﻿26G Dr. J. A. Ewino- on th 



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six neighbours. But it may be conjectured that some of them 

 may take up pyramidal piling (touching twelve others) under 

 the compulsion of strong forces — such forces, for example, as act 

 on the superficial molecules of a surface that is being polished. 



If this also occurs at a surface of slip, it gives us a clue to 

 several known facts. It at least assists in explaining the 

 familiar result that metal is hardened by straining in the 

 sense of being made less plastic. Again, it accounts for the 

 geneial increase of density which is found to take place in 

 such an operation as wire-drawing. Further, if a local in- 

 in crease of density occurs in the interior of a grain through 

 piling of some molecules in the closer manner where repeated 

 slips are going on, the concentration of material at one place 

 requires it to be taken from another ; in other words, the 

 closer piling tends to produce a gap or crack in the neigh- 

 bourhood where it occurs. This is consistent with what we 

 know of the development of cracks through repeated alterna- 

 tions of strain. 



Recourse to the model shows that with pyramidal piling 

 the polar axes point in so random a manner that the aggre- 

 gate may fairly be called amorphous. To illustrate this a 

 group is shown with centres fixed at the corners of equilateral 

 triangles. 



It is obvious that any pyramidal piling at a surface of slip 

 tends to bar further slip at that particular surface. Hence 

 not only the augmented hardness due to strain, but the 

 tendency in repeated alternations to lateral spreading of the 

 region on which slip occurs. The hardness due to straining- 

 is, of course, removed when we raise the metal to such a tem- 

 perature that complete recrystallization occurs, normal piling- 

 being then restored in the new grains. 



Taking a previously unstrained piece, it is clear that the 

 facility with which slip will occur at any particular surface 

 of slip in any particular grain depends not only on the nature 

 of the metal and on the orientation of the surface in question 

 to the direction of the stress, but also on the amount of support 

 the <>rain receives from its neighbours in resisting slip there. 

 In other words, for a given orientation of surface the resist- 

 ance to slip may be said to consist of two parts ; one is in- 

 herent in the surface itself, and the other is derived from the 

 position of the grain with reference to other grains. 



To make this point clear, think of a grain (under stress) in 

 which there is a gliding surface oriented in the most favour- 

 able direction for slipping. Slip on this surface can take place 

 only when its yielding compels the neighbours (which are also 

 under stress) to yield with it, and the surfaces in these on 

 which slip is compelled to occur are, on the whole, less 

 favourably situated. Hence the original grain cannot yield 



