THE METALLOGRAPHY OF METEORIC IRON 



27 



panied by reciystallization, the gamma grains changing into smaller 

 alpha grains. This is the last structural change, the iron now hav- 

 ing reached its final microscopic structure; but at 760° a further 

 change in physical properties takes place, the iron becoming ferro- 

 magnetic. These processes are indicated diagrammatically in figure 1. 



c. 



1600 -n 



■l53Cf SOLIDIFICATION BEGINS, BODY -CENTERED 

 DELTA CRYSTALS FORMING FROM THE 

 MELT. 



l400Trl4OO'* DELTA-GAMMA TRANSFORMATION; BODY- 

 CENTERED DELTA CRYSTALS BECOME FACE- 

 CENTERED GAMMA IRON. 



1200 



1000 



800 



600-^ 



- 910° GAMMA- ALPHA TRANSFORMATION; FACE- 

 CENTERED GAMMA GRAINS CHANGE TO 

 BODY-CENTERED ALPHA GRAINS, THE 

 FINAL STRUCTURE. 



760° IRON BECOMES MAGNETIC, BUT NO 

 CHANGE IN MICROSTRUCTURE. 



Figure 1. — Solidification of pure iron. 



Thermal critical points. — Each of these transformations corresponds 

 with a thermal critical point — that is, a point where the uniform 

 rate of cooling is briefly retarded, after which it resumes its regular 

 rate of progress. These points are designated, in doAvnward order. 



