THE METALLOGKAPHT OF METEiORIC IROiN" 



31 



Equilibrium diagrams. — -As the positions of the various critical 

 points in an alloy vary according to the proportion of its components, 

 these ranges of temperature vary in width as the composition changes. 

 An equilibrium diagram shows the phase corresponding with any 

 given combination of temperature and composition. It is con- 

 structed by determining all the critical points for a given alloy in 

 all proportions of composition, then plotting them to form a diagram 

 in which the ordinates represent temperatures and the abscissae the 

 percentages of the components. Lines are then drawn connecting 

 all the Ai points, A3 points, etc. Such diagrams are also called 

 constitution diagrams. 



1200 



1000 



800 



< 600 



400 



200 



X 100 

 Y 



90 80 70 



10 20 30 



PERCENT OF X AND Y 

 Figure 2. — Schematic equilibrium diagram. 



60 



40 



50 

 50 



Figur6 2 is an equilibrium diagram in its simplest form for a hypo- 

 thetical alloy of X and Y, 9 binary alloy, having two phases M and 

 N, showing the position of the transformation points for all tempera- 

 tures and compositions. The line AJ represents the beginning of 

 the M-N transformation, the line AK the end of it. 



Traiisformatiov of a typical alloy. — Assume that the alloy in the 

 proportions P has solidified at some temperatm-e above 1,000°. It 

 is now a solid solution of X and Y in the M phase. On further cool- 

 ing it continues unchanged to 700° where the M-N transformation 

 starts. It has been homogeneous, but now it starts to become a 



