Micro-mechanisms and precipitation kinetics of delta (d) phase in Inconel 718 superalloy during aging Mohsen Rafiei, Hamed Mirzadeh * , Mehdi Malekan School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran article info Article history: Received 11 March 2019 Received in revised form 23 April 2019 Accepted 1 May 2019 Available online 4 May 2019 Keywords: Nickel-based superalloy Cold rolling d phase precipitation Kinetic studies Activation energy abstract The formation of d phase in Inconel 718 superalloy during aging was quantitatively studied based on improved analyzing techniques to unambiguously determine the mechanism of d phase precipitation and its kinetics. Based on the general transformation rate expressed in terms of the time corresponding to the transformed fraction of 0.5 or 50% and also the modified JohnsoneMehleAvramieKolmogorov (JMAK) equation, the average activation energy for the formation of d phase was determined as 212.4 kJ/ mol and 197.5kJ/mol, respectively. These values are comparable to the activation energy for diffusion of Nb in Ni (202.59 ± 4.71 kJ/mol). Therefore, the micro-mechanism for the formation of d phase was characterized as the diffusion of Nb in the matrix. Based on the kinetic studies, the temperature of 1055.7 K (782.7  C) was found as the lower bound for the formation of d phase, which was consistent with the precipitation-temperature-time (PTT) plot developed for this material. Finally, a formula was obtained to quantitatively show the effects of aging temperature and cold rolling reduction on the transformation rate for the precipitation of d phase. © 2019 Elsevier B.V. All rights reserved. 1. Introduction Due to its desirable mechanical properties at elevated temper- ature and low cost, Inconel 718 is known to be one of most common nickel-based superalloys [1e4]. The FCC Ni 3 (Al,Ti) g 0 phase, the BCT Ni 3 Nb g } phase, and the orthorhombic Ni 3 Nb d phase precipitate during aging of Inconel 718 superalloy [5e8]. The g 0 and g } phases are known as the main strengthening phases but d phase is nor- mally regarded as an undesirable phase. The g }and d phases are Nb-based intermetallics, where g }phase is a metastable phase and the d phase is the equilibrium one. As a result, the growth of the d phase occurs at the expense of g }phase [9, 10]. Therefore, formation of d phase is associated with the decreased precipitation hardening effect by depletion of the coherent g }precipitates. The increased susceptibility to hot cracking has been also noted in the d phase containing alloys [9]. However, this phase with appropriate morphology is noted for inhibition of grain growth during thermal processing [11], increased resistance to grain boundary creep failure [12], and providing the possibility to design efficient thermomechanical processing by affecting the dynamic recrystallization (DRX) [13e18], post-dynamic recrystal- lization [19,20], and interaction with the recrystallization of the cold rolled alloy [21 ,22]. Whether the detrimental or beneficial effects of the d phase are taken into account, the study of its pre- cipitation kinetics [9, 10,23e27] or dissolution [28,29] is vital. It has been shown that the prior cold rolling before aging for the for- mation of precipitate phases can significantly accelerate the ki- netics of precipitation [10,21 ,23,24,26]. The so-called apparent activation energy for precipitation of d phase (Q) in the cold rolled Inconel 718 superalloy has been re- ported in the range of 1113 to 577 kJ/mol by Liu et al. [23,24], where increasing the rolling reduction from 25 to 65% resulted in the decreased Q in this range. However, the same authors have noted that the value of Q should be less than 500 kJ/mol if the precipi- tation of d phase is controlled by diffusion of alloying elements. Therefore, they concluded that the formation of d phase involves a structure change, and hence, requires higher activation energy. As a result, it seems that this subject needs more attention with improved analyzing techniques to unambiguously determine the mechanism of d phase precipitation and its kinetics. Accordingly, in the present work, the micro-mechanisms and precipitation kinetics of delta phase in Inconel 718 superalloy during aging were investigated, which are important for the in- dustrial production. For this purpose, the precipitation mechanism * Corresponding author. E-mail address: hmirzadeh@ut.ac.ir (H. Mirzadeh). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2019.05.001 0925-8388/© 2019 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 795 (2019) 207e212