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Pb-Sn Phase Diagram Laboratory

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Introduction

Pb-Sn Phase Diagram Laboratory

by Justin Jones

Introduction

The aim of this laboratory session is obtain cooling curves for lead-tin alloys at various compositions.  By plotting the decrease in temperature against time, as the liquid alloy cools, on a graph we can spot plateaus and changes in gradient.  The phase diagram in the handout tells us roughly where to look for plateaus and changes in gradient.  These represent the temperatures at which the alloy changes phase.  This is because as an alloy changes phase energy is released in the form of heat.  When the heat energy released due to a change in phase is equal to heat energy lost to its surroundings due to cooling we get a plateau in the cooling curve.  When the energy generated from a phase change does not fully compensate for the heat lost to its surroundings we get a change in gradient on our cooling curve.  We can identify these plateaus/changes in gradient as areas of ‘thermal arrest’ thus telling us when the alloy changes phase.

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Middle

327

318

274

184

200

186

Line crossed

Liquidus

Liquidus

Liquidus

Eutectic Isotherm

Liquidus

Liquidus

Phase change

L  α

L  L + α

L  L + α

L  α + β

L  L + α

L  β

2nd plateau

Temp (°C)

299

180

185

Line crossed

Solidus

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Conclusion

To plot a more accurate cooling curve you could cool the alloy down as gradually as possible.  This would make plateaus in the cooling curves more apparent and more accurate points for the phase diagram would be easier to determine.

Finally, when the molten alloy in the crucible is cooling the temperature of the metal nearer the outside is cooler than that in the core.  It would be better to homogenise the mixture while it cools to give a more accurate picture.

In conclusion, the most significant source of error is the lack of points available to draw a line on the phase diagram.  By taking measurements for more compositions the accuracy of the phase diagram could be greatly improved.  

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