42 



BULLETIN 18 4, UNITED STATES NATIONAL MUSEtUM 



2. The structural changes, which m artificial irons are confined 

 to a very limited range of carbon content, extend from 6 to 30 per- 

 cent nickel. 



3. The line AB, marking the beginning of the gamma-alpha inver- 

 sion, is greatly depressed. 



4. The line AC is still more depressed, thus leaving a much wider 

 gap than in the iron-carbon diagram. 



The lines Ai and A3 in the iron-carbon diagram do not appear in 

 the iron-nickel diagram, which shows only the beginning and the end 

 of the gamma-alpha transformation. 



Hysteresis increased. — In pure iron the critical points mark 

 practically definite temperatures; but in alloys there is a more or 



1000 



800 



.600 





400 



200 



10 20 30 



PER CENT NICKEL 



Figure 6. — Iron-nickel constitution diagram. 



40 



less pronounced lag (hysteresis) in the transformation, which takes 

 place somewhat above the normal pomts on heating and somewhat 

 below them on cooling. 



Hysteresis is comparatively small in the iron-carbon system, but 

 is greatly increased by the presence of nickel. In an iron-nickel alloy 

 it becomes very pronounced, as shown in figure 6, which indicates 

 relative positions of the gamma-alpha range on heating and on cool- 

 ing. The lines AB and AC mark the beginning and the end of the 

 gamma-alpha transformation on cooling, the shaded area marking 

 the position of the corresponding range on heating. 



