Thermal Stability of the Structure and Microhardness of the Al–0.05 vol % Al2O3 Nanocomposite Fabricated by Accumulative Roll Bonding

Abstract

The evolution of the microstructure and microhardness of the Al–0.05 vol % nAl₂O₃ nanocomposite (where nAl₂O₃ are aluminum nanoparticles) and aluminum without nanoparticles fabricated by accumulative roll bonding with annealing in a temperature range of 373–573 K is investigated. Ball-shaped Al₂O₃ nanoparticles with an average diameter of 50 nm are introduced between rolled plates of technically pure aluminum from the first to fourth rolling cycles, while the fifth to the tenth rolling cycles are performed without nanoparticles. The average grain size and aspect ratio of elements of the material grain–subgrain structure in the initial state and after annealing at 473 K are measured using transmission electron microscopy. It is shown that the nanocomposite microhardness is 5–13% higher than the corresponding value of HV for aluminum over the entire studied annealing temperature range. The main factor of the higher nanocomposite microhardness is precipitation hardening due to Al₂O₃ nanoparticles. The contribution of the substructural and grain-boundary hardening is identical in both materials. The thermal stability of the nanocomposite microhardness is only ~25 K higher than that of aluminum, which is caused by the nonuniform distribution of nanoparticles in the matrix and their low volume fraction. The high thermal stability of the fine-grain structure itself, formed by accumulative roll bonding, when compared with other methods of intense plastic deformation, also plays a role. It is established that most Al₂O₃ nanoparticles remain at the grain boundaries of nanocomposite after annealing at 473 K; therefore, the ability to fasten the boundary by Al₂O₃ nanoparticles under the conditions under study is retained at least to 473 K.