THE METALLOGRAPHY OF METEORIC IRON 29 



Most structures, both in natural and artificial irons, develop in 

 the solid state, but at temperatures high enough to give the solid 

 solution the necessary atomic mobility. When the temperature 

 has fallen to a point where diffusion practically ceases, structural 

 changes also practically cease. 



Solidification of alloys. — At its melting (or freezing) point a pure 

 metal passes directly from the liquid to the solid state. The same 

 is not true of an alloy, that temperature only marking the point 

 where it becomes completely liquid on heating, and where solidification 

 begins on cooling. 



At the beginning of solidification the liquid alloy becomes pasty 

 from the incipient formation of crystals. The line where this proc- 

 ess begins, and the line where it ends and the alloy has become 

 completely solid, are termed respectively the liquidus and the solidus. 



As the mass continues to cool, though it has become solid, it is 

 not yet rigid, and its continued atomic mobility makes possible 

 various changes and rearrangements of its crystalline structure. It 

 is to this process that the varied structures of meteoric iron are 

 chiefly due. 



Eutectics; eutectoids. — An alloy solidifies at varying temperatures, 

 according to the proportion of its components. A certain propor- 

 tion results in the lowest fusion point, often much lower than that 

 of either of its components. For example, silver melts at 962° and 

 copper at 1,084°, but an alloy of 72 percent silver and 28 percent 

 copper melts at 778°. This lowest fusion point for a given alloy is 

 termed its eutectic temperatiu-e, and the combination in that pro- 

 portion is called the eutectic of its components. 



For iron and carbon the eutectic proportion of carbon is 4.3 per- 

 cent, and an iron-carbon alloy of that composition solidifies at 1,130°. 

 It is called eutectic cast iron (pi. C). 



The terms eutectic and eutectoid should not be confused. A 

 eutectic arises directly from the melt; it marks the eutectic tempera- 

 ture — the point where the alloy passes directly from the liquid to 

 the solid state without going through any intermediate phase change. 

 A eutectoid arises in the solid state, the product of a rearrangement 

 of the components by atomic diffusion. 



In each case the process is completed at a certain fixed tempera- 

 ture, and the structure consists of two components with definite 

 compositions. Indeed, the two structures are essentially the same, 

 except for the fact that the eutectoid arises in the solid state. 



While the eutectic point for iron and carbon is 1,130° and the 

 eutectic ratio 4.3 percent carbon, the eutectoid temperature is 723° 

 and the eutectoid ratio 0.80 percent carbon. The eutectoid struc- 

 ture produced at that temperature is called pearlite, a very finely 

 laminated aggregate of ferrite and cementite (pi. A). 



