Effect of grain boundary misorientation on η phase precipitation in Ni-base superalloy 718Plus

Materials Characterization 151 (2019) 53–63

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Effect of grain boundary misorientation on η phase precipitation in Ni-base superalloy 718Plus

T

⁎

Bader Alabbad , Sammy Tin Illinois Institute of Technology, 10 W. 32nd street, Chicago, IL 60616, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Ni-base superalloys 718plus Serration η phase Misorientations

In an effort to engineer the morphology of the grain boundaries in Alloy 718Plus to be more damage tolerant, the primary aim of this study was to investigate the effect of the grain boundary misorientation on the precipitation of η phase precipitates. Two types of η phase precipitates were observed to form within the microstructure which formed either lamellar structures or were present as discrete η phase precipitates. The lamellar η phase precipitates were able to induce the formation of serrated grain boundaries, but their precipitation was not correlated to the grain boundary misorientation as they were observed to reside along approximately half of all low angle grain boundaries. However, the average length of the η phase precipitates was found to be affected by the grain boundary misorientation, especially when the precipitates have a low inclination angle with respect to the grain boundary. The presence of discrete η phase precipitates along the grain boundary was found to be a function of the grain boundary as they were preferentially found to form along grain boundaries with high misorientations.

1. Introduction The superior properties of nickel base superalloys at elevated temperature make them an attractive choice to be used in the hot sections of gas turbine engines. Polycrystalline nickel base superalloys are known to possess high temperature strength, good crack growth resistance and corrosion resistance at high temperatures. These desirable attributes can largely be attributed to a combination of solid solution strengthening and the precipitation of a coherent second phase that serves to restrict dislocation motion. For most conventional Ni-base superalloys, the main strengthening precipitate phase is γ′ which has an ordered L12 crystal structure. These coherent γ′ precipitates generally form in the solid state and are uniformly dispersed in the disordered FCC γ matrix [1–7]. Nickel‑iron‑chromium-based superalloys, such as Inconel 718, are typically used in a variety of cast and wrought structures located in the rear section of the turbine engine where the service temperatures tend to be lower. By volume, Inconel 718, it the most widely used superalloy for aerospace and power generation turbines due to its unique combination of balanced properties in parallel with the ease of its processibilty. The typical microstructure of Inconel 718 consists of a disordered face center cubic (FCC) γ matrix and a variety of ordered intermetallic precipitates. The most commonly observed phases are the Ni3Al γ′ phase precipitates that possess a L12 crystal structure, the tetragonal ⁎

Ni3Nb γ″ precipitates possessing a D022 crystal structure and the orthorhombic (D0a) Ni3Nb δ phase precipitates [8–11]. The γ′ and γ″ phases are coherent with the FCC matrix and distributed intragranularly, while the δ phase are not fully coherent with the matrix and tend to reside along grain boundaries. Compared to most conventional Ni-base superalloys that rely on γ′ precipitates for strengthening, Inconel 718 tends to be rather unique as the microstructure is primarily reliant on the γ″ phase for strengthening [8,12]. Although Inconel 718 exhibits high strength, the temperature capability of the alloy is limited to temperatures below ~650 °C as higher temperatures accelerate the transformation of the γ″ phase precipitates to δ phase and weakens the microstructure [13]. As the operating temperatures of advanced gas turbines are continuously being increased in an effort to improve the overall efficiency, the limitations of existing structural materials used in the hot section of the engine are rapidly being approached. Thus, there is significant demand for developing innovative classes of Ni-based superalloys that can maintain phase stability and operate at temperatures higher than Inconel 718 while maintaining phase stability. In recent years, various studies have been conducted to modify the chemistry of Inconel 718 and develop cast and wrought superalloys with improved temperature capability. Although γ′ strengthened superalloys such as Waspaloy and Rene 41 possess higher temperature capabilities than Inconel 718, their processability and workability is comparatively more difficult, thereby

Corresponding author. E-mail address: [email protected] (B. Alabbad).

https://doi.org/10.1016/j.matchar.2019.02.038 Received 15 November 2018; Received in revised form 23 February 2019; Accepted 23 February 2019 Available online 23 February 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.

