Microstructural Evolution and Coarsening in Ni–Al–Cr–Co Model Superalloys with Nano γ′-Precipitates: An Experimental and Thermodynamic Investigation

The microstructural evolution, including precipitate size, volume fraction, morphology, and distribution of γ′ precipitates in the γ matrix, as well as coarsening kinetics and hardness of Ni–11Al–9Cr, Ni–11Al–9Cr–2Co, and Ni–11Al–9Cr–4Co model superalloys aged at 800 °C for 1–256 h, are studied experimentally and thermodynamically. Thermodynamic investigations show that Co addition slightly changes the solvus, solidus, and liquidus temperatures. For early aging times (t < 64 h), spheroidal γ′-precipitates with particle sizes smaller than 60 nm is observed for all compositions. For the longer aging times (t ≥ 64 h), the γ′-precipitates become slightly coarser, the precipitate radius increases, and their morphology starts to transform into cuboidal. For almost all compositions and aging conditions, the average γ′-precipitate radius is lower than 100 nm. Co refines the microstructure, reduces the coarsening rate, and increases the microhardness. Among investigated compositions, Ni–11Al–9Cr–4Co superalloy exhibits the lowest γ′-precipitate size, slowest coarsening rate and highest microhardness.