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

Abstract

We examine by transmission electron microscopy the precipitation state in the as-built microstructure of a hot-cracking sensitive nickel-based superalloy fabricated by laser powder bed fusion. Most observations are carried out on carbon extraction replica to isolate the precipitate signals from those of the matrix. Chemical 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 chemical analyses are completed by electron energy-loss spectrometry measurements made on thin foils. The large fields of view investigated allow for achieving a representative picture of the second-phase particles in the as-built microstructure. 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 precipitates themselves are also complex in terms of their inner structure: carbonitrides can present a core-shell structure of composition and are frequently twinned, alumina particles serve as nucleation sites for carbonitride aggregates, and several crystallographic forms of borides may coexist next to each other. These observations provide the necessary information to retrace the solidification path of the microstructure during fabrication and to discuss the precipitation mechanisms under the extreme processing conditions associated with laser powder bed fusion.