Influence of heat treatment on the distribution of Ni ₂Nb and microsegregation in cast inconel 718 alloy

The influences of solution and homogenization heat treatments and grain size on the microstructure and mechanical properties of cast Inconel 718 alloy have been investigated. The microstructure of as cast In? 18 alloy consists of primary γ, eutectic (γ + NbC) and eutectic (γ + Ni ₂Nb). Fine and coarse grain structure specimens were achieved by controlling casting temperature. The fine grain specimen has a higher volume fraction of Ni ₂Nb, 4.99%, than the coarse grain one, 3.45%. The volume fraction of NbC is 1.0 to 1.3% in both fine and coarse grain specimens, and almost unchanged by solution heat treatment. In case of the fine grain specimen, the as cast volume fraction of Ni ₂Nb decreases by solution treatment to 1.26% at the temperature of 1403 K for 0.5 h holding time and completely dissolute after 4h at the same temperature. By increasing temperature to 1440 K, it takes only 2h to vanish Ni ₂Nb from the microstructure. The coarse grain structure gives lower tensile strength for both as cast and as solution treated in compare with fine grain ones. The volume fraction of Ni ₂Nb phase has a significant influence on the tensile strength and strain values, while it has no effect on the yield stress and hardness measurements. The fracture crack during tensile test is selectively initiated at Ni ₂Nb phase and propagated along it in interdendritic zone.