THE METALLOGRAPHY OF METEORIC IROiN" 



37 



Whereas the saturation of cementite m iron occurs at a point 

 corresponding with about 0.01 percent carbon, the saturation of 

 taenite in kamacite at room temperature is at a point corresponding 

 with about 6 percent nickel. Therefore a meteoric iron with less 

 than 6 percent nickel (kamacite) is a homogeneous solid solution 

 analogous to an artificial iron containing less than 0.01 percent of 

 carbon and, like it, is structureless except for granulation. 



Early iron-nickel diagrams. — The equilibrium diagram of the iron- 

 nickel system has been a subject for conflicting views, and even now 

 it is not free from doubt. 



The origin of a two-phase iron-nickel structure by transformation 



10 



20 



30 40 



PERCENT NICKEL 



50 



60 



Figure 4. — Early iron-nickel diagram. 



in the solid state was first made clear in the classic diagram of Osmond 

 and Cartaud (1904), modified by Osmond and Roozeboom in 1911. 

 The diagram is shown in simplified form in figure 4, embodying the 

 temperatures of the earlier diagram and the nickel percentages of 

 the later one. Comparing this diagram with the iron-carbon dia- 

 gram, we may note that the line AC, marking the beginning of the 

 gamma-alpha transformation, is much lower and ends at about 350° 

 instead of 723.° 



An alloy with 5 percent nickel in cooling would enter the gamma- 

 alpha range at a little above 700° and the alpha phase would begin to 

 separate from the gamma phase, producing a heterogeneous structure. 

 Gamma would first predominate, the proportion of alpha increasing 



