Growth behavior of the δ-Ni₃Nb phase in superalloy 718 and modified KJMA modeling for the transformation-time-temperature diagram

The δ-Ni₃Nb phase is indispensable in achieving the uniform microstructure containing refined grains during the hot-forging process of superalloy 718. The growth behavior of the δ-Ni₃Nb phase at temperatures ranging from 1213 to 1283 K was investigated. The heat-treatments were performed in the argon atmosphere, and annealing time for the precipitation of the δ-Ni₃Nb phase varies from 10.8 to 93.6 ks. The measurement of the δ-Ni₃Nb phase was performed on the backscattered electron (BSE) image. The positive influence of small grain size on accelerating the precipitation process of the δ-Ni₃Nb phase was confirmed. The δ-Ni₃Nb phase was found to nucleate at grain boundary preferentially, and the evaluation of the δ-Ni₃Nb phase strongly depends on the grain size. A grain-size-dependent Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation, [Formula presented] where ξ is the transformed phase fraction over the equilibrium fraction, K(T) is the temperature-dependent term, ρ_l is the length of the grain boundary per unit area, m is a constant, t is the time in second, and n is a nucleation index, was used by considering proper nucleation and growth behaviors. The predicted TTT curve agrees well with the reported experimental data. The presently proposed KJMA model and plotted TTT curve provide the valuable information for the microstructure control of superalloy 718, accelerating the development of superalloy 718.