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

TL;DR

This study uses density functional theory to analyze how various alloying elements affect carbon diffusion pathways and energies in austenite and ferrite phases, revealing element-specific impacts on diffusion rates.

Contribution

It provides a detailed first-principles analysis of how different alloying elements influence carbon migration energies and pathways in steel phases, highlighting their roles in diffusion control.

Findings

Attractive binding elements increase carbon migration energy barriers.

Repulsive binding elements decrease migration barriers, accelerating diffusion.

Ni and Mn accelerate C diffusion in austenite; Cu decelerates it.

Abstract

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.