Avaliação dos precipitados em compósitos de matriz metálica de Al-Zn-Mg-Cu (AA7050) reforçados com partículas de alumina e consolidado por HPT (High-Pressure Torsion)

Abstract

The Al alloys of the 7xxx series are age-hardenable Al-Zn-Mg-(Cu) alloys that have several applications in the aerospace industry. Aircraft structures undergo demanding operating conditions and must comply with strict safety and performance requirements. For this reason, new materials have been developed, among them, the metal matrix composites of Al alloy reinforced with ceramic material. In order to produce a metal matrix composites of age-hardenable Al alloy reinforced with ceramic material, AA7050 gas atomized powders and Al2O3 particles (10% vol.) were consolidated by HPT (High Pressure Torsion) for manufacturing of high hardness samples. In addition, to understand the influence of dispersed phases (reinforcement) on the morphology of the precipitates, the sample was homogenized and aged at peak hardness conditions of AA7050 alloy. The AA7050 alloy and alumina randomly sized powders were mixed and consolidated by HPT (P = 6 GPa, N = 50 turns), and 10 mm diameter and 1mm thickness discs were manufactured. Three conditions were chosen for characterization of the discs: As-HPT; homogenization at 470 °C for 30 minutes; homogenization followed by aging at 120 °C for 24 hours for further microscopy and mechanical characterization. A composite with ultra-fined grains was produced and this As-HPT MMC reached hardness values of ~ 300 Hv. New intermetallic phases were identified for the consolidated and later homogenized samples, especially in the grain boundaries. As for the aged samples, it was observed the presence of GP zones finely dispersed within the grain of the composite matrix. The metastable phase η’ was predominantly found in regions with high dislocation density, such as the matrix/reinforcement interface (MPZ - Matrix Plastic Zone). The equilibrium phase η was mainly seen in the grain boundary and in the MPZ, approximately with 200 nm size. It was also noted for the heat-treated discs that after homogenization, the microstructure was recovered and the grains were recrystallized, which led to a decrease in hardness of the discs processed by HPT. In addition, the hardening generated by aging treatment was not high enough to “compete” with the hardening by grain refinement caused by HPT processing.