Abstract
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 high-strength and high-toughness aluminium alloys.
Highlights
Aluminium alloys are re-evaluated as most feasible way to satisfy the industrial needs of light-weight structural materials
The quasi-cleavage fracture observed in Al alloys differs from that of well-known fracture modes in metals such as hydrogen-induced cracking[16], which is generally caused by hydrogen-enhanced decohesion (HEDE) at cleavage planes and grain boundaries[17,18,19,20], hydrogen-induced local plasticity (HELP)[21,22,23], and hydrogen-mediated micropore distribution[10,24,25]
We focus on the hydrogen embrittlement of high-strength Al-Zn-Mg alloys and estimate the equilibrium partitioning of hydrogen in this Al alloys using a thermodynamic model based on first-principles density functional theory (DFT) calculations
Summary
Aluminium alloys are re-evaluated as most feasible way to satisfy the industrial needs of light-weight structural materials. We prepared tensile specimens containing a variety of defects (vacancy, dislocation, grain boundary and precipitate) in which hydrogen diffuses to the trap sites (see the methods section for details). These experimental observations suggest that hydrogen trapping at the Al–MgZn2 precipitate interface induces decohesion and originate quasi-cleavage crack, promoting HE behaviour.
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