Microstructural Assessment of a Single-Crystal Ex-Service Land-Based Gas Turbine Blade

Abstract

In this study, we examine an ex-service, Ni-base single-crystal blade made of alloy PWA1483, which was in service for 6000 h. Using light optical, scanning, and transmission electron microscopy, we analyzed the microstructure at the blade’s tip, middle, and root. Key focus areas included surface features, dendrite spacings, γ’-particle sizes, and dislocation densities. The findings reveal that the bulk microstructure hardly evolved. Dendrite spacings exhibited a consistent microstructure across all locations and there were no significant differences between the local alloy chemistries of dendritic and interdendritic regions, indicating high-quality processing. A bimodal γ’-particle distribution was observed. Variations in γ’-sizes and γ-channel widths were noted, with the tip showing rounded γ’-particles. Small spherical particles occurred only in the root and middle of the blade. The middle location exhibited the highest hardness. Dislocation densities were low and uniform, with the highest density correlating with the highest hardness.

1. Introduction

Ni-base single crystal superalloys (SXs) are used to make first-stage blades for gas turbines which operate in aeroengines and powerplants [1,2,3,4]. Prior to high-temperature service, the base material exhibits a γ/γ’-microstructure, where a high number of cuboidal γ’-particles (volume fraction: ≈65%, ordered L1₂ phase, average cube edge length: 0.5 µm) is separated by thin γ-channels (volume fraction: ≈35%, fcc solid solution, average channel width: 0.1 µm). In the present work, we analyze an ex-service SX blade made of PWA1483 [5], an alloy which was developed for use in large industrial gas turbines (IGT). The blade was taken out of service after 6000 h, corresponding to a substantial condition before usual replacement or end-of-life. The blade was exposed to temperatures between 500 °C (maximum substrate temperature at the root) and 940 °C (maximum substrate temperature at the tip) and experienced close to 1000 start-up and shut-down events. Figure 1 shows the typical distribution of temperature and stress from the cooler root to the hotter tip of a turbine blade. Temperature increases toward the tip, while mechanical stress peaks near the root due to centrifugal loading [6,7].

These conditions lead to microstructural changes and damage processes, such as coarsening of the γ/γ’-structure and damage to the thermal barrier coating (TBC), which ultimately limits the service life of the blade [4,8].

TBCs are applied to protect blades from working fluid temperatures which can reach temperatures close to the solidus temperature of the metallic substrate [9,10,11]. TBCs do provide some protection for the substrate, but their service life is limited by microcracks, delamination, oxidation, and thermal cycling [12].

The combined degradation of the coating system and the underlying microstructure alters internal stress states, reduces mechanical performance, and ultimately determines component life [11,12,13,14]. Detailed ex-service examinations have shown that the distributions of microstructure and lattice parameters in SX blades are governed predominantly by centrifugal stress and high material temperatures [8], consistent with the strong coupling of stress fields and temperature to γ/γ’-stability and defect kinetics.

In addition, service-like thermomechanical fatigue (TMF) testing for blades and vanes has been developed to reflect realistic duty cycles and to validate creep–fatigue life assessments under combined thermal and mechanical loading [15]. Such approaches provide valuable insights but cannot fully capture the complexity of real operating environments, specifically not the inhomogeneous distribution among engineering components.

Generally, in Ni-based superalloys, γ’-particle coarsening, rafting, and partial dissolution lead to degradation of tensile strength and hardness. Eutectic microstructures, carbides, and casting pores act as crack initiation sites and influence damage evolution [13]. Complementary work on blades with different service durations has shown that γ-channel width and γ’ volume fraction strongly correlate to creep damage and can predict remaining life. Degradation is most pronounced in high-temperature regions such as the leading edge. Longer service times lead to greater microstructural evolution and reduced creep life [16].

