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Study on the microstructural degradation and rejuvenation heat treatment of directionally solidified turbine blades
Journal Pre-proof Study on the microstructural degradation and rejuvenation heat treatment of directionally solidified turbine blades Peiyu Zhang, Xin Zhou, Xuede Wang, Yuwen Lu, Xing Cheng, Wenqian Zhang PII:
S0925-8388(20)30837-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.154474
Reference:
JALCOM 154474
To appear in:
Journal of Alloys and Compounds
Received Date: 9 January 2020 Revised Date:
21 February 2020
Accepted Date: 22 February 2020
Please cite this article as: P. Zhang, X. Zhou, X. Wang, Y. Lu, X. Cheng, W. Zhang, Study on the microstructural degradation and rejuvenation heat treatment of directionally solidified turbine blades, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154474. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Study on the microstructural degradation and rejuvenation heat treatment of directionally solidified turbine blades Peiyu Zhanga, Xin Zhoua,*, Xuede Wanga, Yuwen Lub, Xing Chenga,c, Wenqian Zhangb a
Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, 710038
Xi’an, China b
Aeronautics Engineering College, Air Force Engineering University, Xi’an 710038, China
c
Aero-space Engine & Smart Manufacturing Institute, 710038 Xi’an, China
*Corresponding Author: Xin Zhou; e-mail: [email protected] Abstract: The directionally solidified Ni-based superalloy is used in gas turbine blades for advanced aircraft engines because of its high stability and favorable properties at elevated temperatures. The microstructures of superalloys degrade when they are exposed to long-term services, and the recovery of these damaged microstructures in a real turbine blade remains a considerable challenge. In this paper, the microstructural degradation in the leading edge of the serviced turbine blades and its microstructural evolution during the rejuvenation heat treatment are analyzed using scanning transmission electron microscopy. The results show that the service-exposed turbine blade suffers from significant microstructural changes, including the coalescence and coarsening of the γ'-phase, transformation of carbides, and precipitation of a topological close-packed phase. The microstructure of the γ'-phase varies between the dendritic arm and the interdendritic region, while the carbide evolution displays regional differences caused by the complex cooling patterns in blades. After the improved rejuvenation heat treatment, most degraded γ'-phases are restored in size, morphology, and volume fraction, accompanied by an increase in microhardness. Despite the variability in microstructural degradation, it is possible to recover the microstructure and microhardness of a severely overaged blade to meet the requirements of continued service. Keywords: Turbine blade; directionally solidified alloy; microstructural degradation; heat treatment; microhardness; restoration 1. Introduction Ni-based superalloys have been widely used as hot-section parts in turbine engines because of their superior tensile strength, creep strength, and corrosion resistance at elevated temperatures [1-3]. These high-quality performances are generally attributed to the combination of strengthening effects arising from γ'-phase and solid-solution-strengthening elements. Owing to the emergence of the directional solidified (DS) technique, the horizontal grain boundaries perpendicular to the stress axis can be eliminated [4]. The DZ125 superalloy is one of the DS casting Ni-based alloys that possesses better resistance to creep and thermal fatigue when than poly-crystal casting alternatives. Therefore, high-pressure gas turbine blades in advanced aircraft engines are partly made of DS alloys. However, turbine blades are normally exposed to aggressive service environments characterized by high temperatures, large temperature gradients, oxidizing and corroding atmospheres, and high pressure and stress [5, 6]. Despite the elimination of the horizontal grain boundaries of DS nickel-based superalloys, the microstructural degradation caused by creep behavior is the primary cause of failure with long-term thermal exposure [7-11]. Previous papers have reported such microstructural changes in DZ125 and other DS alloys. Sun et al. [12] investigated the microstructure heterogeneity and creep damage of DZ125, revealing that the cuboidal γ'-phase would transform into a rafted structure aligned perpendicular to the stress axis during the primary creep stage, which may later induce micro-cracks 1
along the boundaries. Huang et al. [13] evaluated the microstructural damage of the DZ125 blade after actual services. They considered that the coarsening and rafting of the γ'-phase significantly contribute to microstructural damage and the degradation of mechanical properties. However, the decomposition of carbide and the formation of topological close-packed (TCP) phases were not observed in their study.
Turazi et al. [14] analyzed the microstructural evolution of the GTD-111 turbine blade after long-time service, showing spheroidizing of the γ'-phase and the precipitation of film-shaped secondary M23C6 carbides at grain boundaries. Recently, Wang et al. [15] verified the existence of a
phase (one of the
TCP phases) in the second generation DS alloy during high temperature creep, which would not degrade the creep-rupture life under those experimental conditions. In contrast, Volek et al. [16] found that the formation of TCP phase in IN 792 alloy may cause creep and rupture that decrease its lifespan. According to these studies, the microstructural degradation of the DS alloy includes coarsening and rafting of the γ'-phase, transformation of carbides, and formation of TCP phases, which degrade the mechanical properties and hence reduce the lifespan of the turbine blades. In practical applications, severely overaged DS blades are often discarded during maintenance and replaced with expensive new blades. It would be more economical if the microstructure and the related mechanical properties of these blades can be restored to the conditions of new materials. Heat treatment has proved to be an effective method for microstructure and property rejuvenation. For instance, Lang et al. [17] verified that the degraded γ'-phase in the serviced GDT111 turbine blades could be recovered and even improved using a full solution followed by a two-step aging heat treatment. Yang et al. [18] investigated the parameters of the heat treatment on microstructure evolution, showing that the heat temperature mainly affected the solubility of the carbides and γ'-phase, while the cooling rate has an effect on the morphology of the secondary γ'-phase. Jiang et al. [19] successfully recovered the stress rupture property of the degraded DZ411 alloy using the reheat treatment. They attributed the improvement to the restoration of the γ'-phase with respect to morphology, size, and chemical composition. Despite continuous efforts enhance the performance of rejuvenation heat treatment, it is still a challenge to recover the microstructures and properties of serviced DS turbine blades. In general, gas turbine blades have complex shapes and a hollow profile caused by internal cooling channels, as well as complex profiles to optimize external hot gas flow [20]. Such complex geometries combined with various service conditions (e.g., aging time and ambient environment) lead to heterogeneous microstructural degradations in different parts of the blades[21-23]. In fact, the microstructural evolution may produce significant variations over small distances, which makes it difficult to develop an effective heat treatment process that considers all types of microstructural degradation. For the aforementioned reasons, current studies focus on the rejuvenation of DS alloy samples, rather than the actual DS turbine blades. In this study, we tried to investigate the microstructural degradation and rejuvenation heat treatment for directionally solidified DZ125 turbine blades after actual services. First, the degraded microstructure in the leading edge of the blades tip was compared with that of the new blades, and the microstructural diversity in different degraded regions was also analyzed. Then, a three-step heat treatment process was developed based on the degradation mechanism, and the microstructural evolution during each step was investigated. Finally, we compared the new, serviced, and rejuvenated blades in terms of microstructure and hardness, verifying the effect of the heat treatment rejuvenation. 2. Materials and methods 2
