Lab Report: Cooling Curves of Lead-Tin Alloys

Categories: Chemistry

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Abstract

The aim of this laboratory session was to obtain cooling curves for lead-tin alloys at various compositions in order to draw a phase diagram. By plotting temperature against time as the liquid alloy cools, we can identify plateaus and changes in gradient, which correspond to phase changes in the alloy. The data collected from six different compositions of the Pb-Sn alloy was used to create cooling curves, and the temperatures at which phase changes occurred were recorded. This data was then analyzed to draw a phase diagram.

Introduction

Phase diagrams are essential tools in material science and engineering, providing valuable information about the relationships between temperature, composition, and phase transformations in a given material.

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In this experiment, we focused on lead-tin (Pb-Sn) alloys, which are widely used in various applications due to their desirable properties. The goal was to obtain cooling curves for Pb-Sn alloys with different compositions, ultimately enabling us to construct a phase diagram.

Phase changes in materials are associated with the release or absorption of energy, typically in the form of heat.

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When the heat energy released during a phase change balances the heat lost to the surroundings due to cooling, a plateau appears on the cooling curve. Conversely, when the energy generated from a phase change does not compensate fully for heat loss, a change in gradient occurs on the cooling curve. These plateaus and changes in gradient are referred to as 'thermal arrest' points, indicating phase transitions. By identifying these points, we can construct a phase diagram for the Pb-Sn alloy system.

Materials and Methods

We were provided with six different compositions of Pb-Sn alloys, ranging from 0wt% Sn (pure Pb) to 100wt% Sn (pure Sn).

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Each composition provided essential data for constructing the phase diagram. The experimental procedure involved the following steps:

Heating the alloys to a molten state in a crucible, reaching temperatures between 350-400°C using a Bunsen burner while wearing safety glasses and lab coats.
Placing an electronic thermometer in the molten alloy and allowing it to cool down to room temperature, recording temperature measurements every 10 seconds.
Plotting the temperature measurements to create cooling curves for each alloy composition.

Results

The collected data was used to plot cooling curves for the different alloy compositions. The following table summarizes the temperatures at which phase changes occurred, including the corresponding phase transitions:

Composition (wt% Sn)	1st Plateau	2nd Plateau	3rd Plateau
0	327°C	299°C	177°C
10	318°C	180°C	-
20	274°C	185°C	-
61.9 (Eutectic Isotherm)	184°C	185°C	-
80	200°C	-	-
100	186°C	-	-

Discussion

The constructed phase diagram closely resembles the one provided in the handout, with most points and lines in the correct positions, including the eutectic isotherm. However, some discrepancies were observed. Notably, the points marking the liquidus, solidus, and solvus lines at 10wt% Sn were slightly higher than expected. Additionally, the point for 100wt% Sn on the handout phase diagram showed a value of 232°C, while the measured value was 186°C, indicating a significant difference of 46°C. The solidus and solvus lines for 10wt% Sn are marked in red on the phase diagram to indicate their potential inaccuracies.

These discrepancies can be attributed to various sources of error encountered during the experiment. To improve the accuracy of the phase diagram, the following strategies could be employed:

Using a more extensive range of compositions, such as every 5wt% (e.g., 100, 95, 90, 85wt% Sn), to provide multiple data points for drawing lines on the phase diagram.
Collecting temperature measurements at smaller time intervals, such as every 2 seconds, to create more detailed cooling curves, which would result in a more accurate phase diagram.
Gradually cooling the alloy to enhance the visibility of plateaus in the cooling curves, making it easier to determine accurate phase transition points.
Homogenizing the alloy while it cools to achieve a more uniform temperature distribution throughout the sample, reducing temperature variations near the surface.

Conclusion

In conclusion, the experiment aimed to obtain cooling curves for various compositions of lead-tin alloys and construct a phase diagram. While the phase diagram closely resembled the handout, some discrepancies were observed due to sources of error. The most significant source of error was the limited number of data points available for drawing lines on the phase diagram. To enhance the accuracy of future experiments, it is recommended to collect data for a more extensive range of compositions and at smaller time intervals. These improvements will contribute to a more precise and reliable phase diagram for the Pb-Sn alloy system.

Recommendations

Based on the findings and discussions in this experiment, the following recommendations are proposed for future investigations:

Expand the range of compositions tested, including additional data points between existing compositions, to improve the accuracy of phase diagrams.
Collect temperature measurements at smaller time intervals, preferably with automated data collection methods, to create more detailed and precise cooling curves.
Implement controlled and gradual cooling techniques to enhance the visibility of plateaus on cooling curves, facilitating the identification of phase transition points.
Consider methods to homogenize the alloy during cooling to minimize temperature variations within the sample.

By implementing these recommendations, future experiments in this area can yield more accurate and reliable phase diagrams for lead-tin alloys, advancing our understanding of their material properties and applications.