Microstructural evolution of cryomilled nanocrystalline Al-Ti-Cu alloy

The microstructural evolution during processing of a nanocrystalline Al-Ti-Cu alloy was investigated using transmission and scanning electron microscopy. Grain refinement was achieved by cryomilling of elemental powders, and powders were consolidated by hot isostatic pressing (HIP) followed by extrusion to produce bulk nanocrystalline Al-Ti-Cu alloys. In an effort to enhance ductility and toughness, multi-scale structures were produced, which included nanocrystalline grains, elongated coarse-grains of pure Al, and intermediate grains. Pure aluminum grains were elongated along the extrusion direction and formed coarse-grain bands comprised of sub-grains. Nanocrystalline second phases were distributed in the intermediate grains and nanocrystalline regions. The distribution and identity of these phases were determined by analytical and high-resolution microscopy. Examination of bulk tensile fracture specimens revealed unusual failure mechanisms and interactions between ductile coarse-rains and nanocrystalline regions.