
 
 

Procedure

  1. With your TA's help, calibrate the strip chart recorder and thermocouple.
  2. One at a time, remove the alloys from the furnace and place it securely into the cooling unit.
  3. Insert the thermocouple into the narrow protection tube immersed in the alloy. Turn on the strip chart recorder.
  4. Allow the alloy to cool undisturbed. Note the changes in slope of the cooling curve as the temperature of the alloy decreases.
  5. Once the temperature drops below 110oC turn off the recorder.
  6. Repeat steps 2 through 5 for your second alloy.
     

    Note: The chart converting the thermocouple output in millivolts to temperature assumed a cold junction of 0oC. With your junction at room temperature a correction must be added. The correction can be done by either:

    1. Adding 0.04 mV per oC above 0oC to each mV reading on the chart.
    2. Adding the room temperature to the temperature obtained from the chart.

    Look at OMEGA's technical support for Type K Thermocouple Reference Data and Using Thermocouples

Data

  1. Make an accurate copy of the temperature (ordinate) vs. time (abscissa) curve which were drawn by the recorder. Suitably label axes and points.
  2. Label on your curve the exact temperature where changes in slope occur and indicate what metallurgical changes were occurring. Label all phases occurring in each of the regions of the cooling curve.
  3. Give the TA a table indicating the alloy composition and the temperatures where each alloy started and finished solidification.
  4. Obtain from the TA a master listing of each alloy composition and temperatures corresponding to the beginning and end of solidification. From this data draw the liquidus and solidus lines for the Pb-Sn eutectic phase diagram.

Background Information

Construction of A Eutectic Phase Diagram

 

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Last Modified 9/30/99 by Patrick Bloomer