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increasing the overall cost of the component. In recent years, a relatively new commercially developed superalloy, ATI 718Plus, has been developed that appears to maintain a similar ease of processing, but possesses a 55 °C higher temperature capability in comparison to Inconel 718 [14,15]. Compositionally, ATI 718Plus is similar to Inconel 718 but contains the addition of 9 wt% Co and a reduced Fe content. Other minor compositional differences between ATI 718Plus and Inconel 718 include slight variations in the Al and Ti content along with the addition of 1 wt% of W in ATI 718Plus. These compositional modifications served to stabilize the γ′ phase precipitates [16–19] that serve as the main strengthening phase in ATI 718Plus whereas the γ″ phase precipitates provide the majority of strengthening in conventional Inconel 718. Due to these variations in microstructure and chemistry, the high temperature mechanical properties of ATI 718Plus was found to be superior to Inconel 718 at 704 °C while maintaining nominally similar levels of room temperature strength [14,15,20,19]. In addition to the precipitation of γ′ in ATI 718Plus, grain boundary η and/or δ phase precipitates also form during thermal – mechanical processing or following long term thermal exposures. The precipitation behavior in ATI 718Plus has been extensively investigated in various studies and found out those plate-like precipitates are η phase which possesses a chemistry of Ni3Nb0.5(Al, Ti)0.5 and a hexagonal crystal structure [16,21,22]. The orientation relationship between η phase and the matrix is {111} γ ∥ (0001)η, < 110>γ ∥ < 2110>η [22]. In some cases, orthorhombic δ phase was observed to be dispersed within the η phase [22,23]. Studies have also shown that the precipitation of those precipitates along the grain boundaries alters the grain boundary morphology as serrations form due to the protrusion of the precipitates into neighboring grains. The formation of lamellar η precipitates along the grain boundaries in 718Plus appears to occur via a discontinuous cellular reaction where the grain boundary morphology becomes modified and serrations form along the grain boundaries. [18,22,28]. The presence of these grain boundary serrations have been reported as being beneficial to high temperature mechanical properties in several alloying systems [24–27] and may potentially be an innovative method of extending the temperature and creep resistance of structural materials. In general, the discontinuous cellular reaction is a solid state transformation that leads to the formation of a two-phase lamellar structure [29,30]. To reduce the free energy of the system, the supersaturation ahead of a mobile interface or grain boundary is consumed by the moving boundary and solute atoms can be effectively redistributed to the progressing lamellar structure. Thus, the growth kinetics of the discontinuous precipitates is largely governed by grain boundary diffusion. When the lamellar structure maintains an orientation relationship with its host grain and grow into the adjacent grain, the resulting grain boundary morphologies become serrated and perturbed. As Ni-base superalloys are being used at ever increasing temperatures to extend the performance and efficiency of advanced gas turbines, understanding the mechanisms by which creep and fatigue interact in polycrystalline structures is becoming increasingly important. Serrated grain boundaries may potentially provide higher resistance to grain boundary sliding and crack growth during deformation of polycrystalline Ni base superalloys. In γ-γ′ Ni-base superalloys, the slower cooling rate promotes heterogeneous nucleation of grain boundary γ′ precipitates that can grow and induce serrations along the grain boundaries [31–36]. Ni-base superalloys containing serrated grain boundaries within the microstructure were found to have higher resistance to grain boundary sliding and crack growth [33,37–40]. Improvements in mechanical properties by inducing serrated grain boundaries have been demonstrated in various Ni-base superalloy where serrations were induced by the formation of either M23C6 carbides or coarse grain boundary γ′ precipitates [33,41–43]. Nucleation and growth of the δ precipitates along the grain boundaries can induce serrations within the microstructure of Inconel 718 which have been associated with longer creep rupture times [44]. While large volume

Table 1 The chemical composition of 718Plus (wt%). Ni

Cr

Fe

Co

Mo

W

Nb

Al

Ti

C

B

P

Bal.