The materials used in this experiment were obtained from the DZ125 turbine blades before and after service, and all the serviced blades were from the same batch of aircraft engines with a service time of more than 500 h. The nominal composition of the directionally solidified DZ125 alloy (made in China) is given in wt.%: Ni-10Co-8.9Cr-7W-5.1Al-3.8Ta-2Mo-1.5Hf-0.1Ti-0.095C-0.015B. The most relevant phases in the alloy include (1) an austenitic γ-Ni matrix with Cr, Co, Ti, Al, and Ta in a solid solution, (2) primary (cuboidal) and secondary (spherical) γ'-Ni3Al precipitates, (3) primary and secondary carbides (MC, M6C and M23C6), and (4) possible deleterious TCP phases promoted by Cr, Mo and W [24]. The density of the alloy is ~8.25 g/cm3, and the liquidus temperature is ~1230–1240 ℃. The microstructural degradation features of the serviced turbine blades are heterogeneous owing to the complex geometry of the internal cooling structures. Therefore, it is important to select a reasonable observation region in this experiment. As shown in Fig. 1a, the blade tip (Region 1) may go through the most severe microstructural damage under the combined effect of having the largest centrifugal effects and the highest temperature during actual service [13]. The tested samples were cut from the tip region along the (001) crystal plane, with a thickness of 1.5 mm. These samples can be divided into two main regions (Fig. 1b). Region 2 contains the main channel of the internal cooling flow, where the thermal stresses are relatively small. Meanwhile, the internal flows and thermal stresses in Region 3 are rather complicated owing to the presence of the inlet and exhaust edges. Thus, Region 3 was selected as the main observation region for this experiment. To analyze the microstructural degradation after service, the new blades were used as a reference. The test zone (blue dots in Figs. 1c-d) was selected in the same area of the new and serviced blades, ensuring that the experimental results were comparable.
Fig. 1 Samples for analysis. (a) a DS gas turbine blade; (b) the blade tip; (c), (d) show the test zone in the serviced and new blades, respectively.
A three-step heat treatment process of the serviced DZ125 turbine blades was performed in a tubular muffle furnace under the argon atmosphere. Microstructures were analyzed in each step via the following procedure: In the first step, six serviced blades (same batch) were selected for pretreatment (1180 ° C for 2 h + 1230 ° C for 3 h + air cooling); then, four of the pretreated blades were processed in the solution treatment (1100 ° C for 4 h + air cooling); finally, two samples after the solution-treated were subjected to precipitation aging (870 ° C for 14 h + air cooling). All samples were prepared for microstructural characterization using standard metallographic 3
techniques, including mirror-like polish using colloidal silica. The specimens were etched with solutions of 20 g CuSO4·5H2O, 5 ml H2SO4, 100 ml HCl, and 80 ml H2O to reveal the γ'-phase microstructure. The microanalysis used a Hitachi SU6600 field-emission scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The average size of the γ'-phase was measured using the equivalent diameter (square root of the γ´-phase area), and the volume fraction of the γ'-phase was measured in the Image-J software [25]. As the quantitative metallographic investigation is mainly a statistical measurement, at least 5 images for each condition were analyzed. The microhardness of the samples was measured using the HXD-1000TMC/LCD microhardness tester equipped with the Vickers diamond indenter. A load of 200 g was applied for 15 s. Ten random measurements were performed for each sample. 3. Results and discussion 3.1 Microstructural degradation of the serviced blades 3.1.1 Microstructure of the new blades Figures 2a-c show the morphology of the γ'-phase in the new blades under high magnification. In Fig. 2a, the phase configuration of the dendritic arm is identified as quasi-cuboidal γ'-phase (indicated by the red circle) connected by γ-channels where ultrafine secondary γ' particles (indicated by the yellow frame) are distributed. Such grid structures (cuboidal γ' + γ-channels) can be observed in most of the Ni-based DS or single-crystal superalloys. The average size of γ'-phase in the new blade is calculated as ~0.4 µm, while its volume fraction is ~70 %. In general, the evolution of the γ'-phase during casting includes origination, growth, and differentiation. Therefore, the emergence of the incompletely differentiated γ'-phase (indicated by the blue circle) in the new blades is a normal phenomenon. The morphology of interdendritic γ'-phase (see Fig. 2b) is different from that in the dendritic arm, where the growth of some γ'-phases deviated from the [001] direction. In the junction zone between the inter-dendrite and dendritic arms (blue dashes), the two types of γ'-phases coalesce together, forming a large coarser structure of size ~1 µm. In addition to the γ'-phase, γ-γ' eutectic is also one of the common microstructures in Ni-based superalloys, which is characterized by the cooperative growth of two constituent phases within the same liquid alloy [26]. The eutectic γ-γ' phases (see Fig. 2c) in the new blades present a radial shape with a strip eutectic structure (red dotted frame) pointing to the center of the radiation. The boundary between the strip structures becomes very blurred in the region marked by the yellow dotted frame, which may be attributed to the anisotropic characteristics of γ-γ' eutectic during the solidification process [27]. 3.1.2 Degradation of the γ'-phase After long-term service the γ'-phase degrades via changes in size, morphology, and volume fraction. In the dendritic arm (Fig. 2d), previously cuboidal γ'-phases have changed into a spherical morphology, with the average size increased to ~0.6 µm (red-dotted circles). Some spheroidized γ'-phases coalesce together (blue-dotted circle), forming into rafted structures. The overall volume fraction of the dendritic γ'-phase is reduced to ~60 %. The morphology of the interdendritic γ'-phase is much more complex than the dendritic arm (Fig. 2e), with labyrinth-shaped structures (blue-dotted circles) and coarse particles (red-dotted circles). The formation of such complex structures may result from the coarsening and coalescence of previous γ'-phases, which presented a skewed growth and disordered distribution (Fig. 2b). The eutectic γ-γ' shown in Fig. 2f also suffers from the coarsening behavior, with its well-defined, radial morphology becoming dense blocky.
