Excellent cryogenic mechanical properties of a novel medium-entropy alloy via vanadium-doping and hetero-grain/precipitation engineering

Abstract

A novel vanadium-doped f.c.c. medium-entropy alloy (MEA) Ni42.4Co24.3Cr24.3Al3Ti3V3 was designed to achieve excellent cryogenic mechanical properties. The vanadium was added in order to increase the temperature-dependent friction stress, and aging of the MEA, which had been given a homogenization anneal at 1150 °C followed by cold-rolling to 80% reduction, at 700–900 °C was used both to produce partial recrystallization and precipitate L12 nanoparticles. Aging produced a heterogeneous microstructure composed of fine, soft recrystallized grains surrounded by harder, relatively-large non-recrystallized grains, containing L12-precipitates. Increasing the aging temperature from 700 °C to 900 °C promoted V partitioning into the f.c.c. matrix with the partitioning coefficient increasing from 0.8 to 1.9; the grain size increasing from 2.3 μm to 4.6 μm; an increase in the precipitate size from 10.5 nm to 80.0 nm; a reduction in the residual geometrically-necessary dislocation (GND) density from 9.0 × 1014 m−2 to 2.8 × 1014 m−2; and a decrease in the volume fraction of precipitates fraction from 28% to 14%. All three aged MEAs showed extraordinary cryogenic mechanical properties with an increase in the aging temperature resulting in an increase in strength but a decrease in ductility, viz, a yield strength (YS) of 1929 MPa, an ultimate tensile strength (UTS) of 2147 MPa, and an elongation to failure (ε) of 6.6% at 700 °C, YS ∼ 1735 MPa, UTS ∼ 1976 MPa, ε ∼ 9.0% at 800 °C, and YS ∼ 1274 MPa, UTS ∼ 1694 MPa, ε ∼ 24.8% at 900 °C. The dislocation back stress rose progressively with increasing strain and was invariably higher than the effective stress, accounting for 51–59% of the flow stress i.e. the back stress played a dominant role in the ultrahigh flow stress. Increasing the aging temperature also produced a greater abundance of nanoscale deformation twins, stacking faults, and dislocation networks, which are responsible for excellent strain hardening capacity and greater ductility. After deformation, the density of low-angle grain boundaries increased by 3–6 times to 0.6–1.3 μm/μm2, and the GND density increased by 4–6 times to 1.2–2.0 × 1015 m−2.