In situ observation of temperature-dependent atomistic and mesoscale oxidation mechanisms of aluminum nanoparticles

Oxidation is a universal process causing metals’corrosion and degradation.While intensive researches have been conducted for decades,the detailed atomistic and mesoscale mechanisms of metal oxidation are still not well understood.Here using in situ environmental transmission electron microscopy(E-TEM)with atomic resolution,we revealed systematically the oxidation mechanisms of aluminum from ambient temperature to^600℃.It was found that an amorphous oxide layer formed readily once Al was exposed to air at room temperature.At^150℃,triangle-shaped Al2O3 lamellas grew selectively on gas/solid(oxygen/amorphous oxide layer)interface,however,the thickness of the oxide layer slowly increased mainly due to the inward diffusion of oxygen.As the temperature further increased,partial amorphous-to-crystallization transition was observed on the amorphous oxide film,resulting in the formation of highly dense nano-cracks in the oxide layer.At^600℃,fast oxidation process was observed.Lamellas grew into terraces on the oxide/gas interface,indicating that the high temperature oxidation is controlled by the outward diffusion of Al.Single or double/multi-layers of oxide nucleated at the corners of the terraces,forming denseγ’-Al2O3,which is a metastable oxide structure but may be stabilized at nanoscale.

Brittle-to-ductile transition in Ti–Pt intermetallic compounds

Phase transformation changes numerous properties of materials. Ti–Pt alloys have received much interest because of high martensitic transformation temperature. However, the intrinsic brittleness of these intermetallic compounds with low crystal symmetry and complicated phase structure limit their applications,especially when composition deviates from stoichiometry ratio. By performing in situ heating high-resolution scanning transmission electron microscopy experiment and micro-mechanical testing on Ti-35 at% Pt that contained majorly Ti3Pt and αTiPt phases, it was found that precipitating herringbone twinned αTiPt islands within Ti3Pt could occur upon heating, significantly refining mixed-phase structure. The refinement of multi-intermetallic mixed-phase structure endowed brittle material with remarkable capacity for plastic deformation and strain hardening. The plastic deformation mechanisms include phase transformation upon yielding and dislocation slips during hardening, which rarely occurs in intermetallic compounds with low symmetry. The strong interaction between different deformation modes even caused nano-crystallization along slip bands. The results demonstrate that brittle-toductile transition in intermetallic compounds can be achieved by tuning mixed-phase structure through phase transformations.