196 DR. S. W. J. SMITH AND MR. J. GUILD: A THERMOMAGNETIC STUDY OF 



Although the eutectoid disappears at the same temperature as in the hypo- 

 eutectoid steels of those figures, the reappearance now occurs at a much higher 

 temperature. It is interesting to notice also that there is now no appreciable return 

 of magnetism prior to the crystallisation of the eutectoid. 



The process of crystallisation in such an alloy may be regarded as follows : 

 Suppose that the alloy, still at a temperature above that at which the excess carbide 

 begins to deposit, is cooling down. Eventually it will reach the temperature at 

 which it could exist in equilibrium with free carbide if the latter were present 

 in bulk. Precipitation will not begin at this temperature, however, if there is 

 appreciable surface energy between carbide crystals and solid solution. 



Every re-crystallisation is characterised by the fact that, in the minute crystals 

 first formed, the ratio of surface area to mass is relatively very great. Consequently 

 the surface energy may be an appreciable fraction of the total energy of these 

 crystals. This surface energy virtually increases the chemical potential of the 

 separated material and therefore tends to make the temperature at which crystals 

 can form, in a given solution, lower than it would be otherwise. 



The disturbing effects of surface energy become less important as the crystals 

 enlarge. Hence crystallisation, once begun, tends to proceed around the nuclei first 

 deposited. At the same time the concentration in carbide of the solution in contact 

 with the crystals approaches the equilibrium value. 



This value decreases as the temperature falls, and is attained by deposition from 

 the solution upon the crystals. As before, owing to the slowness of diffusion, the 

 equilibrium will not be complete. The pai-ts of the solution remote from the crystals 

 will be too rich in carbide unless the rate of cooling is infinitely slow. 



When the eutectoid temperature is reached the solution in contact with the 

 crystals will have the eutectoid composition ; but the eutectoid will not form without 

 lag of a similar nature to that which delayed the appearance of the carbide. 



In this case the lag will be due mainly to the iron, since carbide crystals are 

 already present. 



15. An Effect Due to the Excess Carbide. 



Indirectly the presence of the carbide crystals will accelerate the crystallisation of 

 the iron. For when the temperature falls below the eutectoid point the carbide 

 continues to deposit and the solution in contact with the crystals continues to get 

 richer in iron. The chemical potential of this iron therefore diminishes less rapidly 

 as the temperature falls than it would do if carbide crystals were not present. Hence 

 the amount by which this potential exceeds that of the iron crystals in bulk at any 

 temperature below the eutectoid point is greater than it would be in the absence of 

 the carbide. Therefore the resistance to the formation of the iron crystals and hence 

 of the eutectoid will be overcome at a higher temperature when carbide crystals are 

 present beforehand, as in the case of fig. 9, than in the eutectoid steel. 



