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Acta Metall Sin  2018, Vol. 54 Issue (11): 1683-1692    DOI: 10.11900/0412.1961.2018.00341
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Hydrogenated Vacancy: Basic Properties and Its Influence on Mechanical Behaviors of Metals
Jun SUN(), Suzhi LI, Xiangdong DING, Ju LI
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
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Hydrogen embrittlement, the degradation of mechanical behaviors due to the existence of hydrogen, is an industrially and environmentally critical problem in metals and alloys. Yet the fundamental mechanism(s) of embrittlement are still controversial, and the molecular-level damage events shrouded in mystery. In hydrogen embrittlement phenomena, the molecular-level agents of damage are hypothesized to be hydrogen-vacancy complex (Va+nH→VaHn), hereupon called hydrogenated vacancy. Contrary to vacancy, hydrogen-vacancy complex has good thermal stability and low diffusivity. When metals undergo plastic deformation at low homologous temperature in the presence of hydrogen, the mechanically driven out-of-equilibrium dislocation processes produce extremely high concentrations of hydrogen-vacancy complexes. Under such high concentrations, these complexes prefer to grow by absorbing additional vacancies and act as the embryos for the formation of proto nano-voids. Our work provides the insight on the microscopic mechanism of hydrogen embrittlement. Moreover, this work also helps understanding some unique mechanical behaviors induced by hydrogen.

Key words:  hydrogen embrittlement      hydrogenated vacancy      nano-void      plastic deformation     
Received:  20 July 2018     
ZTFLH:  TB303  
Fund: Supported by Funds for Creative Research Groups of National Natural Science Foundation of China (No.51621063) and the Major International (Regional) Joint Research Program of China (No.51320105014)

Cite this article: 

Jun SUN, Suzhi LI, Xiangdong DING, Ju LI. Hydrogenated Vacancy: Basic Properties and Its Influence on Mechanical Behaviors of Metals. Acta Metall Sin, 2018, 54(11): 1683-1692.

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Method VaH VaH2 VaH3 VaH4 VaH5 VaH6
MD[16] 0.603 0.603 0.425 0.346 0.217 0.158
DFT[15] 0.559 0.612 0.399 0.276 0.335 -0.019
Table 1  Averaged binding energy of VaHn (n=1~6) in α-Fe[15,16]
Fig.1  Migration paths of hydrogen-vacancy complex, vacancy and hydrogen in α-Fe. As indicated in red, blue and green lines, the migration barriers of corresponding point defects are 0.76 eV, 0.64 eV and 0.04 eV, respectively[16]
Fig.2  Four independent events illustrating the high stability of hydrogen-vacancy complex when interacting with dislocations in α-Fe. Crystals are oriented along x-[111], y-[1?01], z-[12?1]. Spheres with blue, gold and black colors refer to iron atoms in dislocations, vacancies and hydrogen, respectively. The radius of vacancy and hydrogen are enlarged for clarity[16]
(a) when a a/2<111>-type edge dislocation interacts with a vacancy in the slip plane, the dislocation absorbs the vacancy
(b) in contrast, the hydrogen-vacancy complex is very stable when colliding with an edge dislocation
(c) the stability of hydrogen-vacancy complex is further confirmed when lattice hydrogen grabs an absorbed vacancy from an edge dislocation and stabilizes it in the form of a hydrogen-vacancy complex
(d) the hydrogen-vacancy complex can even grow displacively by capturing more vacancies that were absorbed by edge dislocations
Fig.3  Stable dislocation configurations of prismatic loops (PLs) (a) and dislocation network evolved from shear loops (SLs) (b). Spheres with blue and gold color refer to the iron atoms in dislocations and vacancies, respectively[16]
Fig.4  Variation of concentration of vacancies and hydrogen-vacancy complexes (CVCVa+CVaHn) (a) and concentration of hydrogen-vacancy complexes (CVaHn) with applied strain (ε) (b) under different hydrogen concentrations (CH) in iron. CV, CVa, CVaHn and CH are in the units of atomic ratio[16]
Fig.5  Stress-strain (σ-ε) curves of uniaxial tension for both systems containing SLs and PLs under CH=10-2 (a), and variation of concentration of CV, including both unhydrogenated vacancies and hydrogen-vacancy complexes (b). The yellow and cyan solid lines are the linear fit to the two curves in strain range 0.06~0.6 [16]
Fig.6  The accumulation of vacancy and hydrogen-vacancy complexes by diffusion in kinetic Monte Carlo simulations. Spheres with gold and black colors refer to vacancies and hydrogen. Taking hydrogen-vacancy complexes as nuclei, vacancies are accumulated as indicated by the red circles (t—time)
(a) t=0 s (b) t=0.002 s (c) t=0.021 s (d) t=0.188 s
Fig.7  Mean radius distributions of proto nano-voids at different ageing time at room temperature by using cluster dynamics simulations. As time increases, the most probable size of cluster increases, indicating the accumulation of vacancies and hydrogen-vacancy complexes during long-time evolution[16]
Fig.8  Experimental observations on hydrogen embrittlement in steel[16]
(a) TEM image of deformation microstructure underneath a hydrogen embrittled quasi-brittle fracture facet in a tensile tested X65 grade pipeline steel
(b) SEM image of a typical hydrogen embrittled quasi-brittle fracture surface of X80 grade steel fracture toughness tested in 3000 psi H2 gas pressure
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