Strength and ductility enhancement in nanostructured Al6063 with a bimodal grain size distribution

A bimodal grain structure comprising of ultrafine-grained (UFG) and coarse-grained (CG) regions was produced in Al6063 by means of powder metallurgy route. Nanocrystalline Al alloy powder was synthesized by high energy mechanical milling using a planetary ball mill under Ar atmosphere for 22.5 h. The as-milled powder was mixed with 30 vol.% unmilled powder and consolidated by extrusion at 450 degrees C with an extrusion ratio of 9:1. UFG and CG billets were also fabricated at the same condition by using milled and unmilled Al6063 powders, respectively. The grain structure was studied by scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) techniques. The deformation behavior under uniaxial tension was investigated. The bimodal Al6063 showed balanced mechanical properties, including enhanced yield and ultimate strength comparable UFG alloy with reasonable ductility analogous CG material. The fracture surfaces demonstrated a ductile fracture mode, in which the dimple size was related to the grain structure. The ductility enhancement mechanisms in nanostructured Al6063 with bimodal grain structure were discussed.