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

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 degrees C followed by cold-rolling to 80% reduction, at 700-900 degrees 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 degrees C to 900 degrees 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 mu m to 4.6 mu m; 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 x 1014 m(2) to 2.8 x 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 (epsilon) of 6.6% at 700 degrees C, YS 1735 MPa, UTS 1976 MPa, epsilon 9.0% at 800 degrees C, and YS 1274 MPa, UTS 1694 MPa, epsilon 24.8% at 900 degrees 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 nano scale 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 mu m/mu m(2), and the GND density increased by 4-6 times to 1.2-2.0 x 10(15) -2.