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

Abstract

The investigations described in the present paper were begun in pursuance of a scheme for the systematic investigation of the constitution and properties of eutectic alloys. The first steps in such an investigation naturally consist in the preparation of some samples of typical eutectic bodies in a state approaching purity, followed by the determination of their chemical composition and constitution. Since a large number of systems of binary alloys have been closely studied and diagrams claiming to set forth their equilibrium conditions have been published, it was thought at the outset that there would be no difficulty in finding a number of typical eutectic alloys whose chemical composition was accurately known and whose constitution could be deduced from the corresponding equilibrium diagrams. The eutectic of the lead-tin series was chosen as a suitable example for early study partly because the alloys of this series are very easily prepared and manipulated, but principally because this series has hitherto been regarded as the typical example of the simplest class of binary alloys, viz., those in which the two metals are mutually insoluble in the solid state. In the preparation of the eutectic alloy of lead and tin on the basis of the data given by Roberts-Austen it was found that the results of the present experiments did not agree well with the data given by that author. Thus, while Roberts-Austen gives the composition of the eutectic alloy as approximately 31 per cent, of lead and 69 per cent, of tin, the present investigation shows the composition of this alloy to be very nearly 37 per cent, of lead and 63 per cent, of tin. Further, the diagram given by Roberts-Austen indicates that solid lead and tin are nearly mutually insoluble in the solid state, or, in other words, that the eutectic alloy is present in alloys quite near the two ends of the series. As regards the lead end of the series, such serious discrepancies from these views were found when the attempt was made to verify them, that it became evident that the investigation of this point had not been pushed far enough. It has accordingly been thought desirable to undertake a complete redetermination of the equilibria of the lead-tin system with a view to placing our knowledge of the constitution of the eutectic on a surer footing. This task was rendered more difficult, as well as more interesting, by the discovery of a trans­formation which occurs in the alloys rich in lead at temperatures below that of complete solidification.