542 C. H. Mathewson — Metallograpliic Description 



tion values, as well as the temperatures of the two horizontals, 

 are written directly into the diagram.. Bronzes which, under 

 equilibrium conditions, should be composed exclusively of the 

 alpha constituent immediately after solidification invariably 

 show a zonal structure after ordinary casting and, even when 

 the tin-content is less than 2 per cent, still contain liquid metal 

 when the lowest temperature of alpha crystallization is reached, 

 whereupon solidification is completed in the form of a 

 secondary, or beta constituent. 



The minute detail of the transformations which occur in this 

 beta constituent on further cooling cannot be fully described 

 at present. The principal features are, however, well under- 

 stood and corresponding effects upon the working properties 

 of the metal are not difficult to determine. Probably most of 

 the unstable beta constituent, whose presence in these alloys 

 is due to rapid solidification, is formed in the normal manner 

 by reaction between the residual liquid (after crystallization 

 of alpha) and the neighboring alpha constituent. This would 

 yield a beta of minimum tin-content (22), at least along the 

 line of contact with alpha. The interior of this constituent 

 may be protected from reaction with alpha (by envelopment), 

 in which case, its tin-content will be higher. In view of the 

 facts that the entire beta is small in amount, that it must form 

 within the enveloping alpha, high in copper, and that its 

 corresponding solidus and liquidus concentrations are not far 

 removed from one another, it does not appear that the tin-con- 

 tent at any point, in spite of rapid cooling, can be high enough 

 to permit the separation of anything but alpha (along the 

 line 22-28) on subsequent cooling. During such cooling, the 

 prevailing tendencies are (1) for the enveloping alpha to absorb 

 the beta and to homogenize generally, (2) for the copper-rich 

 beta at the boundaries to separate alpha according to the 

 curve 22-28, and (3) for the peripheral and central portions 

 of the beta to effect a concentration adjustment. However 

 these individual tendencies may operate in conjunction, when 

 the temperature has fallen to 500°, or thereabouts, (4) the 

 residual beta of eutectoid composition will undergo transforma- 

 tion into alpha and delta of distinct eutectoid appearance. 

 Such a structure is represented by fig. 43. The white delta is 

 considerably more abundant than the dotted enclosures (dark) 

 of alpha. The present experiments have shown that this trans- 

 formation always occurs so as to produce a distinct eutectoid 

 structure in small chill-cast specimens which must have cooled 