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The elastic coherent stress caused by the lattice mismatch between the γ matrix and the γ'-phase has a significant effect on the coarsening behavior [28, 29]. High lattice mismatch leads to high elastic strain and low interface stability energy, which contributes to the coarsening of the γ'-phase. For the face-centered cubic (FCC) Ni-based superalloys, the six ˂100˃ directions are the preferred directions of growth in high elastic-stress condition [30]. Thus, at the given (001) crystal plane studied in present work, the interface stability energy is low in these four directions: [100], [100], [010] and [010]. During long-term service, the original cuboidal γ'-phase in the dendritic arm tends to creep along directions with low interface stability, increasing the lattice mismatch between the γ matrix and the γ'-phase. As a result, the cubic γ'-phases gradually coarsen into spherical particles under lattice stress. For the interdendritic structures, the size of the γ'-phase is not homogeneous. The solute concentration in the γ matrix channel around the small γ' particles is higher than that around the large γ' particles. The imbalance in the concentration generates a diffusion-driving force that causes solute atoms to diffuse from around the small-sized particles and to the vicinity of the large ones, which is known as Ostwald ripening [31, 32]. As a result, the small-sized γ'-phases are continuously dissolved while the large ones grow coarser. During this process, some γ' particles will coalesce with each other and form a dislocation network at the γ-γ' coherent interface, which may release the two-phase coherent stress and further change the original elastic stress field. Therefore, elastic stress gradients from the center to the edge of the γ'-phase disappear, leading to the formation of rafted structures.
Fig. 2 serviced Morphology of γ'-phase in the new blade (a-c) compared with their counterparts in the serviced one (d-f). (a), (d) dendritic arm; (b), (e) interdendritic region; (c), (f) eutectic γ-γ'.
3.1.3 Evolution of carbides in different regions The morphology of carbides after service shows regional differences within the blade tip. In the region near the exhaust edge (indicated by the green point in Fig. 3b), plenty of fishbone-shaped carbides are precipitated along the grain boundaries (Fig. 3a). Such carbides (Fig. 3d) mainly comprise a diffusely distributed bulky precipitate (dotted by red frame) and a parallel strip of elongated precipitate (dotted by blue frame), occupying ~30 % of the microstructure in this region. The EDS analysis shows that the bulky precipitate (Point 1), which is supposed to be the MC-type carbide, contains W (71.36 %), Co, Cr and C. The fishbone-shaped carbide surrounds a dense film, where the 5
Ni content (Point 2) is only about 29.96 % without the existence of the spheroidized or drafted γ'-phase. In the middle region (indicated by the purple point in Fig. 3b), the precipitates contain several bulky MC carbides, with some strip-shaped M6C carbides distributed in linear features (Fig. 3e). Compared to the MC carbide, the content of W in M6C carbide (Point 3) is reduced to 59.51 %, while the Mo content is increased to ~5.32 %. Similarly, the matrix around the carbides does not depict the γ'-phase, except for the region on the right of the yellow line. However, the Ni content in Point 4 is increased to 41.13 % compared to that in Point 3. Regarding the region near the outer surface of the turbine blade (indicated by the orange point in Fig. 3b), plenty of needle-shaped precipitates (orange frames in Fig. 3c) are observed in the crystal. Similar results have also been reported by Wang et al. [33] for a Ni-Fe-Cr base alloy, in which they attributed the formation of needle-shaped precipitates to the segregation of alloy elements. Such structures comprise several parallel precipitates (size of 0.5×1.5 µm) enveloped by the γ'-film, which is supposed to be a type of TCP phase. EDS analysis shows the TCP phase (Point 5) mainly contains W (55.88 %), Cr (12.57 %), and Mo (17.87 %), while the surrounding γ'-film (Point 6) is rich in Ni (63.00 %) and Al (9.07 %). The hardness results (Fig .6) show that the microhardness of the serviced blade is higher than that of the new blade, which may be related to the precipitation of the brittle TCP phase [34]. The formation of the carbides or TCP phases during service may consume refractory elements (W, Cr, Co, Mo, etc.) in the matrix, significantly weakening the solid-solution strengthening effect of these elements.
Fig. 3 Evolution of carbides in different regions after service. (a), (d) near the exhaust edge; (e) in the middle region; (c), (f) near the blade surface; (b) diagram of the regions.
The spatial variation in the evolution of carbides is determined by the service environment. The DZ125 turbine blades have complex inner cooling structures, where the cooling effect and the resultant temperature history of different regions are not the same [35, 36]. The variations in carbide evolution in the selected part (dotted blue box in Fig. 3b) is comparable to different durations of aging. In general, 6
the cooling airflow will form a gas film on the blade surface when the turbine engine works, which will reduce the service temperature. Therefore, the equivalent aging time in the region near the outer surface is short. In contrast, the airflow temperature can be very high near the exhaust edge, accelerating the aging effect. The sequential evolution of carbides during service begins when (1) W, Mo, Cr, Co, and other refractory elements begin to precipitate, forming Mo-rich TCP phases in the crystal and W-rich MC carbides at the grain boundary, then (2) as the aging time increases, a part of the bulky MC carbides may be decomposed into the strip-shaped M6C carbides, and (3) the most refractory element (W) will continue to precipitate for a long time. In this process, MC and M6C carbides may gradually grow and connect to each other, forming a fishbone-shaped structure. 3.1.4 Overall comparison between the new and serviced blades As shown in Fig. 4, both the new and serviced DZ125 blades depict the typical dendritic structures, including the dendritic arm (A) and the interdendritic structures (B); some γ/γ' eutectic (C) can be observed in the interdendritic region. The dendritic arm is the main structure of the DZ125 blade, occupying more than 60 % of the entire morphology. Previously mentioned microstructural degradations, such as γ'-phase coarsening, carbide transformation and TCP phase precipitation, can be observed in Fig .4b. In addition to various microstructural changes, the results of the EDS analysis revealed a significant change in the content of W, Co, and other elements after service; the concentration of W (wt. %) increased from 9.82 % to 13.05 %, while for Co (wt. %), it decreased from 6.04 % to 3.84 %.
Fig. 4 Overall comparison in microstructure and element composition: (a) new blade, (b) serviced blade.
