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Experiments in Micro-metallurgy. 



time, and in this respect the aggregate effect is not easily distinguish- 

 able from the deformation of a viscous liquid. 



We infer from the experiments that " flow " or non-elastic deforma- 

 tion in metals occurs through slip within each crystalline grain of 

 portions of the crystal on one another along surfaces of cleavage 

 or gliding surfaces. There is no need to suppose the portions which 

 slip to be other than perfectly elastic. The slip, when it occurs, in- 

 volves the expenditure of work in an irreversible manner. 



It is because the metal is an aggregate of irregular crystals that it is 

 plastic as a whole, and is able to be deformed in any manner as a result 

 of the slips occurring in individual crystals. Plasticity requires that 

 each portion should be able to change its shape and its position. Each 

 crystalline grain changes its shape through slips occurring within itself, 

 and its position through slips occurring in other grains.* 



From what is known about the break-down in elasticity which occurs 

 as the immediate effect of overstrain and the subsequent recovery of 

 elasticity after a period of rest, it would seem that the surfaces over 

 which slip has occurred are at first weak, but heal with the lapse of 

 time. To discuss these points, however, lies beyond the scope of a 

 preliminary notice. 



The experiments were made in the engineering laboratory at Cam- 

 bridge, and are being continued. We wish to take this opportunity of 

 thanking Sir W. Eoberts- Austen and Mr. T. Andrews for the great 

 kindness with which, at the outset of our work, they gave us the 

 benefit of their experience in preparing and observing microscopic 

 specimens of metals. 



[Note added April 14, 1899. — In a specimen of cast nickel, which 

 showed after etching a crystalline structure much resembling that of iron, 

 but on a considerably smaller scale, straining developed minute slip- 

 bands, which are clearly apparent under a power of 1,000 diameters. 



We have also examined a specimen of pure gold by compressing a 

 cast button with a polished face, not etched. The straining reveals 

 crystalline structure on a large scale, and in each of the crystalline 

 grains there is a superb development of slip-bands. They are long, 

 nearly straight, exceedingly numerous, and very closely spaced. A 

 power of 1,000 diameters is required to see them well. Two inter- 

 secting systems are common, and three are well seen in some of the 

 grains, forming a regular geometrical network. The intergranular 

 boundaries are sharply denned by the meeting of the slip-bands on 

 each grain with those on its neighbours. The slip-bands in adjacent 

 crystals meet in a way which demonstrates the absence of any appre- 

 ciable quantity of foreign matter in the intergranular junctions.] 



* Attention should be called in this connection to the experiments of Messrs. 

 MeConnel and Kidd on the plasticity of glacier ice (' Roy. Soc. Proc.,' vol. 44, 

 p. 331). They found that bars cut from glacier ice, which is an aggregate of 

 irregular crystals, are plastic. 



