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Original Article
DOI: 10.1016/j.jmrt.2018.03.006
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The effect of a very high overheating on the microstructural degradation of superalloy 718
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Leonardo Sales Araujoa,
Corresponding author
lsales@metalmat.ufrj.br

Corresponding author.
, Clarissa Hadad de Melob, Rodrigo Pereira Gonçalvesa, Amanda de Vasconcelos Varelaa, Luiz Henrique de Almeidaa
a Programa de Engenharia Metalúrgica e de Materiais, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
b Departamento de Engenharia Metalúrgica e de Materiais, Universidade do Porto, Porto, Portugal
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Tables (3)
Table 1. Chemical composition (weight %) of the samples.
Table 2. Semi-quantitative EDS point analysis of the following regions of the microstructure (in weight %).
Table 3. Fraction of Nb-rich regions and grain sizes for various time intervals at 1300°C.
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Abstract

Under the harsh conditions imposed by the processing or due to an “overheating event”, the superalloy 718 may be exposed to very high temperatures, which can result in degradation of the microstructure. The time under such high temperatures can be short, as during the welding process, or long, as during reheating or homogenization of billets. Operational events or accidents that induce very high temperatures can also be a source of microstructural degradation. The present work aims to study the effect of a very high temperature on the degradation of the microstructure of the nickel base superalloy 718. Samples were heated up to 1300°C under air at different time intervals from 1 to 480min. The microstructural changes were evaluated by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction and compared with thermodynamic calculations. It was evidenced that even for a very short time as 1min, Nb segregation at grain boundaries induces constitutional liquation and oxidation at those regions, as well as the formation of Laves phase and Nb-rich MC carbides with “Chinese script” morphology. With the continuous grain growth, a more intense liquation of the grain boundaries is induced, up to circa 60min. After this threshold value, the intergranular liquation is “channeled out” via the interconnected grain boundary network, resulting in the disruption of grain boundaries and accelerated structural collapse of the material.

Keywords:
Superalloy 718
Grain boundary liquation
Oxidation
Full Text
1Introduction

Nickel-based superalloys are high strength materials finding applications as structural components. The operational environment usually involves high temperatures and/or degrading atmospheres. The superalloy 718 is a precipitation hardened alloy with good formability and weldability, able to operate from cryogenic temperatures up to 650°C [1]. The alloy presents significant additions of alloying elements such as Fe, Cr, Mo, Nb, Al and Ti and it is prone to intense segregation [2,3]. Various phases can be formed during processing or after specific heat treatments. For fabrication processes such as welding the alloy is exposed to temperatures up to the melting point. Specifically in the heat affected zone, liquation along grain boundaries and phase interfaces are likely to occur. But, for almost all the welding processes, the heating rate is very high, meaning that the residence time at this temperature is short and, consequently, the liquation is limited to a narrow region. Operational events or accidents can also lead to very high temperatures, resulting in material degradation. Such occurrences can present durations longer than the time related to welding processes. As a consequence, more intense grain boundary liquation is expected. Furthermore, high temperature oxidation can aggravate the degradation process. For example, Greene and Finfrock [4] presented the case of clads for water cooled tungsten targets. Overheating accidents can lead to temperatures exceeding 1200°C with a duration of tens of minutes. Reheating processes previous to thermomechanical processing or high temperature heat treatments such as the homogenization can be another source of overheating [5,6]. The process temperatures are usually close to 1200°C, but variations on the temperature profile inside the reheating furnaces can lead to overheating occurrences [7–9]. Dupont et al. [10] related the liquation phenomena with segregation, constitutional liquation or melting of eutectic constituents. Greene and Finfrock [4] conducted very high temperature oxidation evaluations and showed that the oxidation behavior of the superalloy 718 can change, depending on the temperature range. In this regard, the objective of the present work is to evaluate the degradation of the microstructure via an emulation of an overheating event at 1300°C from 1min to 480min.

2Methods

Samples of superalloy 718 with dimensions of 12mm×12mm×4mm were cut from a 4-mm thick hot rolled plate. Table 1 presents the chemical composition of the samples, determined by optical emission spectroscopy. The samples were submitted to a solution annealing heat treatment at 1030°C for 2h, to promote full recrystallization and dissolution of the δ, γ′ and γ″ phases.

Table 1.

Chemical composition (weight %) of the samples.

Ni  Cr  Fe  Nb  Mo  Al  Ti  Co  Mn  Si  Cu 
53.70  18.10  18.10  5.15  3.03  0.47  1.01  0.13  0.24  0.09  0.05  0.002  0.005  0.006 

