Strain-rate dependence of hydrogen-induced defects in pure α-iron by positron annihilation lifetime spectroscopy

Abstract

The formation of defects in hydrogen-charged pure α-iron deformed at room temperature at different tensile strain rates was investigated by variable-temperature positron annihilation lifetime spectroscopy to clarify the role of hydrogen-induced defects in driving the hydrogen embrittlement mechanism. A distinct strain-rate dependence of the hydrogen-related defects was observed and a clear difference between the defects formed in hydrogen-embrittled iron and non-embrittled iron was detected. In hydrogen-embrittled slowly strained iron, the low-temperature measurements showed the formation of smaller vacancy clusters than in non-embrittled fast strained iron. With increasing annealing temperature, these defects grew into larger vacancy clusters. These results suggest that, in hydrogen-embrittled iron, vacancy clusters stabilized by hydrogen accumulated locally in high concentrations. This condition is found to be the a sufficient and critical condition to trigger the hydrogen embrittlement. This result represents a crucial step forward towards a comprehensive understanding of the hydrogen embrittlement mechanism in iron.

Introduction

In recent years, hydrogen has attracted significant attention as a clean energy vector for the development of a sustainable society, but hydrogen embrittlement (HE) of the structural materials used in the hydrogen infrastructure represents a major limitation. HE is a phenomenon characterized by a significant reduction in ductility and brittle fracture of susceptible metals due to the diffusion of hydrogen in the lattice [1]. Despite numerous experimental studies and a variety of proposed HE mechanisms (see Ref. [2] and references therein), a comprehensive understanding of the HE process and underlying defects has not yet been obtained. One of the most commonly mentioned approaches is the hydrogen-enhanced strain-induced vacancy formation (HESIV) model: hydrogen stabilizes atomic vacancies and promotes their aggregation and clustering, thus fostering the plastic instability [3].

Iron is the fundamental material to investigate the HE mechanism, as it is the main component of steels, does not form complex microstructures and contains almost no impurities. The interaction between atomic vacancies and hydrogen plays a very important role in the HE mechanism [4]. Calculations have shown that hydrogen reduces the vacancy formation energy in α-iron and that monovacancies are in the most stable state when two hydrogen atoms are trapped (VH₂ complex) [[5], [6], [7]]. Hence, hydrogen trapping and stabilization of monovacancies promotes the formation of vacancies. The important role of vacancies in the HE mechanism is confirmed by studies evaluating the strain-induced defects that act as trapping sites of hydrogen [1]. A ductility reduction was observed in α-iron pre-strained in a hydrogen environment, even though hydrogen was absent in the subsequent deformation stage. This suggests that HE is due to the hydrogen-related defects created during the plastic deformation, rather than to hydrogen itself. The strain rate and temperature of tensile tests strongly affects the HE susceptibility in α-iron [8]. Stretching at slow strain rate (~10⁻⁵/s) reduces the fracture strain and the HE susceptibility is high, whereas at faster strain rates (~10⁻³/s) neither ductility reductions nor HE are observed [9].

Low-temperature thermal desorption analyses (TDA) [[10], [11], [12]] showed that hydrogen trapping sites in hydrogen-charged strained α-iron are enhanced. The main desorption peak at around 50 °C is thought to originate from strain-induced dislocations. A significant peak at 100 °C is also observed, but only at slow strain rates. The origin of this peak is unclear but is believed to be HE related, possibly hydrogen detrapped from vacancies. Defect analysis in hydrogen-charged α-iron (see e.g. Refs. [[13], [14], [15], [16]]) by positron annihilation lifetime spectroscopy (PALS) [17] showed that the presence of hydrogen enhances the generation and accumulation of vacancies. The strong strain-rate dependence suggested that the interaction of hydrogen with vacancies generated by dislocation reactions is important. It is expected that at slow strain rate hydrogen becomes trapped and stabilizes vacancies, whereas at fast strain rate the fast dislocation motion inhibits hydrogen trapping and vacancies cannot be stabilized. However, the exact mechanism remains unclear. The large diffusion coefficient of hydrogen and monovacancies in α-iron at room temperature (RT) [18] represents the major limitation to the detection of hydrogen-related defects. In those earlier studies [[13], [14], [15], [16]], the plastic deformation was carried out after the hydrogen charge, so that most of the hydrogen desorbed during the straining and the generated defects might not have been fully hydrogen-related. Moreover, PALS measurements normally take several hours and RT aging inevitably changes the state of the strain-induced defects.

This study presents variable-temperature PALS measurements on pure α-iron deformed at slow and fast strain rates to determine the role of defects in the HE susceptibility dependence on the strain rate. Iron was quenched in liquid nitrogen after simultaneous hydrogen charging and tensile testing to preserve the state of the hydrogen-induced defects. A clear strain-rate dependence of the hydrogen-induced defects in α-iron was detected by PALS. In hydrogen-embrittled slowly strained iron, the localization of a high concentration of small vacancy clusters stabilized by hydrogen was observed and is thought to be crucial in initiating the HE process.