Aluminium alloys are re-evaluated as most feasible way to satisfy the industrial needs of light-weight structural materials. However, unlike conventional structural metals such as iron and titanium, aluminium does not have easily accessible secondary phases, which means that aluminium-based alloys cannot be strengthened by harnessing multiple phases. This leaves age hardening as the only feasible strengthening approach. Highly concentrated precipitates generated by age hardening generally play a dominant role in shaping the mechanical properties of aluminium alloys. In such precipitates, it is commonly believed that the coherent interface between the matrix and precipitate does not contribute to crack initiation and embrittlement. Here, we show that this is not the case. We report an unexpected spontaneous fracture process associated with hydrogen embrittlement. The origin of this quasi-cleavage fracture involves hydrogen partitioning, which we comprehensively investigate through experiment, theory and first-principles calculations. Despite completely coherent interface, we show that the aluminium-precipitate interface is a more preferable trap site than void, dislocation and grain boundary. The cohesivity of the interface deteriorates significantly with increasing occupancy, while hydrogen atoms are stably trapped up to an extremely high occupancy over the possible trap site. Our insights indicate that controlling the hydrogen distribution plays a key role to design further highstrength and high-toughness aluminium alloys.
High angle annular dark field scanning transmission electron microscopy has been employed to observe precipitate structures in Al-Zn-Mg and Mg-Zn alloys. h 1 precipitate structures in Al-Zn-Mg are commonly formed by MgZn 2 Penrose bricks, but also frequently observed to incorporate Mg 6 Zn 7 elongated hexagons via two different modes. Tilings of MgZn 2 and Mg 6 Zn 7 building blocks in both b ′ 1 in Mg-Zn and h 1 in Al-Zn-Mg alloys, create overall patterns which deviate from the chemical and structural configuration of solely monoclinic Mg 4 Zn 7 or MgZn 2 unit cells. Precipitate morphologies were found to be correlated to their internal sub-unit cell arrangements, especially to Mg 6 Zn 7 elongated hexagons. Systematic or random arrangements of Mg 6 Zn 7 elongated hexagons inside precipitates and therefore compositional and structural patterns, were found to be strongly related to the aspect ratio of the precipitates and altering of the precipitate/matrix interfaces.
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