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) 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 x 10(-24) m(3)/s at 700 degrees C, 4.78 x 10(-23) m(3)/s at 800 degrees C, and 4.32 x 10(-22) m(3)/s at 900 degrees C), which mainly results from the strong pinning from the high density of precipitates and high activation energy for grain growth (similar to 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 x 10(-30) m(3)/s at 700 degrees C, 2.03 x 10(-28) m(3)/s at 800 degrees C, and 7.16 x 10(-27) m(3)/s at 900 degrees 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 degrees C. In comparison, both the YS and UTS of MEA aged at 800-900 degrees 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.