MT4 Phase Diagram of Pb-Sn Alloys Name：SHI Tai Student Number:52888227 1. Purposes：1. Define component, phase, diagram phase, and cooling curve. 2. Describe the phase change taking place at different point on a cooling curve. 3. Construct a diagram from cooling curves. 2. Instruments and specimens： Crucibles Thermoelectric couple Heating-furnace Multi-channel graph recorder Stirrer bar Specimens： Channel 0 1 2 3 4 5 Sn 100% 80% 62% 40% 10% -- Pn -- 20% 38% 60% 90% 100%

3. Introduction: A phase diagram is a plot of the equilibrium state of a system. A eutectic system can occur when terminal solid solutions exist on both end of the binary equilibrium phase diagram. In the Pb-Sn alloys system, there are two solid solutions-α and β. The α phase indicates a solid solution of tin in lead, whereas β presents in the opposite way. The eutectic invariant point appears at 61.9 wt% Sn. The maximum solid-state solubility both occur at 183℃ which is referred to as the eutectic temperature. At this temperature, there exists a point on the phase diagram (a single combination of composition and temperature) where three phases (the two solids and a solid) can exist simultaneously in equilibrium. This combination of temperature and composition is an invariant point on the binary diagram like the freezing point of water on the single component system the eutectic reaction where upon cooling L→α+β represents the isothermal transformation of liquid into two different solids. Depending upon the overall bulk composition of the system, a variety of different equilibrium microstructures are possible. However, as mentioned above, equilibrium requires sufficient time for the system to find the minimum in free energy. In real systems, this is not always possible and non-equilibrium microstructures are common. When this same type of reaction occurs in the solid state where one solid decomposes into two new solid phases isothermally, this is called a eutectoid reaction. γ→α+β. These relationships are determined by these principles of the thermodynamics and have practical applications in many fields of science and engineering.

4. Procedure: 1. Setting up the apparatus. 2. Switch on the bottoms to heat coil until the temperature up to around 400℃. 3. Switch on Multi-channel graph recorder and set the chart speed. 4. Control the cooling rate under 5℃ per minute. 5. Turn off the bottoms when the temperatures of the specimens fall down to 100℃. 0 1000 2000 3000 4000 0 50

100

150 200 250 300 350 400 450 0 1 2 3 4 5

Amplified Cooling Curves of Pb-Sn Alloys

Time(s)

5. Results and Discussions Figure 1 Figure 2 Temperature(℃) Table1. The composition of Pb-Sn Alloys Sample 0 1 2 3 4 5 Content (wt%) Sn 100 80 62 40 10 0 Pb 0 20 38 60 90 100

Table2. Arrest points of Pb-Sn Alloys Sample 0 1 2 3 4 5 Temperature ℃ 237 204 186 243 303 326 186 186

Figure 3 The experimental and standard Pb-Sn phase graph As can be seen from the experimental and standard diagram of Pb-Sn alloys, there are some diversities between them. This experiment studys the binary Pb-Sn system dominated of eutectic alloy system. It is clearly finds the two different elements are absolutely soluble in each other under the liquid condition, while only partially soluble in the solid phase. Alloys in which solid-to-solid convention occurred are easily analyzed for phase graph through the cooling curve method of thermal analysis. This is because the solid condition transformation is often sluggish and the thermal exchange is too small that we can hardly discover by cooling curves.

6. The answer to the questions 1. What is the expected difference in cooling curves for Pb-10 percent Sn and Pb-40 percent Sn alloys? Explian. The cooling curve of 10 percent Sn in Pb has a constant melting piont at 303℃, while the curve for Pb-40 percent Sn ranging from 186℃ to 243℃. This is mainly due to the composition of the alloy. 2. Discuss the arrest point in Pb-62 percent Sn alloy. The point is the Eutectic point and the transformation through this point is called Eutectic reaction: L→α+β. It is clearly see the arrest temperature of Pb-Sn alloys is about 186℃, which is a bit different from the data shown in the standard Pb-Sn phase graph (183℃). The reasons may be the follow three. First, a member of our group touched the heat-sensitive line during the experiment, which leads to inaccuracy of measurement. Second, the specimens of Pb-Sn alloys have been used for many times so that they may be partly oxidation by the air, that is to say, the samples is