Nb rich precipitates in Inconel 718 produced by selective laser melting

Microstructure of nickel-based superalloy Inconel 718 produced by selective laser melting (SLM) was studied by electron microscopy. Multiple cycles of heating and rapid cooling during SLM lead to the formation of a complex microheterogeneous structure in the alloy. At the same time, a chemical analysis of such a microstructure showed a high homogeneity. The microstructure consisted of gamma grains with a size of about 10 mu m banded with delta phase particles and small carbides. Scanning and transmission electron microscopy examination showed that the. grains consisted of columnar and equiaxed subgrains. The intermetallic phases based on Ni3Nb (gamma '' and delta) were found to be homogeneously precipitated. The volume fraction of these precipitates was experimentally defined to be about 20%. The carbides with dimensions of 2 - 10 mu m typically observed in Inconel 718 manufactured by conventional ingot metallurgy were not detected. The untypical microstructure of the SLM material presumably resulted from fast cooling during SLM process. The maximum volume fraction of the intermetallic phases gamma ''' and delta was also calculated on the assumption that niobium was not dissolved in the matrix gamma phase. The calculated volume fraction of the niobium containing phases was determined to be 13.6%. Energy dispersive X-Ray analysis of niobium containing precipitates showed that the Ni3Nb based intermetallic phases also contained molybdenum and titanium. The maximum volume fractions of virtual intermetallic phases Ni3Ti and Ni3Mo were calculated on the assumption that these elements were not dissolved in the matrix gamma phase as well. The calculated volume fractions of these virtual phases were shown to be equal to 4.3 and 7.5%, correspondingly. The defined volume fraction of niobium containing phases was less than the calculated volume fraction (20% against 25.4%). This suggests that Nb, Ti and Mo were not completely presented in the niobium containing phases and were partially dissolved in the matrix gamma phase during SLM process.