Uncovering the supersaturation limit of Cu in Fe during additive manufacturing

Additive manufacturing illustrates great potential in breaking the supersaturation limit of Cu in steels fabricated via conventional casting processes. Considering that the maximum solubility of Cu in γ-Fe at elevated temperatures is approximately 14 wt%, we selected a Cu content slightly above this value and fabricated an Fe15Cu (wt%) immiscible alloy via laser powder bed fusion (LPBF). Microstructural analysis reveals that Cu atoms exist in four different forms in the as-built alloy, viz., intergranular Cu phase, intragranular Cu nanoparticles (25–140 nm), coherent B2-FeCu phase (2–5 nm) and solute Cu in the Fe matrix. Specifically, the intergranular Cu phase forms due to the continuous enrichment of Cu atoms in the remaining liquid during the solidification of γ-Fe. In contrast, the intragranular Cu nanoparticles precipitate out within the supersaturated Fe matrix upon cooling. The formation of B2 phase within the matrix is attributed to the thermal cycles inherent to LPBF. In general, up to 12 wt% of Cu has been incorporated into the Fe matrix, approaching the equilibrium high-temperature limit of 14 wt%. This work may provide some guidance for the development of high-performance Cu-containing steels and similar immiscible systems via additive manufacturing.