INSIDE S OF METALS — ZAPFFE 259 



For the first concept, "dislocations" cuiTently provide the most pop- 

 ular picture. These are the result of vacant or improperly filled atomic 

 positions in an otherwise regular lattice; and their propagation and 

 motion throughout the body of the crystal are believed to develop the 

 observed subdivided structure. 



For both concepts, the term "mosaic" has been widely used, ex- 

 pressing a picture of a gross form built from small fragments, the 

 misfits of the mosaic blocks creating the subdivisions in question. The 

 mosaic block is usually pictured as the result of microcracking, but 

 it has also been related to a preexistence in the liquid. 



Recently a theory has been proposed by the author in which the 

 mosaic block is described as a inicelle specifically originating in the 

 liquid and having fundamental thermodynamic reasons for its sepa- 

 rate existence. This word is borrowed from the organic chemists, 

 and means a small repetitive arrangement of a given atomic or mo- 

 lecular species, having the form of a tiny crystallite. Such clusters 

 are believed to be present, according to the micellar theory, in the 

 liquid and even in the gaseous phase prior to solidification. The 

 theory particularly postulates their existence within single homoge- 

 neous phases, such as that of a pure metal. A phenomenon of this 

 type is known in colloid chemistry, the liquid being called an isocolloid. 



As early as 1907, one of the founding scientists of colloid chemistry, 

 P. P. von Weimarn, proposed a somewhat similar concept, and it has 

 since been discussed by Alexander in America, Klyatchko in Russia, 

 and Yoshida in Japan. The present micellar theory, published in 

 1949, differs in certain respects from those earlier described. It was 

 designed specifically to explain the problem of imperfection structure 

 in the solid state. 



Without going into any of its technical details, the theory can be 

 described as postulating the formation of clusters of atoms (or mole- 

 cules) in the homogeneous liquid state as the result of a balance among 

 four principal thermodynamic variables: 1, temperature; 2, pressure; 

 3, composition; and 4, surface tension. The net result is the produc- 

 tion of a liquid which in effect is a mass of tiny solid particles swim- 

 ming in their own debris. The size and form of the particles are 

 determined by thermodynamic and crystallographic factors. When 

 the temperature is reduced to what is known as the freezing point, 

 these minute crystallites attach to one another, orienting their own 

 atomic alignments with respect to one another as far as allowed by 

 the freezing conditions, and thus form the solid. The mosaic block 

 is now the micelle ; and the subdivisional structure is the result of the 

 persisting micelle boundaries. 



Returning to fractography, plate 3, figure 2, shows the pattern of 

 a fracture which passed through a grain of cast molybdenum — in 



