Journal of Japan Institute of Light Metals

Al-Zn-Mg alloys in T6 temper are susceptible to stress corrosion cracking based on hydrogen-induced intergranular cracking. Over-aging is effective to suppress hydrogen embrittlement, but results in loss of strength. In this study, sliding friction treatment (SFT) and subsequent aging treatment were applied to an Al-4.4mass%Zn-1.4mass%Mg ternary alloy in order to suppress hydrogen embrittlement without decreasing strength. Mechanical properties and hydrogen embrittlement susceptibility were evaluated by tensile tests in two environments: dry nitrogen gas and humid air. Hardness measurement showed that SFT resulted in increment of the hardness near the specimen surface. The specimen processed by SFT and subsequent aging showed the higher strength compared to the T6-tempered one. Moreover, hydrogen embrittlement susceptibility of the specimen processed by SFT and aging was lower than that of T7-tempered one. Fracture surface observation revealed that intergranular cracking was suppressed by SFT and aging. Therefore, SFT followed by aging was effective for the concurrence of high strength and resistance to hydrogen embrittlement.

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While plate-like Guinier-Preston (GP) zones are formed during aging process in Al-Cu alloys, spherical nanocluster formation occurs in the early stage of aging in Al-Mg-Si alloys. Unlike well-known GP (I) zone in Al-Cu, there is no specific configurations within the nanocluster. However, the solute concentration and local configuration should play decisive role in subsequent formation of precipitates. In the present study, the first-principles calculations were performed to investigate the factors determining the stable shape during the formation process of GP zones and clusters in Al-Cu and Al-Mg-Si alloys. As a result of formation energy calculation of three-body bonds, the Cu-Cu-Cu triplet with the bond angle of 90° was the most stable. Monte Carlo simulations with newly developed machine-learning potential were then performed, and consequently the segregation of Cu atoms with the bond angle of 90° was observed more frequently. In contrast, three-body triplet with the bond angle of 60°was most stable without any specific directional anisotropy in Al-Mg-Si alloy, resulting in the formation of spherical nanoclusters. These results suggest that the intrinsic feature of the stability of local bonding dominates the shape of GP zones and nanoclusters, in which planar- or spherical-like cluster is formed.

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