Post on 22-Oct-2015
The Pb – Sn Phase Diagram
Dendritic α-Pb face-centered cubic dendrites of cast pure lead, Pollack’s reagent, DIC
Cast Pb – 20% Sn, as-polished: α-Pb dendrites (white) and α-Pb - β-Sn eutectic (dark)
Pb – 30% Sn, as-polished; α-Pb dendrites and an α-Pb/β-Sn eutectic
Pb – 40% Sn, Pollack’s etch; α-Pb dendrites (white) and an α-Pb/β-Sn eutectic (magnification bar is 20 μm)
Pb – 50% Sn, Pollack’s etch; dark α-Pb dendrites and a fine α-Pb/β-Sn eutectic
Pb – 60% Sn, Pollack’s etch, DIC; near eutectic microstructure of α-Pb and β-Sn
Pb – 61.9 % Sn eutectic, as above but DIC;α-Pb/β-Sn eutectic
Pb-70% Sn, Pollack’s etch, polarized light; primary β-Sn dendrites (with twins) and the α-Pb/β-Sn eutectic
Pb – 80% Sn, Pollack’s etch; note the mechanical twins in β-Sn dendrites and the α-Pb/β-Sn eutectic
Pb – 90% Sn, as polished, polarized light; B-Sn dendrites with mechanical twins and an α-Pb and β-Sn eutectic
Pure Sn, as-polished, polarized light; twinned β-Sn (tetragonal) grains
Cast Pb – 10% Sn; fine precipitates of β-Sn in a α-Pb matrix; Pollack’s reagent
Pb-61.9 % Sn, slow cool, 2% nital etch, BF; α-Pb/β-Sn eutectic matrix
Surface Abrasive SizeLoad
lb. (N)Platen Speed**
(rpm)
Time (min.)
CarbiMet® Paper 240-grit SiC* 4 (18)240
1
CarbiMet® Paper 320-grit SiC* 4 (18) 240 1
CarbiMet® Paper 400-grit SiC* 4 (18) 240 1
CarbiMet® Paper 600-grit SiC* 4 (18) 240 1
MicroCloth® pad 5-μm Al2O3 4 (18) 150 7
MicroCloth® pad 1-μm Al2O3 4 (18) 150 5
MicroCloth® pad 0.3-μm Al2O3 4 (18) 150 4
MicroCloth® pad0.05-μm MasterPrep®
Alumina4 (18) 150 3
MicroCloth® pad0.05-μm MasterMet®
Colloidal Silica- VibroMet® 2 ≥ 60
The binary Pb-Sn phase diagram has been studied for over 100 years and is a classic eutectic. Lead (Pb) has an atomic number of 82 and a high density. Its crystal structure is face-centered cubic (fcc). At 50 °C, 2% Sn is soluble in Pb and at the eutectic temperature, 183 °C, the maximum solubility of Sn in α-Pb is 19%. The melting point of pure Pb is 327.46 °C. Tin (Sn) has an atomic number of 50 and exists in the β form at room temperature. β-Sn has a tetragonal crystal structure. At room temperature, there is almost no solubility of Pb in Sn and at the eutectic temperature, 183 °C, there is a maximum solubility of 2.5% Pb in β-Sn. The eutectic reaction, L ↔α-Pb + β-Sn, occurs at 61.9% Sn and at 183 °C. The shape of the eutectic Pb constituent varies from lamellar to spheroidal with Sn as the continuous phase, as shown in the examples below.
High-purity Pb and Sn are very difficult metallographic subjects and the alloys of Pb and Sn are somewhat easier to prepare, but still rather difficult. The above table lists our preferred preparation procedure. Vibratory polishing is essential for best results. Pollack’s reagent (100 mL water, 10 g citric acid, 10 g ammonium molybdate) is one of the best etchants for Pb and Pb-Sn alloys; 2% nital is a good etch for pure tin.
*Coat the SiC paper with paraffin wax; use water as a coolant ** Use contra rotation and a low head speed for best resultsReprinted with permission of ASM International® All rights reserved.
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Pb – 60% Sn, Pollack’s etch, BF; near eutectic microstructure of α-Pb and β-Sn, 200x (20 μm magnification bar)
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