Many studies have characterized the microstructural degradation in ex-service blades and their heterogeneity. Lu et al. [17] reported that the most pronounced microstructural degradation occurred during the intermediate creep stage in an ex-service SX turbine blade from the first stage. The microstructure in this region is characterized by a high dislocation density and an increased density of stacking faults. Studies on industrial blades have further shown that maximum microstructural and mechanical degradation can occur in the blade tip, not necessarily at the nominally hottest regions, due to the combined effects of temperature, local stresses, and thinner geometry. In these regions, rafted γ’-particles near MC carbides and bimodal γ’-particle coarsening have been observed [18]. Analyses by Błachnio et al. [14] carried out after service also consistently show significant carbide precipitation in dendritic areas and directional γ’-coagulation in the tip section of the blade, indicating prolonged exposure to high temperatures. It has been found that casting defects accelerate local wear and increase microstructural changes.

Liburdi and Stephens [19] published an early paper on how to analyze an ex-service gas turbine blade, which had suffered creep damage by cavitation. They conducted creep tests on specimens taken from different blade locations and analyzed rupture surfaces. Biermann et al. [20,21] used advanced diffraction techniques to study an ex-service blade in a synchrotron beam-line. They interpreted their results in terms of heterogeneous local stress and strain states associated with interdiffusion processes which occurred in service. They used line profile analysis to measure the lattice spacings of the γ/γ’-phases.

Despite continuous advances in internal cooling concepts and high-temperature materials that improve turbine efficiency and reduce failure rates, the dominant degradation mechanisms in service that remain are hot corrosion, creep, thermal fatigue, erosion, oxidation, and carbide precipitation. Operating factors, including the choice of working fluid and fuel, have a significant impact on thermal loading, performance, and environmental impact [22]. From a reliability perspective, most blade failures are intercepted during inspections, and blade or vane rejections are more often linked to ingested materials and sustained high operating temperatures rather than to sudden mechanical breakdown [23,24].

Studies of Miura et al. [25] on single-crystal HPT turbine blades show that analysis of deformed microstructures at the tip, midspan, and root enables mapping of thermal and mechanical stresses otherwise inaccessible in service. Based on these observations, the present study focuses on an ex-service, not end-of-life, turbine blade with the aim of providing a characterization of microstructural degradation at an intermediate stage of use. The analyses cover all relevant length scales from the dendrite morphology all the way down to γ’ characteristics as their distribution among the entire component. By investigating microstructural features known to be sensitive to creep damage, a strengthened basis for residual life assessment and repair decision-making for gas turbine blades is provided. Research on ex-service blades remains necessary to better understand material failure under real operating conditions. It also provides important data for optimizing alloys for longer service life and safety.

Figure 2 illustrates a schematic of the received turbine blade from both front and top perspectives, with the gas and air flow paths indicated. The microstructure of the blade was investigated using light optical microscopy (dendrite structure), scanning electron microscopy (surface features and particle shapes) and transmission electron microscopy (dislocation/particle interactions). Special emphasis was placed on determining whether microstructural evolution in different regions of the blade results in varying mechanical properties.

2. Materials and Methods

2.1. Ex-Service Blade

For metallographic investigation, the blade was cut up into smaller pieces using waterjet cutting, a cutting technique that allows precise cutting without local heating. For reference, the individual parts of the blade were numbered, as shown in Figure 3. For metallographic investigation of the dendritic microstructure, cross-sections perpendicular to the longitudinal direction of the blade were prepared, which were ground and polished. Images were taken in the center of the cross-section, with maximum possible distance from both sides of the blade. For the final polishing step, a 1 µm diamond paste was used. After polishing, the specimens were etched for 8 s using Kalling’s solution [26], consisting of 100 mL Ethanol, 25 mL HCl und 1 g CuCl₂. The polished and etched cross sections were investigated using an Axio optical microscope (Carl Zeiss GmbH, Oberkochen, Germany) equipped with high-quality lenses, a high-resolution Leica DFC320 CCD camera and a Tango Desktop stepper-motor-driven sample stage (Märzhäuser, Wetzlar, Germany) and software imagic ims client (V23H2) which combine individual images to montages.

