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All issues Volume 3 / No 4 (2024) Natl Sci Open, 3 4 (2024) 20230042 Full HTML
Open Access
Issue
Natl Sci Open
Volume 3, Number 4, 2024
Article Number 20230042
Number of page(s) 11
Section Materials Science
DOI https://doi.org/10.1360/nso/20230042
Published online 27 December 2023

© The Author(s) 2023. Published by Science Press and EDP Sciences.

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

INTRODUCTION

Ni-based single crystal superalloys are an irreplaceable material for aero-engine turbine blades because of their excellent ability to retain mechanical strength and resist oxidation at high temperatures [1,2]. Creep resistance is the most crucial performance aspect in such applications [3]. The creep resistance in these alloys is significantly affected by the microstructure of γ′ cuboids embedded in the γ matrix [4,5]. The behavior and activities of dislocations within this microstructure are crucial in determining creep behavior [6–9]. The progression of the dislocation structure during the creep phenomenon primarily consists of three distinct stages [10–12]. These stages include: (1) the movement and the multiplication of dislocations within the γ matrix, (2) the accumulation of dislocations at the γ/γ′ interface, and (3) the penetration of the dislocations into the γ′ phase cuboids. Among the three, stage (2) generally accounts for most of the service time and the penetration of dislocations into the γ′ phase marks the start of rapid failure of the alloy [13–15]. In this regard, enhancing the interface strength and retarding the accumulation of dislocations at the γ/γ′ interfaces are the two key design considerations for improving the creep life of the alloys.

One approach to increasing the γ/γ′ interface strength is to promote the segregation of some stronger alloying elements such as Re in the γ/γ′ interface, as the researcher has found the segregation of Re will inhibit dislocations and cracks to propagate into γ′ phase [16,17]. To control the accumulation activities of dislocations at the γ/γ′ interface, the traditionally used method is to increase the density of the γ/γ′ interfacial network by increasing the misfit. However, it is difficult to control and optimize the interface misfit by controlling the compositions and lattice parameters of the γ′ and γ phases, as the distribution coefficient of elements is sensitive to adding other elements. In addition, the accumulation activities of dislocations at the γ/γ′ interface can also be implied from the classic Hall-Petch effect and heterostructure strategy [18,19] that the stresses for accumulation dislocations would be reverse propositional to the sizes of γ/γ′ interface and γ channels. Therefore, this study proposes a heterostructure γ/γ′ interface size control route for tuning the dislocation activities in the creep lives of Ni-based superalloys, i.e., the narrower the γ channel is, the stronger the alloy will be. Under a fixed volume fraction of γ′ phase, the size of γ′ phase will determine the number of γ/γ′ interfaces, i.e., the sizes of γ/γ′ interfaces and the γ channels. Finally, retarding the penetration of dislocations into the γ′ cuboids can also be realized by the decrease of the size of the γ′ phase, as the resistance of the strong pair dislocation cutting mechanism increases with the reduction of precipitated phase size [20,21].

RESULTS AND DISCUSSION

To select the alloy with a smaller size of γ′ phase, 60 Ni-based superalloys were designed. They are fabricated into button-shaped ingots and measured the γ′ cuboid dimensions in the dendrite and inter-dendrite regions of the as-cast state. Ni-based single crystal superalloys generally contain seven main alloying elements in addition to Ni, including Al, Ti, Ta, Cr, Co, W and Mo. With consideration of further retarding dislocation activities above 1100°C, Re and Ru have been considered in higher generation and proved to be effective [22–25]. The compositions of the designed 60 alloys are in the content ranges of these elements in common alloys [3,9,11,13,14,26–55]. The button-shaped ingots were fabricated using the electric arc melting technique. The dendrite structures are depicted in Figure 1. The figures demonstrate that the alloys possess distinct dendrite microstructures, suggesting that the composition of the alloy has a significant impact on their microstructural characteristics. The primary focus of our strategy is to track the sizes of the γ′ cuboids. Figure 2 presents the statistical data regarding the sizes of the γ′ cuboids in the as-cast state for 60 button ingots. This includes measurements from both the dendrite and inter-dendrite regions. Following the guidelines for small γ′ cuboid dimensions, three alloys containing 2.5 wt.% Re, 5.5 wt.% Re, and 5.0 wt.% Ru have been chosen from the bottom left corner. These alloys will be referred to as 2.5Re, 5.5Re, and 5.0Ru hereafter.

thumbnail Figure 1

The dendrite structure of the 60 button ingot alloys.
thumbnail Figure 2

The size of γ′ phases between dendrite and inter-dendrite in the as-cast alloys of the 60 button ingot alloys. The black, red and blue five-pointed stars represent the selected 2.5 wt.% Re, 5.5 wt.% Re, and 5.0 wt.% Ru alloys, respectively.

