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We report experimental observation of a deformation mechanism in nanocrystalline face-centered-cubic Al, partial dislocation emission from grain boundaries, which consequently resulted in deformation stacking faults (SFs) and twinning. These results are surprising because (1) partial dislocation emission from grain boundaries has not been experimentally observed although it has been predicted by simulations and (2) deformation stacking faults and twinning have not been reported in Al due to its high SF energy.

Increasing the strength of metallic alloys while maintaining formability is an interesting challenge for enabling new generations of lightweight structures and technologies. In this paper, we engineer aluminium alloys to contain a hierarchy of nanostructures and possess mechanical properties that expand known performance boundaries -an aerospace-grade 7075 alloy exhibits a yield strength and uniform elongation approaching 1 GPa and 5 % , respectively. The nanostructural architecture was observed using novel high-resolution microscopy techniques and comprises a solid solution, free of precipitation, featuring (i) a high density of dislocations, (ii) subnanometre intragranular solute clusters, (iii) two geometries of nanometre-scale intergranular solute structures and (iv) grain sizes tens of nanometres in diameter. Our results demonstrate that this novel architecture offers a design pathway towards a new generation of super-strong materials with new regimes of property-performance space.

A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T mp . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al 2 O 3 particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crackblunting objects.

Due to its high stacking fault energy, no deformation twin has ever been observed in coarse-grained Al. Recent molecular dynamic ͑MD͒ simulations predicted the formation of deformation twins in nanocrystalline ͑nc͒ Al. Here, we report transmission-electron-microscopic observations of two types of twins in nc Al processed by cryogenic ball milling. They were formed via mechanisms suggested by the MD simulations. We also observed curved twin boundaries caused by partial dislocations. These results indicate that deformation mechanisms not accessible to coarse-grained Al are active in nc Al. They could be responsible for some unique mechanical properties of nc materials.

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