Hydrogen trapping and quasi-cleavage fracture in Al-Zn-Mg alloy

Tsuru, Tomohito   ; Shimizu, Kazuyuki*; Yamaguchi, Masatake   ; Itakura, Mitsuhiro  ; Ebihara, Kenichi   ; Bendo, A.*; Matsuda, Kenji*; Toda, Hiroyuki*

7xxx series alloys are the high strength Al alloys which contain Zn and Mg as the major alloying elements for precipitate hardening. In these alloys, a small portion of dissolved hydrogen has been found to cause an anomalous brittle fracture called "quasi-cleavage" fracture. In the present study, we focus on the hydrogen embrittlement of high-strength Al-Zn-Mg alloys and estimate the equilibrium partitioning of hydrogen. We explored an unprecedented HE mechanism related to the quasi-cleavage fracture in Al alloys through experimental observation and theoretical model combined with electronic structure calculations. First-principles calculations were carried out to evaluate the binding energy between hydrogen and various defect structures such as vacancy, edge/screw dislocations, grain boundary, and -MgZn precipitate. The results show that the binding energy at the Al-MgZn interface is higher than that of the grain boundary and vacancy, even if the interface remains completely coherent and the free volume around the interface is small. The Al-MgZn interface is therefore one of the most favourable trap sites among possible defect structures in Al alloys. Subsequently we evaluated the relationship between cohesive energy and occupancy at the interface. According to the comprehensive investigation of hydrogen partitioning, we advocate a new process of hydrogen-induced quasi-cleavage fracture in Al-Zn-Mg alloys.