On the variety and formation sequence of second-phase particles in nickel-based superalloys fabricated by laser powder bed fusion

International audienceWe examine by transmission electron microscopy the precipitation state in the as-built microstructure of a nickel-based superalloy fabricated by laser powder bed fusion. Most observations are carried out on carbon extraction replica to isolate the different signals of the precipitates from those of the matrix. Composition maps measured by energy dispersive X-ray spectrometry are compared to phase and orientation maps obtained on the same precipitates by automated crystal orientation mapping. The composition analyses are completed by electron energy-loss spectrometry measurements made on thin foils. A large variety of nano-particles is found: (Ti,Nb)(C,N) carbonitrides, (δ) Al2O3 alumina, (Cr,Mo)3B2 and (Cr,Mo)5B3 borides, and the intermetallic phase Ni7Zr2. The precipitation state is also complex in terms of its structure: carbonitrides can present a core-shell structure of composition and are frequently twinned, alumina particles serve as nucleation sites for carbide agregates, and several crystallographic phases of borides may coexist next to each other. These observations allow one to retrace the genesis of the microstructure during fabrication and to discuss the precipitation mechanisms under the extreme processing conditions associated with laser powder bed fusio