Grain boundary segregation of light elements and their effects on cohesion in ferritic steels

TL;DR

This study provides a comprehensive ab initio analysis of light element segregation at grain boundaries in ferritic steels, revealing their effects on cohesion and embrittlement.

Contribution

It offers a large dataset and analysis of segregation energies and cohesive effects for multiple light elements in ferritic steels using DFT, addressing previous data scarcity.

Findings

B and C enhance grain boundary cohesion at the same concentration.

He, O, S act as strong decohesive agents and embrittlers.

Sampling both substitutional and interstitial sites is crucial for accurate segregation spectra.

Abstract

Light elements play an important role in influencing the macroscale properties of engineering alloys through grain boundary (GB) segregation phenomena. However, the scarcity and scattered nature of ab initio datasets for light elements in steels makes reproduction and extraction of general trends from the literature difficult. Here, we present a comprehensive ab initio evaluation of the segregation energies and cohesive effects for H, He, B, C, N, O, P, S, extensively sampling both substitutional and interstitial sites in six model coincident site lattice (CSL) ferritic iron GBs using density functional theory (DFT). Cohesive effects are evaluated in both a quantum-chemistry bond-order and rigid Rice-Wang interfacial cohesive strength framework. Our calculations indicate that, compared at the same concentration, B and C enhance GB cohesion, N, P, H are mildly detrimental, and He, O, S as powerful decohesive agents/embrittlers. Sampling both interstitial and substitutional starting positions is necessary to accurately capture segregation spectra. Commonly utilised sampling criteria such as site volumes prove insufficient for identifying deepest GB binding sites. Solutes placed in either kind of site can induce large relaxations to the same final configuration, resulting in site classification ambiguity. The nearest neighbour distance of a solute to its neighbours after relaxation is shown to be a controlling factor for the lower threshold of segregation energies at sites. The freely available DFT dataset and analysis repositories are expected to advance understanding of GB segregation behaviours of light elements in steels and serve as a resource for developing machine learning interatomic potentials.