According to the highest temperature at which the alloy can endure creep testing for 1000 h at 137 MPa is usually used as a criterion to divide different generations of superalloys. It increases from ~1000 to ~ 1120°C. From the chemical composition viewpoint, the main difference between the first and second generations is the addition of ~3.0 wt.% Re. The third-generation usually increases the Re content to >5.0 wt.%, and the temperature resistance of the first three generations of alloys is also increased by ~30°C [28–30,35–38]. The addition of Ru element to the fourth-generation alloy increases the temperature resistance to ~1080°C [15,47,48,55]. The fifth-generation alloy increases the temperature resistance to ~1100°C when the Ru content exceeds 5.0 wt.% [23,44,56]. By optimizing the optimum proportion of elements, the temperature resistance of the sixth-generation alloy can break through to ~1120°C [45,51]. In these regards, these three selected alloys are compared with second, third and fifth-generation superalloys, respectively. The selected alloys were fabricated using the directional solidification investment casting method into cylindrical single crystal bars with a [001] axial orientation. The bars underwent a solution treatment process in order to dissolve the inter-dendrite eutectic structure with a low melting temperature, thereby achieving compositional homogeneity within the matrix. Following the solution treatment, the alloy underwent an aging process to facilitate the controlled precipitation and formation of cuboidal γ′ phase structures. The optimized heat treatment procedure is selected from more than 20 procedures based on γ′ size and interfacial segregation.

Figure 3A presents a representative example that demonstrates the overall microstructures of these alloys and the characteristics of the γ/γ′ interface. Specifically, it showcases the microstructures of the 5.0Ru alloy following the aging heat treatment. The micrograph presented in this study depicts a cross-sectional view of the alloys, specifically oriented perpendicular to the growth direction along the [001] crystallographic axis. The microstructure of the material is characterized by the presence of the standard γ′ cuboidal –γ channel configuration. The size distribution of the γ′ cuboids is illustrated in Figure 3B. The estimated average size is approximately 288 nm. The average size of the γ channel is about 60 nm. The high-angle annular dark field (HAADF) image across the γ-γ′ interface is depicted in Figure 3C. The provided image displays the HAADF intensity profiles across the interfaces, revealing that the γ-γ′ interfaces exhibit a higher concentration of high-Z elements, where Z represents the atomic number. Figure 3D depicts the energy dispersive X-ray spectrometry (EDS) analysis conducted on the Re element. The Re element exhibits an enrichment phenomenon at the interface between the γ and γ′ phases. The image presented in Figure 3E depicts the HAADF image at the atomic scale, specifically focusing on the γ-γ′ interface. The lower portion of the images displays the HAADF intensity profiles across the interfaces, and the width of the γ/γ′ interface is definite as the peak area of the line scan contrast in Figure 3E. It exhibits a spatial distribution of approximately 3 nm of heavy elements, indicating a fuzzy interface. Table 1 exhibited the average chemical compositions of the γ′, γ and γ/γ′ interface from 20 measurements at different locations. Re and Mo were enriched at the γ/γ′ interface. This observation suggests that the segregation of the Re element has been achieved.

thumbnail Figure 3

The microstructure of the newly designed 5.0Ru alloy. (A) The initial microstructure. (B) The size distribution of γ′ phase. (C) and (D) The HAADF and EDS analyses of Re distribution across the γ-γ′ interface, respectively. (E) The HAADF image in atomic scale across the γ-γ′ interface.
Table 1

The chemical compositions of the γ′, γ and γ/γ′ interface (wt.%)

Subsequently, the creep life experiments were conducted on the three alloys at temperatures ranging from 1100 to 1110°C and under stress of 137 MPa to validate the suggested approach. The creep behavior of the 5.0Ru alloy, subjected to a temperature of 1110°C/137 MPa, is illustrated in Figure 4A. The creep life is approximately 748 h, with a creep strain of ~6%, as denoted by the blue line. Considering this test temperature is 10°C higher than the typically tested temperature of 1100°C, this value is converted using the Larson-Miller formula, which is a typical method in the literature [57]. The formula is indicated by the equation (1).

thumbnail Figure 4

(A) The creep curves of three alloys. (B) Strain rate versus strain curve. The strain rate is represented by its logarithmic value. (C) The size effect in these three alloys. The blue line represents the new design 5.0 wt.% Ru alloy tested at 1110°C/137 MPa, and the black and red lines represent other newly designed 2.5 wt.% Re and 5.5 wt.% Re alloys tested at 1100°C/137 MPa.

(1)

where LMP is the Larson-Miller parameter, T is the test temperature, tr is the creep rupture time, and C is a constant. The Larson-Miller parameter (LMP) is a combined indicator of creep resistance, including the creep test temperature T and the creep rupture time tr. At a testing stress of 137 MPa, the LMP of this alloy is 31.63. This is below those of TMS-238 (31.99) and TMS-196 (31.83), comparable to those of TMS-162 (31.62), but well above that of the fourth-generation TMS-138A (31.38) and DD22 (30.78) [58,56,59,60]. This indicates the creep resistance of this alloy can reach the fifth-generation alloy.

