INTERMETALLIC COMPOUNDS 75 



soft and ductile form a compound to which the 

 chemical symbol Al 2 Cu has been assigned, which 

 consists of long, sharp needle-like crystals which 

 are extremely hard and brittle. If, now, we add 

 copper to aluminium as is frequently done, with 

 a view to rendering it stronger and harder, the 

 first additions really produce the desired result 

 to a certain extent; the hard crystals of the com- 

 pound, present as minute particles in the inter- 

 stices of the softer aluminium crystals, serve to 

 strengthen and stiffen their neighbours, while their 

 soft and ductile surroundings save the brittle 

 crystals of the compound from easy fracture. As 

 soon, however, as the amount of copper is increased 

 and the brittle crystals begin to appear as an 

 important part of the structure, the alloy as a 

 whole also becomes brittle and, therefore, useless 

 for practical purposes. Here we see once again 

 the scientific explanation of facts readily observed 

 and well known in practice. Again, however, we 

 find advance made possible by the utilisation of 

 a further scientific discovery. In this case the 

 discovery in question is that of a system of 

 aluminium alloys in which, although there is not 

 an entire absence of compound formation, yet up 

 to a very considerable limit the first type or class 

 of alloys is followed, so that the two metals solidify 

 in the form of homogeneous crystals without the 

 presence of any distinct brittle constituent. Such 

 a series is found in the alloys of aluminium with 

 zinc, and their utilisation for practical purposes 



