Interplay between vacancy-induced hydrogen segregation and stress-induced vacancy redistribution causing embrittlement of alpha-iron

This study proposes a novel mechanism of intergranular fracture in alpha-iron, focusing on the effects of trapped vacancies, H atoms, and their synergistic interplay under tensile strain. We present a methodology for the introduction of H into grain boundaries (GBs) resulting in a realistic distribution by considering H-H interactions. Accordingly, optimal H concentrations were determined under specific environmental conditions for GBs with and without vacancy-induced segregation under zero and 2% tensile strain, respectively. Subsequently, the reduction in cohesive energy at GBs was evaluated at the optimal H concentration under these conditions. In the case of H segregation without vacancies at zero applied strain, the reduction in the cohesive energy ranged approximately from 15% to 35% for all the GB configurations. Eventually, vacancy segregation increased H concentration at the GBs, defined as vacancy-induced H segregation. The vacancy-induced H segregation resulted in a 60-117% increase in H concentration and a 70-80% decrease in cohesive energy at a vacancy concentration of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>7.49</mml:mn> <mml:mrow><mml:mrow><mml:mi> </mml:mi></mml:mrow> </mml:mrow> <mml:mn>1</mml:mn> <mml:mrow><mml:mo>/</mml:mo></mml:mrow> <mml:mrow><mml:mrow><mml:mi>n</mml:mi></mml:mrow> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow><mml:mrow><mml:mi>m</mml:mi></mml:mrow> </mml:mrow> <mml:mn>2</mml:mn></mml:msup> </mml:mrow> </mml:math> under zero applied strain. The proposed vacancy-induced H-segregation mechanism explained the delayed fracture in steel. Furthermore, the effect of tensile strain on embrittlement was elucidated, with strain-induced vacancy redistribution and vacancy-induced H segregation synergistically promoting GB decohesion, resulting in a 73-93% reduction in cohesive energy at the same vacancy concentration.