Abnormal secondary strengthening phenomenon induced by multiple fracture behavior of δ phases in IN718 alloy fabricated by arc-based directed energy deposition

The room-temperature tensile strength of Inconel 718 alloy (IN718), fabricated by arc-based directed energy deposition (DED), exhibited an obvious "M"-shaped trend with increasing δ phase precipitation time, and this "M"-shaped trend was confirmed at 900, 930, and 960 °C within the δ phase precipitation temperature window of 890 - 980 °C. Experimental results indicated that the first tensile strength peak was attributed to the discrete precipitation of fine granular δ phases at grain boundaries, while the second tensile strength peak (i.e., secondary strengthening) was primarily due to the occurrence of multiple fracture behaviors of the slendered δ phases. The characterization results of scanning electron microscopy showed that the δ phase precipitation time corresponding to the second peak could ensure that the δ phase precipitation was maximized without affecting the γ′′ strengthened phases, resulting in a significantly higher number of granular δ phases formed by slendered δ phases after multiple fracture behaviors than that at the first peak. In addition, transmission electron microscopy characterization results revealed that the unique lattice orientation relationship between δ/γ prevented crack sources introduced by the multiple fracture behavior of δ phases from penetrating the high-energy interface between δ/γ, effectively avoiding the fracture of the nearby γ matrix. In summary, this new discovery that the multiple fracture behavior of δ phases can effectively improve the mechanical properties of the alloy provides new ideas and insights for designing new post-treatment processes for DED IN718 alloy to comprehensively improve its strength and plasticity.