From light micrographs, average dendrite spacings were obtained by manually marking dendrite core points and calculating nearest neighbor spacings using Delaunay triangulation [27]. Along with the analysis of metallographic cross sections by light optical microscopy, hardness measurements were performed at the three locations of the blade using HV10, with the KB30 device (KB Prüftechnik GmbH, Hochdorf-Assenheim, Germany), applying a load for 7 s and a dwell time of 10 s.

Additionally, from selected blade locations, miniature creep specimens were extracted. The corresponding miniature creep tests will be published elsewhere.

2.2. Scanning Electron Microscopy (SEM)

For SEM investigations of the γ/γ’-microstructure, polished cross-sections were etched for 3 s in a solution consisting of 40 mL distilled H₂O, 20 mL HCl and 10 mL H₂O₂. SEM was conducted using a Leo Gemini 1530 from Carl Zeiss (Oberkochen, Germany), equipped with a field emission gun and an Inlens detector. Secondary electron (SE) and backscatter electron (BSE) images were taken using acceleration voltages of 12 (SE images) and 20 kV (BSE images). For each blade location, 5 SEM micrographs of 110 µm² were taken to determine particle sizes. In each micrograph, the image contrast was adjusted and then evaluated using the procedure of Horst et al. [28], as shown in Figure 4. Incorrectly detected particles were manually excluded (Figure 4d red). Particles located at the image boundaries (white) were not considered in the evaluation (Figure 4d white).

For one measurement, a total of 1000 particles were considered. The particles were analyzed by determining their horizontal and vertical Feret sizes [29]. The mean value of these two parameters yielded particle size s. A line intersection method was used to measure channel widths w [28], as shown in Figure 5. Five SEM micrographs were considered, and from each, about 100 interparticle spacings were evaluated, considering vertical and horizontal <100> directions.

In this preliminary SEM evaluation, only the secondary γ’-particles (index: 2nd) were considered. From average particle sizes $\overline{s}$ and average channel widths $\overline{w}$, volume fractions f_V^2nd were evaluated using the following formula [4]:

$$f_V^{2nd} = {\overline{s}}^3/{\left(\overline{s}+\overline{w}\right)}^3$$

(1)

To characterize the particle shape, a roundness parameter R was calculated from the average particle areas A (round particle: r²π, cuboidal particle: a²) and average particle perimeters P (round particle: 2·r·π, cuboidal particle: 4·a). R is 1 for a circular and π/4 for a quadratic particle; it increases with increasing roundness.

$$R = 4\pi ·A/P^2$$

(2)

An Octane Elite energy-dispersive x-ray spectroscopy (EDX) detector from EDAX (Mahwah, NJ, USA) was used for local chemical analysis in the SEM. Element maps were recorded for one surface region of the blade (location 1a). For the measurement of the chemical composition of the dendritic and interdendritic regions, 5 point analyses were performed. Examples for two individual EDX spectra for one interdendritic (location 1a) and one dendritic region (location 7a) are presented in Figure 6.

For a beam/surface contact region of 3 µm diameter, the individual measurements showed little scatter. All alloying elements were measured and recorded during EDX analysis. However, only Al and W are presented here, as these elements exhibited the most pronounced compositional variations, whereas the remaining elements showed less distinct segregation. Mean values and corresponding standard deviations for the Al and W from

Table 1. EDX results for Al from interdendritic region (ID) and W from dendritic region (D) from 5 point measurements for each element.

Blade Location	Microstr. Region	Element	Concentration/wt.%
1a	ID	Al	3.2
“	“	“	3.4
“	“	“	3.8
“	“	“	3.6
“	“	“	3.6
7a	D	W	4.5
“	“	“	4.7
“	“	“	4.7
“	“	“	4.8
“	“	“	4.7

are 3.5 ± 0.2 and 4.7 ± 0.1 wt.%. The maximum variations between the highest and lowest results are 0.6 and 0.3 wt.%, respectively. The scatter observed for the other elements is of a similar magnitude.

