Segregation at prior austenite grain boundaries: the competition between boron and hydrogen

TL;DR

This study explores how boron and hydrogen interact at grain boundaries in martensitic steels, revealing boron's ability to repel hydrogen and its impact on hydrogen trapping and steel failure.

Contribution

It combines experimental and computational methods to elucidate the competition between boron and hydrogen at grain boundaries in steels, highlighting boron's role in hydrogen trapping.

Findings

Boron segregates strongly at prior austenite grain boundaries.

Hydrogen is attracted to grain boundaries but can be repelled by boron.

Hydrogen traps saturate at low concentrations, leading to failure initiation.

Abstract

The interaction between boron and hydrogen at grain boundaries has been investigated experimentally and numerically in boron-doped and boron-free martensitic steels using thermal desorption spectrometry (TDS) and ab initio calculations. The calculations show that boron and hydrogen are attracted to grain boundaries but boron can repel hydrogen. This behavior has also been observed using TDS measurements, with the disappearance of one peak when boron is incorporated into the microstructure. Additionally, the microstructure of both steels has been studied through electron backscattered diffraction, electron channeling contrast imaging, synchrotron X-ray measurements, and atom probe tomography. While they have a similar grain size, grain boundary distribution, and dislocation densities, a pronounced boron segregation into PAGBs is observed for boron-doped steels. Then, the equilibrium hydrogen concentration in different trapping sites has been evaluated using the Langmuir-McLean approximation. This thermodynamic model shows that the distribution of hydrogen is identical for all traps when the total hydrogen concentration is low for boron-free steel. However, when it increases, traps of the lowest segregation energies (mostly PAGBs) are firstly saturated, which promotes failure initiation at this defect type. This finding partially explains why PAGBs are the weakest microstructure feature when martensitic steels are exposed to hydrogen-containing environments.