550 C. H. Mathewson — Metallogmphic Description 



by cold work which is necessary in order to bring about this 

 result, it may be safely asserted that, with this class of material, 

 any reduction above 15 per cent (in area of section) by cold 

 rolling or hammering will suffice to bring about uniform grain 

 characteristics on anneal for moderate periods of time above 

 700° C. As we pass to lower temperatures of anneal, the 

 visible grain is not in a natural condition throughout, but is 

 constrained, in part, owing to the presence of strained patches 

 or grain fragments which are potentially subject to recrystal- 

 lization. It is thus evident, that the count cannot be extended 

 to the low temperatures of incipient recrystallization and still 

 retain its original significance. The prolongation of all 

 annealing curves in the direction of their origin must, therefore, 

 be based largely upon theoretical considerations. 



A discussion of the lower, as well as the upper ranges of the 

 temperature-grain size curves has been undertaken by Tammann 

 (1. c, p. 548). The principal arguments are as follows : Recrys- 

 tallization in a conglomerate containing grains artificially 

 reduced by deformation will start at a. temperature where the 

 tension at some of the inner surfaces overcomes the rigidity at 

 these points. At other points where the rigidity is greater, 

 recrystallization will not occur until a higher temperature is 

 reached. At any of these equilibrium temperatures, the mean 

 grain size is smaller than would correspond to a natural grain 

 size for the same temperature, owing to increased rigidity. A 

 temperature is ultimately reached where the tensions at the 

 inner surfaces become equal to the rigidity of the mean size of 

 grain for the natural condition. At this point, all evidences of 

 deformation will have disappeared and the curve merges into 

 the natural curve. 



I have attempted in the following remarks to give a general 

 outline of recrystallization phenomena with the aid of a pro- 

 visional diagram, fig. 6, which seems to account for the main 

 facts developed by experiment. 



This diagram is based upon the assumption that the mean 

 size of the new grains which form at any given temperature 

 by coalescence of existing grains or by local break-down of 

 internal surfaces within the original strain-hardened grains 

 is determined by the temperature of anneal. Thus, the size 

 of the recrystallized grain will be some function of the anneal- 

 ing temperature and the conglomerate will be composed of 

 recrystallized grains along with unrecrystallized fragments of 

 the mechanically altered grains. Such structures are com- 