2.3. Transmission Electron Microscopy (TEM)

Thin foils for TEM investigation were prepared using an Accutom-10/100 cut-off machine from Struers GmbH (Willich, Germany) equipped with a Al₂O₃ (corundum) cutting disk. This device allowed us to cut off slices of 400 µm thickness, which were then ground down to a thickness of 100 µm. The foils were sectioned in the [001] direction, providing cross-sectional views perpendicular to the blade direction. From these slices, electron transparent foils were produced by electrochemical thinning in a Tenupol-5. Good thinning conditions were obtained at a working temperature of −5 °C, a voltage of 12 V, a flow rate of 16 and an electrolyte consisting of 70 vol.% methanol, 20 vol.% glycerin and 10 vol.% perchloric acid. After thinning, the specimen was cleaned in water and isopropanol.

TEM investigations were conducted using two Jeol JEM-2100 microscopes (JEOL Ltd., Tokyo, Japan) one equipped with a field emission gun and another with a LaB₆ cathode. The microscope with the field emission gun from the Institute of Physics of Materials in Brno was used for the analysis of dislocation processes and for stereo microscopy [29]. The LaB₆-TEM was used at the Ruhr University Bochum for the analysis of particle compositions by EDX. Figure 7 shows two TEM micrographs taken from the root section of the blade. Figure 7a presents an overview micrograph of a larger region. The higher magnification (S)TEM image in Figure 7b documents the presence of small tertiary γ’-particles, highlighted by a small dashed white arrow. Figure 7b also shows the presence of a dislocation (black contrast) in the center of the image, highlighted by a small white arrow pointing to the upper right.

An effort was made to determine dislocation densities ρ using the line intersection method originally proposed by Ham [30]. The foil thickness t_F required in Ham’s formula was determined using the stereo method from Agudo [29], as shown in Figure 8a.

Figure 8b shows the system of reference lines which were used to count the intersections with dislocations. Dislocation densities were obtained using Ham’s formula:

$$\rho ={\displaystyle \frac{1}{t_F}}·\left({\displaystyle \frac{n_v}{L_v}}+{\displaystyle \frac{n_h}{L_h}}\right)$$

(3)

In Equation (3), n_v and n_h represent the total number of intersections between the dislocations’ vertical (v) and horizontal (h) reference lines, respectively. L_v and L_h are the total lengths of all vertical (v) and horizontal (h) reference lines.

Figure 9a shows dislocations in the microstructure at blade location 5a. To quantify the volume fraction (index: 3rd) of small, round tertiary γ’-particles within the γ-channels, we performed image analysis on high-magnification (S)TEM micrographs as, for example, shown in Figure 9b. Measurements were conducted at blade locations 5a (middle) and 7a (root).

For this purpose, the diameters d_i of all n_P particles were measured assuming a spherical particle shape, and the volume V_P of the tertiary particle was calculated. The test volume V_T where the particles were evaluated is given by the product of the projected micrograph investigation area A_P times the foil thickness t_F:

$$V_T=A_P·t_F$$

(4)

The volume fraction f_V^3rd of small tertiary particles in the γ-channels is then obtained as

$$f_V^{3rd}=V_P/V_T$$

(5)

The total volume fraction f_V of γ’-particles is calculated from the volume fraction of secondary particles f_V^2nd and the volume fraction of tertiary particles f_V^3rd in the γ -channels multiplied by the channel volume fraction 1 − f_V^2nd.

$$f_V=f_V^{2nd}+(1-f_V^{2nd})·f_V^{3d}$$

(6)

3. Results

3.1. Primary Dendrite Arm Spacings (PDAS)

A montage of optical micrographs of the tip region (location 1a) is presented in Figure 10. The dendritic metal region can be clearly distinguished from the TBC (gray).

Primary dendrite arm spacings (PDAS) were measured at blade locations 1a (tip), 5a (middle) and 7a (root). At each location, close to 150 dendrites from a micrograph, shown in Figure 10, were considered. The PDAS is a microstructural quantity which shows high inherent scatter, as can be seen in the histograms of Figure 11 where PDAS between 300 and 800 µm are observed.

Average PDAS and the corresponding standard deviation ∆PDAS were determined; the results are presented in

Table 2. Primary dendrite arm spacings at three blade locations.