18

10

9

2.7

1

5.4

1.45

0.7

0.025

0.004

0.007

fractions of η phase precipitates are likely to be detrimental, small volume fractions of η phase precipitates along grain boundaries should induce serrations in 718plus superalloy. It is interesting to note, however, that in Inconel 718 and ATI 718plus, the formation of grain boundary precipitates such as δ or η does not occur uniformly at all grain boundaries. Some grain boundaries are populated with precipitates and are serrated, while other boundaries are free of precipitates and appear as linear segments. Since only a few studies have investigated the effect of the grain boundary structure and character on the precipitation behavior in these alloys, an improved understanding of the influence of the grain boundary structure and character on the precipitation would be important and useful. 2. Experiment Results from this study were obtained from a forging of 718Plus that has a chemical composition listed in Table 1. To better examine and understand the parameters affecting the nucleation and growth of η precipitates, a super-solvus heat treatment was chosen to dissolve all pre-existing γ′ and η precipitates back into the matrix and minimize prior thermal-mechanical processing effects on the precipitation. Hence, this investigation used a super-solvus heat treatment at 1100 °C for 2 h to ensure the dissolution of all intermetallic precipitates back into the matrix. The super-solvus solution treatment was followed by direct aging at 850 °C for 16 h in which the sample was cooled at a rate of 0.1 °C/s cooling rate from solution. The aging temperature and time was selected to allow for the precipitation of a small fraction of η phase precipitates at grain boundaries which can generate serrated grain boundaries without strongly affecting the subsequent precipitation of γ′ precipitates in the system. For microstructural characterization, standard metallographic techniques were used finishing with a final polish using 0.06 μm colloidal silica. Electrolytic etching was performed using a solution of 10% Oxalic acid in water for 5 s. Microstructure examinations were conducted using a JEOL JSM 6701-F field emission scanning electron microscope (FESEM) with an accelerating voltage of 10 kV. The grain boundary misorientation was characterized using an Oxford Instruments Nordlys Nano electron backscatter diffraction (EBSD) detector that attached to a JEOL JSM 5900-LV scanning electron microscope (SEM). All of the EBSD scans were captured at magnification of X300 and step size of 1.3 μm. Oxford Instruments Channel 5 data processing software was used to characterize the grain boundary misorientation for each grain boundary. Standard noise reduction methods from the Channel 5 software package was performed to interpolate non-indexed regions based on the characteristics of the neighboring points. To examine the influence of the grain boundary misorientation on the precipitation of η precipitates, micro-hardness indents were made on the sample surface to serve as fiducial reference markers. The fiducial marks allowed for subsequent superposition and location correlation of the microstructural features characterized by the EBSD and FESEM analyses. This was necessary since the EBSD samples were examined in the as-polished condition prior to etching of the sample for characterization of the precipitate fraction and morphologies in the FESEM. This approach allows the grain boundary character distribution quantified from the EBSD scan to be coupled and correlated with the etched microstructure revealing at which grain boundary the precipitation of η precipitates occurred. Moreover, this technique also allows quantification of the average length and inclination angle between 54

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Fig. 1. Microstructure of direct aged ATI 718Plus showing the (a) the precipitation η and γ′, (b) the morphology of γ′ precipitates.

η precipitates and the grain boundary as a function of the grain boundary misorientation. Image-J software was used for image processing and image analysis to determine the average length of η precipitates and the their inclination angle with respect to the grain boundary.

dimensions of the precipitates. The growth of the lamellar η precipitates produces serrations on the grain boundary as can be seen in Fig. 4. The mechanism responsible for the formation of the serrated grain boundaries appeared to vary within the microstructure. One class of serrations occurs through the formation of “steps” along the grain boundary as shown in Fig. 4a. Here, a protrusion is created by a group of η precipitates and the following group of η phase precipitates forms the subsequent protrusion in the grain boundary. This phenomenon occurs on grain boundaries containing relatively high densities of thin η phase precipitates and becomes more apparent with the reduction of the inter-particle spacing between η precipitates. The other type of grain boundary serration has a characteristic “stitched” or “sewing” pattern, Fig. 4b. This type of serrated grain boundary morphology occurs at grain boundaries containing relatively low number density of η phase precipitates. In this scenario, each individual η precipitate produces a protrusion at the grain boundary. In addition to the precipitation density and the thickness of the precipitates, another major factor influencing the formation of serrated grain boundaries in Alloy 718Plus is the inclination angle between the η precipitates and the grain boundary line. From Fig. 4a and b, the effect of the inclination angle on the formation of serrated grain boundary is apparent. The precipitation of η phase precipitates with low inclination angles oriented with respect to the grain boundary would limit the ability to develop pronounced serrations along the grain boundary. Hence, both the inclination as well as the length of the η precipitates could impact the formation of serrated grain boundaries in Alloy 718Plus. From the observed microstructures, the occurrence of the η phase precipitation, the number density of precipitates, their length, width, thickness, and their angle relative to the grain boundary appear to vary significantly along different grain boundaries. Such a behavior suggests that the grain boundary structure/character influences the precipitation of the η phase. To further investigate the details of the precipitation behavior of η precipitates, characteristic attributes, such as the precipitation frequency, length of the precipitate, and inclination angle of η precipitates with respect to the grain boundary were examined as a function the grain boundary structure and misorientation. The grain boundary misorientation is known for having a strong influence on the grain boundary energy and consequently on the precipitation kinetics of heterogeneously distributed phases. A summary of the procedure for correlating grain boundary precipitate details as a function of grain boundary misorientation is presented in Fig. 5. For this investigation, over 65 distinct regions within the microstructure containing nearly 2000 grain boundaries were investigated in detail to ensure statistically significant results. The overall grain boundary character distribution in Alloy 718Plus is presented in Fig. 6 where the distribution frequency is based on the total number of grain boundaries assessed. Based on these

