Insights into Nb-silicide-based in-situ composite processed by electron beam powder bed fusion

Abstract

This is the first report introducing the idea of processing Nb-silicide-based in-situ composite through electron beam powder bed fusion (PBF-EB). A high energy input together with a line order strategy resulted in cracking, whilst a lower energy input without the line order led to microstructural inhomogeneity characterised by alternating dark- and bright-contrast bands. In samples printed using the line order strategy, long cracks initiated close to the melt pool boundary and propagated into the previous layer. These cracks were due to the weak interface between the primary Nb3Si columnar grains and the equiaxed (Nb,Ti) solid solution phase or Nb3Si-(Nb,Ti) solid solution eutectic structure. The porosity level increased from 0.122 % to 0.547 % with the increase of area average energy input from 2.22 J/mm2 to 6.67 J/mm2. The density decreased to 6.80 g/cm³ in the line order strategy samples due to the presence of cracks, compared to 6.84 g/cm³ in samples without employing the line order strategy. The dark-contrast band consisted of (Nb,Ti) solid solution, Nb3Si and Nb5Si3, while the bright-contrast band was composed of (Nb,Ti) solid solution and Nb3Si. Within the bright-contrast band, a recurring layered microstructural inhomogeneity appeared, with columnar grains at the bottom, whilst fine- and coarse-grained eutectic networks in the middle and upper regions. The Nb3Si phase exhibited a pronounced {001} texture, whereas the (Nb,Ti) solid solution phase displayed a weaker texture. The Nb-silicide-based in-situ composite fabricated through PBF-EB had a microhardness of ∼700 HV0.5. This value is higher than that achieved through conventional means but falls within the mid-range for laser-based additive manufacturing counterparts.