CONSTITUTION OF THE ALLOYS OF ALTMINII'M \NI> ZINC. 335 



definitely duplex, is granular and patchy, recalling the structure of sorbitic and 

 troostitic steels rather than the pearlitic structure shown in fig. 27. Although the 

 rates of cooling and heating involved are decidedly slower in the present case, the 

 behaviour of the /3 meteral appears to present some interesting analogies to that of 

 carbon steels. 



To illustrate the behaviour of the group of alloys lying somewhat further to the 

 right of the line GFK, the photomicrographs of figs. 28 to 31 represent the micro- 

 structure of alloy No. 75 (74'5 per cent, of zinc). Fig. 28 (x!50) represents the 

 structure of a specimen of this alloy which has been cooled from fusion to 470 C., 

 held at that temperature 30 minutes and then quenched. This temperature lies just 

 above the line CH, and the micro-structure accordingly consists of crystals of y 

 embedded in a finely-granulated matrix which represents that part of the alloy which 

 was liquid at the moment of quenching. In fig. 29 ( x 150) we have the structure of 

 a specimen of the same alloy which has been cooled from fusion to 430 C., held at 

 that temperature for 30 minutes and then quenched. In this structure we see an 

 approach to homogeneity ; the black areas represent holes or cavities in the 

 specimen, but there is no sign of the presence of liquid at the moment of quenching, 

 nor is any eutectic present. This structure represents a stage of the reaction 

 y + liquid = (5, where the reaction, although still incomplete, has progressed to a 

 considerable extent. Fig. 30 ( x 150) represents the structure of a specimen of the 

 same alloy which has been kept at 430 C. for about 2 hours, this treatment being 

 followed by quenching from that temperature ; here the reaction named above has 

 been allowed to complete itself, while the decomposition which would otherwise have 

 set in at 256 C. has been prevented by quenching ; the resulting structure is 

 therefore homogeneous and is typical of the alloys consisting of S. The same alloy, 

 when slowly cooled after prolonged heating at 430 C., again shows the typical 

 pearlitic structure resulting from the decomposition of S or ft. This is illustrated in 

 fig. 31 under a magnification of 300 diameters. 



The persistence with which meta-stable eutectic is found in the alloys of this group 

 when slowly cooled is illustrated by fig. 32 (x 150) which represents alloy No. 70 in 

 t his condition, which corresponds closely to that illustrated for alloy No. 78 in fig. 24 ; 

 we have cores of primary y with rims of decomposed S enclosing areas of eutectic. 

 The same alloy, after heating for a considerable time at 430 C. followed by slow 

 cooling, is illustrated in fig. 33 (x!70), where the pearlitic structure can again be 

 seen while there is total absence of eutectic. 



The micro-structures of alloys lying to the right of the point H present few points 

 of interest so far as the constitution of the alloys is concerned ; they exhibit the 

 characteristic features of alloys consisting of a single solid solution possessing a long 

 range of solidification. The typical formation of cores in such alloys is illustrated in 

 fig. 34 (x 150), which represents the structure of a slowly-cooled specimen of alloy 

 No. 1st ; it' heated for any length of time to a temperature below that of the s.. I id us. 