Blade Location	Number of Dendrites	Average PDAS/µm	∆PDAS/µm
1a	155	545	82
5a	156	432	47
7a	116	589	81

.

3.2. Local Alloy Compositions

Chemical compositions were measured for dendritic and interdendritic regions using SEM EDX point analysis at the blade tip (location 1a) and at the blade root (location 7a). The average values from five point analyses are presented in

Table 3. Comparison of chemical composition (in wt.%) of interdendritic (ID) and dendritic (D) regions of the microstructure in different blade locations (1a and 7a). Mean value M and standard deviation ∆M were determined.

Blade Location	Region	Al	Co	Cr	Fe	Mo	Ta	Ti	W
1a	ID	3.7	8.4	11.5	0.3	1.6	7.9	4.5	3.2
7a	ID	2.9	8.8	11.9	0.5	1.3	7.2	4.1	3.2
1a	D	3.7	8.5	11.8	0.4	1.3	7.9	3.7	5.2
7a	D	2.9	9.0	12.2	0.5	1.7	6.7	3.5	4.4
M	–	3.3	8.7	11.9	0.4	1.5	7.4	4.0	4.0
∆M	–	0.4	0.2	0.2	0.1	0.2	0.5	0.4	0.8

.

Only the heavy elements Ta and W show larger differences between dendritic and interdendritic regions. From the four measurements, a mean value M and a mean deviation from ∆M were evaluated. As a result, it was found that there was neither a significant difference between the dendritic and interdendritic regions nor between the tip and the root of the blade. From the four measurements, we determine a mean value which represents the alloy concentration of PWA1483. Note that the mean deviation from this mean value from the four measurements is always below 1 wt.%, which represents the resolution limit of the SEM EDX analysis. For the alloy elements Al, Ti, Cr, Fe, Co, and Mo, the observed differences are insignificant. This is in line with the EDX mappings shown in Figure 12, where the elements Ni, Al, W and Ti are homogeneously distributed. Only in the case of W can a slight enrichment in the dendrite center be recognized.

3.3. Particle Sizes and Channel Widths—SEM Results

Figure 13a–c show three SEM micrographs which were taken from dendritic regions at different blade locations (locations 1a, 5a and 7a).

γ’-particle sizes s and γ-channel widths w in Ni-base SX superalloys are distributed quantities which exhibit high intrinsic scatter. This is shown in Figure 14 and Figure 15. Figure 14 shows the results of particle size distributions in the three blade locations investigated. Particle size distributions are presented in histograms, where the relative frequency is plotted against the particle size s (50 nm size classes between 0 and 800 nm).

From the distributions presented in Figure 14, mean values and mean deviations from the individual size measurements were determined ($\overline{\mathrm{s}}$ and ∆s). Together with the calculated volume fractions f_V (Equation (1)) and roundness parameters R (Equation (2)), these are listed in

Table 4. Mean values and mean deviations from mean values of particle sizes as measured by SEM for the blade locations 1a, 5a and 7a. Area fractions f_V and roundness parameters of particles R are also reported.

Blade Location	No. Particles	$\overline{\mathit{s}}$/nm	$\Delta \mathit{s}$/nm	fV2nd/%	R
1a	1144	330	110	56	0.99
5a	1317	259	117	61	0.96
7a	1115	289	98	58	0.97

.

The SEM results of the γ-channel width distributions from the blade locations 1a, 5a and 7a are shown in Figure 15.

From the distributions presented in Figure 15, mean values and mean deviations from the individual channel widths measurements were determined ($\overline{w}$ and ∆w). These are listed in

Table 5. Mean values and mean deviations from mean values of channel widths as measured by SEM for the four blades segments (blade positions 1a, 5a and 7a).

Blade Location	No. of Channels	$\overline{\mathit{w}}$/nm	$\Delta \mathit{w}$/nm
1a	214	66	26
5a	187	46	13
7a	216	58	16

.

