It has generally been assumed that metals usually fail as a result of microvoid nucleation induced by particle fracture. Here we concentrate on high-density micropores filled with hydrogen in aluminium, whose existence has been largely overlooked until quite recently. These micropores exhibit premature growth under external loading, thereby inducing ductile fracture, whereas the particle fracture mechanism operates only incidentally. Conclusive evidence of a micropore mechanism is provided by the observation of an instantaneous release of gas at failure. We can therefore conclude that the growth of micropores dominates ductile fracture. Since the material we used has a standard pore density, we can assume that an identical fracture mechanism operates in other aluminium alloys. This finding suggests that intense heat treatment, which is generally believed to enhance the mechanical properties through homogenisation, may have entirely the opposite effect. This revelation will have a major impact on the engineering design of metals.

Synchrotron X-ray microtomography was used to observe hydrogen micropores. Competitive growth between pre-existing high-density micropores and voids originating from damage during loading was observed in an aluminum alloy during a tensile test. Extensive and premature growth of pre-existing hydrogen micropores has been observed during tension, while the ordinary damage initiation increased rapidly more later. According to the estimation on the areal fraction of dimple patterns originating from the pre-existing hydrogen micropores, it has been concluded that the hydrogen micropores more or less make contributions to ordinary ductile fracture.

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