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

Abstract

The microstructural evolution, including precipitate size, volume fraction, morphology, and distribution of gamma ' precipitates in the gamma 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 degrees 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 gamma '-precipitates with particle sizes smaller than 60 nm is observed for all compositions. For the longer aging times (t >= 64 h), the gamma '-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 gamma '-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 gamma '-precipitate size, slowest coarsening rate and highest microhardness.