3.4. TEM Results

Figure 16 shows TEM micrographs which were taken at the three locations 1a (tip), 5a (middle) and 7a (root). The upper row of Figure 16 (Figure 16a–c) shows individual images. The lower row of Figure 16 (Figure 16d–f) presents the corresponding 3D anaglyphs. With colored glasses (left: red, right: cyan [29]), these images allow us to appreciate the 3D nature of the microstructure. All images were taken using g = (200) two-beam conditions. The three locations feature regular arrangements of larger secondary γ’-particles, which do not appear to differ significantly in size. However, there is a difference regarding the presence of small tertiary γ’-particles, which can be clearly detected in the middle of the blade (location 5a, Figure 16b) and at the root (location 7a, Figure 16c).

Figure 17 shows two (S)TEM micrographs taken at higher magnification, where the round tertiary particles are indicated by dashed white arrows. Significantly smaller tertiary γ’-particles are observed in the root location (Figure 17b) than in the middle location (Figure 17a) of the blade. No small tertiary γ’-particles can be detected in the tip location of the blade, as shown in Figure 16a,d.

The results of STEM-EDX measurements are presented in Figure 18. The multiple-beam contrast image in Figure 18a contains a region highlighted by a rectangle. The outer parts of this capture parts of large γ’-particles which exhibit a L1₂ ordered crystal structure. Figure 18b–e were taken using a TEM with a LaB6 cathode. These mappings are of limited resolution, but they clearly demonstrate that the small particles within the channels exhibit the same chemical composition as the large cuboidal particles, as shown in Figure 18b–e. The small particles are rich in Al and Ti and contain less Co and Cr, which also characterizes the large cuboidal particles. Therefore, it is concluded that they also exhibit the L12 crystal structure (γ’-phase).

Table 6. TEM evaluation of tertiary particles and correction of SEM-volume fractions. L—blade location, n_P number of tertiary particles considered, d—average size of tertiary particles, V_P—total volume of all tertiary particles, V_T—size of test volume (Equation (4)), f_V^3rd—volume fraction of tertiary particles in % (Equation (5)), f_V—final volume fraction (secondary and tertiary particles (Equation (6)).

L	nP	d/nm	VP/µm3	VT/µm3	fV3rd/%	fV/%
5a	118	46 ± 15	88 × 10−4	1.06	0.8	61.8
7a	145	14 ± 9	3 × 10−4	0.06	0.6	58.6

shows calculated values which characterize the tertiary γ’-particle distribution in the middle of the blade (location 5a) and in the blade root (location 7a). These particles have an average size of 46 ± 15 nm at location 5a and 14 ± 9 nm at location 7a. In both locations, the tertiary volume fraction was calculated to be below 1%.

Dislocation densities which were measured at three blade locations (1a, 5a and 7a) are presented in

Table 7. Dislocation densities ρ evaluated from different blade locations 1a, 5a and 7a.

Location	Area/µm2	Foil Thickness/µm	Eval. Volume/µm3	ρ/1012m−2
1a	49	0.14	6.8	3.7
5a	49	0.18	8.7	4.3
7a	49	0.13	6.3	1.9

. The HV10 hardness values for these three blade locations are given in

Table 8. Hardness (HV10) values at the three blade locations 1a, 5a and 7a.

Location	1a	5a	7a
HV10	430 ± 6	450 ± 6	420 ± 2

. The dislocation density is highest in the middle of the blade (location 5a) with 4.3 × 10¹² m⁻², where the hardness also reaches a maximum of 450 ± 6 HV10.

4. Discussion

4.1. Alloy Homogeneity and Crystal Mosaicity

In the present work, the chemical composition was measured using EDX in the SEM. An effort was made to determine how much individual point analyses differ. Table 1 shows that the individual point analyses of Al concentrations in the interdendritic regions (3.21–3.84 wt.%) and of W in the dendritic regions (4.46–4.76 wt.%) are very close, with statistical variations of the order of ± 0.2 wt.%. Evaluations of element distribution maps (Table 3 and Figure 12) show only minor differences in local alloy chemistry between dendritic and interdendritic regions. The variations in concentrations obtained for the lighter alloy elements (Al, Ti, Cr, Fe, Co, Mo) are insignificant (<0.5 wt.%). Only for the heavy elements (W and Ta), variations between 0.5 and 1 wt.% are shown.

