366 PROFESSOR J. A. EWING AlfD MH. \V i:< iSKMIAIX 



If we assume that plastic strain takes place solely by these slips, it follows that the 

 ultimate crystalline elements should always remain parallel to themselves. The 

 orientation of the elements would remain uniform throughout the mass of one grain, 

 however much the outline of that grain were distorted by slips occurring within it. 

 In other words, the crystalline structure of a metal should survive even the severest 

 strain. 



This conclusion is borne out by the fact that in metals which have been much 

 strained we find evidence of crystalline structure similar to that which is found in 

 unstrained metal. 



Taking this evidence in the same order as before, we would refer first to the 

 appearance which is presented in severely strained metal, by a polished and etched 

 section when seen under oblique light. We have examined a bar of fine Swedish 

 iron (kindly supplied by Messrs. EDGAR ALLEN and Co.), which had been for the 

 purpose of these experiments rolled in the cold state from a diameter of three-quart e rs 

 of an inch to a diameter of half an inch, and had not been subsequently heated. A 

 longitudinal section of this cold-rolled bar showed a great lengthening of the crystalline 

 grains in the direction of rolling, and even in the transverse section it was obvious 

 that the grains had been much distorted, though there was no direction of predominant 

 length. Under oblique light both sections (longitudinal and transverse) exhibit the 

 effect described above for unstrained metal the grains are distinguished from one 

 another by differences of brightness, which vary when the incidence of the light is 

 altered, and this brightness is uniform over the entire surface of each grain. As in 

 the case of unstrained metal, we regard this as evidence of the uniform orientation of 

 the crystalline elements throughout each grain. Fig. 31, Plate 23, is a photograph under 

 oblique light of a specimen of this bar cut transversely, and polished and etched. It 

 illustrates the uniform brightness of each grain, due to uniform orientation. Similar 

 characteristics have been observed in many other specimens of severely strained 

 metal. 



Another line of evidence in proof of the persistence of crystalline structure after 

 severe straining is afforded when a polished specimen of, say, cold-rolled iron is 

 subjected to a slight further strain. The slip-bands appear as they would have done 

 had the specimen never been strained before. The general features are much the 

 same as in annealed metal, but they show on the whole a greater tendency towards 

 sudden steps and branches. This difference in the character of the slips may be 

 connected with the well-known fact that such strained material is considerably harder, 

 in the sense of having a higher yield-point and less capability of plastic deformation 

 than it shows in the virgin or annealed state, and also with our own observation that 

 the slip-bands are much more straight and regular in very plastic metals, such as lead, 

 gold, and copper, than in harder metals like iron and nickel. The mere fact that 

 finite slips occur at intervals throughout the grain implies that it is easier for sliding 

 to take place over certain surfaces than it is over others. The surface on which 



