THE STRUCTURE OF METALS 97 



extremely fine division as to be at the limits of microscopical 

 vision and prolonged annealing is necessary in order that 

 segregation may proceed far enough to give rise to a visibly 

 duplex structure. This interesting discovery has thrown much 

 light on the changes of properties undergone by these alloys 

 during heat-treatment and serves further to call attention to the 

 fact that the simplicity of constitution of some of our best known 

 alloys is only apparent and that subjection to long annealing 

 processes at a comparatively low temperature may produce very 

 far-reaching modifications of structure. In view of the extensive 

 use of alloys for engineering purposes in positions in which 

 they are exposed to the prolonged influence of temperatures 

 above that of the atmosphere, the technical importance of this 

 and similar observations is obvious. 



A large proportion of the alloys in general use thus fall into 

 one of two classes from the point of view of crystalline structure. 

 The first class comprises alloys in which crystals of a single 

 type compose the whole of the alloy, which has thus, at least in 

 the annealed condition, the structure of a pure metal. This 

 class includes the true brasses, the alloys of copper with small 

 quantities of nickel, arsenic, manganese, iron and other metals, 

 used whenever toughness and resistance to high temperatures 

 are required, as in the fire-boxes of locomotives, the lower tin 

 bronzes, etc., Monel-metal (an alloy of copper and nickel, with 

 the latter in excess), German silver, manganese steel, nickel 

 steel and many other alloys, including the standard gold and 

 silver used for coinage. The second class, in which two types 

 of crystalline material are necessarily present as structural 

 constituents, includes Muntz-metal and manganese bronze, the 

 principal aluminium bronzes, naval brass and other similar 

 alloys. In most gun-metals and in bearing-bronzes, the one 

 material during cooling undergoes resolution into other con- 

 stituents and is therefore present as a complex. This is also 

 the case with carbon steels. 



The class of alloys so frequently encountered in laboratory 

 investigations, in which the primary crystals are surrounded by 

 an eutectic alloy, 1 is relatively ol much less importance in technical 



1 An eutectic alloy is an intimate mixture of two or more kinds of crystal 

 characterised by the fact that its melting point is lower than that of alloys 

 containing more of either the one or the other constituent and that it solidifies 

 at a definite temperature. 



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