3.2 Rejuvenation heat treatment A three-step rejuvenation heat treatment was designed to recover the degraded microstructure and performance of the serviced DZ125 blades. The alloy pretreatment (1180 ° C for 2 h + 1230 °C for 3 h + air cooling) is a process of atomic thermal activation, with which the alloy structure and composition can be homogenized. The 1180 °C stage eliminates the low-melting-point phase (such as the TCP phase) in the alloy, and 1230 °C stage re-dissolves the degraded γ'-phase and refractory carbide elements into the γ matrix. To precipitate the γ'-phase using the second-step solution treatment (1100 ° C for 4 h + air 7
cooling), the designed temperature (1100 °C) should be higher than the solid-solution temperature of the γ'-phase. The third-step aging treatment (870 °C for 14 h + air cooling) aims to adjust the morphology, size, and volume fraction of the γ'-phase. The aging time is reduced from 20 h (standard heat treatment of the DZ125 alloy) to 14 h to avoid initial melting. Figure 5 shows the evolution of the γ'-phase during the three-step rejuvenation heat treatment. After the pretreatment process, the formerly degraded microstructures are well homogenized. However, morphology of the dendritic γ'-phase (dotted red box) shows regional differences due to various degrees of degradation. In the region near the exhaust edge (Fig. 5a), several spherical γ'-phases (about 0.1 µm in diameter each) cluster together, forming a petal-shaped structure with a volume fraction of ~45 %. In the region near the blade surface (Fig. 5b), the γ'-phase presents an irregular square shape, with a size of ~0.4 µm and a volume fraction of ~50 %. The morphology of the interdendritic γ'-phase (Fig. 5c) presents a skewed distribution on the γ matrix, which may alter the preferred growth directions in the (001) crystal plane. Apart from the precipitation of several γ'-phases in this process, numerous ultrafine secondary γ' particles (dotted green box) can be found in the γ matrix. The size, morphology and volume fraction of the dendritic γ'-phases have changed significantly after the solution treatment. In the region near the exhaust edge (Fig. 5d), the petal-shaped γ'-phases merged into a spherical structure with a diameter of ~0.2–0.4 µm and a volume fraction of ~60 %, morphologically similar to the vertical section of a multiphase Ni3Al-based superalloy after annealing treatment [37], which can be explained by the phase growth in manner of dissolution and aggregation by γ'II → γ'I + γ during the solution procedure. In the region near the blade surface (Fig. 5e), the morphology of the dendritic γ'-phase is recovered to cuboidal shape (0.3–0.5 µm), with its volume fraction increased to ~65 %. Additionally, the granular secondary γ' particles within the γ-channels are nearly absent due to the solid solution behavior. In the interdendritic region (Fig. 5f), the skewed γ'-phases coalesced with each other with anisotropic grain growth, transforming into elongated particles. The morphology of the dendritic γ'-phases can be further adjusted after the aging treatment. In the region near the exhaust edge (Fig. 5g), the shape of the spheroidized γ'-phases gradually changed to cubical forms. In the region near the surface of the blade, more cubical γ'-phases are precipitated from the γ'-channels, with the volume fraction increased to ~70 % (Fig. 5h). In addition to the precipitation of new γ'-phases, the previous precipitates continue to grow along the preferred [001] direction. Thus, the dendritic γ'-phase in this region presents a height difference on the (001) plane, which is similar to the morphology of the new blade shown in Fig. 2a. The interdendritic γ'-phases also show the growth in the preferred direction, deviating from the [001] direction due the original skewed distribution shown in Fig. 5c. Such interdendritic γ'-phases, indicated by yellow line (Fig. 5i), stand out when compared with the surrounding dendritic precipitates, which may be contributed to growth competition between the two types of γ'-phases.
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Fig. 5 Evolution of γ'-phases in different regions during a three-step rejuvenation heat treatment.
In this experiment, most microstructures in the serviced blades have been significantly restored after the three-step heat treatment. On the right side of the yellow line (Fig. 6), the size, morphology, and volume fraction of the γ'-phases in the dendritic arm (A) and the interdendritic region (B) recover to a similar level to new blades. However, it is worth noting that the morphology of the dendritic γ'-phase in the region near the exhaust edge (region C) is not completely restored. This region suffers the most severe microstructural degradation as discussed above, with a many refractory elements precipitated during service, which requires more energy to rearrange and precipitate the cubical γ'-phases. Generally, the dendritic morphology evolution has a close relationship with the hardness [38], and hence, the recovery effect of the heat treatment can also be verified by hardness testing. As shown in Fig. 6, the microhardness of the serviced blades has decreased after the heat treatment, which is nearly the same as the new ones (~410 HV0.02). The reported relationship between the material strength and hardness can be expressed by the empirical equation Hv = Kσ, where K is a constant related to deformation rate [39]. Thus, the mechanical properties of the rejuvenated blades are largely similar to new ones.
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Fig. 6 Recovery effect in terms of microhardness and microstructure.
4. Conclusion The degradation behavior of a directionally solidified gas turbine blade after real service was studied and a rejuvenation heat treatment was performed to recover the microstructure and microhardness of the blade. The results of this study led to the following conclusions: (i) The leading edge in the blade tip showed various microstructural changes, including the degradation of the γ'-phase, transformation of carbides, and precipitation of the TCP phases. (ii) The degradation mechanism of γ'-phases were characterized by the spheroidizing, coalescing and coarsening behavior. The evolution of carbides displayed spatial variation owing to complex cooling patterns. (iii) Despite the variation in microstructural degradation, the present study showed the possibility to recover a severely overaged turbine blade in terms of microstructure and microhardness. Despite the possibility of restoring a severely overaged turbine’s microhardness and microstructure, full restoration of microstructures and properties after the present heat treatment process is still challenging. Future studies will be focused on improving the rejuvenation heat treatment and evaluating the recovered mechanical properties (e.g., tensile strength, fatigue performance and creep property) to meet the requirements of continued service. Acknowledgements This work was supported by the National Natural Science Foundation of China (grant number 91860136 and 51801231), the Key R&D plan of Guangdong Province (grant number 2018B090905001) and the Key Science and Technology project of Shaanxi Province (grant number 2018zdzx01-04-01). Author statement Peiyu Zhang: Writing- Original draft preparation, Methodology Xin Zhou: Writing- Reviewing and Editing, Supervision Xuede Wang: Project administration Yuwen Lu: Data Curation Wenqian Zhang: Visualization Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Reference [1] P. Caron, T. Khan, Evolution of Ni-based superalloys for single crystal gas turbine blade applications, Aerospace Science and Technology 3(8) (1999) 513-523. [2] J.T. Guo, C. Yuan, H.C. Yang, V. Lupinc, M. Maldini, Creep-rupture behavior of a directionally 10
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Highlights ● The microstructural degradation and rejuvenation heat treatment for a directionally solidified turbine blades after actual services are investigated. ● The degraded microstructure shows reginal differences due to of the complex cooling structures of blades. ● The microstructure and microhardness of a severely overaged blade can be partially recovered by the rejuvenation heat treatment.