Except for the 5.0Ru alloy marked by the blue line, the creep behaviors at 1100°C/137 MPa of the two alloys without Ru addition are depicted in the same figure. These behaviors are represented by the black line for the 2.5% Re alloy and the red line for the 5.5% Re alloy. The creep lives and creep strain for the 2.5% Re alloy are 188 h and ~10%. Whereas the creep lives and creep strain for the 5.5% Re alloy are 388 h and ~12%.

These are also shown in Figure 4A. Figure 4B illustrates the relationship between the creep strain rate and the applied strain during the creep process. The creep process can be categorized into three stages, each corresponding to distinct dislocation activities. The minimum strain rate follows 2.5Re > 5.5Re > 5.0Ru. This makes the average stain rates in stages II and III also follow this regular. The dislocation densities of these three alloys after the creep tests are listed in Table 2. The value increased from 5.8×10−5 to 1.12 ×10−4 (1/nm2). By considering the creep time, the evolution rate of dislocation density is consistent with the strain rate. This implies that the interface strength and their ability to prevent the dislocations’ penetration into the γ′ cuboids are enhanced. The relationship between the size of γ′ and the creep life in these three alloys is illustrated in Figure 4C. The size of γ′ particles follows the reduction from 315 to 297 nm and further to 288 nm. This decrease in size increased their creep life in accordance with the size effect as designed. According to the first, second and third-generation alloys in the literature that have been taken into application [61], their improvement of creep life is of great significance. As a comparison, Figure 5 collects the data of the creep lives at 1100°C/137 MPa for the alloys without Ru addition for those CMSX series, RENE, TMS and DD series [38,43,46,49,62–75]. It is worthwhile to notice that ~188 h with 2.5Re is located in the medium level of these alloys, whereas the one with 5.5Re stands at the highest level by ~388 h among all those reported data, which is 58 h longer than the one of CMSX-10k and 28 h longer than that of TMS-82+.

thumbnail Figure 5

The comparison of the creep life tested at 1100°C/137 MPa of the current research and those of the first three-generation alloys in the literature [38,43,46,49,62–75].
Table 2

The dislocation density of the three alloys

CONCLUSION

In summary, this work provides a microstructural design strategy for Ni-base single crystal superalloy based on heterostructure γ′/γ interfacial structure control on two considerations: heterostructure γ′/γ size effects and interfacial strengthening by key alloy composition design to promote the creep lives. Verified by this strategy, without the expensive Ru, the creep life of a third-generation alloy reaches 388 h at 1100°C/137 MPa, which stands at the highest level from the first-generation to the third-generation superalloys.

Data availability

The original data are available from corresponding authors upon reasonable request.

Funding

This work was supported by the National Key Research and Development Program of China (2021YFA1200201), the Natural Science Foundation of China (91860202, 51988101, 52171001, 52071003 and 52001297), the R&D Program of Beijing Municipal Education Commission (KM202210005003), the Beijing Outstanding Young Scientists Projects (BJJWZYJH01201910005018), the Beijing Nova Program (Z211100002121170), and the Overseas Expertise Introduction Project for Discipline Innovation (“111” project) (DB18015).

Author contributions

H.L. and X.H. conceived the project and designed the details of the alloy composition and heat treatment experiments. J.Z., X.Y. and H.F. did the experiments by the supervision of H.L. and X.H. Y.Z. and Y.L. supervised the growth of superalloy single crystals. All authors participated in the discussions, analyzed the data and wrote the manuscript. H.L. and X.H. finalized the manuscript.

Conflict of interest

The authors declare no conflict of interest.

Supplementary information

The supporting information is available online at https://doi.org/10.1360/nso/20230042. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.

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All Tables

Table 1

The chemical compositions of the γ′, γ and γ/γ′ interface (wt.%)

In the text
Table 2

The dislocation density of the three alloys

In the text

All Figures

thumbnail Figure 1

The dendrite structure of the 60 button ingot alloys.
In the text
thumbnail Figure 2

The size of γ′ phases between dendrite and inter-dendrite in the as-cast alloys of the 60 button ingot alloys. The black, red and blue five-pointed stars represent the selected 2.5 wt.% Re, 5.5 wt.% Re, and 5.0 wt.% Ru alloys, respectively.
In the text
thumbnail Figure 3

The microstructure of the newly designed 5.0Ru alloy. (A) The initial microstructure. (B) The size distribution of γ′ phase. (C) and (D) The HAADF and EDS analyses of Re distribution across the γ-γ′ interface, respectively. (E) The HAADF image in atomic scale across the γ-γ′ interface.
In the text
thumbnail Figure 4

(A) The creep curves of three alloys. (B) Strain rate versus strain curve. The strain rate is represented by its logarithmic value. (C) The size effect in these three alloys. The blue line represents the new design 5.0 wt.% Ru alloy tested at 1110°C/137 MPa, and the black and red lines represent other newly designed 2.5 wt.% Re and 5.5 wt.% Re alloys tested at 1100°C/137 MPa.
In the text
thumbnail Figure 5

The comparison of the creep life tested at 1100°C/137 MPa of the current research and those of the first three-generation alloys in the literature [38,43,46,49,62–75].
In the text

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