On the Ordered Arrangement of Interstitial Atoms in Interstitial Solid Solution. I. The Equilibrium between Supersaturated Ferrite and Martensite in Iron-Carbon Alloys

Abstract

The concept of superlattice which had generally been considered only in the substitutional solid solution was extended to the case of the interstitial solid solution. What becomes the object of the ordered arrangement here is the distribution of interstitial atoms among the interstitial sites. Since for such an atom not only the scattering power of X-rays is small, but also the solubility range is narrow, the effect of distributional change of interstitial solute atoms cannot easily be detected only with experimental methods and theoretical interpretation is indispensable for such a problem. Here the case of α-Fe including C atoms was treated as an example of the case for b.c.c. lattice. Martensite obtained by quenching Austenite is a sort of superlattice in which all C atoms are existing on only one sort of (\frac12,\frac12,0) sites among three sorts of such sites. To discuss the equilibrium between this and supersaturated ferrite with carbon atoms distributed at random among three sorts of sites is important also for the thermodynamics of Martensite and the interpretation of the mechanism of low temperature tempering of Martensite, etc. The results are as follows. (1) Martensite is more stable than supersaturated ferrite at low temperatures. (2) The transition point between these two states rises lineary with the carbon content and this line meets the room temperature at about 0.2%C. (3) The preferential distribution of C atoms, and consequently the tetragonality, appear spontaneously. These effects vanish discontinuously at the transition point if no stress is produced. From these results some interesting conclusions were drawn.