Author statement Peiyu Zhang: Writing- Original draft preparation, Methodology Xin Zhou: Writing- Reviewing and Editing, Supervision Xuede Wang: Project administration Yuwen Lu: Data Curation Wenqian Zhang: Visualization
S0925-8388(20)30837-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.154474
Reference:
JALCOM 154474
To appear in:
Journal of Alloys and Compounds
Received Date: 9 January 2020 Revised Date:
21 February 2020
Accepted Date: 22 February 2020
Please cite this article as: P. Zhang, X. Zhou, X. Wang, Y. Lu, X. Cheng, W. Zhang, Study on the microstructural degradation and rejuvenation heat treatment of directionally solidified turbine blades, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154474. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Study on the microstructural degradation and rejuvenation heat treatment of directionally solidified turbine blades Peiyu Zhanga, Xin Zhoua,*, Xuede Wanga, Yuwen Lub, Xing Chenga,c, Wenqian Zhangb a
Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, 710038
Xi’an, China b
Aeronautics Engineering College, Air Force Engineering University, Xi’an 710038, China
c
Aero-space Engine & Smart Manufacturing Institute, 710038 Xi’an, China
*Corresponding Author: Xin Zhou; e-mail: [email protected] Abstract: The directionally solidified Ni-based superalloy is used in gas turbine blades for advanced aircraft engines because of its high stability and favorable properties at elevated temperatures. The microstructures of superalloys degrade when they are exposed to long-term services, and the recovery of these damaged microstructures in a real turbine blade remains a considerable challenge. In this paper, the microstructural degradation in the leading edge of the serviced turbine blades and its microstructural evolution during the rejuvenation heat treatment are analyzed using scanning transmission electron microscopy. The results show that the service-exposed turbine blade suffers from significant microstructural changes, including the coalescence and coarsening of the γ'-phase, transformation of carbides, and precipitation of a topological close-packed phase. The microstructure of the γ'-phase varies between the dendritic arm and the interdendritic region, while the carbide evolution displays regional differences caused by the complex cooling patterns in blades. After the improved rejuvenation heat treatment, most degraded γ'-phases are restored in size, morphology, and volume fraction, accompanied by an increase in microhardness. Despite the variability in microstructural degradation, it is possible to recover the microstructure and microhardness of a severely overaged blade to meet the requirements of continued service. Keywords: Turbine blade; directionally solidified alloy; microstructural degradation; heat treatment; microhardness; restoration 1. Introduction Ni-based superalloys have been widely used as hot-section parts in turbine engines because of their superior tensile strength, creep strength, and corrosion resistance at elevated temperatures [1-3]. These high-quality performances are generally attributed to the combination of strengthening effects arising from γ'-phase and solid-solution-strengthening elements. Owing to the emergence of the directional solidified (DS) technique, the horizontal grain boundaries perpendicular to the stress axis can be eliminated [4]. The DZ125 superalloy is one of the DS casting Ni-based alloys that possesses better resistance to creep and thermal fatigue when than poly-crystal casting alternatives. Therefore, high-pressure gas turbine blades in advanced aircraft engines are partly made of DS alloys. However, turbine blades are normally exposed to aggressive service environments characterized by high temperatures, large temperature gradients, oxidizing and corroding atmospheres, and high pressure and stress [5, 6]. Despite the elimination of the horizontal grain boundaries of DS nickel-based superalloys, the microstructural degradation caused by creep behavior is the primary cause of failure with long-term thermal exposure [7-11]. Previous papers have reported such microstructural changes in DZ125 and other DS alloys. Sun et al. [12] investigated the microstructure heterogeneity and creep damage of DZ125, revealing that the cuboidal γ'-phase would transform into a rafted structure aligned perpendicular to the stress axis during the primary creep stage, which may later induce micro-cracks 1
along the boundaries. Huang et al. [13] evaluated the microstructural damage of the DZ125 blade after actual services. They considered that the coarsening and rafting of the γ'-phase significantly contribute to microstructural damage and the degradation of mechanical properties. However, the decomposition of carbide and the formation of topological close-packed (TCP) phases were not observed in their study.
Turazi et al. [14] analyzed the microstructural evolution of the GTD-111 turbine blade after long-time service, showing spheroidizing of the γ'-phase and the precipitation of film-shaped secondary M23C6 carbides at grain boundaries. Recently, Wang et al. [15] verified the existence of a
phase (one of the
TCP phases) in the second generation DS alloy during high temperature creep, which would not degrade the creep-rupture life under those experimental conditions. In contrast, Volek et al. [16] found that the formation of TCP phase in IN 792 alloy may cause creep and rupture that decrease its lifespan. According to these studies, the microstructural degradation of the DS alloy includes coarsening and rafting of the γ'-phase, transformation of carbides, and formation of TCP phases, which degrade the mechanical properties and hence reduce the lifespan of the turbine blades. In practical applications, severely overaged DS blades are often discarded during maintenance and replaced with expensive new blades. It would be more economical if the microstructure and the related mechanical properties of these blades can be restored to the conditions of new materials. Heat treatment has proved to be an effective method for microstructure and property rejuvenation. For instance, Lang et al. [17] verified that the degraded γ'-phase in the serviced GDT111 turbine blades could be recovered and even improved using a full solution followed by a two-step aging heat treatment. Yang et al. [18] investigated the parameters of the heat treatment on microstructure evolution, showing that the heat temperature mainly affected the solubility of the carbides and γ'-phase, while the cooling rate has an effect on the morphology of the secondary γ'-phase. Jiang et al. [19] successfully recovered the stress rupture property of the degraded DZ411 alloy using the reheat treatment. They attributed the improvement to the restoration of the γ'-phase with respect to morphology, size, and chemical composition. Despite continuous efforts enhance the performance of rejuvenation heat treatment, it is still a challenge to recover the microstructures and properties of serviced DS turbine blades. In general, gas turbine blades have complex shapes and a hollow profile caused by internal cooling channels, as well as complex profiles to optimize external hot gas flow [20]. Such complex geometries combined with various service conditions (e.g., aging time and ambient environment) lead to heterogeneous microstructural degradations in different parts of the blades[21-23]. In fact, the microstructural evolution may produce significant variations over small distances, which makes it difficult to develop an effective heat treatment process that considers all types of microstructural degradation. For the aforementioned reasons, current studies focus on the rejuvenation of DS alloy samples, rather than the actual DS turbine blades. In this study, we tried to investigate the microstructural degradation and rejuvenation heat treatment for directionally solidified DZ125 turbine blades after actual services. First, the degraded microstructure in the leading edge of the blades tip was compared with that of the new blades, and the microstructural diversity in different degraded regions was also analyzed. Then, a three-step heat treatment process was developed based on the degradation mechanism, and the microstructural evolution during each step was investigated. Finally, we compared the new, serviced, and rejuvenated blades in terms of microstructure and hardness, verifying the effect of the heat treatment rejuvenation. 2. Materials and methods 2
