334 DR. WALTER ROSENHAIN AND MR. SYDNEY L. ARCHBUTT ON THE 



( x 300) which represents an alloy in which the formation of ft has been. completed by 

 p o old heating just helow 430 C, but the decompose of ft has been prevented 

 q3ching thf specimen just above 256" C. The grey areas seen to this pho o- 

 graph are perfectly nlogeneous and show no signs of pearhtic struc ure, even under 

 hfehJT magnification. These five photomicrographs, showing the variation m 

 Zto of an alloy lying just to the left of the line GFK, confirm the digram 

 of fig. 4 and its explanation as put forward by the authors. 



We now pass to the alloys lying just to the right of the line GFK. Fig 

 represents alloy No. 78 containing 78'48 per cent, of zinc magnified 150 diameters. 

 The specimen in this case has been slowly cooled from fusion and is therefore in a 

 meta-stable condition ; dark cores of primary y are surrounded by sheaths of decon 



osed B leaving a residue of eutectic. The decomposition of the ft body is not 

 obvious in this photograph, but is very clearly seen in fig. 25 representing a port, 

 of the same specimen as fig. 24 but magnified by 500 diameters. Here the laminated 

 structure of the rims of decomposed ft is very evident. In fig. 26 ( x 150) we have 

 the micro-structure of alloy No. 77, which in this respect is typical of all alloys 

 containing less than 78 per cent, of zinc, after annealing for a considerable time 

 430 C. Mowed by quenching from a temperature just above 256 C. The specime. 

 exhibits a perfectly homogeneous structure merely diversified by a few holes and 

 cracks. If this structure were met with only in the alloy of the composition of 

 compound Al,Zn,, the structure might be described as that of the pure ft meteral, but 

 as a matter of fact this same homogeneous structure is exhibited by all the alloys 

 containing less than 78 per cent, of zinc which have been treated in the same way. 

 It follows from this observation that the ft body forms a solid solution with the y 

 body, since to the right of the line GFK the reaction y + liquid = ft must leave a 

 residue of y. For purposes of distinction the authors have termed this solid solution 

 of ft and y the $ meteral. The ft which is dissolved in this S body, at any rate so 

 long as it is present in any considerable quantity, still undergoes decomposition at the 

 temperature of the line JKL, and the phases present below this line are again y + , 

 but of course the amount of a decreases rapidly with decreasing concentration of 

 zinc. It has not, however, been found possible to determine microscopically the 

 concentration at which a ceases to appear in the alloys. 



Fig. 27 shows, under a magnification of 600 diameters, the appearance of fully 

 decomposed ft or S rich in ft. The photograph represents alloy No. 77 (7 7 '3 per cent. 

 of zinc) after prolonged heating, first at 430 C. and then just below 256 C. The 

 beautifully laminated structure and its striking resemblance to the pearlitic structure 

 of steel will be recognized at once. It is interesting to notice that this laminated 

 structure is only produced when the alloy is cooled slowly through the decomposition 

 temperature and is then held for some time just below that temperature. If the 

 specimen be quenched from above and be then heated to a temperature just below 

 256 C. and held there for a considerable time, the resulting micro-structure, although 



