Hydrogen Embrittlement at Cleavage Planes and Grain Boundaries in Bcc Iron-Revisiting the First-Principles Cohesive Zone Model.

Abril Azócar Guzmán,Rebecca Janisch,Alexander Hartmaier,Jeongwook Jeon

doi:10.3390/ma13245785

Abstract

Hydrogen embrittlement, which severely affects structural materials such as steel, comprises several mechanisms at the atomic level. One of them is hydrogen enhanced decohesion (HEDE), the phenomenon of H accumulation between cleavage planes, where it reduces the interplanar cohesion. Grain boundaries are expected to play a significant role for HEDE, since they act as trapping sites for hydrogen. To elucidate this mechanism, we present the results of first-principles studies of the H effect on the cohesive strength of α-Fe single crystal (001) and (111) cleavage planes, as well as on the Σ5(310)[001] and Σ3(112)[10] symmetrical tilt grain boundaries. The calculated results show that, within the studied range of concentrations, the single crystal cleavage planes are much more sensitive to a change in H concentration than the grain boundaries. Since there are two main types of procedures to perform ab initio tensile tests, different in whether or not to allow the relaxation of atomic positions, which can affect the quantitative and qualitative results, these methods are revisited to determine their effect on the predicted cohesive strength of segregated interfaces.

Highlights

Hydrogen embrittlement (HE) is one of the main challenges in modern materials science, notably affecting structural materials, such as iron and steel
The excess energy from the decohesion in the (111) and (001) cleavage planes is illustrated in Figure 5a, including the corresponding universal binding curves
The cohesive strength of the Σ5 is reduced by 3% at a coverage of 0.08 atom/Å2 and the one of the Σ3 symmetrical tilt grain boundary (STGB) by 6% at 0.05 atom/Å2

Summary

Introduction

Hydrogen embrittlement (HE) is one of the main challenges in modern materials science, notably affecting structural materials, such as iron and steel. An important mechanism is hydrogen enhanced decohesion (HEDE), which has been observed in ferritic microstructures [1]. Decohesion is defined as a sequential tensile separation of atoms ahead of a crack tip when a critical crack-tip-opening displacement (in the order of half the interatomic spacing) is reached. In the HEDE process, hydrogen accumulates between crystallographic planes, for example, due to strain fields, or by segregation to interfaces such as grain boundaries (GBs). Atomistic studies are the method of choice to enhance our understanding of HEDE. They have not produced a clear picture of intraand intergranular HEDE

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