CONSTITUTION OF THE Al.I.oys OF ALUMINIUM AND ZINC. 333 



reaching the temperature of the line CGH the y crystals react with the liquid 



according to tin- equation 



y + liquid = (Al,Zn,). 



Up to the line GFK, which represents the composition of the compound 

 this iv.-i.'tion, even if completed, leaves a residue of liquid which solidifies as eutectic. 

 The amount of this eutectic, however, decreases until it vanishes at the composition 

 of the compound. 



The microscopic evidence supporting this statement is of considerable importance 

 because it constitutes a strong confirmation of the existence of the definite compound 

 AlZna ; it is, therefore, given in some detail. We have first in h'g. 19 (x!5()) an 

 alloy containing 80 per cent, of zinc and, therefore, lying just to the left of the line 

 (rFK; this has IM-CII maintained for Hve hours at a temperature just below the line 

 CH and has then been slowly cooled. The photomicrograph shows the presence of 

 small quantities of eutectic. In this respect this alloy is in contrast with one 

 containing 78 per cent, of zinc, in which the eutectic disappears entirely under the 

 same treatment, the resulting structure being perfectly homogeneous, like that shown 

 in h'g. 26. 



The structure shown in fig. 19 represents an aggregate of /3+ eutectic, except that 

 in consequence of slow cooling the reaction along JK has taken place and the dark- 

 etched ft Ixxly is in reality duplex, consisting of a. and y. 



The reaction y + liquid = ft which takes place along the line CH, being a reaction 

 between a solid and its mother liquor, results in the formation of sheaths of ft 

 surrounding the y crystals. In the alloy of fig. 19 the prolonged heating at 430 C. 

 has obliterated the sheaths, but if a similar alloy is cooled comparatively quickly the 

 existence of these sheaths is very clearly seen. In fig. 20 ( x 150) we have an alloy 

 of the same group which has been quickly cooled down to a temperature just above 

 256 C., and has then been quenched in order to prevent the decomposition of the 

 ft sheaths. These are very clearly seen in the photograph as dark edges surrounding 

 the relatively light bodies of primary y. The same alloy slowly cooled from fusion to 

 the ordinary temperature is shown in fig. 21 ( x 150) ; the cooling having been slower 

 m this case the original sheaths of ft are hardly visible, almost the whole of the dark 

 dendrites having l>een transformed into ft when the alloy passed the line CH. At 

 256 C., however, the rate of cooling was not slow enough to allow the decomposition 

 of ft to be completed, and the structure therefore shows dendrites of ft merely 

 decomposed at the edges. In fig. 22 ( x 300) we have the same alloy, this time after 

 prolonged heating just below 430 C., and then cooled more slowly than in the 

 previous example. Here both the formation and decomposition of ft have been 

 completed, and a higher magnification would reveal the completely pearlitic structure 

 of the dark areas of this photograph ; in the photograph the duplex character of 

 the dark constituent is evident in many places. In contrast to this we have fig. 23 



