1. Introduction
The nickel-based superalloy was widely used in gas turbines due to its excellent resistance to corrosion and oxidation under severe high-temperature conditions [
1]. The different element ratios and heat-treatment conditions have significant influence on the properties of the alloy. Therefore, there is still a lot of room to improve the performance of superalloy turbines. To further upgrade the strength and resistance to corrosion and oxidation of superalloy, the strengthening process or mechanism has been extensively investigated.
According to previous studies, the different elements ratios and heat treatment temperature can influence the lattice constants of the matrix and precipitated phase and then affect the properties of the alloy [
2,
3,
4,
5,
6,
7]. The thermal corrosion resistance and high-temperature oxidation resistance of superalloy can be improved by adding trace grain boundary strengthening elements of B and Zr or refractory elements such as W, Mo, Nb, and Hf [
2]. Changes in the microstructure by a suitable heat treatment process can further improve the performance of the alloy. As is well known, the microstructure of casting nickel-based superalloy consists of the γ′ phase, carbides, borides, nitrides, TCP phases, γ/γ′ eutectic structure, and other precipitated phases. The heat-treatment process often includes three steps: the solid solution treatment, the intermediate treatment, and the aging treatment. The research shows that the cast structure of a coarse γ′ phase, the carbides, and γ/γ′ eutectic can be dissolved in the matrix through the high-temperature solution treatment [
3,
4,
5,
6], while the following intermediate and aging treatments contribute to the formation of a fine γ′ phase [
7]. Therefore, the heat treatment plays an important role in controlling the shape, size, distribution, and volumes of γ′ phase, γ/γ′ eutectic, and carbides.
In addition, the solid-solution treatment can also improve the elements’ segregation. It promotes the separation of W elements in dendrites and diffusion to interdendrites. Moreover, elements of Hf, Nb, Ti, and Cr diffused into the dendrites, the segregation degree of each component of the alloy was greatly reduced, and the mechanical properties of the alloy were improved [
8]. However, the excessively high solution treatment temperature will cause the “incipient melting” phenomenon [
9,
10]. Incidentally, the cooling has an important influence on the γ′ phase morphologies, and with decreases in the cooling rate, the γ′ phase cubic degree increases [
11,
12].
Recently, most work was focused on the relationship between deformation behavior and the precipitates of the cast nickel-based superalloys after heat treatment. L. Shi et al. [
13,
14,
15] found that the deformation was dominated by dislocation shearing in the second phase when the temperature was low, and in contrast, the dislocations by passing the second phase when on the high temperature. Jian Wang [
16] researched the primary MC decomposition and its effects on the tensile test behaviors at 900 °C in a high-Cr nickel-based superalloy when the solution treatment temperature was 1160 °C and found that the cracks mainly initiated in the grain boundaries, while during the stress rupture test at 900 °C/274 MPa, the cracks merely formed in the primary MC decomposition region. Yuan et al. [
17] found that the alloy after 1050–1150 °C solution treatment did not have significant changes in the morphology of grain and grain boundary, but with an increase in the solution treatment temperature, the γ′ phase of the interdendritic region becomes cubic gradually; simultaneously, the secondary γ′ phase is precipitated in the γ channel. Mao et al. [
18] found the alloy composition was of 8% Cr, 25% Mo and 62% Ni and found that the TCP phase precipitated in Re 5% alloy was σ and in Re 10% were σ and P phases, and that the size of TCP-phases increases and the volume fraction of the TCP phases decreases with the increase in the heat-treatment temperatures.
In summary, the low-heat treatment temperature (≤1160 °C) has a great influence on the precipitation phase and mechanical properties of the alloy. However, the alloy in use often suffers the high-temperature heat treatment phenomenon and severely deteriorated mechanical properties, and the effect of high heat-treatment temperature on high-Cr superalloy precipitates has not been studied in detail. Therefore, it is necessary to systematically investigate the mechanism whereby high-temperature heat treatment influences the microstructure and properties of high-Cr nickel-based superalloy.
In this paper, a type of high-Cr superalloy is designed which contains many solid solution-strengthening elements aimed at the gas turbines. Moreover, the influences of the high-heat treatment process on the microstructure, the behavior of deformation, and the mechanism of the second γ′ phase of the high-Cr superalloy have been investigated and discussed in detail. Finally, this work has provided an experimental basis for an appropriate heat-treatment schedule.
Author Contributions
Conceptualization, H.L.; methodology, H.L., M.Y., B.C. and F.Y.; validation, H.L., Z.M. and D.L.; formal analysis, F.Y. and L.Z.; writing—original draft preparation, H.L.; writing—review and editing, L.Z. and F.Y. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Data Availability Statement
Not applicable.
Conflicts of Interest
The authors declare no conflict of interest.
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