The materials used in this experiment were obtained from the DZ125 turbine blades before and after service, and all the serviced blades were from the same batch of aircraft engines with a service time of more than 500 h. The nominal composition of the directionally solidified DZ125 alloy (made in China) is given in wt.%: Ni-10Co-8.9Cr-7W-5.1Al-3.8Ta-2Mo-1.5Hf-0.1Ti-0.095C-0.015B. The most relevant phases in the alloy include (1) an austenitic γ-Ni matrix with Cr, Co, Ti, Al, and Ta in a solid solution, (2) primary (cuboidal) and secondary (spherical) γ'-Ni3Al precipitates, (3) primary and secondary carbides (MC, M6C and M23C6), and (4) possible deleterious TCP phases promoted by Cr, Mo and W [24]. The density of the alloy is ~8.25 g/cm3, and the liquidus temperature is ~1230–1240 ℃. The microstructural degradation features of the serviced turbine blades are heterogeneous owing to the complex geometry of the internal cooling structures. Therefore, it is important to select a reasonable observation region in this experiment. As shown in Fig. 1a, the blade tip (Region 1) may go through the most severe microstructural damage under the combined effect of having the largest centrifugal effects and the highest temperature during actual service [13]. The tested samples were cut from the tip region along the (001) crystal plane, with a thickness of 1.5 mm. These samples can be divided into two main regions (Fig. 1b). Region 2 contains the main channel of the internal cooling flow, where the thermal stresses are relatively small. Meanwhile, the internal flows and thermal stresses in Region 3 are rather complicated owing to the presence of the inlet and exhaust edges. Thus, Region 3 was selected as the main observation region for this experiment. To analyze the microstructural degradation after service, the new blades were used as a reference. The test zone (blue dots in Figs. 1c-d) was selected in the same area of the new and serviced blades, ensuring that the experimental results were comparable.
Fig. 1 Samples for analysis. (a) a DS gas turbine blade; (b) the blade tip; (c), (d) show the test zone in the serviced and new blades, respectively.
A three-step heat treatment process of the serviced DZ125 turbine blades was performed in a tubular muffle furnace under the argon atmosphere. Microstructures were analyzed in each step via the following procedure: In the first step, six serviced blades (same batch) were selected for pretreatment (1180 ° C for 2 h + 1230 ° C for 3 h + air cooling); then, four of the pretreated blades were processed in the solution treatment (1100 ° C for 4 h + air cooling); finally, two samples after the solution-treated were subjected to precipitation aging (870 ° C for 14 h + air cooling). All samples were prepared for microstructural characterization using standard metallographic 3
techniques, including mirror-like polish using colloidal silica. The specimens were etched with solutions of 20 g CuSO4·5H2O, 5 ml H2SO4, 100 ml HCl, and 80 ml H2O to reveal the γ'-phase microstructure. The microanalysis used a Hitachi SU6600 field-emission scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The average size of the γ'-phase was measured using the equivalent diameter (square root of the γ´-phase area), and the volume fraction of the γ'-phase was measured in the Image-J software [25]. As the quantitative metallographic investigation is mainly a statistical measurement, at least 5 images for each condition were analyzed. The microhardness of the samples was measured using the HXD-1000TMC/LCD microhardness tester equipped with the Vickers diamond indenter. A load of 200 g was applied for 15 s. Ten random measurements were performed for each sample. 3. Results and discussion 3.1 Microstructural degradation of the serviced blades 3.1.1 Microstructure of the new blades Figures 2a-c show the morphology of the γ'-phase in the new blades under high magnification. In Fig. 2a, the phase configuration of the dendritic arm is identified as quasi-cuboidal γ'-phase (indicated by the red circle) connected by γ-channels where ultrafine secondary γ' particles (indicated by the yellow frame) are distributed. Such grid structures (cuboidal γ' + γ-channels) can be observed in most of the Ni-based DS or single-crystal superalloys. The average size of γ'-phase in the new blade is calculated as ~0.4 µm, while its volume fraction is ~70 %. In general, the evolution of the γ'-phase during casting includes origination, growth, and differentiation. Therefore, the emergence of the incompletely differentiated γ'-phase (indicated by the blue circle) in the new blades is a normal phenomenon. The morphology of interdendritic γ'-phase (see Fig. 2b) is different from that in the dendritic arm, where the growth of some γ'-phases deviated from the [001] direction. In the junction zone between the inter-dendrite and dendritic arms (blue dashes), the two types of γ'-phases coalesce together, forming a large coarser structure of size ~1 µm. In addition to the γ'-phase, γ-γ' eutectic is also one of the common microstructures in Ni-based superalloys, which is characterized by the cooperative growth of two constituent phases within the same liquid alloy [26]. The eutectic γ-γ' phases (see Fig. 2c) in the new blades present a radial shape with a strip eutectic structure (red dotted frame) pointing to the center of the radiation. The boundary between the strip structures becomes very blurred in the region marked by the yellow dotted frame, which may be attributed to the anisotropic characteristics of γ-γ' eutectic during the solidification process [27]. 3.1.2 Degradation of the γ'-phase After long-term service the γ'-phase degrades via changes in size, morphology, and volume fraction. In the dendritic arm (Fig. 2d), previously cuboidal γ'-phases have changed into a spherical morphology, with the average size increased to ~0.6 µm (red-dotted circles). Some spheroidized γ'-phases coalesce together (blue-dotted circle), forming into rafted structures. The overall volume fraction of the dendritic γ'-phase is reduced to ~60 %. The morphology of the interdendritic γ'-phase is much more complex than the dendritic arm (Fig. 2e), with labyrinth-shaped structures (blue-dotted circles) and coarse particles (red-dotted circles). The formation of such complex structures may result from the coarsening and coalescence of previous γ'-phases, which presented a skewed growth and disordered distribution (Fig. 2b). The eutectic γ-γ' shown in Fig. 2f also suffers from the coarsening behavior, with its well-defined, radial morphology becoming dense blocky.