Furthermore, chemical compositions were also measured at the blade tip (location 1a) and at the blade root (location 7a). The average values from five point analyses are presented in Table 3. The point analyses show that the differences between different blade locations (1a and 7a) and between dendritic and interdendritic regions are small. In all cases, the scatter is below 1 wt.%. The EDX analysis performed in the present work shows that the alloy chemistry of the investigated blade is homogeneous, both in terms of different microstructural locations (D and ID) as well as regarding different blade locations. Furthermore, all measured elemental concentrations fall within the prescribed compositional limits for the alloy PWA1483 [5].

It is well known and has been discussed in the literature that dendrite spacing is a quantity which shows high inherent scatter [31,32]. No significant differences in PDAS were detected in the different blade locations. The average dendrite spacing of the investigated cast component was determined to be 520 ± 140 µm. The optical light micrograph in Figure 10 suggests that dendrites are well aligned with no obvious misorientations. This indicates a high quality of the technical single crystal achieved through well-controlled processing conditions.

4.2. Particles

The blade material exhibits a bimodal γ’-particle distribution, featuring large cuboidal particles (average sizes: 260–330 nm) and small spherical particles (average size: 17–45 nm). Large cuboidal secondary γ’-particles were characterized by SEM (Figure 13), whereas the small spherical tertiary γ’-particles are only resolved in the TEM (Figure 17).

Analysis of the secondary γ’-particles and the γ-channel widths reveals microstructural scatter across the blade, which is reflected in the measured γ’ volume fractions (Table 4): 56% (tip, location 1a), 61% (middle, location 5a), and 58% (root, location 7a). Both SEM and TEM images suggest that the large γ’-particles in the blade tip (location 1a) have rounded edges, an effect less pronounced in the lower parts of the blade. This is also weakly reflected in the roundness parameter R (Table 4), which is 0.99 for the tip region (location 1a), 0.96 in the middle (location 5a), and 0.97 at the bottom of the blade (location 7a). γ’-coagulation in the tip section indicates prolonged exposure to elevated temperatures, consistent with localized reductions in the elastic modulus that can facilitate particle coarsening [14,33].

The small spherical tertiary particles are detected only in blade locations 7a (root) and 5a (middle) and are absent at tip location 1a. Their largest sizes occur in the middle (location 5a). TEM-based estimation of the volume fraction of the small spherical γ’-particles yields less than 1% (Table 6). The EDX analysis confirms that their chemical composition matches that of the large cuboidal γ’-particles as presented in Figure 18. The middle section (5a), which contains the largest spherical tertiary γ’-particles, exhibits the highest Vickers hardness (Table 8).

We attribute the absence of small spheroidal γ’-particles and the lower γ’ volume fraction in the blade tip region to higher maximum temperatures [7], as suggested in Figure 1a. The nucleation and growth of small spheroidal γ’-particles have been discussed in the literature [34,35,36]. The formation of fine γ’-precipitates in the γ-channels has been attributed to the high supersaturation of Cr and Al [34], and intermediate cooling rates provide the appropriate combination of undercooling and short diffusion paths for their formation [37]. Furthermore, when γ’-particles are small, their morphology is determined solely by interfacial energy, rendering them spherical. The differences in elastic strain energies associated with elastic anisotropy are not yet significant enough to promote cuboidal shapes [38].

The short-service exposure of the blade investigated in the present study did not cause the larger-scale microstructural changes reported for service blades that operated under more severe conditions [16,17].

