Thermal stability of microstructure and their influences on mechanical properties of precipitation-hardened medium-entropy alloy Ni43.4Co25.3Cr25.3Al3Ti3.

The thermal stability of the coherent, nanoscale, L1 2 precipitates and fine grains in a strong, ductile medium-entropy alloy (MEA) Ni 43.4 Co 25.3 Cr 25.3 Al 3 Ti 3 were analyzed and related to the room-temperature mechanical properties. Increasing the aging temperature and prolonging the soaking time produced more L1 2 precipitates and reduced the matrix lattice constant. The fine grains exhibit excellent thermal stability with a very low coarsening rate (1.73 × 10−24 m3/s at 700 °C, 4.78 × 10−23 m3/s at 800 °C, and 4.32 × 10−22 m3/s at 900 °C), which mainly results from the strong pinning from the high density of precipitates and high activation energy for grain growth (∼272 kJ/mol). The L1 2 precipitates remain spherical and have a coherent relationship with the matrix during ripening from tens to hundreds of nanometers. They exhibit better thermal stability (6.45 × 10−30 m3/s at 700 °C, 2.03 × 10−28 m3/s at 800 °C, and 7.16 × 10−27 m3/s at 900 °C) than the L1 2 precipitates in nickel-based superalloys by 1–2 orders of magnitude with an activation energy of 341 kJ/mol. The critical size for the transition from dislocation shear of the L1 2 precipitates to dislocation looping is 23.5–30.4 nm. No significant coarsening of either the grain size or L1 2 precipitates caused a small decrease in the YS and UTS of MEA aged at 700 °C. In comparison, both the YS and UTS of MEA aged at 800–900 °C dramatically decreased, which is caused by the coarsening of the L1 2 nanoparticles and the average grain size. This decrease in strength was accompanied by a slight increase in ductility at all three aging temperatures. • Activation energy for L1 2 precipitates growth of 341 kJ/mol caused good thermal stability. • High dense precipitates resulted in strong pinning pressure for fine grains coarsening. • Activation energy for grain growth is as high as 272 kJ/mol. • Transition from dislocation shearing/looping L1 2 precipitates is 23.5–30.4 nm. • Significant coarsening of grains and L1 2 precipitates occurs at 800–900 °C. [ABSTRACT FROM AUTHOR]