Absorption of hydrogen in iron and steel can result in hydrogen embrittlement and severe deterioration of mechanical properties. Hydrogen diffuses through the material and is retained in different types of traps. Depending on their binding energy, these traps can be classified as reversible or irreversible.
The aim of this research was to investigate the influence of microstructure on hydrogen trapping in Armco iron by systematically analyzing the trapping ability of grain boundaries and dislocations. High purity Armco iron was used as sample material and subjected to a combination of various heat treatment and deformation processes (high pressure torsion).
High resolution characterization of the specimens was done by HR-SEM, EBSD and HR-TEM. Hydrogen permeation experiments were carried out in a Devanathan-Stachurski permeation cell to evaluate the apparent diffusion coefficient and determine the proportion of reversibly and irreversibly trapped hydrogen by multiple loading.
Results were very similar for specimens submitted to recrystallization annealing and specimens annealed after a high pressure torsion process, showing a predominance of irreversible traps. The microstructure of these specimens had a certain degree of similarity, presenting only little misorientation of grains and low density of dislocations. In contrast, irreversible traps prevailed in specimens subjected only to intense mechanical distortion by high pressure torsion. The microstructure showed heavy deformation spreading over the grain boundaries, strong misorientation of grains and high density of dislocations. Consequently, severe plastic deformation results in a high number of dislocations, representing reversible traps. Annealing causes the annihilation of dislocations, resulting in a high fraction of irreversible traps, namely grain boundaries.