66 BITLLETIN 184, imiTED STATES NATIONAl. MTJiSEiUM 



is an alpha-gamma mixture formed during transformation — a coarse 

 structure if formed in a high thermal range, an extremely fine struc- 

 ture if formed at a low temperature. Plessite fields are remnants 

 of the gamma phase left in the interstices of the octahedral network, 

 and their structure was developed in the solid state. 



Coarse plessite prevails in the coarser octahedrites, being excep- 

 tionally found in the fine ones and never in the finest. The coarse 

 and perfect separation of kamacite and taenite, the former usually 

 predominating, reflects a low or moderate nickel content, and there- 

 fore transformation in a relatively high thermal range where atomic 

 mobility makes possible a rapid and complete separation. 



The absence of a marked taenite border along the interface of 

 coarse plessite fields (e.g., Seelasgen, Kenton County, Bear Creek, 

 Smith's Mountain, Roebourne) also is an indication of formation 

 at a relatively high temperature, the segregation having been so 

 complete as to leave little residual gamma phase in the surrounding 

 mass to form a border later. In the fine and finest octahedrites, 

 which being nickel-rich transformed at much lower temperatures, 

 the fine to dense plessite fields have distinct, often wide, taenite 

 borders. 



Vogel's views. — ^Vogel, who held that the Widmanstatten structure 

 originated immediately after the primary solidification, assigned the 

 formation of light plessite to the same cause. His original explana- 

 tion of dense plessite (1927) was that it is identical with dense 

 ataxites and that its structure is due largely to the presence of phos- 

 phorus — that is, that it is a eutectoid representing the equilibrium 

 of a ternary iron-nickel-phosphide alloy with more than 0.2 percent 

 of phosphorus. (According to his view there could not be such an 

 equilibrium of two phases in a binary iron-nickel alloy.) This 

 explanation seems untenable, for dense plessite is found in irons 

 that are nearly or wholly phosphorus-free — e. g., San Francisco 

 Mountains, which has none. 



In a subsequent treatment of the subject (1932) he ascribed the 

 origin of dense plessite to transformation in the solid state, a second- 

 ary crystallization due to the gamma-alpha transformation. Accord- 

 ing to that view the very fine structure, which shows black with 

 ordinary etching but is not affected by sodium picrate, represents an 

 alpha-gamma mixture — not a two-phase equilibrium structure, but 

 an intimate mixture of gamma and alpha solid solutions, sometimes 

 with transformation figures (gamma needles in alpha kamacite); 

 and that it indicates spots not of phosphorus but of nickel enrichment. 



This view the author also regards as untenable, for the existence 

 of two phases in artificial irons has been proved, and the structures in 

 both plessite and ataxites are consistent with that fact. 



