30 BULLETIN 18 4, UNITED STATE'S NATIONAL IMUSEUM 



True eutectoid structures do not occur in meteoric iron, but there 

 are structures that approximate them, for which the author suggests 

 the designation paraeutectoid. 



Plates A, B, and C show the eutectic and eutectoid structures of 

 artificial iron-carbon alloys, and many examples of paraeutectoid 

 structures will be found among the illustrations of nickel-rich meteoric 

 irons. 



Phase changes in alloys. — It has already been pointed out that an 

 alloy, unlike a pure metal, does not solidify instantly on cooling but 

 passes through a range in which the liquid and solid phases coexist. 

 There are differences in the other phase changes of an alloy, as com- 

 pared with those of a pure metal. For example, pure iron on cooling 

 passes directly from the delta to the gamma phase, and from the 

 gamma to the alpha. An alloy of iron, on the other hand, does not 

 change directly from one phase to the next but at each such trans- 

 formation passes tlirough a thermal range in which both of the 

 phases temporarily coexist. This range is sometimes termed a 

 miscibility gap (Mischungsliicke). The position and the width of 

 such a range or gap vary with the proportions of the alloying 

 components. 



Phase equilibrium. — The fact that phase changes in an alloy are 

 different from those in a pure metal is the basis of the "equilibrium 

 diagram," an understanding of which is essential to the study of 

 metallography. 



An alloy is stable when either its temperature or the proportion 

 of its components may vary without producing any structural change. 

 But there are certain intermediate thermal ranges in which (if it be 

 a binary alloy) two phases coexist — one rich in the component X, 

 the other in the component Y. In such a range the alloy is unstable, 

 for a variation in either temperature or composition causes a change 

 in the two phases and therefore in the structure. 



Equilibrium in an alloy has been defined as a state of rest, in 

 which neither chemical change nor atomic transformation is taking 

 place. It would be more strictly accurate to say that the alloy is 

 in equilibrium when the tendency to the separation of the two phases 

 is in balance. Thus in an unstable range there is equilibrium at 

 any given point representing a certain temperature and composition. 



With an alloy of a given composition, in which two phases are 

 present, the composition of the phases must vary as the temperature 

 varies, in order that the two may at all times be in equilibrium. 

 The beginning and the end of the transformation mark the thermal 

 range within which such a shifting of phase compositions takes 

 place. 



