Phase transformations and grain growth behaviors in superalloy 718

The phase transformation and grain growth behaviors of commercial Ni-Fe-base superalloy 718 at 1323, 1373, 1423 and 1473 K were investigated through DSC and SEM methods. The transformation temperatures of γ + NbC→L and γ→L were determined as 1495 and 1555 K, respectively, and TiN was shown to gradually dissolve into the formed L. The pinning effect caused by Nb-rich carbide (NbC) and Ti-rich nitride (TiN) was observed, and the mean size of the grains increased with holding time at each temperature. The grain growth exponent, m, was confirmed to be 4, indicating that grain growth was controlled by the coarsening of the precipitate through grain boundary diffusion. The grain boundary diffusion of Nb and N was confirmed to control the coarsening of NbC and TiN, respectively. The major factor for γ grain growth was confirmed to be the coarsening of NbC, and the activation energy for grain growth was calculated as 417 kJ·mol⁻¹. The equations for grain growth were determined as G⁴-G₀⁴ = 0.10 × 10⁶ × t (1323 K), G⁴-G₀⁴ = 0.49 × 10⁶ × t (1373 K), G⁴-G₀⁴ = 2.01 × 10⁶ × t (1423 K) and G⁴-G₀⁴ = 3.89 × 10⁶ × t (1473 K), for all four temperatures respectively. The predicted grain growth curves were in good agreement with the experimental data.