# Calculation of Austenite Generalized Stacking Fault Energy in M50NiL Steel

## TL;DR

This study calculates how alloying elements and carbon atoms affect the stacking fault energy in M50NiL steel, explaining how these changes influence the formation of sub-micron crystal regions.

## Contribution

A novel GSFE model for austenite in M50NiL steel was developed using first-principles calculations to explore the effects of alloying elements and carbon atoms.

## Key findings

- Alloying elements (excluding nickel) reduce unstable stacking fault energy by weakening Fe–Fe bonds in the L0 layer.
- Interstial carbon atoms near the L0 layer increase unstable stacking fault energy by strengthening Fe–Fe bonds.
- The combined effect of alloying elements and carbon atoms maintains low stacking fault energy and barrier, promoting dislocation slip and austenite stability.

## Abstract

By optimizing the carburizing heat treatment process, the grain size of the carburized layer of M50NiL steel was successfully refined to the sub-micron level. The mechanism for the generation of a large number of sub-micron crystal regions (SMCR) is that dislocations are entangled and linked due to the pinning effect of nanometer-sized carbides. In this study, a stacking fault energy (SFE) model for austenite in M50NiL steel was established. First-principles calculations were employed to investigate the effects of alloying elements, as well as the position and quantity of carbon (C) atoms, on the generalized stacking fault energy (GSFE). The variations in SFE were further analyzed in combination with differential charge density calculations. The simulation results revealed that the addition of alloying elements excluding nickel led to a reduction in the unstable stacking fault energy. Differential charge density analysis indicated that this decrease was associated with the weakening of Fe–Fe bonds in the L0 layer, where stacking faults occurred. When C atoms are interstitially dissolved near the L0 layer, the Fe–Fe bonds near the L0 layer are enhanced, and the unstable stacking fault energy is correspondingly increased. Compared with the pure iron system, the combined effect of alloying elements and C atoms in M50NiL steel maintained a relatively low level of both the unstable stacking fault energy and the stacking fault formation barrier, provided that C atoms were not dissolved in the L1 layer. This condition was favorable for dislocation slip. Meanwhile, the stable stacking fault energy significantly increased, enhancing the stability of austenite. Based on these simulation results, the relationship between the GSFE of austenite in M50NiL steel and the formation of subgrains and twins within the submicron crystalline regions of the carburized layer was discussed.