CONSTITUTION OF THE ALLOYS OF ALUMINIUM AND ZINC. 331 



magnifications there is a distinct difference l>etweeii such a quenched alloy and one 

 which has been slowly cooled. It is found that while the ft, or dark-etching, 

 constituent of the eutectic is homogeneous in the quenched alloy, in the slowly cooled 

 specimens the ft meteral has undergone decomposition and exhibits a duplex structure 

 which may take the fonn either of parallel lamellae or of minute granules. Fig. 9 

 shows the structure of this decomposed eutectic under a magnification of 1000 

 diameters ; the large dark areas represent the structure of the eutectic while the 

 minute granulation in these areas indicates the decomposition of the ft constituent. 

 The photomicrograph in fig. 9 is actually taken from an alloy containing 95 per cent, 

 of zinc, but the same features are found in all the alloys in which the eutectic is 

 present. 



The typical structure of the eutectic alloy, seen under moderate magnification 

 (200 and 150 diameters respectively), is shown in figs. 10 and 11. It will be seen 

 that the eutectic exhibits in a very beautiful way the characteristic features of well- 

 defined eutectic alloys, and it will further be noted that the relative areas of the dark 

 and light constituents are not very widely different. When it is realized that this 

 structure is found in an alloy containing only 5 per cent, of aluminium, this obser- 

 vation alone strongly suggests that the components of the eutectic are not in reality 

 zinc and aluminium, but zinc and a compound of zinc and aluminium containing a 

 considerable proportion of zinc. Eutectics lying near one end of a binary series are 

 not unknown in other groups of alloys, but they do not exhibit a well-balanced 

 laminated structure unless a compound comes iuto play. 



The micro-structures found in alloys containing less than 95 per cent, of zinc, that 

 is to say, lying just to the right of the eutectic concentration, further support the 

 view that the ft meteral, which forms the dark-etching constituent of these alloys, is 

 a definite compound. These micro-structures are illustrated under a magnification of 

 200 diameters in figs. 13 and 14. These two figures show the presence of strikingly 

 characteristic dendritic crystals, which possess a strong tendency to assume six-rayed 

 forms in which angles closely approximating to 60 degrees are of frequent and typical 

 appearance. 



This is a striking characteristic, particularly in view of the fact that in the great 

 majority of metals the dendritic branches tend to fonn rectangular systems. The 

 microscopic evidence thus strongly supports the pyrometric evidence for the existence 

 of a definite compound, and grounds have been given above for the view that this 

 compound is represented by the formula A L /n 



The close relationship between the six-rayed crystals just described and the eutectic 

 surrounding them is also illustrated in an interesting manner by fig. 14. It is there 

 clearly evident how the crystallization of the eutectic has radiated from the various 

 branches of tin- denilrite, resulting in the formation of an interesting pattern. 



The photomicrographs, figs. 13 and 14, having been taken from slowly cooled 

 specimens of the alloys, are not free from signs of the reaction represented in the 



2 U 2 



