Revealing the true partitioning character of zirconium in additively manufactured polycrystalline superalloys

Abstract

• Experimental evidence of the migration of zirconium away from grain boundaries during heat-treatment of nickel-based superalloys fabricated by laser powder bed fusion. This contradicts the classic literature on the subject. • Experimental evidence of the reduction of alumina oxides by zirconium during heat-treatment. • Experimental evidence of zirconium dissolved in the γʹ precipitates and the γ matrix after heat-treatment.. • Quantification of the zirconium partitioning after heat-treatment. Minor addition of zirconium is common in polycrystalline nickel-based superalloys, where it is believed that it segregates at grain boundaries and contributes to increase the creep resistance. However, in superalloys produced by additive manufacturing, zirconium may become detrimental as it promotes hot-cracking during the fabrication stage. Here, we clarify the controversial role of this element by studying its distribution at near atomic scale in the as-built and heat-treated microstructures. In the as-built microstructure, zirconium is almost exclusively found at grain boundaries. However, after heat-treatment, zirconium is no longer found at grain boundaries. Instead, it partitions in γʹ precipitates and zirconium oxides particles. The formation of zirconia is shown to originate from the reduction of nano-particles of alumina by zirconium during heat-treatment. The absence of zirconium at grain boundaries in this state challenges the classic view often reported in the literature for superalloys.