3. Results Alloy 718Plus was subjected to a heat treatment starting with supersolvus heat treatment at 1100 °C for 2 h followed by controlled cooling with a cooling rate of 0.1 °C/s to the aging temperature of 850 °C where it was then held for 16 h. The coarse grain microstructure consisted of fine intragranular γ′ precipitates that are distributed uniformly within the matrix along with a heterogeneous distribution of η phase precipitates, Fig. 1a. The η phase precipitates are predominately located along the grain boundaries, but are not distributed uniformly as many grain boundaries were observed to not contain η precipitates. Direct aging of the material at 850 °C resulted in a microstructure containing uniform distribution of fine, spherical γ′ precipitates averaging approximately 68 nm in diameter, Fig. 1b. This particular heat treatment permitted the nucleation of grain boundary η phase precipitates along the grain boundary which grew and adopted a thin plate-like morphology as can be seen in Fig. 2a, c. The aligned and lamellar morphology of these η phase precipitates are characteristic of a discontinuous precipitation reaction. It is worthwhile to note that the inter-precipitate spacing between the η phase precipitates was found to vary substantially from grain boundary to grain boundary. Within a particular grain boundary segment, however, the inter-lamellar spacing of η precipitates did not vary and those precipitates were observed to maintain the same orientation. Discrete “individual” η phase precipitates were also observed in the microstructure and found to form along the grain boundary parallel to the grain boundary plane. These discrete precipitates were not uniformly distributed and were largely oriented along and parallel to the grain boundary with a comparatively low density, Fig. 2b. In the direct aged specimen, nucleation of the lamellar η phase precipitates occurred at the grain boundaries. Since the η phase precipitates maintain a specific orientation relationship with the grain from which they nucleated from, they were observed to maintain the same inclination angle along individual grain boundaries. In general, the η precipitates adopt a thin plate morphology where the length ≫ width ≫ thickness. The typical dimensions of the η precipitates are illustrated in Fig. 3. However, it is important to mention that most η precipitates have a limited thickness making the measurement of their thickness difficult in most cases. Therefore, the dimensions of the η phase precipitates shown in Fig. 3 are not quantitative measurements, but are intended to schematically illustrate the characteristic 55

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Fig. 2. (a) The precipitation of η phase in the form of the lamellar structure, (b) the precipitation of η phase in the form discrete precipitates, and (c) SEM image highlighting the lamellar structure of η phase precipitates forming along the grain boundary.

precipitates is overlaid onto the plot showing the total frequency of grain boundaries in the system shown in Fig. 6. It should be noted that twin boundaries were excluded from the results. The highest frequency of presence of η phase precipitates is along grain boundaries possessing misorientations between 50°–60° with 5% followed by 40°–50° with 2.5%. The lowest frequency occurred at low angle grain boundaries 0°–10°. Even though the overall frequency of occurrence of the lamellar structure of η precipitates is the lowest along low angle grain boundaries, the results show that the proportion is high as nearly half of the low angle grain boundaries in the system exhibit them. On the other hand, the occurrence of lamellar η phase precipitates along high angle

results, twin boundaries are the most common and are present with the highest frequency of 37%, followed by high angle grain boundaries with misorientations between 50°–60° and 40°–50°, with frequencies of 18% and 17%, respectively. The frequency of grain boundaries with misorientations between 30°–40° is 12% and decreases to 9% for boundaries with a misorientation of 20°–30°. Grain boundaries with misorientations of between 10°–20° are present with only 5% frequency, while low angle grain boundaries with misorientations of 0°–10° has the lowest frequency around 2%. The frequency of grain boundaries containing lamellar η phase precipitates is shown in Fig. 7 where the fraction of the grain boundary containing the lamellar η

Fig. 3. Characteristic dimensions of η phase precipitates. 56

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Fig. 4. Grain boundary serrations produced by (a) closely packed η phase precipitates, (b) η phase precipitates with large interspacings.

Fig. 5. EBSD map showing the grain boundary character and the method of statistically correlating the precipitation of grain boundary η phase precipitates as a function of grain boundary misorientation. 57

