37 
1911-12.] Note upon the Structure of Ternary Alloys. 
examined more closely, dark skeleton growths are detected in the fringe 
surrounding each primary crystal of bismuth. These are crystals of tin, 
which have been attacked by the hydrochloric acid used for etching the 
section. The presence of the crystals of tin shows that the fringe is not 
the simple binary eutectic which Charpy supposed it to be. 
It is advantageous to consider for a moment the conditions which 
prevail during the solidification of an alloy of only two metals, before 
going on with the case in which three are present. In the simple type to 
which the present observations are restricted, such a binary alloy, when 
solid, consists of primary crystals of one of the metals (containing a small 
proportion of the other in solution), in a matrix of the binary eutectic. 
During the course of solidification the primary crystals have grown within 
the liquid throughout a certain interval of temperature, at the end of which 
period the eutectic has crystallised around them at constant temperature. 
Primary crystals are free to migrate over small distances through the 
liquid portion of the alloy, so that a slow rate of cooling allows of the 
formation of large aggregates. But during the solidification of the eutectic 
each small crystal is virtually locked in place by those deposited around 
it, and the opportunity for migration is very small ; the structure of a 
eutectic, therefore, is on a fine scale as compared with the arrangement of 
the primary constituent. If the rate of cooling is slow, however, some 
migration of the particles of the eutectic takes place, the relatively 
large primary crystals attracting tiny crystals of the same substance from 
the eutectic to themselves. A section of the cold alloy generally shows 
that the movement has been arrested, by the completion of solidification, 
before the smaller crystals have been wholly absorbed by the large ones. 
In alloys of strongly crystalline metals, like bismuth and antimony, the 
phenomenon is particularly marked. Fig. 1 is a section, somewhat im- 
perfectly polished, of an alloy containing 60 per cent, of bismuth and 40 
per cent, of tin ; it shows primary crystals of bismuth in a matrix of 
bismuth-tin eutectic. The bismuth crystals are surrounded by a fringe of 
the same material, attracted from the immediately surrounding eutectic, 
which is accordingly left relatively rich in tin. The appearance of fig. 1 
differs but little from that of Charpy’s section mentioned above, so that 
this type of structure cannot be considered as a special feature of a 
ternary alloy, or one which represents the process of solidification of a 
ternary alloy. 
When the alloy is one of three metals the primary crystals solidify 
under conditions similar to those in an alloy of two metals. The ternary 
eutectic freezes at constant temperature, like the binary eutectic of a binary 
