Thermal Stability of Nanostructured Internally Oxidized Vanadium Alloy with Combined Dispersion and Substructural Hardening

Abstract

The paper studies the effect of the annealing temperature on microstructural transformation and micro-hardness variation in an internally oxidized vanadium alloy V—Cr—ZrO₂ deformed by high-pressure torsion in Bridgman anvils. It is shown that the development of large plastic strains and subsequent annealing of the particle-reinforced V—Cr—ZrO₂ alloy lead to the formation of a fine-grained structure (with about 1 µm grain size) with a high density of high-angle boundaries pinned by ZrO₂-based nanoparticles. Such high-defect states are characterized by a more than twofold increase in microhardness with the major contribution of grain boundary hardening. The conducted research has revealed the main relaxation features of V—Cr—ZrO₂ alloy deformed by high-pressure torsion at room temperature. The heat treatment of the studied material at 800°C is shown to activate recovery and polygonization. Primary recrystallization is observed upon temperature increase to S00°C. A further increase in temperature in the interval 950–1050°C intensifies collective recrystallization, due to which the fraction of equi-axed grains increases significantly. Secondary recrystallization is activated at 1200°C and, as a result, individual grains grow in size. These processes are accompanied by a decrease in the V—Cr—ZrO₂ alloy microhardness from 3500 to 2000 MPa. Dispersion and substructural hardening are analyzed, and their contribution to the strength is studied. It is shown that the high thermal stability of the nanostructural and fine-grained states is ensured by the high density of uniformly distributed ZrO₂-based nanoparticles (of size 3–10 nm) that pin the high-angle grain boundaries.