Structural aspects of annealing and hot deformation of Al-Nb 2 O 5 mechanically alloyed composite

Abstract

A mechanical alloying and hot extrusion method was used to manufacture an Al based composite reinforced by 10 wt-% addition of Nb₂O₅. Annealing experiments were performed to test the effect of chemical reaction between components on the material structure. After annealing at 873 K for 12 h the material hardness was found to have increased from HB 121 (as extruded) to HB 186 (annealed). The hardness maximum was followed by material softening at prolonged aging at 873 K. Transmission electron microscopy observations revealed a very well consolidated nanoscale structure for the as extruded composite. Annealing of the material resulted in an increase in porosity that was ascribed to Nb oxide reduction by aluminium. Due to diffusion of the released Nb element into the surrounding Al matrix, formation of new intermetallic grains of Al₃Nb was accompanied by very fine needlelike Al oxide particle growth within the aluminium matrix. Chemical reaction between aluminium and Nb oxide at solid state was related to local specific volume reduction being responsible for an increase in the material porosity. Hot compression tests were performed on as extruded and samples annealed at 873 K for 12 h. The good mechanical properties of the composite were ascribed to the highly refined structure of the material. In compression tests, samples were not fractured within the experimental strain range used. In hot deformation tests at lower temperatures and a strain rate of 5 10⁻³ s⁻¹, some fine microcavities were observed by TEM for samples deformed to ɛ _t≈0·3.