Eutectic research. No. 1.—The alloys of lead and tin

Abstract

Attempts to prepare pure eutectic alloys of known constitution led to the discovery of discrepancies between the authors’ experiments and the data on lead-tin alloys published by Roberts-Austen. A complete redetermination of the equilibria of the lead-tin system was therefore undertaken, by both pyrometric and microscopical methods. Cooling-curves of the alloys taken by both inverse-rate and differential methods are given; these, together with the microscopic data, lead to the equilibrium diagram shown in the figure. This differs principally from that given by Boberts-Austen, in that the eutectic point is placed at a concentration of 63 per cent, of tin instead of 69, that the eutectic line towards the lead end of the series terminates at a concentration close to 16 per cent, of tin, and that a series of transformations (along the line EFG) have been found in the solid alloys near the lead end of the series. The discrepancies of these results, as regards the eutectic composition, arise from the more delicate method employed in the present research. While alloys within 1 per cent, on either side of the true eutectic composition show no detectable difference in freezing- or melting-point, the presence of small amounts of excess of either constituent can be detected by the microscope, and in this manner the composition of the pure eutectic has been ascertained. As regards the solubility of tin in solid lead, it was found that the occurrence of the eutectic arrest-point in alloys containing less than 16 per cent, of tin depended upon the rate of cooling; by maintaining alloys at a temperature of 175ºC. for periods up to six weeks, approximately complete equilibrium conditions can be attained, and the study of the cooling-curves and micro structure of such alloys has led to the conclusions embodied in the diagram. These conclusions are much strengthened by the fact that the authors have succeeded in preparing alloys very rich in lead for microscopic examination by polishing with levigated oxide of chromium.