560 G. H. Mathewson — Metallographic Description 



Three theories of deformation have lately come into promi- 

 nence. A brief characterization of each follows : 



The Translation Theory, proposed by Tammann, (1. c. (16), 

 p. 56-74), rests upon a purely crystallographic basis. All 

 deformational properties are explained by movements of the 

 above character, in which the original molecular space lattices 

 are maintained ; every movement proceeding strictly in accord- 

 ance with the crystallographic relationships within the primitive 

 grain. 



The Displacement Theory (cf. Mollendorff and Czochralski 

 (17)) is based mainly upon ideas of molecular aggregation 

 developed by Lehmann (18) and associates a gradual destruc- 

 tion of the space lattices with the progress of deformation 

 whereby a final condition of "forced homotropy" results, in 

 which the molecules of a primitive grain are not thrown utterly 

 into disorder but are forced into a modified configuration 

 governed by their own mutual attractions and the play of 

 imposed forces. 



A Modificational Theory elaborated by Bosenhain (19) from 

 ideas originally advanced by Beilby (20) builds upon the 

 ordinary conception of translation by assuming that thin layers 

 of an amorphous cementing material are formed wherever 

 intercrystalline sliding movements occur. Similar layers are 

 supposed to exist wherever two different grains meet, whether 

 these are recrystallized grains or primary grains of solidifica- 

 tion. This theory is especially serviceable in explaining the 

 remarkable adhesive properties of the grains whereby, at 

 ordinary temperature, a fracture normally crosses rather than 

 follows the grain boundaries ; while, at elevated temperature, 

 the tendency to pull apart along the boundaries begins to assert 

 itself. These facts are easily explained by attributing suitable 

 properties to the intercrystalline cement, i. e., it is assumed to 

 be rigid and strong at ordinary temperatures, but weakens 

 as the temperature rises. There is no difficulty in proving 

 that a thin, seemingly isotropic, layer of metal is formed at the 

 surface during the operation of fine polishing. This constitutes 

 the experimental basis upon which this theory was developed. 

 Direct proof that such layers are actually amorphous, or indeed, 

 that metal can be transformed into an amorphous modification 

 has not yet been forthcoming. Lehmann believes that these 

 layers are semi-isotropic after the manner of certain liquid 

 crystals. 



It is clear that modificational and displacement theories can- 

 not be sharply differentiated without the aid of molecular- 



