An investigation on microstructure and mechanical properties of strong yet ductile nanoparticle-strengthened, partially recrystallized, medium-entropy alloy

Influences of both aging temperature and vanadium addition on the microstructure and mechanical properties of a nanoparticle-strengthened, partially-recrystallized, medium-entropy alloy (MEA) Ni43.4Co25.3Cr25.3Al3Ti3 were probed in the present work. Aging (700 degree celsius or 900 degree celsius) produced both a high density of nano-sized L1(2) precipitates, and a heterogeneous grain structure, which consisted of fine soft recrystallized grains surrounded by harder relatively large unrecrystallized grains. The volume fraction and size of the nanoparticles were strongly dependent on the heat treatment, as was the partitioning of the V, i.e., almost evenly distributed between the f.c.c. matrix and L1(2) precipitates at 700 degree celsius but partitioned more to the f.c.c. matrix with a coefficient of 1.9 at 900 C. The yield strength (YS), ultimate tensile strength (UTS), and elongation to failure (epsilon) simultaneously increased with decreasing temperature from 298 K to 77 K. The V-doped MEA shows an excellent combination of strength and elongation, viz. YS of 1035 similar to 1637 MPa, UTS of 1587 similar to 1983 MPa, and epsilon of 17.6-37.7 % for the V-free MEA, and YS of 1274 similar to 1929 MPa, UTS of 1694 similar to 2147 MPa, and epsilon of 6.6-24.8 % for V-doped MEA. The excellent strength of the MEA resulted from a combination of back-stress hardening (1108-1195 MPa corresponding to 51-60 % of the flow stress), Hall-Petch strengthening, and hardening from pre-existing dislocations (199-355 MPa) at 298 K and 77 K. Increasing the aging temperature promoted a transition in the deformation behavior from heterogeneous to homogeneous, and produced a greater abundance of nanoscale deformation twins, stacking faults, and dislocation networks, leading to the exceptional monotonic decreased strain-hardening rates during tensile tests at 293 K and 77 K.