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

Abstract

The thermal stability of the coherent, nanoscale, L12 precipitates and fine grains in a strong, ductile medium-entropy alloy (MEA) Ni43.4Co25.3Cr25.3Al3Ti3 were analyzed and related to the room-temperature mechanical properties. Increasing the aging temperature and prolonging the soaking time produced more L12 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 L12 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 L12 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 L12 precipitates to dislocation looping is 23.5–30.4 nm. No significant coarsening of either the grain size or L12 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 L12 nanoparticles and the average grain size. This decrease in strength was accompanied by a slight increase in ductility at all three aging temperatures.