Experimental and theoretical evidence for carbon-vacancy binding in austenite

Our research and results from the literature all consistently suggest a binding energy of nearest-neighbor carbon-vacancy (C-V) pairs of the order 35 to 40 kJ/mole in austenitic alloys. Results examined include point-defect anelasticity, self-diffusion, high-temperature creep, strain aging, strain-age hardening, radiation damage, and point-defect structure modeling. Increases in the height of carbon-based anelastic peaks by quenching, cold work, and electron irradiation are consistent with a substantial nonexclusive contribution of C-V complexes. Increased carbon content in austenite increases the iron self-diffusivity and the high-temperature creep rate of fcc Fe, implying a C-V binding energy of ∼40 kJ/mol. Dynamic strain aging of carbon-containing austenites occurs in temperature ranges too low to involve interstitial solute mobility and requires an interpretation of large C-V binding wherein the vacancy is the more mobile component. Strengthening in heavily deformed austenitic stainless steels associated with strain aging or long-term aging near room temperature implies solution hardening by tetragonal-like C-V complexes formed at these temperatures. Results on radiation damage of austenitic steels show effects of carbon on irradiation susceptibility. Finally, we have performed first-principles gradient-corrected density functional calculations to determine the binding energy of nearest-neighbor C-V pairs in fcc iron; a value of ∼35 kJ/mol is obtained. This article is based on a presentation in the symposium “Terence E. Mitchell Symposium on the Magic of Materials: Structures and Properties” from the TMS Annual Meeting in San Diego, CA in March 2003.