Bulk Nanocrystalline Al 7050 Prepared via Cryomilling

Abstract

The operation with a combination of three processing routes: cryomilling, hot isostatic pressing (HIPping) and hot extrusion was adopted in the present study for preparation of the bulk nanocrystalline Al 7050. The microstructure and fractography of the bulk material were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Furthermore, the chemical composition, density and tensile properties of the material were also measured. Microstructural investigation showed that the grain size of the bulk nanocrystalline Al 7050 ranged from 100nm to 200nm. Numerous dispersoids with a diameter/length of ~50nm were observed on grain boundaries and inside the grains. Besides, one phase of these dispersoids existed in the bulk nanocrystalline Al 7050 was identified as Al6(FeMn). These dispersoids dispersed within the bulk nanocrystalline Al 7050, to some extent, increased the mechanical properties and thermal stability of the material. The resulted sample exhibited ultimate strength of 412MPa with an elongation of 5.2% when tested under tensile load, which was a bit lower than that of the traditionally wrought Al 7050-T6. The present results suggested that improper selected starting powder and milling parameters resulted in the flake-like morphology of the cryomilled powder. The flake-like morphology made it difficult for the cryomilled powder to fill the can entirely and achieve a high density material, which led to the weak interface within the bulk material and in turn degraded the mechanical properties of the bulk nanocrystalline Al 7050 prepared in the present work.