4.3. Dislocations

As can be seen in Figure 16 and Figure 17, the dislocation densities are low. The dislocation densities for the three blade locations are given in Table 7. Dislocation densities in Ni-base superalloys show a large lateral scatter; reasons for this have been discussed [39]. A maximum value of 4.3 × 10¹² m⁻² was found in the middle of the blade. This is also where the alloy has its highest hardness value (Table 8) and the highest stress occurs [6], as suggested in Figure 1b. Low dislocation density in an ex-service superalloy turbine blade has also been observed by other researchers [40,41]. It has been reported that the dislocation configuration in an ex-service blade varies in a transverse cross-section [19], a result which was not found in the present work. The formation of dislocation networks in more severely exposed ex-service blades, which has been reported in TEM work [18,39,41], was not observed in the present study.

4.4. Comparative Assessment of Microstructural Condition

The microstructural characteristics of the blade after service were evaluated with reference to available experimental data, literature reports, and common manufacturing standards. Since the initial as-manufactured microstructure was not available, the root region is assumed to most closely represent the original condition. Minor differences between different blade regions are related to different solidification conditions associated with the blade geometry.

Overall, no large microstructural differences were observed between the root, middle, and tip regions of the blade. Pronounced rafting, coarsening, or other degradation phenomena commonly reported in the literature [16,17,18,19,42,43] were absent. Severe damage mechanisms described by Leopold [24], such as high-temperature corrosion or fatigue cracking, were not observed. Similarly, cavitation damage and the high, heterogeneous dislocation densities reported by Biermann et al. [20,21] were not present. The work of Lu et al. [17] demonstrates that microstructural degradation is influenced not only by temperature but also by mechanical stress, which can lead to different rafting behavior at the same temperature.

In contrast to other ex-service blades which were closer to their end of lives, the present blade exhibits only minimal degradation, consistent with continued safe operation under standard service conditions. It should be noted that the typical minimum service life of power-generation turbine blades is 30,000 h [44]. With 6000 h, the consumed life fraction of the blade investigated in the present work is only 20%.

Ongoing miniature creep tests on specimens from the current blade follow the methodology of Wen et al. [7] and will provide a quantitative assessment of residual creep properties. Creep is one of the life-limiting factors of a turbine blade, and therefore, creep tests are important to complement the microstructural results obtained in the present work.

5. Summary and Conclusions

In the present work, we investigate an ex-service Ni-base single-crystal blade of the alloy PWA1483, of which service was terminated after 6000 h (moderate exposure). We use optical microscopy as well as scanning and transmission electron microscopy to characterize the microstructure of the blade at three locations (tip, middle and bottom). Emphasis was placed on dendrite spacings, γ’-particle sizes, and dislocation densities. From the results obtained in the present work, the following conclusions can be drawn:

(1) Chemical compositions of dendritic and interdendritic regions were measured using SEM EDX, showing little differences in local alloy chemistry across all three blade locations. Variations in lighter elements were insignificant, while heavier elements showed only slight variations. Dendrite spacings of 520 ± 140 µm were consistently found at all blade locations, further indicating high-quality single-crystal processing.

(2) The blade material has a bimodal γ’-particle distribution with large cuboidal particles and small spherical particles, the latter found only in the root and middle locations of the blade. TEM EDX analysis suggests that these small particles share the same composition as the large cuboidal ones and constitute less than 1% of the overall volume fraction. The absence of small spheroidal γ’-particles at the tip is attributed to the fact that the blade tip is exposed to higher temperatures.

(3) The blade exhibits large microstructural scatter in γ’-sizes and γ-channel widths, reflected in varying particle volume fractions: 56% at the tip, 61% in the middle, and 58% at the root. SEM and TEM images show rounded edges of large γ’-particles at the tip, less pronounced in lower blade locations. It appeared that the blade’s bulk microstructure did not evolve far from the initial processing condition. The blade’s short-service exposure prevented larger-scale microstructural evolutions seen by other researchers in blades exposed to longer-service conditions.

(4) Dislocation densities in the blade are low and show no increase from bottom to top, with the highest density of 4.3 × 10¹² m⁻² found in the middle, correlating with the highest hardness. This contrasts with other studies reporting different dislocation configurations in ex-service blades. The formation of dislocation networks seen in more severely exposed blades was not observed in the present study. This allows us to conclude that the blade which was investigated had not accumulated significant amounts of plastic strain.