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grain boundaries with 40°–50° misorientations. These results appear to indicate that there appears to be some correlation between the average length of the η phase precipitates and grain boundary misorientation. The shortest η phase precipitates are located predominately along low angle grain boundaries and the average length of the particles increases modestly with increases in the grain boundary misorientation. The relative inclination angle of η precipitates was also measured along each grain boundary exhibiting lamellar structure of η phase precipitates. Correlation of these results with the grain boundary misorientation are shown in Fig. 8b. Precipitates at low angle grain boundaries (0°–10°) have the lowest average inclination angle of 15.23° followed by grain boundaries possessing misorientation between 50°–60° with 21.84°. The highest average inclination angle is observed at grain boundaries where the grain boundary misorientation varies from between 20°–50°. Since the inclination angle is an important factor that contributes to increasing the protrusion amplitude resulting in more pronounced serrations along grain boundaries, these results indicate that grain boundaries with misorientations of 20°–50° are likely to possess serrations with the highest amplitudes. The average length of the lamellar η phase precipitates should also be influenced by inclination angle of the precipitates where the movement of the moving boundary will affect the length differently based on the contact angle or inclination angle. Therefore, an inclination angle of 20° was chosen as the threshold to assess the average length as a function of the grain boundary misorientation for precipitates by either having an inclination angle higher than 20° or lower than 20°. The 20° inclination angle was chosen based on observations that grain boundary η phase precipitates possessing inclination angles larger 20° were found to result in larger protrusions along the grain boundary resulting in more pronounced serrations. A representative grain boundary containing η phase precipitates with a low inclination angle relative to the grain boundary is presented in Fig. 9a while Fig. 9b shows a representative grain boundary with η phase precipitates oriented with a high inclination angle relative to the grain boundary. The influence of the grain boundary misorientation and the relative inclination angles associated with the precipitation of η precipitates is presented in Fig. 10. The average length of η phase precipitates having inclination angles < 20° was found to always be larger than the average length of η phase precipitates possessing high inclination angles at all misorientations, Fig. 10a. For the grain boundaries that possess η phase precipitates oriented at low inclination angles, the average length of the precipitates was observed to vary as a function of misorientation. Low angle grain boundaries that are between 0°–10° have the lowest average length of η precipitates at 2.1 μm. The average length starts to gradually increase with increasing the grain boundary misorientation and reaches the maximum average length of 4.2 μm at grain boundaries with misorientations between 40°–50° followed by 30°–40° with an average length of 4 μm. For grain boundaries where the η phase precipitates were oriented with high inclination angles, the smallest average length precipitate of 1.8 μm was found along low angle grain boundaries, Fig. 10a. The average precipitate length increased slightly to 2 μm at grain boundaries with misorientations of 10°–20° misorientations, but did not seem to change significantly for grain boundaries with misorientations between 20°–60° where the average η phase precipitate lengths fluctuated between 2.5 μm and 2.6 μm. The above results clearly show that there is a strong influence of the grain boundary misorientation on the length of the lamellar η phase precipitates in instances where the inclination angle is < 20°. The frequency of grain boundaries containing η phase precipitates as a function of the “two segments” of the inclination angle and the grain boundary misorientation is presented in Fig. 10b. Low angle grain boundaries that have misorientations between 0°–10° are the only population that were found to possess a higher frequency of grain boundaries containing precipitates with low inclination angles. Grain boundaries with misorientations ranging between 10°–20° and 50°–60° were observed to have marginally higher occurrences for containing precipitates oriented

Fig. 6. The distribution of grain boundary misorientation based on the number of grain boundaries.

Fig. 7. The frequency of the lamellar structure of η precipitates with respect to the grain boundary misorientation.

grain boundaries appears to be relatively limited as the proportion of these grain boundaries observed to contain lamellar η phase precipitates is comparatively smaller. Therefore, the grain boundary misorientation does not appear to be one of the major parameters influencing the nucleation of the lamellar η phase precipitates in the 718Plus. The average length of the lamellar η phase precipitates was measured at each individual grain boundary containing lamellar η phase precipitates. It should be noted, the inclination angle of the grain boundary does affect the observed length and interparticle spacing of the η phase precipitates. Deep etching of the microstructure has revealed that many of the precipitates are inclined and are not oriented edge-on and normal to the viewing plane. It was not possible for these inclination effects to be accounted for in the measurements of the precipitate length. Due to the limited thickness and large aspect ratio of the lamellar η phase precipitates, the associated errors are likely to me minimal. Moreover, a large statistically significant dataset was used to provide averaged values for the characteristic descriptors of the grain boundary precipitates. These precipitate length measurements were correlated with the grain boundary misorientation to provide detailed statistical results showing how the average length of the η phase precipitates varies as a function of the grain boundary misorientation, Fig. 8a. The lowest average length was 2.1 μm and found along grain boundaries with 0°–10° misorientations. The average length increases with increasing the grain boundary misorientation with the highest average length of η phase precipitates measuring 3.2 μm at grain boundaries with misorientations of 50°–60°. The average length went up from 2.1 μm at the grain boundaries with 0°–10° misorientations to 2.2 μm at 10°–20° and to 2.7 μm at 20°–30°. The average length reached 2.7 μm at grain boundaries with 30°–40° misorientations and 2.8 μm at 58

