Secondary strengthening phenomenon and δ phase precipitation behavior of wire arc additive manufactured Inconel 718 alloy by room temperature tension

The room tensile strength of Inconel 718 alloy (IN718), fabricated by wire arc additive manufacturing, exhibited an obvious "M"-shaped trend (i.e. secondary strengthening) with increasing solution time within 890 - 980 ℃, and two strength peak positions showed obvious backward trend with the increasing solution temperature. The experimental results indicated that first strength peak obtained some fine granular δ phases at grain boundaries, but did not affect the volume fraction ratio of γ'' strengthening phases. However, when solution time was extended to second strength peak, the volume fraction of γ'' strengthening phases decreased slightly but its average particle size was invariant, accompanied by a notable increase in the number of fine granular δ phases at grain boundaries. Based on the fracture model established for δ phase, it is demonstrated that the multiple fractures of slender needle-shaped δ phase are the main reason for the obvious rise in the number of fine granular δ phases at grain boundaries of the second strength peak. Relative to MC carbide fracture, TEM results finally demonstrate that multiple fractures of δ phase introduce more fine granular δ phases, but this fracture behavior cannot cause for IN718's failure because of its lower lattice mismatch.