96 SCIENCE PROGRESS 



considerable extent by the /3-constituent when the temperature is 

 raised. By heating and rapidly quenching, therefore, most ot 

 the alloys of this series having a duplex structure may be 

 rendered homogeneous. Such treatment increases the tensile 

 strength of the alloys in question at the sacrifice of much of 

 their ductility. The quenched alloys are in a more or less 

 unstable condition and the duplex structure is restored by 

 annealing at a moderate temperature. 



Aluminium forms alloys with copper which, in some cases, 

 resemble in a very striking manner those containing zinc but a 

 smaller quantity of aluminium is required to produce the effect. 

 Thus the proportions of the a- and /3-constituents in an alloy of 

 60 per cent. Cu and 40 per cent. Zn are almost the same as in an 

 alloy of 90 per cent, of copper and 10 per cent, of aluminium. 

 The latter is an alloy of very high technical value and is well 

 known under the name of aluminium bronze. As in the case 

 of the zinc alloys the 7-constituent, which appears when the 

 aluminium exceeds 12 per cent., is brittle and its presence is 

 fatal to the mechanical properties of the alloy. 



The true bronzes are alloys of copper with tin to which 

 smaller quantities of other elements are very frequently added. 

 The a-constituent richest in copper resembles in all essential 

 properties the corresponding alloys with zinc and with 

 aluminium. Most technical tin bronzes, however, contain a 

 small proportion of the /3-constituent at high temperatures ; as 

 the temperature falls the /3-crystals become unstable and are re- 

 solved into a characteristic complex of finely divided a and a hard, 

 brilliantly white substance, the S-constituent. Small areas of this 

 complex occur in many bronzes and form a large part of the hard 

 bronzes used for bearings. One of these areas is shown in fig. 7. 



The resolution of the /3-constituent of tin bronzes into a 

 complex, which takes place on cooling below 500 and proceeds 

 rapidly to completion, has a remarkable parallel in the alloys of 

 copper and zinc, it having been shown quite recently x that the 

 /3-constituent in this case also is unstable when cooled below 

 470 , being resolved into a complex of a and 7. The main 

 difference lies in the velocity of transformation and of recrystal- 

 lisation. Even when the development of heat during cooling 

 has indicated that resolution into two constituents has taken 

 place, the products remain for some time in a state of such 

 1 H. C. H. Carpenter,/. Inst. Metals, 1912, 7, 70. 



