Experiments in Micro -metallurgy \ 



89 



width of the slip bands themselves is too small to be measured 

 with any accuracy ; it does not appear at any place here to exceed 

 1/2000 mm. 



The slip-bands are developed by compression as well as by extension. 

 Fig. 9 is a photograph (at 400 diameters) from the polished and 

 etched side of a block of Low Moor iron compressed in the testing 

 machine sufficiently to give it a considerable amount of permanent set. 

 The bands developed by compression have apparently all the character- 

 istics which they present in stretched pieces, and we could not, by 

 microscopic examination of the surface, distinguish, in this respect 

 between the effects of compression and extension. The irregular dark 

 patches in fig.- 9 are streaks of slag. 



By twisting an iron bar well beyond the elastic limit the slip-bands 

 are made to appear, for the most part in directions parallel and per- 

 pendicular to the axis of twist. 



A strip of sheet metal, such as iron or copper, in the soft state, when 

 bent and unbent in the fingers shows them well developed by the 

 extension and compression of the surface. 



We have developed the slip-bands in iron, steel, copper, silver, lead, 

 bismuth, tin, gun-metal, and brass. In silver they show particularly 

 well, the crystalline structure being large and the lines straight. In 

 copper also the lines are straighter and more regularly spaced than is 

 general in iron. Most of these metals have been tested in the form 

 of blocks under compression. A beautiful development of slip-bands 

 may readily be produced by pinching a button of polished silver or 

 copper in a vice. 



In carbon steels we have found the slip-bands considerably more 

 difficult to observe than in wrought iron. The smaller granular struc- 

 ture of steel apparently makes the slip-bands correspondingly minute. 

 In mild steel they are seen readily enough, but in a rather high carbon 

 steel we succeeded in seeing them only with difficulty in the " ferrite " 

 areas under a magnification of 1,000 diameters. A cast piece of the 

 nearly pure iron used for dynamo magnets showed a relatively very 

 large granular structure and well marked slip-bands. 



These experiments throw what appears to us to be new light on the 

 character of plastic strain in metals and other irregular crystalline 

 aggregates. Plasticity is due to slip on the part of the crystals along 

 cleavage or gliding surfaces. Each crystalline grain is deformed by 

 numerous internal slips occurring at intervals throughout its mass. In 

 general these slips no doubt occur in three planes, or possibly more, 

 and the combination of the three allows the grain to accommodate 

 itself to its envelope of neighbouring grains as the strain proceeds. 

 The action is discontinuous : it is not a homogeneous shear but a series 

 of finite slips, the portion of the crystal between one slip and the next 

 behaving like a rigid solid. The process of slipping is one which takes 



