Mechanical properties and deformation mechanisms of a Ni2Co1Fe1V0.5Mo0.2 medium-entropy alloy at elevated temperatures

Abstract

A Ni2Co1Fe1V0.5Mo0.2 medium-entropy alloy (MEA) with a single-phase face-centered cubic (fcc) crystal structure are casted. The as-cast MEA possesses remarkable work hardening ability, yield strength (σy), ultimate tensile strength (σUTS) and uniform elongation (σu) at high temperatures up to 800 °C. Up-turn of strain hardening rate after elasto-plastic transition (secondary strain hardening) is observed for the MEA during tensile deformation in the temperature range of 25-800 °C. A high density of dislocation forests with solute atmospheres in concentrated solid solution is considered sufficient for back stress strengthening, thus causing the up-turn of strain hardening rate. Post-mortem microstructural analysis reveals that the temperature effects on mechanical properties of the MEA are closely related to dislocation structures, dislocation densities and dynamic-strain aging (DSA). With increasing deformation temperature from 25 °C to 800 °C, the dislocation density undulates and the fraction of screw dislocations increases. Strong solute pinning effect in conjunction with dominant forest strengthening mechanism (indicated by negative strain rate sensitivity and large activation volume) boost the strain hardening rate at the temperatures from 400 ℃ to 700 ℃. At 800 °C, the dominant deformation mechanism changes from the forest dislocation cutting mechanism to dislocation cross-slip with increasing strain, resulting in the sharp up-turn of strain hardening rate. On the other hand, DSA poses plastic instability to deteriorate the strain hardening of the MEA. These phenomena are considered to have material impacts on mechanical properties and fracture mechanisms of MEAs and high-entropy alloys at high temperatures.