A study was made of the thermodynamic stability of dilute C, N, O, B and C+N interstitial distributions in body-centered cubic iron, combining parameter-free density functional theory in the generalized gradient approximation and microscopic elasticity theory. This scheme permitted the complete capture of the long-range elastic impurity-impurity interactions using moderately sized density functional theory calculations. Employing this approach, temperature-concentration phase diagrams were computed which included the effects of external pressure and provided direct insight into the formation mechanisms of martensite. For all of the investigated impurities, except B, tetragonal states were predicted to be preferred even at low impurity concentrations. The preference was shown to originate from a thermodynamically driven orientational ordering of the interstitials.

Orientational Ordering of Interstitial Atoms and Martensite Formation in Dilute Fe-Based Solid Solutions. A.Udyansky, J.von Pezold, A.Dick, J.Neugebauer: Physical Review B, 2011, 83[18], 184112