The effect of additional severe plastic deformation at elevated temperatures on the microstructure and functional properties of the ultrafine-grained Al–0.4Zr alloy

The effect of high pressure torsion (HPT) at elevated temperatures of 230 and 280$^{\circ}$C on the microstructure, mechanical properties and electrical conductivity of ultrafine-grained (UFG) Al–0.4Zr alloy was studied. The initial UFG structure in the material of the study was preliminarily formed by HPT-processing at room temperature. It was shown that the additional deformation of the UFG Al–0.4Zr alloy at elevated temperatures leads to a simultaneous significant increase in strength from 140 to 230–280 MPa and electrical conductivity from $\sim$47.5% to 52–54% IACS. The obtained results are compared with the effect of annealing at the same temperatures on the microstructure and properties of the UFG Al–0.4Zr alloy. It was found that, compared with annealing, severe plastic deformation at the same temperature leads to more efficient formation of nanoscale precipitates of the Al$_3$Zr secondary phase and, consequently, to a larger decrease in the Zr concentration in the solid solution, which provides a significant increase in electrical conductivity. Based on the obtained microstructural parameters, the contributions of various hardening mechanisms to the total hardening and electron scattering mechanisms to electrical resistivity are estimated. Comparison of the theoretical estimates with the experimental results indicates that the hardening in the UFG structure of the Al–0.4Zr alloy caused by additional SPD at elevated temperatures cannot be described only by the action of hardening mechanisms traditional for UFG materials. Possible reasons for the colossal hardening obtained are discussed.