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Fig. 8. The influence of the grain boundary misorientation on (a) the average length, (a) the inclination angle of η precipitates with respect to the grain boundary.

misorientation is 17% and 22%, respectively. Grain boundaries with 30°–40° misorientations exhibited a higher frequency of 26% and grain boundaries with misorientations of 40°–50° were found to have the highest frequency of this kind of η phase precipitates with 32%.

with high inclination angles. It is valuable to note that grain boundaries with misorientations between 20° to 50° were observed to have a significantly higher tendency to contain η phase precipitates with an inclination angle larger than 20°. As mentioned in the beginning of the Results section, η phase precipitates in 718Plus were found to exist as lamellar structures or as discrete “individual” precipitates. Discrete “individual” η phase precipitates were infrequently observed to form at grain boundaries containing the lamellar structures. The occurrence of those η phase precipitates was found to be irregular. They were observed to be oriented parallel to the grain boundary as can be seen in Figs. 2b, 11. The length of a discrete η phase precipitate appears to be independent from the other discrete η precipitates along the same grain boundary indicating that they form individually which is different from the lamellar structure η precipitates. In many instances, the discrete η phase precipitates were observed to be distributed non-uniformly along the grain boundary where few precipitates might be gathered together while other precipitates form away from them along the same grain boundary, Fig. 11. The quantity of these discrete precipitates along a grain boundary is usually small and in some cases there will be only two or three precipitates along a grain boundary segment measuring > 40 μm. They are typically surrounded by a γ′ denuded region, Figs. 2b and 11b, and their irregular occurrence along the grain boundaries appear to indicate that the grain boundary energy may potentially influence their precipitation. Therefore, the frequency of the occurrence of these discrete η phase precipitates as a function of the grain boundary misorientation was investigated and the results are presented in Fig. 12. Interestingly, no low angle grain boundaries were observed to contain any discrete η precipitates and their occurrence frequency is only 3% at grain boundaries with misorientations of 10°–20°. Their frequency on the grain boundaries with misorientations of 20°–30° and 50°–60°

4. Discussion Slow cooling from the supersolvus temperature followed by direct aging at 850 °C resulted in the microstructure of ATI 718Plus being comprised of γ′ and η precipitates, Fig. 1a. Overall, the γ′ precipitates were found in fine sizes, averaging 68 nm in diameter, while adopting a spherical morphology and were distributed uniformly through the matrix with high number density, Fig. 1b. The spherical morphology of the fine γ′ precipitates is attributed to the low misfit value between the γ′ and the γ matrix [16,45]. The η phase precipitates were distributed non-uniformly along grain boundaries where many of the grain boundaries were not observed to contain any η phase precipitates. Direct aging from the supersolvus temperature produced two different types of precipitation behavior for η where they form as lamellar structures or as discrete precipitates, Fig. 2. The lamellar η phase precipitates form as a result of a discontinuous cellular precipitation reaction where the parent γ matrix and γ′ phase ahead of the moving boundary transform into the η + γ [18,22,28]. Growth of the lamellar η phase is enhanced by solute diffusion along the interface of the cellular precipitate. When the η phase precipitates are not parallel or aligned with grain boundaries, growth of these precipitates results in the formation of protrusions along the grain boundary where the amplitude becomes more pronounced as the inclination angle between the grain boundary and the precipitates increases. Therefore, high inclination angles appear to be beneficial for developing pronounced meso-scale features in the microstructure by forming serrated grain boundaries

Fig. 9. SEM image of η phase precipitates with (a) low inclination angle with respect to the grain boundary, (b) high inclination angle. 59

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Fig. 10. The effect of the grain boundary misorientation and the inclination angle on (a) the average length, (b) the frequency of lamellar structure of η precipitates for each of the two segments.

Fig. 11. SEM images of discrete η phase precipitates under (a) low magnification and (b) high magnification, the arrow pointing to a twin boundary.

