Tensile behavior of the medium-entropy alloy Ni$_{42.4}$Co$_{24.3}$Cr$_{24.3}$Al$_3$Ti$_3$V$_3$ at 4.2 K

In this paper, we studied the microstructure of the partially- and fully-recrystallized medium entropy alloy (MEA) f.c.c. Ni$_{42.4}$Co$_{24.3}$Cr$_{24.3}$Al$_3$Ti$_3$V$_3$, as well as the tensile properties and deformation mechanism at 4.2 K, which were compared to that at 77 K in the literature. Annealing of the cold-rolled (85 % thickness reduction) MEA at 900 °C produced a partially-recrystallized, L1$_2$-type nanoparticle-hardened MEA containing recrystallized grains with an average size of 3.2 μm, a geometrically-necessary dislocation (GND) density of 1.9 × 10$^{14}$ m$^{−2}$ and a {110}<112> texture, while the unrecrystallized grains show an average size of 7.6 μm, a GND density of 8.9 × 10$^{14}$ m$^{−2}$, and a {110}<111> texture. Annealing the cold-rolled MEA at 1100 °C produced a fully recrystallized supersaturated single-phase f.c.c. microstructure with an average grain size of 364 μm, a GND density of 6.9 × 10$^{12}$ m$^{−2}$, and a {110}<112> annealing texture. The 900°C-aged MEA exhibited a yield strength (YS) of 1463 MPa and an ultimate tensile strength (UTS) of 1960 MPa along with a strain to failure (ε) of 27.7 % at 4.2 K, which were higher than the YS of 1274 MPa, UTS of 1694 MPa, and ε ∼of 24.8 % at 77 K. ... mehrBy contrast, the 1100°C-annealed MEA exhibited a YS of 751 MPa, UTS of 1305 MPa, and ε of 54.1 % at 4.2 K, which were higher than the YS of 658 MPa, UTS of 1172 MPa, and ε ∼of 52.2 % at 77 K. Serrations occurred on the stress-strain curves only for specimens tested at 4.2 K. The stress drops increased with increasing strain, from 41 MPa to 127 MPa for the 900°C-aged MEA, and from 30 MPa to 68 MPa for the 1100°C-annealed MEA. Dislocation slip, stacking fault (SF) formation, and deformation twinning occurred during straining of the 900°C-aged MEA at 4.2K, whereas only dislocation slip and the formation of a few SFs occurred for the 1100°C-annealed MEA, indicating that the solutes in the latter increased the SF energy. The reasons for the increased strength with decreasing temperature and serrations upon straining at 4.2 K are discussed.