4
The elastic coherent stress caused by the lattice mismatch between the γ matrix and the γ'-phase has a significant effect on the coarsening behavior [28, 29]. High lattice mismatch leads to high elastic strain and low interface stability energy, which contributes to the coarsening of the γ'-phase. For the face-centered cubic (FCC) Ni-based superalloys, the six ˂100˃ directions are the preferred directions of growth in high elastic-stress condition [30]. Thus, at the given (001) crystal plane studied in present work, the interface stability energy is low in these four directions: [100], [100], [010] and [010]. During long-term service, the original cuboidal γ'-phase in the dendritic arm tends to creep along directions with low interface stability, increasing the lattice mismatch between the γ matrix and the γ'-phase. As a result, the cubic γ'-phases gradually coarsen into spherical particles under lattice stress. For the interdendritic structures, the size of the γ'-phase is not homogeneous. The solute concentration in the γ matrix channel around the small γ' particles is higher than that around the large γ' particles. The imbalance in the concentration generates a diffusion-driving force that causes solute atoms to diffuse from around the small-sized particles and to the vicinity of the large ones, which is known as Ostwald ripening [31, 32]. As a result, the small-sized γ'-phases are continuously dissolved while the large ones grow coarser. During this process, some γ' particles will coalesce with each other and form a dislocation network at the γ-γ' coherent interface, which may release the two-phase coherent stress and further change the original elastic stress field. Therefore, elastic stress gradients from the center to the edge of the γ'-phase disappear, leading to the formation of rafted structures.
Fig. 2 serviced Morphology of γ'-phase in the new blade (a-c) compared with their counterparts in the serviced one (d-f). (a), (d) dendritic arm; (b), (e) interdendritic region; (c), (f) eutectic γ-γ'.
3.1.3 Evolution of carbides in different regions The morphology of carbides after service shows regional differences within the blade tip. In the region near the exhaust edge (indicated by the green point in Fig. 3b), plenty of fishbone-shaped carbides are precipitated along the grain boundaries (Fig. 3a). Such carbides (Fig. 3d) mainly comprise a diffusely distributed bulky precipitate (dotted by red frame) and a parallel strip of elongated precipitate (dotted by blue frame), occupying ~30 % of the microstructure in this region. The EDS analysis shows that the bulky precipitate (Point 1), which is supposed to be the MC-type carbide, contains W (71.36 %), Co, Cr and C. The fishbone-shaped carbide surrounds a dense film, where the 5
Ni content (Point 2) is only about 29.96 % without the existence of the spheroidized or drafted γ'-phase. In the middle region (indicated by the purple point in Fig. 3b), the precipitates contain several bulky MC carbides, with some strip-shaped M6C carbides distributed in linear features (Fig. 3e). Compared to the MC carbide, the content of W in M6C carbide (Point 3) is reduced to 59.51 %, while the Mo content is increased to ~5.32 %. Similarly, the matrix around the carbides does not depict the γ'-phase, except for the region on the right of the yellow line. However, the Ni content in Point 4 is increased to 41.13 % compared to that in Point 3. Regarding the region near the outer surface of the turbine blade (indicated by the orange point in Fig. 3b), plenty of needle-shaped precipitates (orange frames in Fig. 3c) are observed in the crystal. Similar results have also been reported by Wang et al. [33] for a Ni-Fe-Cr base alloy, in which they attributed the formation of needle-shaped precipitates to the segregation of alloy elements. Such structures comprise several parallel precipitates (size of 0.5×1.5 µm) enveloped by the γ'-film, which is supposed to be a type of TCP phase. EDS analysis shows the TCP phase (Point 5) mainly contains W (55.88 %), Cr (12.57 %), and Mo (17.87 %), while the surrounding γ'-film (Point 6) is rich in Ni (63.00 %) and Al (9.07 %). The hardness results (Fig .6) show that the microhardness of the serviced blade is higher than that of the new blade, which may be related to the precipitation of the brittle TCP phase [34]. The formation of the carbides or TCP phases during service may consume refractory elements (W, Cr, Co, Mo, etc.) in the matrix, significantly weakening the solid-solution strengthening effect of these elements.
Fig. 3 Evolution of carbides in different regions after service. (a), (d) near the exhaust edge; (e) in the middle region; (c), (f) near the blade surface; (b) diagram of the regions.
The spatial variation in the evolution of carbides is determined by the service environment. The DZ125 turbine blades have complex inner cooling structures, where the cooling effect and the resultant temperature history of different regions are not the same [35, 36]. The variations in carbide evolution in the selected part (dotted blue box in Fig. 3b) is comparable to different durations of aging. In general, 6
the cooling airflow will form a gas film on the blade surface when the turbine engine works, which will reduce the service temperature. Therefore, the equivalent aging time in the region near the outer surface is short. In contrast, the airflow temperature can be very high near the exhaust edge, accelerating the aging effect. The sequential evolution of carbides during service begins when (1) W, Mo, Cr, Co, and other refractory elements begin to precipitate, forming Mo-rich TCP phases in the crystal and W-rich MC carbides at the grain boundary, then (2) as the aging time increases, a part of the bulky MC carbides may be decomposed into the strip-shaped M6C carbides, and (3) the most refractory element (W) will continue to precipitate for a long time. In this process, MC and M6C carbides may gradually grow and connect to each other, forming a fishbone-shaped structure. 3.1.4 Overall comparison between the new and serviced blades As shown in Fig. 4, both the new and serviced DZ125 blades depict the typical dendritic structures, including the dendritic arm (A) and the interdendritic structures (B); some γ/γ' eutectic (C) can be observed in the interdendritic region. The dendritic arm is the main structure of the DZ125 blade, occupying more than 60 % of the entire morphology. Previously mentioned microstructural degradations, such as γ'-phase coarsening, carbide transformation and TCP phase precipitation, can be observed in Fig .4b. In addition to various microstructural changes, the results of the EDS analysis revealed a significant change in the content of W, Co, and other elements after service; the concentration of W (wt. %) increased from 9.82 % to 13.05 %, while for Co (wt. %), it decreased from 6.04 % to 3.84 %.
Fig. 4 Overall comparison in microstructure and element composition: (a) new blade, (b) serviced blade.
