THE METALLOGRAPHY OF METEORIC IRON 



41 



homogeneous alpha (kamacite), whereas at all higher percentages it 

 shows a two-phase structure of alpha and gamma (taenite) . 



Because of the marked hysteresis or lag in the transformation of 

 iron-nickel alloys, there is a considerable thermal range between the 

 beginning and the end of transfonnation, either in heating or cooling. 

 (See figure 6.) It is therefore impossible to determine the actual 

 equilibriiun temperature line for iron-nickel alloys. It is possible 

 in iron-carbon alloys, in which hysteresis is slight and the gap is 

 practically negligible. Figure 5 indicates the range of transfonna- 

 tion in cooling. As shown in figure 6, transformation in heating 

 begins and ends at higher temperatures. 



5 10 15 20 



PERCENT NICKEL 

 Figure 5.— Eclectic iron-nickel diagram. 



Comparisons with the iron-carbon diagram. — Comparing this dia- 

 gram with that of the iron-carbon system, several differences are 

 apparent of which the most striking are these: 



1. Whereas the iron-carbon system ends at 9.60 percent carbon, 

 that being its saturation point in melted iron, the iron-nickel diagram 

 (if presented fully) would extend to 100 percent nickel, there being 

 no saturation point for nickel m iron. 



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