THE METALDOGRiAiPHY OF METEO'RIC IROiN' 



57 



Vogel's theory. — The foregoing outline of the process reflects the 

 current prevailing views of metallographers and is consistent Vvith 

 the early iron-nickel equilibrium diagram of Osmond and Cartaud 

 (fig. 4), upon which the now generally accepted diagrams are based, 

 which postulated the existence of two phases although at that time 

 two phases had not been produced in artificial alloys. That was 

 accomplished first by Benedicks (1910), who succeeded in producing 

 characteristic Widmanstatten structures, in some cases visible to 

 the eye. A macroscopic structure of great perfection was recently 

 produced by Mehl and Derge (1937). Vogel, however (1925, 1927), 

 did not accept the production of an artificial Widmanstatten struc- 

 ture as confirmation of Cartaud's hypothesis, arguing that the exist- 



r 



o 



o 



a: 



a: 



UJ 

 Q. 



LU 



1500 



1400 



20 

 PER CENT 



30 



NiCKEL 



Figure 9. — The delta-gamma transformation in nickel-iron. 



ence of a eutectoid horizontal line extending from 6 to 30 percent 

 nickel, which he deemed essential to that theory, had not been estab- 

 lished. He ascribed the origin of the structure (both natural and 

 artificial) not to the gamma-alpha transformation but to an earlier 

 delta-gamma transformation. J^igure 9 shows the range in which 

 this transformation takes place, which is brief and at a very high 

 temperature. 



The Vogel mechanism. — In artificial irons containing carbon the 

 delta range between the melt and gamma phase is small and its 

 effects are negligible; but in iron-nickel alloys its width is greater 

 and it extends to about 30 percent nickel. If the percentage of 

 nickel is more than that, the alloy passes directly from the melt to 

 the gamma phase. 



507394 — 44 5 