3.2 Rejuvenation heat treatment A three-step rejuvenation heat treatment was designed to recover the degraded microstructure and performance of the serviced DZ125 blades. The alloy pretreatment (1180 ° C for 2 h + 1230 °C for 3 h + air cooling) is a process of atomic thermal activation, with which the alloy structure and composition can be homogenized. The 1180 °C stage eliminates the low-melting-point phase (such as the TCP phase) in the alloy, and 1230 °C stage re-dissolves the degraded γ'-phase and refractory carbide elements into the γ matrix. To precipitate the γ'-phase using the second-step solution treatment (1100 ° C for 4 h + air 7
cooling), the designed temperature (1100 °C) should be higher than the solid-solution temperature of the γ'-phase. The third-step aging treatment (870 °C for 14 h + air cooling) aims to adjust the morphology, size, and volume fraction of the γ'-phase. The aging time is reduced from 20 h (standard heat treatment of the DZ125 alloy) to 14 h to avoid initial melting. Figure 5 shows the evolution of the γ'-phase during the three-step rejuvenation heat treatment. After the pretreatment process, the formerly degraded microstructures are well homogenized. However, morphology of the dendritic γ'-phase (dotted red box) shows regional differences due to various degrees of degradation. In the region near the exhaust edge (Fig. 5a), several spherical γ'-phases (about 0.1 µm in diameter each) cluster together, forming a petal-shaped structure with a volume fraction of ~45 %. In the region near the blade surface (Fig. 5b), the γ'-phase presents an irregular square shape, with a size of ~0.4 µm and a volume fraction of ~50 %. The morphology of the interdendritic γ'-phase (Fig. 5c) presents a skewed distribution on the γ matrix, which may alter the preferred growth directions in the (001) crystal plane. Apart from the precipitation of several γ'-phases in this process, numerous ultrafine secondary γ' particles (dotted green box) can be found in the γ matrix. The size, morphology and volume fraction of the dendritic γ'-phases have changed significantly after the solution treatment. In the region near the exhaust edge (Fig. 5d), the petal-shaped γ'-phases merged into a spherical structure with a diameter of ~0.2–0.4 µm and a volume fraction of ~60 %, morphologically similar to the vertical section of a multiphase Ni3Al-based superalloy after annealing treatment [37], which can be explained by the phase growth in manner of dissolution and aggregation by γ'II → γ'I + γ during the solution procedure. In the region near the blade surface (Fig. 5e), the morphology of the dendritic γ'-phase is recovered to cuboidal shape (0.3–0.5 µm), with its volume fraction increased to ~65 %. Additionally, the granular secondary γ' particles within the γ-channels are nearly absent due to the solid solution behavior. In the interdendritic region (Fig. 5f), the skewed γ'-phases coalesced with each other with anisotropic grain growth, transforming into elongated particles. The morphology of the dendritic γ'-phases can be further adjusted after the aging treatment. In the region near the exhaust edge (Fig. 5g), the shape of the spheroidized γ'-phases gradually changed to cubical forms. In the region near the surface of the blade, more cubical γ'-phases are precipitated from the γ'-channels, with the volume fraction increased to ~70 % (Fig. 5h). In addition to the precipitation of new γ'-phases, the previous precipitates continue to grow along the preferred [001] direction. Thus, the dendritic γ'-phase in this region presents a height difference on the (001) plane, which is similar to the morphology of the new blade shown in Fig. 2a. The interdendritic γ'-phases also show the growth in the preferred direction, deviating from the [001] direction due the original skewed distribution shown in Fig. 5c. Such interdendritic γ'-phases, indicated by yellow line (Fig. 5i), stand out when compared with the surrounding dendritic precipitates, which may be contributed to growth competition between the two types of γ'-phases.
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Fig. 5 Evolution of γ'-phases in different regions during a three-step rejuvenation heat treatment.
In this experiment, most microstructures in the serviced blades have been significantly restored after the three-step heat treatment. On the right side of the yellow line (Fig. 6), the size, morphology, and volume fraction of the γ'-phases in the dendritic arm (A) and the interdendritic region (B) recover to a similar level to new blades. However, it is worth noting that the morphology of the dendritic γ'-phase in the region near the exhaust edge (region C) is not completely restored. This region suffers the most severe microstructural degradation as discussed above, with a many refractory elements precipitated during service, which requires more energy to rearrange and precipitate the cubical γ'-phases. Generally, the dendritic morphology evolution has a close relationship with the hardness [38], and hence, the recovery effect of the heat treatment can also be verified by hardness testing. As shown in Fig. 6, the microhardness of the serviced blades has decreased after the heat treatment, which is nearly the same as the new ones (~410 HV0.02). The reported relationship between the material strength and hardness can be expressed by the empirical equation Hv = Kσ, where K is a constant related to deformation rate [39]. Thus, the mechanical properties of the rejuvenated blades are largely similar to new ones.
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Fig. 6 Recovery effect in terms of microhardness and microstructure.
4. Conclusion The degradation behavior of a directionally solidified gas turbine blade after real service was studied and a rejuvenation heat treatment was performed to recover the microstructure and microhardness of the blade. The results of this study led to the following conclusions: (i) The leading edge in the blade tip showed various microstructural changes, including the degradation of the γ'-phase, transformation of carbides, and precipitation of the TCP phases. (ii) The degradation mechanism of γ'-phases were characterized by the spheroidizing, coalescing and coarsening behavior. The evolution of carbides displayed spatial variation owing to complex cooling patterns. (iii) Despite the variation in microstructural degradation, the present study showed the possibility to recover a severely overaged turbine blade in terms of microstructure and microhardness. Despite the possibility of restoring a severely overaged turbine’s microhardness and microstructure, full restoration of microstructures and properties after the present heat treatment process is still challenging. Future studies will be focused on improving the rejuvenation heat treatment and evaluating the recovered mechanical properties (e.g., tensile strength, fatigue performance and creep property) to meet the requirements of continued service. Acknowledgements This work was supported by the National Natural Science Foundation of China (grant number 91860136 and 51801231), the Key R&D plan of Guangdong Province (grant number 2018B090905001) and the Key Science and Technology project of Shaanxi Province (grant number 2018zdzx01-04-01). Author statement Peiyu Zhang: Writing- Original draft preparation, Methodology Xin Zhou: Writing- Reviewing and Editing, Supervision Xuede Wang: Project administration Yuwen Lu: Data Curation Wenqian Zhang: Visualization Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Reference [1] P. Caron, T. Khan, Evolution of Ni-based superalloys for single crystal gas turbine blade applications, Aerospace Science and Technology 3(8) (1999) 513-523. [2] J.T. Guo, C. Yuan, H.C. Yang, V. Lupinc, M. Maldini, Creep-rupture behavior of a directionally 10
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Highlights ● The microstructural degradation and rejuvenation heat treatment for a directionally solidified turbine blades after actual services are investigated. ● The degraded microstructure shows reginal differences due to of the complex cooling structures of blades. ● The microstructure and microhardness of a severely overaged blade can be partially recovered by the rejuvenation heat treatment.
Author statement Peiyu Zhang: Writing- Original draft preparation, Methodology Xin Zhou: Writing- Reviewing and Editing, Supervision Xuede Wang: Project administration Yuwen Lu: Data Curation Wenqian Zhang: Visualization