Influence of the Mg Concentration on the Plasticization Effect in Ultrafine-Grained Al-Mg-Zr Alloys

Abstract

The influence of additional deformation-heat treatment consisting in annealing at 150 or 230°C and additional deformation by 0.25-revolution high-pressure torsion (HPT) at room temperature on the microstructure, mechanical characteristics, and electrical conductivity of the ultrafine-grained Al-1.17Mg-0.33Zr (wt %) alloy processed by HPT at room temperature is studied for the first time. It is shown that deformation-heat treatment at both annealing temperatures leads to the plasticization effect in the material, i.e. a significant increase in plasticity (by more than an order of magnitude) on retention of high strength (80% of the strength of the untreated alloy). The revealed effect is compared with that in ultrafine-grained Al-Mg-Zr alloys with a lower magnesium concentration. It is shown that the value of plasticity achieved as a result of deformation-heat treatment (annealing at 150°C and additional 0.25-revolution HPT) decreases, and the strength increases as the Mg concentration grows from ~0.5 to ~1.2 wt %. The ultrafine-grained alloy Al-1.17Mg-0.33Zr (wt %) demonstrates a higher thermal stability compared to the ultrafine-grained Al-Mg-Zr alloys with a lower Mg concentration, which allows using a higher annealing temperature (230°C) during deformation-heat treatment. It is found that deformation-heat treatment by 230°C annealing and 0.25-revolution HPT provides the best combination of strength (yield strength ~380 MPa, ultimate tensile strength ~480 MPa) and plasticity (elongation to failure ~9%, uniform strain ~4%), which is not inferior to commercial Al-Mg alloys with ~4% magnesium after conventional strengthening treatment or treatment by equal channel angular pressing. The physical reasons for such combination of properties are analyzed against microstructural changes during deformation-heat treatment.