Magnificent tensile strength and ductility synergy in a NiCoCrAlTi high-entropy alloy at elevated temperature

Abstract

The study reported a novel L12-strengthening NiCoCrAlTi high-entropy alloy (HEA) with an outstanding synergy of tensile strength and ductility at both ambient and high temperatures. The HEA was prepared by arc melting and cold-rolling, followed by isothermal aging at designed precipitation temperatures to achieve coexistence of recrystallized and non-recrystallized grains. Transmission electron microscopy (TEM) characterization revealed a high density of rod-like and spheroidal L12 precipitates distributing in the micro/nanograins and non-recrystallized regions in the annealed specimens. The yield stress, ultimate tensile stress, and ductility of the HEA were ∼1300 MPa, 1610 MPa, and 14 % at room temperature, respectively. Such excellent mechanical properties of ∼1060 MPa, 1271 MPa, and 25 % were maintained to 600 °C, which was superior to most reported HEAs and Co- and Ni-based superalloys to date. Systematic TEM analysis unveiled at low strain, the deformation mechanism was mainly controlled by dislocations and stacking faults (SFs). With the increase in strain, the deformation mode gradually transferred to dislocations, deformation twins (DTs), and SFs. Moreover, the complex interaction between SFs and L12 precipitates, including the shearing and blocking effects, was frequently observed, contributing to the high tensile strength. Thus, the extremely high tensile strength and sustained ductility at 600 °C mainly originate from the cooperation among interaction between L12 precipitation and dislocations and extensive SFs, DTs, immobile Lomer-Cottrell (L-C) locks formed from interactions between SFs and SFs/DTs, hierarchical SFs/DTs networks, as well as hetero-deformation-induced strengthening. Such a unique deformation mechanism breaks the strength-ductility trade-off of the HEA at high temperatures.

Keywords

• High entropy alloys
• High-temperature tensile properties
• Stacking faults
• Lomer-cottrell locks