176 



The Crystalline Structure of Metals. 



given, showing the twin bands as revealed by a cross-hatching of 

 parallel slip lines, the sets of lines being parallel to one another in 

 alternate bands of the twin. The twinning under strain which we 

 have observed in various metals is similar to that which is known to 

 occur in calcite. It may be regarded as a result of slip accompanied 

 by a definite and constant amount of rotation on the part of the 

 molecules. 



From this point of view there are two modes in which plastic yield- 

 ing occurs in a crystalline aggregate. One is by simple slips, where 

 the movements of the crystalline elements are purely translatory and 

 their orientation is consequently preserved unchanged. The other is 

 by twinning, where rotation occurs through an angle which is the same 

 ior each molecule in the twinned group. Both modes are often found 

 not only in a single specimen of metal but in the same crystalline 

 grain. 



At the suggestion of Messrs. Hey cock and Neville, the authors' 

 examination of the effects of strain has been extended to certain 

 •eutectic alloys. The structure of such alloys has already been 

 described by Osmond, with whose observations these are in agreement. 

 The alloy generally exhibits rather large grains, the structure of which 

 is very different from that of pure metals, for it consists of an intimate 

 intermixture of two constituents, one of which appears as separate or 

 dendritic crystals on a field formed of the other constituent. The two 

 .are seen forming an exceedingly minute and complex structure within 

 •each of the large grains of which the alloy is made up. Straining has 

 the effect of making this intimate structure more apparent, by causing 

 slips which set up differences of level between pieces of one and the 

 other constituent. 



A study of the micro-structure of alloys suggests a possible explana- 

 tion of the peculiarities they present in regard to variation of electrical 

 conductivity with temperature. The two constituents may behave 

 individually as pure metals in this respect, but if their coefficients of 

 .expansion are different the closeness of the joints between them will * 

 depend on the temperature. Thus if the more expansible metal exists 

 as plates, or separate pieces of any form within the other, the effect of 

 heating will be to make the joints between the two conduct more 

 readily, with the result of reducing the increase of resistance to which 

 heating would otherwise give rise, and in extreme cases with the effect 

 even of producing a negative temperature coefficient. The high 

 resistance of alloys generally may be ascribed to the large number of 

 joints across which the current has to pass. 



In casting metals against glass and other smooth bodies for the 

 purpose of getting a surface fit for microscopical examination, a surface 

 is occasionally produced which not only shows the true boundaries be- 

 tween the crystalline grains, but also additional markings which simulate 



