THE STRUCTURE OF METALS 207 



Such an alloy is far more brittle than the rolled specimen. If, 

 besides heating it to a high maximum temperature, the alloy be 

 quenched, so that it is entirely in the /3-condition, the brittleness 

 is enormously increased, the /3-grains being separated by 

 rectilinear boundaries without any a-constituent to produce 

 even a partial union. 



The mechanical behaviour of metals at high temperatures also 

 has great technical importance. Such objects as the valves for 

 the admission and regulation of superheated steam or the plates 

 and stays of locomotive fire boxes are exposed to severe 

 mechanical stress at temperatures very considerably above that 

 of the atmosphere. It is well known to engineers that all 

 metals deteriorate in strength as the temperature increases but 

 satisfactory information on the subject is curiously scanty. 

 Generally speaking, the tensile strength, both of pure metals and 

 alloys, diminishes as the temperature rises. The ductility of 

 a cast or annealed metal also diminishes at first, whilst that of a 

 cold-worked metal increases, owing to progressive annealing. 

 At higher temperatures the ductility varies in an apparently 

 capricious manner, finally reaching zero at or near the melting 

 point. A number of factors are evidently concerned in the form 

 of the ductility curve and much work will be needed in order to 

 disentangle them. 



The most satisfactory experiments of this kind are those 

 recently conducted at Liverpool by Mr. G. D. Bengough. 1 

 Considering only the tensile stress under which a specimen 

 breaks, it appears from these tests that the stress falls as the 

 temperature is raised in a manner which is best represented by 

 two intersecting lines one of which is straight whilst the other 

 may be either straight or curved. The general condition pre- 

 sented by pure metals or homogeneous alloys is shown in fig. 5. 

 The line ABC represents the variation of strength with 

 the temperature of a cold-worked metal, whilst DBC represents 

 that of the same metal in a cast or hot-worked condition. The 

 change of direction at B is always well marked in the actual 

 curves. The point B is designated by Mr. Bengough the 

 " temperature of complete recuperation." The curve AB is 

 evidently a range within which annealing of the cold-worked 

 metal, that is, recrystallisation of the amorphous modification, 

 is going on. It is suggested that our ordinary cast or hot- 



1 Journ. Inst. Metals, 1912, 7, 123. 

 H 



