THE STRUCTURE OF METALS 199 



further slipping unnecessary unless the stress be increased. 

 In the case of progressively increasing stress, more and more 

 crystal grains are dislocated in turn and the constantly varying 

 direction of the local stresses causes the opening up of new- 

 cleavages, so that a grain examined microscopically shows 

 two or more intersecting systems of slip-bands corresponding 

 in direction with its systems of cleavage planes. 



If this were all that happened, the hardening effect would 

 remain unaccounted for, as a mere translation of crystal elements 

 does not cause a change of properties. As a matter of fact a 

 more profound structural change occurs as soon as the amount 

 of cold-working is considerable. The slip-bands lose their 

 simple character and become broad and prominent on a smooth 

 surface. Etching no longer removes them completely ; a close 

 examination proves that the surfaces along which slipping 

 took place are now separated by a layer of material which 

 differs in some way, both chemically and physically, from the 

 unaltered crystals. 



An explanation of the hardening of metals has been given 

 by Dr. G. T. Beilby, 1 who has based his conclusions on observa- 

 tions of the effects produced by polishing. Whenever a metal 

 is subjected to friction a superficial layer is formed which 

 possesses peculiar physical and chemical properties, being 

 hard, isotropic and more active chemically than the original 

 metal. A similar layer may be formed in the interior of a 

 metal by cold-working. The first motion of translation along 

 a gliding plane may produce little effect but by repeated 

 rubbing a layer of the hard material is built up between the 

 two surfaces which hinders further slipping ; the process being 

 repeated on successive cleavage planes, eventually the whole 

 mass of the metal is appreciably hardened. 



Hardening by cold-working cannot be continued indefinitely 

 but reaches a limit, which has a definite value in the case of 

 that particular metal under given conditions. Further stress 

 weakens the metal by causing rupture of the hard layer and 

 consequent separation of adjoining crystals. The effect is 

 often seen in hard-drawn wire. If the drawing be continued 

 too long the wire loses its strength ; if a longitudinal section 

 be examined, it is seen that only the outer shell is continuous, 



1 Phil. Mag. 1904 [vi.], 8, 258 ; /. Inst. Metals, 191 1, 6, 5. 



