  TThe 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
6th 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 1st NN sites of
a C atom in an octahedral site introduce asymmetry into the minimum energy
diffusion pathway, causing up to about 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 are C diffusion accelerators, while Cu
decelerates C diffusion in austenite. Among the four ferrite stabilizers, Si is
a C diffusion accelerator, while V and Ti serve as C diffusion decelerators in
ferrite. Aluminum has no significant effect on C's diffusivity, while Mo and Cr
decelerate C diffusion.
