Solidification behavior and Laves phase dissolution during homogenization heat treatment of Inconel 718 superalloy (original) (raw)

Highlights

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The presence of the eutectic structure in Inconel 718 with ∼5 wt% Nb was explained.
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The extension of the eutectic isotherm to 4.25 wt% Nb was scientifically discussed.
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Electroetching in HCl-HNO3-Glycerol was a proper microstructure revealing technique.
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The Laves phase was characterized as Ni2Nb based on EDS elemental analysis.
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Dissolution time of Laves phase at 1150 °C was estimated as ∼28 h.

Abstract

The solidification behavior and microstructural evolutions during homogenization heat treatment of Inconel 718 superalloy were studied. The microstructure of the as-cast alloy was characterized by optical micrographs, scanning electron microscopy (SEM) images, and elemental analysis based on energy dispersive spectroscopy (EDS). The optimum etching techniques, especially electroetching, were characterized to reveal the dendritic nickel matrix, the microsegregation between dendrite arms, and the austenite/Laves eutectic structure. The elemental analysis of different microstructural features, especially the variation of the chemical composition of austenite, was successfully related to the pseudo-binary solidification constitution diagram. Moreover, the extension of the nonequilibrium eutectic isotherm was evaluated to unravel the solidification path of this important engineering alloy. The dissolution of the Laves phase and its kinetics during exposure at elevated temperatures with the purpose of homogenization were also discussed.

Introduction

Nickel-based superalloys are austenitic precipitation hardening alloys, which are important structural materials for many engineering applications [[1], [2], [3], [4], [5], [6]]. Among them, Inconel 718 is more popular than any other superalloy. This alloy is precipitation strengthened by special alloying elements such as Nb and also contains refractory elements such as Mo [[7], [8], [9], [10]]. The presence of these elements makes the alloy susceptible to severe microsegregation between dendrite arms and formation of undesirable Laves phase. The latter is considered as a type of harmful topologically close-packed (TCP) phases. On the one hand, the heavily solute concentration regions are highly detrimental to fatigue life. On the other hand, the Laves phase is associated with decreased mechanical performance due to consumption of a large amount of Nb as the principal element required for precipitation hardening of the alloy. Its own brittle nature and low melting point [[11], [12], [13]] are also problematic. The likelihood of its presence increases as the solute segregation of the ingot increases.

The solidification of Alloy 718 and other Nb-bearing austenitic superalloys begins with the formation of Nb-lean austenitic dendrites. Interdendritic eutectic-type solidification constituents involving MC-type carbides and a Nb-rich Laves phase occur in these alloys. While it has been stated that the austenite/Laves eutectic constituent terminates solidification in these alloys [14], it is not clear how the eutectic reaction takes place in alloys with Nb content lower than the eutectic start point at 9.6 wt%. Niobium is the dominant element in the evolution of solidification microstructure with C and Si affecting the amounts of austenite/MC and austenite/Laves constituent observed. In Alloy 718 with high amount of Nb (4.75–5.5 wt%), austenite/Laves eutectic is the predominant minor constituent observed beside the austenite phase [14].

During reheating, there is an incipient melting temperature (IMT) associated with the presence of undesirable phases, which is reported to be between 1160 and 1180 °C [[15], [16], [17]]. The dissolution of Laves phase occurs during exposure at elevated temperatures (but lower than IMT) by back-diffusion of the solute atoms from the solute-rich regions into the matrix without local melting [15]. This homogenization treatment plays an essential role in obtaining uniform microstructures free from harmful phases [10,[15], [16], [17], [18], [19], [20]]. The exposure temperature is considered close to the IMT to decrease the required homogenization time.

While the solidification behavior of Inconel 718 has been studied so far [[21], [22], [23], [24]], the interpretation of the presence of obtained phases and their chemical composition based on the phase diagram has not been unraveled completely. In fact, it is not clear how the eutectic reaction takes place in the Inconel 718 alloy with Nb content of ∼5 wt%, which is lower than the eutectic start point at 9.6 wt% Nb. Thus, this subject needs more experimental work by consideration of rigorous elemental and microstructural analyses to unravel the solidification path of this alloy. Moreover, the etching techniques for studying the as-cast structure of Inconel 718 for characterization of microstructural features (austenite dendrites, segregated regions, and eutectic structure) based on more appropriate chemical and electroetching techniques need to be reevaluated and compared with the usual procedure. Therefore, the present work aims to deal with the solidification behavior of Inconel 718 superalloy and metallographic etching. Afterward, the homogenization treatment for dissolution of Laves phase will be discussed.

Section snippets

Experimental details

A forged Inconel 718 superalloy was vacuum arc remelted and cast in a water-cooled copper mold (with dimensions of 6 × 2.5 × 1 cm3) to obtain a superalloy with chemical composition as shown in Table 1. It can be seen that the Nb content of the as-cast alloy is close to the nominal composition reported for Inconel 718 alloy (5 wt%) [13]. The homogenization treatment was performed at 1150 °C for holding durations of 1, 2, 3, and 5 h as schematically shown in Fig. 1. The choice of this temperature

As-cast microstructure

SEM images of the as-cast alloy along with part of the representative pseudo-binary solidification constitution diagram are shown in Fig. 2. It can be seen in Fig. 2a that the as-cast alloy is mainly consisted of austenite dendrites as well as some eutectic structures between dendrite arms. A SEM image at higher magnification is shown in Fig. 2b, where the presence of the eutectic constituent is evident. The phase diagram shown in Fig. 2c implies that the eutectic structure should not be

Conclusions

The solidification behavior and the dissolution of the Laves phase during homogenization of Inconel 718 superalloy were studied. The following conclusions can be drawn from this work:

(1)
Chemical etching by the renowned Kalling's reagent number 2 was found to be not the most appropriate technique to clearly distinguish the dendritic nickel matrix, the microsegregation between dendrite arms, and the austenite/Laves eutectic structure. Several other etchants were evaluated, and conclusively,