32 BULIvETTN 1S4, UNITED STATES NATIONAL MUSEIUM 



mixture of the two phases M and N. Since these phases must 

 remain in equilibrium, as the temperature falls the proportion of the 

 N phase must increase in comparison with the M phase. The lower- 

 ing of temperature also is marked by a progressive change in the 

 composition of both the newly formed N phase and of the gradually 

 disappearing M phase. 



Just below 700° two phases coexist — the N phase of the compo- 

 sition a (richer in X) and the M phase of the composition b (richer 

 in Y), the mixture being almost wholly in the M phase. 



At 600° these compositions have shifted respectively to c and d, 

 the two phases about equally divided; at 500° to e and /, the N phase 

 now strongly predominating. 



At 400° the transformation is completed; the alloy is now wholly 

 in the N phase, the last disappearing traces of the M phase having 

 the composition h. From this point downward the alloy remains 

 in the N phase. 



With the proportions Q, the transformation begins just below 

 600°, and the same shifting of phase compositions ensues, until at 

 200° the last of M disappears and the alloy is whollj'- in the N phase. 

 With the composition R, the M-N transformation begins a little 

 above 400° and there still is a mixture of M and N at room temper- 

 atm-e. With the proportions S there is no transformation, the alloy 

 being in the M phase throughout, from the melt to room temperature. 



The foregomg outline describes ideal equilibrium conditions, 

 which are never wholly attained. 



VII. THE IRON-CARBON SYSTEM 



The iron-carbon diagram. — Figm^e 3 represents in simplified form 

 the equilibrium diagram of iron- carbon alloys. It omits the brief 

 delta-gamma transformation (from 1,530° to 1,400° and confined 

 to irons with less than 0.35 percent carbon), and it also omits the 

 debatable beta phase, which has no practical application in this 

 discussion, showing only the gamma-alpha transformation. The 

 corresponding transformation in nickel-iron alloys produces aU the 

 primary structures of meteoric irons. 



As shown by the diagram, an iron containing carbon instead of 

 solidifying directly from the melt, as does pm-e iron, passes through 

 a transitional zone ACB, in which the liquid is mingled with crystals 

 of a solid solution of carbon in gamma iron (termed in metallography 

 austenite) in a pasty or mushy state. If it contains less than 0.80 

 percent carbon, on passing the AC line (and now solid) it is in the 

 gamma phase (austenite). On crossing the A3 line at DE it enters 

 another transitional zone DEF, in which the solid solution is a mix- 

 ture of gamma and alpha phases. Finally at FE, the Ai line, it has 

 completely changed into alpha iron (ferrite) plus FcaC (cementite). 



