1911)] on The Hardening of Steel 505 



responsible for the hardness of quenched steel, and in this respect 

 resembles the theory pnt forward by Professor Edwards and myself. 

 The formation of this phase, however, is attributed to entirely 

 different considerations, which will now be briefly set forth. Mr. 

 Hunifrey considers in the first place what happens when one allotropic 

 form of an element passes into another. He starts from the theory 

 of crystal structure already alluded to in McCance's theory, namely, 

 that the centres of gravity of the molecules are arranged together 

 accordiug to one of a series of geometrical devices, called space- 

 lattices. In each molecule of such a crystal the atoms are similarly 

 situated. 



When an allotropic change occurs in such a crystalline body, 

 there is an internal re-arrangement of the atoms in each molecule, 

 and if the new form which the molecules take involves a corresponding 

 change in the external forces which they exert upon one another, the 

 space-lattice may become unstable and a different one formed. Before, 

 however, the reorganization can be completed, it is assumed that there 

 must be temporarily a state of disorder, and it is during this period 

 that Humfrey considers that the structure must be considered as 

 amorphous. This intermediate state is regarded as corresponding to 

 the liquid " which would be formed by the fusion of the solid phase 

 stable at the lower temperatures, if the conditions could be so adjusted 

 that the subsequent re-crystallization were avoided." The tem- 

 perature-tenacity curves obtained by Eosenhain and Humfrey are 

 regarded as indicatiug that the melting points of a and /? iron would 

 be considerably lower than that of y iron, if they could be preserved 

 unchanged up to their melting points. 



In the next place consideration is given to the fact that physicaJ 

 changes of state do not take place instantaneously throughout the 

 whole mass of any given substance, but that they start from certain 

 nuclei and proceed outwards. From this Humfrey conceives that the 

 intermediate amorphous phase existing during an allotropic phase 

 begins to re-crystallize almost as soon as it is formed, " and is present 

 at any particular instant during the change, merely as thin films or 

 layers between the co-existing crystalline phases, the films moving"^ 

 forward as the change progresses. The course of the breakdown of 

 the one crystalline phase to the amorphous would tend to follow those 

 planes in which the freedom of movement was greatest, namely, the 

 crystal-gliding planes and possibly the intercrystalline boundaries." 



With regard to the formation of a new crystalline phase Humfrey 

 points out that the temperature at which it occurs must not be toa 

 low for the forces of crystallization to overcome the viscosity of the 

 amorphous phase. If the viscosity is sufficient to prevent recrystalliza- 

 tion, the amorphous phase formed will be practically stable. Even 

 in cases, however, where an allotropic change normally takes place at 

 a temperature well above that at which the viscosity is sufficient to 

 prevent recrystallization, abnormal conditions such as rapid cooling 



2 M 2 