Thermodynamic calculations were performed, aiding to define the overheating temperature and the phases present. The calculations were made with the Thermocalc software [11] with the thermodynamic database TCNI8 [12]. Based on the calculations, the temperature defined was 1300°C. At this temperature, it was indicated by the calculations that the Nb-rich MC carbide would be completely dissolved, as presented in Fig. 1. The time intervals set for the tests were 1, 5, 15, 30, 60, 120 and 480min. A muffle furnace was used, with a thermocouple positioned close to the samples, and the temperature was maintained in the 1300±5°C interval. After the heat treatments, the samples were cooled in water. The metallographic preparation consisted of mounting the samples in resin, grinding with emery paper up to 2500meshs and polishing with diamond paste up to 1μm. The microstructural evaluation was conducted with a JEOL JSM 6460 LV and FEI Versa 3D scanning electron microscopes (SEM), using backscattered and secondary electrons modes and energy dispersive spectroscopy (EDS) with 20kV. For each sample, five images were made. The image analysis was performed with the Image J open source software. The grain size measurement procedure was based on the Heyn method, described in the ASTM E112-13 standard [13]. The phase fraction was calculated from binary images, transformed from the backscattered SEM images with 8-bits grayscale. The phases presented were also analyzed by X-ray diffraction using a Shimadzu XRD 6000 equipment (Cu k-α emission, scan speed of 2°/min and 10–80° scan range).

Fig. 1.
(0.14MB).

Diagram showing the mass fractions of phases formed. At 1300°C, the (Nb,Ti)C carbide is completely dissolved. Thermodynamic database used: TTNI8.

3Results and discussion

Fig. 2 shows the microstructures observed via SEM (in backscattered mode) for the samples after solution annealing and no overheating emulation and after the overheating emulation at 1300°C for various time intervals. The brighter areas in the images are associated with Nb-rich phases. For the solution annealed sample, only blocky Nb-rich MC carbides were evidenced, disposed as particle strings following the rolling direction. For the samples exposed to the overheating emulation, a longer residence time at 1300°C resulted in an increasing fraction of Nb-rich regions, preferentially at the grain boundaries, as well as an increase in grain size. Additional regions of Nb-rich phases were observed inside some grains. These morphologies were noticed even for a short time as 1min. Fig. 3 shows the detailed image of the microstructure at the grain boundary region, showing the Nb-rich phases. The lamellar morphology observed is associated with Laves phase, resultant of previous liquation of grain boundaries [10,14]. A “Chinese script” morphology is also observed, and is associated with (Nb,Ti)C carbides [14,15]. At 1300°C there is the dissolution of the bulky MC particles. Upon cooling, the (Nb,Ti)C carbides are re-precipitated with this morphology. Table 2 presents the EDS point analysis performed at: the matrix, inside the grain; grain boundary region; Laves phase and; (Nb,Ti)C particles with “Chinese script” morphology. Fig. 4 shows thermodynamic diagrams indicating that an increasing concentration of Nb gradually reduces the liquidus temperature, as well as promotes the formation of Laves phase during cooling. A Nb concentration of around 8% in weight is sufficient to promote Laves phase between 1140 and 1180°C. With the reduction of temperature, there is a decreasing mass fraction of liquid, but with the remaining liquid being continually enriched by Nb, achieving circa 11% in weight.

Fig. 2.
(1.46MB).

Microstructure of the samples: (a) solution annealed microstructure; after overheating emulation at 1300°C for: (b) 1min; (c) 5min; (d) 15min; (e) 30min; (f) 60min; (g) 120min; and (h) 480min. All images were made in backscattered mode.

Fig. 3.
(0.55MB).

Detailed image of the sample submitted to overheating at 1300°C for 120min. The Nb-rich regions (brighter areas), show lamellar morphology characteristic of the Laves phase and the “Chinese script” morphology (indicated by the yellow arrows), associated with MC carbides. Image in backscattered mode. The numbered crosshairs indicate the points chosen for the EDS analysis (results presented in Table 2).

Table 2.

Semi-quantitative EDS point analysis of the following regions of the microstructure (in weight %).

Region  Nb  Ti  Mo  Cr  Fe  Ni 
Grain boundary (Pt 1)  9.90  1.73  3.21  16.05  14.70  Bal. 
(Nb,Ti)C (Pt 2)  22.85  2.06  3.37  14.31  12.81  Bal. 
Laves phase (Pt 3)  15.53  1.87  3.30  13.78  12.27  Bal. 
Intragranular (Pt 4)  3.19  0.64  2.66  18.92  19.68  Bal. 

Pt 1: grain boundary region with Nb segregation;

Pt 2 (Nb,Ti)C particle with “Chinese script” morphology;

Pt 3: Laves phase;

Pt 4: intragranular region (point not shown in Fig. 3).

Fig. 4.
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(a) Pseudo-binary diagram showing that an increasing fraction of Nb reduces the liquidus temperature and promotes Laves phase formation and (b) diagram showing that with reduction of temperature, the decreasing amount of liquid becomes enriched with Nb. Thermodynamic database used: TTNI8.

