564 G. H. Mathewson — -Metallo graphic Description 



ously homogenize at atmospheric temperatures, even after long 

 periods of time. Garland, 1. c. (2, second paper), p. 331, 

 describes conditions of this sort affecting Egyptian objects some 

 3000 years old. Many of the present Peruvian bronzes have 

 likewise retained their initial inhomogeneity through the 

 centuries which must have elapsed since their preparation. 



Thorough investigation of the diffusional properties of a 

 group of alloys is, in itself, a task of some magnitude. Since 

 time could not be spared for the complete experimental correla- 

 tion of time, temperature and concentration in these alloys, it 

 was necessary to make certain approximations and rely upon 

 a moderate amount of suitably selected experimental work. 



A cast bronze of the present type is composed of grains, each 

 of which possesses an internal dendritic structure representing 

 its development from the original nucleus by thickening, extend- 

 ing and branching from the primary stem. The average 

 thickness of a stem or branch, as well as the difference in con- 

 centration from center to edge, is determined mainly by the 

 rate at which the whole mass solidifies and this is, in turn, 

 determined mainly by the relations between the mass of metal, 

 its temperature at the moment of pouring and the temperature, 

 size, configuration, and heat conductivity of the mold. When 

 the alloy solidifies rapidly, the numerous nuclei, or centers 

 of crystallization, mutually interfere before they can develop 

 to great size; the completed grains will, therefore, be small 

 and their internal structure fine. On the other hand, when 

 cooling is more gradual, fewer nuclei are formed and these 

 grow to greater size and show coarser internal structure. It 

 is true that other factors; supercooling, convection currents, 

 etc., may influence the final structure of a cast alloy. More- 

 over, we know very little about the actual variation in internal 

 concentration from point to point in specimens produced under 

 different conditions. Gulliver (26) (27), in two papers pre- 

 sented before the Institute of Metals, has dealt quantitatively 

 with the concentration changes which occur during solidification 

 with incomplete adjustment of equilibrium in alloys of the solid 

 solution type, but conditions are highly complicated and it does 

 not seem possible to make any practical use of his deductions 

 in the present case. As a matter of fact, the temperature-time 

 data of homogenization may be fairly well defined on the basis 

 of the average distance between centers of adjacent dendritic 

 branches,* without knowledge as to the average difference in 



* This gives an approximate measure of the distance which must be 

 covered in the diffusional migration of the molecules. 



