A first-principles study of Zn induced liquid metal embrittlement at bcc and fcc grain boundaries

TL;DR

This study uses density functional theory to compare how zinc affects liquid metal embrittlement in bcc and fcc grain boundaries, revealing the roles of crystal structure and magnetism in embrittlement processes.

Contribution

It provides a first-principles analysis of Zn-induced embrittlement in different grain boundary structures, highlighting the influence of magnetic disorder and elastic effects.

Findings

Zn segregation is more favorable in bcc grain boundaries.

Magnetic disorder increases the critical concentration for embrittlement.

Surface defect states drive grain boundary weakening at lower Zn concentrations.

Abstract

Zn induced liquid metal embrittlement (LME) is a major concern in particular for advanced high strength steels, which often contain a significant amount of austenite compared to established steel grades. Using density functional theory (DFT) calculations we, therefore, compare the behaviour of Zn in ferrite (bcc) and austenite (fcc) grain boundaries (GBs) with different magnetic ordering to investigate the role of crystal structure as well as magnetism in LME. We address the performance of DFT based paramagnetic calculations by utilizing the spin space averaging relaxation approach. Our results show that both magnetic and elastic contributions have significant influence towards segregation and embrittling behaviour of Zn. The primary requirement is the elastic contribution, while the presence of magnetic disorder increases the critical concentrations for the onset of GB weakening. While Zn segregation is more favourable in bcc compared to fcc GB, larger impact of Zn coverage on GB weakening is observed for fcc. For both structures, the rapid decrease in surface defect state energies is identified as the driving force behind GB weakening. These surface defect states stabilize at lower Zn concentrations than GB defect states.