\ 



33-J DR. WALTER ROSENHAIN AND MR. SYDNEY L. ARCHBUTT ON THK 



diagram by the line UK. The material of the six-rayed dendrite, even under this 

 moderate magnification, is seen to be far from homogeneous, and under higher 

 magnifications it is found to have undergone the same kind of change as that which 

 has already been described and illustrated for the ft constituent of the eutectic. 



Fig. 16 (x!50) shows the micro-structure of alloy No. 90, containing 89'85 per 

 cent, of zinc, as it appears when the alloy is cooled from fusion in the furnace in about 

 45 minutes. The photograph shows a larger proportion of the ft meteral than that 

 seen in fig. 14 embedded in eutectic, but the edges of the ft masses show rims of a 

 changed material arising from the reaction indicated by the line UK of the diagram. 

 The existence of these rims proves that this reaction is rather slow, and the facility 

 with which it can be inhibited by quenching further confirms this view. 



In fig. 17 (x 150) we see the micro-structure of alloy No. 86 (85'52 per cent, zinc) 

 cooled rather more slowly than the specimen represented in fig. 16. This specimen 

 (fig. 17) has, in fact, been cooled at the rate adopted for the observation of cooling- 

 curves. The proportion of ft is larger than in fig. 16, but the change which resulted 

 in the formation of "rims" in fig. 16 has here taken place to a much larger extent, 

 so that the original ft body has almost entirely disappeared. Under the magnification 

 of fig. 17 (150 diameters) the resulting structure of the "changed" ft is not very 

 evident, but a portion of the same specimen is shown in fig. 18 (Plate 5) under a 

 magnification of 500 diameters, and there the uniformly dark ft is seen to have 

 become broken up into a laminated stnicture closely resembling the pearlite seen in 

 steel. On account of this resemblance, the authors propose to describe this structure 

 as " pearlitic " ; it will, however, be seen that in this case the duplex stnicture arises 

 from the decomposition of a compound and not, as in the case of pearlite, from the 

 decomposition of a solid solution. 



The nature of the reaction represented by the line UK can now be understood it 

 is the decomposition of the definite compound ft into two phases which phase-rule 

 considerations prove to be identical with those called a and y on the diagram, the 

 former being solid zinc saturated with aluminium and probably containing less than 

 1 per cent, of that metal, the latter being aluminium saturated with free zinc, the 

 exact proportion not Ixjing quite definitely determined, but probably lying in the 



cinity of 40 per cent. The phase-rule considerations just referred to are that, in 

 any field or area of a binary system, two phases only can exist in equilibrium. In 

 the field BJKF we have the phases a and ft present. Now, it ft is decomposed, as 



e .Microscope proves that it is, into two phases, say X and Y, then in the field below 

 have a + X+Y present, and it follows that either X or Y must be 

 ith a. Similar considerations applied to the fields to the right of the line 

 . show that the other phase must be identical with y. 



The alloys whose micro-structure is shown in figs. 13 to 18 all belong to the group 



the line BC. We now have to consider the alloys to the right of 



In these the phase first separating from the liquid is y and not ft. On 



