EUTECTIC RESEARCH: THE ALLOYS OF LEAD AND TIN. 1U 



Fig. 17 shows one of the typical forms of the eutectic alloy (magnification 300 

 diameters) containing 62 '97 per cent, of tin. The various forms taken by. this alloy 

 will be discussed in detail in the special section at the end of this paper. 



Fig. 18 represents the alloy containing 65 per cent, of tin. As the micro-structure 

 shows, this alloy is just above the eutectic composition, white crystallites of free tin 

 appearing embedded in the eutectic. (Magnification 200 diameters.) 



Fig. 19 represents the alloy containing 74 per cent, of tin, under a magnification 

 of 200 diameters. The crystallites of free tin are seen in larger quantity and are 

 surrounded, or nearly surrounded, by regions of the lead constituent. 



Fig. 20 shows the alloy containing 85 per cent, of tin. Here the white crystals 

 of tin occupy the greater part of the area of the section, the eutectic merely forming 

 a meshwork l>etween the crystals. In the 95 per cent, alloy shown in the next figure 

 (No. 21, magnification 200 diameters) the meshwork of eutectic, or, rather, of the 

 lead constituent of the eutectic, is very thin and no longer continuous. Finally, in 

 fig. 22, the alloy containing only 1 per cent, of lead (99 per cent, of tin) is shown. 

 The eutectic can be definitely traced in this micrograph, and the structure of this 

 alloy is not affected in this respect by prolonged heating at 175 C. This fact, 

 together with the cooling-curves already described, leads to the conclusion that the 

 eutectic extends very nearly to the tin end of the series, and that the solubility of 

 lead in tin is zero or very nearly zero. 



For the purpose of determining the limiting solubility of tin in lead, the alloys in 

 the neighbourhood of a tin content of 15 per cent, were carefully examined after six 

 weeks' exposure to heat at a temperature of 175 C. Examined under moderate 

 magnifications, by whose aid the presence of "primary" tin could not fail to be 

 recognised, the 15 per cent, alloy was found to be entirely homogeneous, the few 

 white spots seen after four weeks' heating (as seen in fig. 14) having entirely 

 disappeared. In the 1 6 per cent, alloy small specks of primary tin could generally be 

 observed, the appearance of this alloy after six weeks' heating being similar to that 

 shown in fig. 14. By powdering this alloy, compressing the powder so as to form a 

 solid, and then further heating at 175 C., the 16 per cent, alloy can, however, be 

 rendered quite homogeneous under moderate magnifications. In alloys with 17 and 

 18 per cent, of tin the specks of primary tin are always clearly present, and in 

 increasing quantity with increasing tin content. The conclusion drawn from these 

 observations, taken together with the data derived from the cooling-curves described 

 above, is that the limiting solubility of tin in lead at a temperature of about 180 C. 

 (which is near the temperature of the freezing of the eutectic, and therefore very close 

 to the solidus in alloys containing nearly 16 per cent, of tin) is very little more than 

 16 per cent. On this view an alloy containing a little less than 16 per cent, of tin, 

 if cooled so slowly that complete equilibrium is attained, would solidify as a 

 homogeneous mass, no eutectic being separated. It will be seen, however, that there 

 is good reason to suppose that some "secondary" tin is separated from this solid 



