CONSTITUTION OF THE ALLOYS OF ALUMINIUM AND ZINC. 323 



determined ; this is such that a continuous curve from B to D would lie further from 

 the observed points than the limits of experimental error would allow, particularly as 

 the points on the liquidus near the point have been repeatedly determined with 

 special care. The existence of a break at C is further confirmed on theoretical 

 grounds by the evidence to be described presently, proving that the phases which 

 commence to crystallize along the branches BC and CD are two distinct bodies. 



The solidus curve of the diagram consists of the lines AE, EB, BF, FG, GH, HD. 



The evidence for this statement is to a large extent microscopical ; the method of 

 prolonged amif.-iling followed by quenching has been adopted throughout and details 

 of the evidence obtained in this way, particularly with regard to the lines GH and 

 HD, will be given below. 



The line EBF, lx>th as regards temperature and as regards the position of the 

 point B, agrees closely with the eutectic line shown in SHEPHERD'S diagram, but ;m 

 important difference is found with regard to the end of this line on the aluminium 

 side. On referring to the cooling-curves in figs. 1 and 2, it will be found that the 

 arrests along the line EBF are still marked in Nos. 78 and 76, yet the eutectic line in 

 the diagram is drawn only to the point F, corresponding to a concentration of 78 '3 

 per cent, of aluminium. 



The reason for this discrepancy lies in the fact that although alloys cooled with 

 moderate rapidity exhibit a eutectic heat evolution to a considerable distance to the 

 right of the point F, yet both microscopically and pyrometrically this eutectic can 

 be made to disappear entirely by exposing the alloys to a temperature of about 

 430 C. for a considerable period of time. 



In order to establish this point, ingots of a series of alloys lying on either side of 

 the point F were maintained for about 100 hours at a temperature near 430 C., and 

 cooling-curves of these ingots were subsequently taken at a rate similar to that 

 described for the series of curves illustrated in figs. 1 and 2, the observations being 

 taken from a temperature of 430 C. to about 200 C. The curves of alloys, Nos. 80, 

 78, and 75, are shown in fig. 5, the curves marked a, b, and c l>eing those taken by 

 ordinary slow cooling from fusion, while those marked d, e, and f are those taken 

 after the heat treatment just described. In No. 80 the eutectic heat evolution has 

 not been entirely removed by the treatment, but in Nos. 78 and 75 the well-marked 

 peaks of the ordinary curves have entirely disappeared after the treatment. This 

 indicates that the existence of stable eutectic is limited by a concentration lying 

 between 78 and 80 per cent, of aluminium, and probably lying quite close to 78 

 per cent. 



In order further to ascertain the concentration at which the eutectic disappears the 

 method of measuring the heat evolutions in successive alloys was adopted. Although 

 every precaution was taken to render these measurements as accurate as possible, it 

 must be recognised that the method itself is not capable of any high degree of 

 accuracy, the reason being that although the cooling-curves generally give a definite 



2x2 



