δ Phase precipitation in Inconel 718 and associated mechanical properties

Microstructures generated by δ phase precipitation in Inconel 718 (IN718) and related mechanical properties were investigated in this paper. Several heat treatments enabling the precipitation of the δ phase in various proportions and morphologies were conducted. Heat treatments were performed in a temperature range between 875 °C and 975 °C with time durations from 0.5 to 24 h. For each test, the microstructures were characterized and the volume fraction of the δ precipitates quantified. In parallel, uniaxial tensile tests were conducted in order to determine the mechanical properties of the alloy, namely the elongation at necking (N%), the yield strength (YS), the Vickers hardness (HV) and strain hardening coefficients (K, n). The results revealed that when γ″ and δ precipitates coexist, the material remains hardened regardless of the amount of δ phase. However, when only δ phase were presented in the matrix, its volume fraction did not significantly affect the formability of the material. It was observed that the ratio between intragranular and intergranular precipitates could be a critical parameter. Intergranular precipitates, when in sufficient amount, led to a better formability of the material. On the contrary, when the intragranular precipitates were maximized, they tend to harden the material. A solution treatment at 975 °C for 2 h was finally suggested as the best compromise to improve the formability of IN718.