The Pb – Sn Phase Diagram

Dendritic -Pb face-centered cubic

dendrites of cast pure lead, Pollack’s

reagent, DIC

Cast Pb – 20% Sn, as-polished: -Pb

dendrites (white) and -Pb - -Sn

eutectic (dark)

Pb – 30% Sn, as-polished; -Pb

dendrites and an -Pb/-Sn eutectic

Pb – 40% Sn, Pollack’s etch; -Pb

dendrites (white) and an -Pb/-Sn

eutectic

Pb – 50% Sn, Pollack’s etch; dark -Pb

dendrites and a fine -Pb/-Sn eutectic

Pb – 60% Sn, Pollack’s etch, DIC; near

eutectic microstructure of -Pb and -Sn

Pb – 61.9 % Sn eutectic, as above but

DIC; -Pb/-Sn eutecticPb-70% Sn, Pollack’s etch, polarized

light; primary -Sn dendrites (with

twins) and the -Pb/-Sn eutectic

Pb – 80% Sn, Pollack’s etch; note the

mechanical twins in -Sn dendrites

and the -Pb/-Sn eutectic

Pb – 90% Sn, as polished, polarized

light; B-Sn dendrites with mechanical

twins and an -Pb and -Sn eutectic

Pure Sn, as-polished, polarized light;

twinned -Sn (tetragonal) grains

Cast Pb – 10% Sn; fine precipitates of

-Sn in a -Pb matrix; Pollack’s

reagent

Pb-61.9 % Sn, slow cool, 2% nital etch,

BF; -Pb/-Sn eutectic matrix

Surface Abrasive Size

Load

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-mm Al2O3 4 (18) 150 7

MicroCloth® pad 1-mm Al2O3 4 (18) 150 5

MicroCloth® pad 0.3-mm Al2O3 4 (18) 150 4

MicroCloth® pad0.05-mm MasterPrep®

Alumina4 (18) 150 3

MicroCloth® pad0.05-mm 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 -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 form at room temperature. -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 -Sn. The eutectic reaction,

L -Pb + -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 resultsReproduced courtesy of ASM International

Pb – 60% Sn, Pollack’s etch, BF; near

eutectic microstructure of -Pb and -Sn

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