i 9 8 SCIENCE PROGRESS 



due to the slipping of certain portions of the crystal over others 

 along the planes of weakness or cleavage planes. When the 

 stress in a crystal grain becomes too great for the metal to 

 yield elastically, slipping along these planes takes place and 

 the shape of the grain is changed not continuously as that 

 of a truly plastic substance would be but by a series of 

 dislocations completely resembling in origin and appearance 

 the " step-faults " of the geologist. After the formation of the 

 slip-bands, provided that no further change take place, the 

 internal structure of the crystal is not affected, since the dis- 

 placement of neighbouring portions of a crystal is only one 

 of translation. Hence, if we examine a polished surface on 

 which slip-bands are obvious and then remove a thin surface 

 layer by grinding and expose the crystalline structure by 

 etching, the slip-bands do not reappear. This fact distinguishes 

 them from the grosser changes of structure which are produced 

 by mechanical means under certain conditions and especially 

 from twinning. Twinning planes reappear after removal of 

 the surface and re-etching and are easily recognised when 

 once the manner in which they differ from slip-bands has 

 been appreciated. Both twinned lamellae and slip-bands may 

 be present in the same crystal, but whereas the latter are 

 universal in metals after cold-working, the former are less 

 frequent and are only developed abundantly in certain classes 

 of metals and alloys, of which austenitic steels (such as man- 

 ganese steel) and copper and its a-alloys, including yellow 

 brass, are familiar examples. The strained surface of lead in 

 fig. i shows twinning lamellae as well as slip-bands. Fig. 2 

 represents a cube of ingot iron after compression ; both slip-bands 

 and crystal boundaries are distinguishable, the latter having 

 been made visible by the strain without any etching process. 



To produce slip-bands it is not necessary that the stress 

 applied should exceed the elastic limit of the specimen. A 

 crystalline metal, even if practically free from impurities, is 

 not a homogenous substance but is built up of distinct grains 

 aggregated to form a mass. When a stress is applied, it is 

 impossible that it should influence every grain equally and 

 it may readily happen that a few individual grains are stressed 

 by an amount exceeding the elastic limit whilst their neighbours 

 are under a much lower stress. Every slip along a cleavage 

 plane brings about a redistribution of stress, tending to make 



