ON GOLD-ALUMINIUM ALLOYS. 



Eutectic Alloys. 



As the photograph of the alloy with 40 atoms of aluminium shows, there is a 

 typical eutectic at F. This alloy contains practically no primary crystals, but consists 

 of two interlacing networks of E and of X, which must have crystallised almost 

 simultaneously. It is an alloy of minimum and constant freezing point. This is 

 probably the only section exactly at a eutectic point that we have polished, but there 

 must be a eutectic corresponding to the point C, and also at a point a little to the left 

 of B. But one can study eutectics in many other sections ; an examination of the 

 mother-substance between the blobs of primary crystallisation enables us to do this. 

 For example, alloys between 36 and 40 atoms show the F eutectic very well, and it is 

 also well seen here and there on the branch FG, and in the slowly-cooled 45 atom 

 alloy a little above G. Again, although we have not absolutely located the eutectic 

 point between A and B, yet the mother-substance of the 18'1 alloy shows the 

 structure of this eutectic ; the 19 '8 atom alloy, which must be on the other side of the 

 eutectic point, also shows a minute network in its golden mother-substance. The 

 mother-substance of the branch CD, for example, at 21*6 atoms shows a eutectic 

 mixture ; but this was one of numerous cases in which surfusion preceded the solidifi- 

 cation of the eutectic. One would expect surfusion to modify the pattern of the 

 eutectic considerably, and, perhaps, to give three stages of solidification altogether, so 

 that the eutectic structure would have that polygonal form characteristic of a nearly 

 pure substance. The alloys on the branch DE show us that a mother-substance can 

 be a pure body. This will occur whenever a branch of the curve cuts the next lower 

 branch at the summit of the latter. The question whether a eutectic can be a pure 

 body is really one of nomenclature. If we use the term eutectic for the mother- 

 substance of the branch DE, the answer is, Yes ; this mother-substance solidifies at 

 a constant temperature, but is not an alloy of minimum freezing point. 



The mother-substance along GH is a mixture, but not a eutectic. It, however, 

 contains the F eutectic as one of its constituents ; this is very well shown in the 

 horizontal section of the 45 atom slowly -cooled alloy (fig. 27), the gold -smeared E of 

 the eutectic having a minute pattern of pure white, X, scattered through it ; this is, 

 of course, in the unetched alloy. The quickly-cooled alloys of the branch GH do not 

 always show the detail of the eutectic very well, but the row of very steady third 

 freezing points makes the matter certain. The fact that these freezing points occur 

 after marked surfusion may explain the scantiness of the F eutectic along the 

 branch GH, and the fact that many of the blobs of E are large. 



