5'4 



NATURE 



[November 4, 1920 



of another ; (5) the change o( orientation is accom- 

 panied by a difference in level of the surface 

 which is the boundary usually observed ; (6) the 

 rate of growth is not constant for any given time 

 at a ])articular temperature. 



The authors tested the generally accepted view 

 that the crystals of castings do not grow upon 

 heating. This view, although widely held, does 

 not rest upon adequate experimental evidence, and 

 the difficulty of finding a suitable method of testing 

 it is considerable. In the first place, all forms of 

 strain in cutting, grinding, and polishing a speci- 

 men must be avoided, because it is known that 

 work will produce recrystallisation of a metal on 

 annealing. The ordinary methods of preparing 

 a metal surface for microscopic examination are 

 quite sufficient to cause recrystallisation of the 

 surface layer on annealing, particularly in soft 

 metals. After experimenting with various 

 methods, the one finally adopted was suggested 

 by the well-known fact that impurities tend to 

 lie in the crystal boundaries of castings, whereas 

 in the worked or annealed metal they either pass 

 into solution or remain scattered throughout the 

 crystals quite independent of the boundaries. It 

 was reasoned that if it could be shown that after 

 prolonged annealing the boundaries coincided with 

 the impurities, it might be considered evidence 

 that they were in the same position after anneal- 

 ing as before. The impurities must be such 

 that they are insoluble in the metal and have a 

 high melting point. Exhaustive tests were 

 carried out with the metal aluminium, which, even 

 in its purest form, contains the compound FeAlg. 

 In the cast metal some of this compound is always 

 found existing in the boundaries. The authors 

 publish a photograph showing that, even after 

 annealing for ten weeks at 550° C, the boundaries 

 of the crystals are still outlined by the impurities, 

 and since the latter cannot have moved, it follows 

 that neither have the boundaries. A specimen of 

 rolled aluminium annealed for the same period 

 showed that, except in a very few cases, the 

 boundaries were quite free from impurities. The 

 authors conclude, then, that the cast metal when 

 free from strain showed neither crystal growth 

 nor recrystallisation on subsequent heating. 



The latter part of the investigation deals with 

 an elaborate study of the quantitative effects of 

 deformation on crystal growth and recrystallisa- 

 tion, and the structural changes produced in a 

 crystal aggregate by deformation followed by heat 

 may be summarised somewhat in the following 

 way : — 



The first effects are slight. They are revealed 

 by slip bands, and in some <:ases by twins. The 

 former are completely, and the latter to some ex- 

 tent, removed by heating. No change is observed 

 in the shape of the crystals. The boundaries 

 appear unaffected, and, apart from the twins, 

 there is no change in orientation. Thus far only 

 the interior of the crystals is affected. No iden- 

 tities are lost. Somewhat greater deformation, 

 however, produces crystal growth, and at this 

 NO. 2662, VOL. 106] 



I stage the boundaries of the crystals become active. 

 1 The activity is shown in the capacity of the grow- 

 I ing crystal to push forward its boundary in certain 

 I directions, thereby invading other crystals, but 

 even at this stage the orientation of the growing 

 crystals is maintained. That of the crystals 

 grown into is, of course, destroyed, lanless it 

 happens to be the same. The third stage, pro- 

 duced by still greater deformation, appears to take 

 place exclusively in the boundaries of deformed 

 crystals. It is here that the new crystals are 

 born, indicating the destruction of the original 

 crystals and a complete change of orientation. 

 Whereas, therefore, the early effects of deforma- 

 tion are manifest in the interior of the crystals, 

 the later ones appear to take place entirely at the 

 boundaries. 



The chief facts established by these experiments 

 are : (i) The largest crystals are always formed 

 after the minimum amount of stress sufficient to 

 produce growth, which minimum is determined by 

 the annealing temperature ; (2) the lower the tem- 

 perature the greater the stress required to produce 

 the large crystals ; (3) there is no gradual 

 increase in size from the original-sized crystals 

 up to the largest which form directly from 

 them. 



It is considered that all the evidence brought 

 forward points to plastic deformation followed by 

 heat as the true cause of crystal growth and re- 

 crystallisation followed by crystal growth. The 

 only exception to this is where an alteration of 

 crystal form occurs when a metal passes through 

 a phase change. Apart from the direct evidence 

 against any growth of crystals in castings, con- 

 firmatory evidence of the following nature has 

 been obtained in this research : (ij Growth and 

 recrystallisation can be induced by work ; (2) the 

 size of the crystals produced on heating at a given 

 temperature after work is entirely dependent on 

 the amount of deformation ; (3) there is always in 

 practice a limit to the crystal growth produced by 

 work and heat in any metal as a result of which 

 a single crystal has not yet been produced by 

 prolonged heating. Such a result, however, is 

 theoretically imaginable, provided that the degree 

 of deformation and subsequent temperature of 

 heating could be adjusted to certain very precise 

 conditions. 



Whatever forms of energy operate during the 

 growth or recrystallisation of a crystal on heating, 

 the authors' conclusion is that the energy is im- 

 parted to the metal when it is deformed. That 

 energy cannot be stored in the amorphous vitreous 

 films, which, according to Sir George Beilby, are 

 the cause of work-hardening in metals, because, 

 as the authors have shown, the growth of crystals 

 in a worked metal proceeds after all mechanical 

 softening has taken place. Hence all the 

 amorphous metal must have recrystallised. It is, 

 moreover, difficult to explain why the least de- 

 formation should produce the largest crystals on 

 heating if growth depends on the presence of 

 amorphous metal. Further, the energy cannot be 



