Impact of chemical composition on precipitate morphology in an additively manufactured nickel base superalloy

Changes in the iron content within the allowable composition range in Inconel^® 625 produce significant differences in the grain size and mechanical properties during laser-based directed energy deposition additive manufacturing. The resulting precipitate types and morphologies, which contribute to these different properties, originate from a complex interplay between iron, silicon, and titanium. While the addition of iron has traditionally been shown to promote Laves phase formation, thermodynamic calculations have shown that silicon contents in excess of 0.05 wt% are also required to promote its formation. For example, relatively low iron (1 wt%), low titanium (0.019 wt%), and high silicon (0.37 wt%) contents led to the formation of Laves phase. On the other hand, high iron (4 wt%), high titanium (0.19 wt%), and low silicon (0.035 wt%) contents produce MN nitrides and no Laves phase. By increasing the titanium concentration, the precipitation of nitrides rich in titanium and niobium is promoted in the liquid, and Laves phase formation is suppressed during solidification. After hot isostatic pressing, the precipitate distribution displayed minimal differences with that found in the as-deposited condition for both material chemistries. However, the precipitates in the as-deposited low iron material grew larger and developed a blocky morphology while the original Laves phase partially transformed to M₂N nitrides rich in Nb, Cr, and Mo. In the high iron material, the MN nitrides became depleted in niobium and further enriched in titanium while maintaining a size similar to those in the as-deposited condition.