Fig. 5 shows the XRDs of the samples. In addition to the γ-matrix, (Nb,Ti)C carbide and Laves phase, different oxides were evidenced, even for the sample exposed to the 1min interval. For 120 and 480min, the peaks related to the Nb-rich phases and oxides become less intense, as compared to the γ-matrix peaks, indicating the gradual loss of such phases, which corroborate the findings based on image analysis. At high temperatures, different types of Nb oxides can be formed, especially NbO and Nb2O5[16,17]. The formation of Nb oxides is related with the affinity to the Nb segregated in regions as grain boundaries and the oxidation and decomposition of Nb-rich MC carbides [16–18]. Estimates of the depth of penetration of the oxidation into the surface were made based on the data for superalloy 718 at 1300°C presented in the work of Greene and Finfrock [4]. The depth of penetration varied from 1.6μm (for 1min) to 35.9μm (for 480min). However, it should be considered that, in addition to the fact that the oxygen diffusion occurs preferentially through the grain boundaries, the liquation of these regions would enhance even further the oxygen penetration and Nb segregation. Additionally to the Nb-rich, other oxides may be formed in superalloy 718, such as TiO2, MnCr2O4, Cr2O3, Nb2TiO7 and MoO3[19].

Fig. 5.
(0.17MB).

X-ray diffractograms of the samples.

As aforementioned, for an increasing overheating time, the grain size and area fraction of Nb-segregated regions and Nb-rich phases increased accordingly, as presented in Table 3. The initial grain size (for the solution annealed sample) was 65.49±5.18μm. The grain size continually increased with time, reaching 154μm for 480min. Concerning the area fraction of Nb-rich regions, even for a short time as 1min a significant amount of Nb segregation is induced, resulting in the liquation of the boundaries and (Nb,Ti)C particles. Subsequent solidification results in the formation of Laves phase and reprecipitation of (Nb,Ti)C with “Chinese script” morphology. The area fraction values of the Nb-rich phases do not change significantly up to 60min. Then, a sharp decrease is observed between 60 and 480min. Fig. 6 shows a diagram in which a ratio between the area fraction of the Nb-rich phases (Laves+ (Nb,Ti)C) to the grain size (“AF/GS ratio”) was established and plotted against the overheating time. As the dominant sites for the liquation and subsequent formation of Laves and (Nb,Ti)C are the grain boundaries, this AF/GS ratio can be related to the amount of previous liquation and consequent formation of Nb-rich phases per length of boundary. The analysis of the AF/GS ratio with overheating time shows a two-stages behavior, with the threshold time of 60min. As the grain size continually increases, the grain boundary extension decreases, resulting in an ongoing reduction of the AF/GS ratio, meaning that the liquid is increasingly concentrated at grain boundaries, especially up to the 60min threshold. After this time, an additional mechanism acts, reducing the amount of liquid along grain boundaries. Greene and Finfrock [4] showed that at the temperature of 1300°C, the oxidation becomes catastrophic and the samples can be deformed by the influence of its own weight. As the grain boundaries form an interconnected network, the increasing amount of liquid segregated at these regions, in conjunction with the oxidation and structural collapse of the sample, can lead to the “channeling” of the intergranular liquid and oxides out at the surface of the sample. This “channeling effect” is similar to the one observed during solidification of superalloy 718 ingots [20]. Initially, the boundaries close to the surface become gradually “segregation-lean” and, with the aid of oxidation along grain boundaries, a sharp disruption of boundaries occurs. This disruption and the partial loss of the intergranular species help to explain the reduction of the oxide and Laves peaks on the diffractograms for the 120 and 480min samples, as the proportion of the matrix to the remaining phases is increased. Fig. 7 shows the 120 and 480min samples, with the “segregation lean” boundaries close to the surface, as well as partial or complete disruption of the boundaries.

Table 3.

Fraction of Nb-rich regions and grain sizes for various time intervals at 1300°C.

Time (min)  Area fraction of Nb-rich phases (%)  Grain size (μm) 
18.79±2.22  69.71±5.18 
15.46±1.39  69.38±4.36 
15  19.36±2.04  91.14±9.28 
30  19.21±0.68  91.31±5.70 
60  19.06±3.47  103.20±5.52 
120  13.56±1.08  113.34±6.43 
480  9.67±1.68  154.04±7.77 
Fig. 6.
(0.15MB).

Relation between the ratio between area fraction to the grain size (AF/GS) versus time at 1300°C.

Fig. 7.
(0.98MB).

Samples exposed at 1300°C for: (a) 120min, showing a “segregation-lean” zone close to surface and (b) 480min, showing the disrupted boundaries. Image in backscattered mode.

4Conclusions

Based on the findings presented above, the following conclusions can be addressed:

  • -

    when the material is exposed to a temperature of 1300°C, extensive and continuous liquation at the grain boundaries was promoted even for a short time as 1min, evidenced by the Nb segregation at grain boundaries and formation of Laves phase and (Nb,Ti)C with “Chinese script” morphology during cooling;

  • -

    the grain size increased with the overheating time, decreasing grain boundary extension in the samples. This, in turn, resulted in more liquid segregated per length of boundary and the consequent precipitation of Nb-rich phases during cooling;

  • -

    however, after a threshold time (60min in the present study), the segregation behavior changed sharply, with an accelerated loss of liquid from the boundaries and the disruption of these boundaries, especially near the surface of the sample which, in turn, was related to the structural collapse of the material due to the intense liquation and the high degree of oxidation of the alloy.

Conflicts of interest

The author declares no conflicts of interest.

Acknowledgements

The authors would like to thank CAPES, CNPq and FAPERJ for the financial support for this study.

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Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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