induced by the cellular precipitation. Therefore, for meso-scale engineering of the grain boundary morphology in Alloy 718Plus, only the lamellar η phase precipitates are able to form pronounced serrations along the grain boundaries. It should be noted that the precipitation of η precipitates does not occur at all grain boundaries. Moreover, the size distribution of η precipitates, the morphology of η precipitates, number density, and inter particle spacings all vary along different grain boundaries emphasizing the need for better understanding of the effect of the grain boundary structure/character on the precipitation of η precipitates. Thus, investigating the effect of the grain boundary misorientation on the precipitation and growth of η is critically important in order to better understand and control meso-scale features within the microstructure. Twin boundaries were excluded from the investigation because they largely possess high density of γ′ as pointed to by the arrow in Fig. 11. The presence of the lamellar η precipitates as function of the grain boundary misorientation is presented in Fig. 7 and the occurrence of η phase with lamellar structures are found to occur on both low and high angle grain boundaries. Interestingly, the lamellar η phase precipitates was observed to occur on nearly half of the low angle grain boundaries observed in this study. Therefore, it would appear that the nucleation of the lamellar η phase precipitates is not only a function of the grain boundary misorientation, but also other parameters. On the other hand, the average length of the lamellar plates seem to be influenced by the grain boundary misorientation with the lowest average values occurring along low angle grain boundaries and increases as a function of the misorientation of the grain boundary, Fig. 8a. Throughout the microstructure, lamellar precipitates were observed to reside along low angle grain boundaries, even though low angle grain boundaries are generally considered to be free of precipitates due to their characteristically low energy. This atypical behavior has also been reported in several other alloying systems where precipitates were found to nucleate along low

Fig. 12. The frequency of the precipitation of discrete η precipitates as a function of the grain boundary misorientation.

where the serrated grain boundaries found to improve the mechanical properties in Inconel 718 superalloys that is similar to 718Plus [39,40,44]. The inter particle spacing between the η precipitates is another parameter that influences the formation of the serrations where in the case of closely spaced precipitates, the η precipitates are thin in thickness and each small group of precipitates cause a protrusion, Fig. 4a. In the case of large inter particle spacings, each individual precipitate causes a protrusion on the grain boundary, Fig. 4b. Therefore, the inter particle spacing between η phase precipitates is also an important factor in influencing the resulting morphology of the grain boundary. In select instances, the η phase precipitates were also observed to nucleate as discrete precipitates and grow parallel to the grain boundary. This also results in protrusions along the grain boundary, but the magnitude the serrations are negligible when compared those 60

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the microstructure and a fixed aging temperature was used, it appears that grain boundary diffusion is the main mechanism responsible for controlling the growth. The misorientation angle of the grain boundary has an impact on the grain boundary energy where the increase in the misorientation angle increases the grain boundary excess free energy. The excess volume associated with high energy grain boundaries permits higher diffusivity and mobility due to the higher vacancy concentration along those grain boundaries. As a result, the high diffusivity of solute atoms at the reaction front allow for rapid redistribution of solute and the formation of cellular structures with large inter-particle spacings between the precipitates. The continuous γ′ precipitates ahead of the reaction front are responsible for slowing the kinetics of the discontinuous precipitates due to the reduced chemical driving force and the physical resistance to the moving boundary. Therefore, the grain boundary misorientation is one of the critical factors that influence the growth characteristics of the lamella η phase precipitates as it influences the grain boundary diffusivity at the specified aging temperature. Discrete “individual” η phase precipitates were also observed within the microstructure following the defined supersolvus heat treatment and direct aging. Those precipitates were largely observed to reside on grain boundaries that do not contain the lamellar structure. This seemingly indicates that the formation of lamellar structure of η precipitates alleviates solute supersaturation and contributes to retarding the formation of the discrete precipitates within their vicinity. The nucleation of these discrete η phase precipitates requires both a critical level of solute saturation and sufficient diffusivity since it appears that the discrete η phase precipitates only form along high angle grain boundaries. This appears to be consistent with the observation that there were no discrete η phase precipitates noted at any of the low angle grain boundaries where those boundaries are known for their low energy and diffusivity. The precipitation frequency of discrete η precipitates is shown on Fig. 12 and it correlates well with the calculated energies of the grain boundaries, where the nucleation barrier reduces with the increase of the grain boundary energy [52,53]. For the denuded regions observed in Fig. 11, γ′ and η precipitates both compete for the same solute atoms (Al, Nb, Ti). If the η precipitates nucleate prior to the γ′ phase, the growth of the η precipitates deplete the surrounding microstructure of γ′ forming elements and suppress their formation. Additionally, it should be mentioned that the discrete precipitates do cause tiny protrusions on the grain boundaries, but the magnitude of the serrations is negligible and the frequency is much more limited than the pronounced serrations caused by the lamellar structure of η precipitates.

