58 BITLLETIN 18 4, UNTTTED STATES NATIOiNiAX, MUSEUM 



According to Vogel, in the range from to 35 percent nickel solid- 

 ification begins on the line AS. When an alloy of the composition 

 of octahedrites (6 to 14 percent nickel) cools to the line MS a heter- 

 ogeneous structure begins to develop; the primary delta solid solution 

 separates into two components in equilibrium — a nickel-poor delta 

 solid solution of the composition M (kamacite) and a nickel-rich 

 (taenite) of the composition S, with about 35 percent nickel. Later 

 the two components in equilibrium have the compositions x and y, 

 and after passing the line BN the alloy is wholly gamma. 



Thus, according to his view, the delta iron, possessing the same 

 crystalline structure as alpha kamacite, crystallizes directly from 

 the melt in dendrites, growing into skeletal forms, which in the delta- 

 gamma range become lamellae having octahedral planes with the 

 taenite segregating between them, producing the Widmanstatten 

 structure. 



On further cooling the delta completely transforms into the gamma 

 phase with a remnant dendritic pattern and the alloy again becomes 

 homogeneous if cooling is slow. But if sufficiently rapid, Vogel 

 held, the structure may be preserved. 



Objections to Vogel's theory. — Meteoric irons, however, cannot be 

 assumed to have generally undergone quick cooling. Many fea- 

 tures of iheir structure, its coarseness, and especially the unigrain 

 character of large masses, indicate extremely slow, even secular, 

 cooling. 



On other grounds also Vogel's theory does not seem acceptable. 

 It is based upon the fact that all the earlier iron-nickel diagrams 

 (before the delta-gamma transformation was established) indicated 

 that an artificial iron-nickel alloy, regardless of its proportions, 

 passes from one phase to another without any miscibility gap — in 

 other words, the alloy always consists of one structural component. 

 Thus the presence of two distinct components in the octahedral 

 structure, representing two coexisting phases, was not explainable. 

 The delta-gamma range, in which two phases were found to coexist, 

 seemed to Vogel to furnish the answer. 



It has been established, however, that two phases do coexist in the 

 gamma-alpha range in artificial iron-nickel alloys. In meteoric irons 

 the high and low nickel components are seen to produce a regular 

 paraeutectoid structure in nickel-rich ataxites. Such structures clearly 

 cannot arise in the delta-gamma range, nor can they be explained by 

 the influence of phosphorus (in accordance with an opinion expressed 

 by Vogel in 1927) because such structures are highly developed in 

 irons that are practically P-free (Deep Springs 0.06 percent, Iquique 

 0.05-0.07 percent P). 



Vogel's view was rejected by Kase (1925) soon after it was an- 



