A CHEMICAL INTERSTITIAL ORDER - BASED MODEL FOR NITROGEN STRENGTHENING OF FE-NI-CR AUSTENITE

Among the essential characteristics of the mechanical behavior of nitrogen strengthened austenitic Fe-Ni-Cr alloys are planar slip in all alloys containing more than a small concentration of N over wide ranges of temperature and strain rate and a linear dependence of the athermal flow stress on the mole fraction of nitrogen. These characteristics of the deformation mechanism are analyzed using a model which assumes that in the undeformed, equilibrium crystal, the metal atoms are randomly arranged on the f.c.c. lattice but that the N atoms are distributed among the interstitial sites so as to minimize the free-energy. The flow stress is identified by equating the increase in free energy resulting from the redistribution of the N atoms with the work done in moving a dislocation through the lattice. Decrease in the atomic order, caused by the passage of dislocations, reduces the friction on their glide planes promoting planar slip.