1905.] 



Slip- Bands in Metallic Fractures. 



561 



of steps or serrations in the boundary in the transverse section. 

 Although the etching had been carried rather far, so that the outlines 

 of the crystals on the initial surface were very well marked, yet the 

 differences of level between adjacent crystals, which are one result 

 of etching, were only barely visible on the transverse section. A 

 third specimen was polished and strained without any preliminary 

 etching, and examination of a transverse section showed steps and 

 serrations in the boundary line similar to those found on specimens 

 which had been etched before being strained, but they appeared to be 

 somewhat smaller and slightly less well-defined. 



It has thus been shown that these steps or serrations in the boundary 

 line are found in previously strained specimens, whether etched or not, 

 but that they are not found in unstrained specimens. The author there 

 fore feels justified in concluding that the steps seen in transverse sections 

 of strained specimens are the sectional views of slip-bands. Fig. 2 

 (Plate 14) is a photomicrograph of an example of these steps, under a 

 magnification of 1000 diameters. It will be seen that the steps, although 

 very minute, are perfectly distinctive, and that they could not be mistaken 

 for generally rounded foldings of the surface ; they possess, in fact, a 

 general geometrical character, which the author regards as conclusive 

 evidence that they are caused by slip on cleavage or gliding planes of 

 the crystals, and not by any folding or crumpling of the metal. On 

 the other hand, these steps as seen in section are so minute that it is 

 not very safe to draw conclusions as to the details of their configura- 

 tion ; under the most critical conditions of observation, the upper 

 edges of these steps do not appear to be sharply angular, but rather to 

 be rounded off slightly, and in some cases they even appear to possess 

 a minute crest or ridge. Such a configuration would readily account 

 for some of the phenomena pointed out by Osmond, but the dimensions 

 of these details are so near the limit of resolving power of the lenses 

 used by the author (Zeiss 3 mm. apochromat. oil-immersion, 1'40 

 N.A.) that no great reliance can be placed upon their apparent shape, 

 particularly as the tendency of microscopic vision is to obliterate 

 minute angles whose dimensions lie near the limits of resolution. 



In general terms, however, there can be no doubt that the sectional 

 views of slip-bands obtained by the author strongly confirm the theory 

 of deformation by slip, and in order to emphasise this confirmation the 

 diagram, Fig. 3, is given for comparison with the photomicrograph, 



FIG. 3. 