angle grain boundaries [46–50]. However, the sizes of the precipitates along low angle grain boundaries were usually smaller in comparison to the precipitates at high angle grain boundaries [46,48,50]. This may be due to the increased grain boundary energy and diffusivity associated with high angle grain boundaries [50,51]. The results of the inclination angle as a function of the grain boundary misorientation show an interesting trend where the η plates oriented with a high inclination angle with respect to the grain boundary were more frequently observed along high angle grain boundaries. The inclination angle of the η phase precipitates with respect to the grain boundary also appears to be an important parameter influencing the magnitude of the resulting serrations. From the results, a correlation exists between the inclination angle and the overall average length of the η precipitates. Precipitates that possess a low inclination angle with the grain boundary are able to increase their overall length faster as a small movement of the reaction front will result in a comparatively longer extension of precipitates oriented with low inclination angles than precipitates with large inclination angles. The 20° inclination angle was defined as a threshold in an attempt to relate the influence of grain boundary misorientation on the length of the η phase precipitates. For grain boundaries containing precipitates with low inclination angles, the average length was the lowest at low angle grain boundaries and increases significantly with the increase of the grain boundary misorientation. The average length reaches its maximum value at grain boundaries with misorientations of 40°–50°. These results are consistent with the calculated grain boundary energy by Olmsted et al., where the calculated energy for general boundaries in polycrystalline Ni increases with increasing the grain boundary misorientation and reaches a maximum between 45° and 50° due to the increase of the excess free volume with the increase of the grain boundary misorientation and mobility [52]. The effect of the grain boundary misorientation on the length of the precipitates possessing large inclination angles is less clear as their average lengths were measured to be comparatively similar along all high angle grain boundaries but still shorter along low angle grain boundaries. In these instances, the large inclination angle might complicate the growth process as the length becomes proportional to the magnitude of the boundary movement. Although the influence of the grain boundary misorientation on the ability of lamellar η precipitates to nucleate was not immediately obvious, there were other key microstructural features where the effect of the grain boundary misorientation was pronounced. The growth of the lamellar η precipitates appears to be diffusion controlled and related to the velocity of the cellular interface. The inter-lamellar spacing between the η phase precipitates as well as the thickness of the precipitates was also clearly influenced by the grain boundary misorientation. The comparatively lower diffusivity that exists along low angle grain boundaries resulted in the formation of closely spaced and thin η phase precipitates, as can be seen in Fig. 13a. The occurrence of these thin, densely packed lamellar precipitates was consistent along all low angle grain boundaries on which precipitation occurred. On the other hand, the higher diffusivity associated with high angle grain boundaries allowed the formation of thicker η phase precipitates accompanied with larger inter-lamellar spacings, Fig. 13b. Thus, The effect of the grain boundary structure was noticeable on the characteristic growth of the lamellar structure of η precipitates due to the effect of the grain boundary on the redistribution of solute atoms at the reaction front that is a function of the grain boundary structure/misorientation. Therefore, the grain boundary misorientation plays a critical role in influencing the growth by affecting the inter-lamellar spacing between η phase precipitates as well as the thickness of the precipitates. The lamellar η phase precipitates form as a result of a discontinuous cellular precipitation reaction where the parent γ matrix and γ′ phase ahead of the moving boundary transform into the η + γ. The kinetics of discontinuous precipitates in an alloy is generally influenced by the stored strain energy, temperature, and grain boundary diffusion. Since the supersolvus heat treatment likely minimized the stored energy in

5. Conclusion Based on the results of this investigation, the following conclusions can be made:

• A slow cooling rate of 0.1 °C/s from the super-solvus temperature • • • • 61

followed by direct aging at 850 °C allows the formation of lamellar η phase precipitates. The lamellar η phase precipitates are able to produce serrated grain boundaries where the increase of the inclination angle with respect to the grain boundaries increases the magnitude of the serrations. The precipitation of lamellar η phase precipitates does not seem to be only a function of the grain boundary misorientation as approximately half of all low angle grain boundaries in the resulting microstructure were observed to contain them. The length of the η phase precipitates was found to be affected by the grain boundary misorientation, especially when the precipitates have a low inclination angle with respect to the grain boundary. High angle grain boundaries have a comparatively higher tendency to nucleate lamellar η precipitates that exhibit a high inclination angle with respect to the grain boundary allowing the formation of

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Fig. 13. The interspacing between η phase precipitates in the lamellar structure along grain boundaries with misorientations of (a) θ = 8°, (b) θ = 32°, (c) EBSD IPF-Z map for the region that contain the grain boundary in (a), and (d) EBSD IPF-Z map for the region that contain the grain boundary in (b).

• •

serrations with higher amplitudes. The effect of the grain boundary misorientation on the growth of the lamellar η precipitates was clear as the characteristically lower diffusivities associated with low angle grain boundaries resulted in thin lamellar precipitates with small interspacings. Formation of the discrete η precipitates was related to grain boundary misorientation as they did not nucleate at low angle grain boundaries and their highest occurrence frequency was at grain boundaries possessing misorientations between 40°–50°.

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Acknowledgments

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The authors gratefully acknowledge provision of material from Rolls-Royce Corporation. Financial support for this work was provided by NSF CMMI-1334998 and NSF CMMI-1537468.

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