74 MODERN SCIENCE OF METALS 



however, a very different state of affairs is met 

 with; here, although the two metals may still 

 dissolve completely in one another when molten, 

 they crystallise separately on cooling, and the 

 resulting solid alloy is a conglomerate of two kinds 

 of crystals. The resulting structures and proper- 

 ties of the two kinds of alloy are, as a result of this 

 diversity in their mode of solidification, very great, 

 more particularly when one of the metals of the 

 duplex type of alloy is hard and brittle. There 

 are, however, further complications ; in some series 

 of alloys there is no separation of the two com- 

 ponent metals when the alloy first solidifies an 

 aggregate of simple homogeneous crystals is formed ; 

 subsequently at some lower temperature when a 

 critical point is reached these homogeneous crys- 

 tals break up and the two constituents separate. 

 This is the change which occurs at the critical point 

 in steel. But even apart from all critical changes, 

 there is another complication which arises when 

 the two metals enter into definite chemical com- 

 bination with one another; in that case the two 

 kinds of crystal which we find in the resulting 

 alloys are not crystals of the two simple component 

 metals, but crystals of the one metal and crystals 

 of the compound of the two. Now it is a striking 

 fact that these definite intermetallic compounds are 

 peculiarly hard and brittle bodies, even when their 

 component metals are soft and ductile. Thus 

 aluminium and copper both of which are very 



