THE STRUCTURE OF METALS 95 



lower brasses, these alloys solidify in the first place in the 

 form of a homogeneous mass of crystals but as the tempera- 

 ture falls changes take place in the solid, much as in the case 

 of an ordinary solution, new materials separating out. In this 

 instance the separated material is itself a homogeneous solid 

 containing relatively more copper than the original crystals. 

 By a convention which has been generally adopted in the case 

 of this and similar alloys, the new crystals are designated 

 the a- constituent, the prefix /3 being assigned to the material 

 of the original crystals and to that part of the " mother 

 crystals" which remains after complete separation of the 

 excess. Alloys of this class have therefore a duplex structure, 

 the a-crystals being outlined on a (fig. 6) background of ft. 

 As the proportion of zinc in the alloys is increased, so the 

 proportion of a-crystals diminishes, until alloys containing 

 nearly 50 per cent, of zinc consist of homogeneous crystals of 

 the /3-constituent, which may be distinguished from the a- 

 crystals of which 70 : 30 brass is composed by its different 

 behaviour towards etching-agents and by the absence of the 

 cores which are so characteristic of brass in the cast con- 

 dition. A further increase of the proportion of zinc beyond 

 50 per cent, brings about the appearance of small bluish-white 

 crystals of the 7-constituent, which composes the whole alloy 

 when 61 per cent, of zinc is present. This substance is un- 

 doubtedly a definite compound, Cu 2 Zn 3 . It is exceedingly brittle 

 and its presence, even in small quantities, is fatal to the good 

 mechanical properties of the alloys. The proportion of zinc 

 which may be alloyed usefully with copper is therefore limited. 



Each of these constituents has its special characteristics. 

 The a-crystals are remarkably tough and may be subjected to 

 very severe mechanical deformation without cracking. This 

 property reaches its maximum in the 70 : 30 alloy, which is 

 frequently known as " cartridge brass " from its use in the 

 manufacture of cartridge-cases in which process it is very 

 severely deformed by forcing through dies. The /3-crystals 

 are less tough and ductile but have a higher tensile strength ; 

 they are malleable at a high temperature, a property which is 

 not inherent in the alloys richer in copper. The presence 

 of a small quantity of the /3-form is essential if the alloy is 

 to be rolled while hot. 



Both the a- and the 7-crystals are reincorporated to a very 



