Effects of alloying elements on carbon diffusion in the austenite (f.c.c.)- and ferrite (b.c.c.)-phases

The effects of alloying elements on diffusion pathways and migration energies of interstitial carbon in austenite (f.c.c.) and ferrite (b.c.c.) are studied using density functional theory first-principles calculations. The binding energies between carbon and alloying elements are determined through 6 th nearest-neighbor (NN) distances. The elements studied are Ni, Mo, V, Cr, Mn, Cu, Al, Ti, and Si, relevant to most high-strength steels. Nickel, Mn, Al, and Si have repulsive binding energies; Mo, V, Cr, Cu, and Ti have attractive binding energies in austenite and ferrite. Alloying elements at 1 st NN sites of a C atom in an octahedral site introduce asymmetry into the minimum energy diffusion pathway, causing up to ∼1 eV changes in saddle-point energies. This pathway goes from one octahedral site to another via intermediate energy states, differing for austenite and ferrite. We find that the elements with attractive binding energies increase the energy barrier for C migration resulting in decelerated carbon diffusion, while the elements with repulsive binding energies decrease the energy barrier for C migration leading to accelerated C diffusion. The magnitude of changes in C migration energies is proportional to the binding energies between C and alloying elements. Among the three austenite stabilizers, Ni and Mn reduce the activation energy for carbon diffusion, whereas Cu increases it. Regarding the four ferrite stabilizers, Si raises the activation energy, while V and Ti lower it in ferrite. Aluminum has no significant impact on C's diffusivity, whereas Mo and Cr increase the activation energy for carbon diffusion.