Detection of Rhenium-Rich Particles at Grain Boundaries in Nickel-Base Superalloy Turbine Blades
2016 Volume 57 Issue 10 Pages 1698-1706
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2016 Volume 57 Issue 10 Pages 1698-1706
Ni-base single crystal superalloy turbine blades containing defect grains were investigated by high resolution electron microscopy. Several different types of defects, such as, stray grains, equiax grains and freckle chains which formed during casting, and recrystallized grains which formed during subsequent heat treatment, were selected. Regardless of the region of the turbine blade, many particles with large amounts of rhenium were detected at the boundary of the stray grain and matrix. The Re-rich particles were also detected at the boundary of the matrix and other defect grains, such as, equiax grains and freckle chain grains, and even at a low angle grain boundary. However, the boundary of the recrystallised grain and matrix which was formed after solution heat treatment showed just one Re-rich particle. Also, the composition of these Re-rich particles is different from any topologically close packed phases which have been reported in Ni-base superalloys. The results suggest that during solidification, the particles are formed from the melt and pushed ahead of each solidification front of the defect grain and the matrix, and piled up at the boundary of the matrix/defect grain.
Nickel base superalloys have superior mechanical strength, creep, surface stability, and corrosion and oxidation resistance at high temperatures due to a face-centered cubic structure matrix of γ phase with a dispersion of an ordered intermetallic precipitate phase of γ′ (Ni3Al)1–4), leading to extensive use in turbine blades for power generation and aircraft propulsion. The excellent high temperature properties of superalloys are optimized with respect to the size and morphology of the γ′ phase5). Furthermore, refractory elements, such as rhenium, are added to enhance the high temperature creep resistance and alloy stability, particularly at very high in-service temperatures (1273 K (1000℃)～1423 K (1150℃))1). Rhenium can increase the solidus and liquidus temperature of the nickel matrix and the volume fraction of γ′ phase, and is primarily chosen as it is more effective than tantalum and tungsten in this role due to its low solubility in the γ′ phase3,6–8). However, there is an upper limit for the alloying content of Re as high amounts of rhenium degrade the creep and rupture properties due to the formation of topological close packed (TCP) phases induced by the low diffusivity of rhenium in the Ni matrix9,10).
Single crystal turbine blades are made through the directional solidification of investment castings which has been developed to avoid the introduction of casting defects11). Therefore, microstructural features, such as dendrite arm spacing, grain size and orientation, can influence the properties of the superalloy casting components12,13). Investment casting reduces manufacturing costs as well as offering many benefits, such as the ability to use a wider range of casting materials, greater design freedom through a simplified production process12,13). Figure 1 shows a schematic diagram of a Ni-base superalloy turbine blade which can be mainly divided into four parts: root, platform, aerofoil and shroud. The solidification direction is along the length of the aerofoil of the blade. As solidification progresses there are interactions between the component geometry and solidification front, typically near the shroud region, where the melt has higher amounts of solute elements with a negative slope of the liquidus line with nickel. As a result, secondary particles (not γ′ phase) over the solubility limit can be precipitated in the shroud region and affect its microstructural development, which is the main focus of this study. A typical commercial foundry specializing in the casting of single crystal turbine blades has an overall yield of 67%4) with grain related defects being a major cause of non-conformance. Several kinds of defects, such as stray grains, equiax grains, freckle chain grains, low angle grain boundaries and recrystallized grains have been detected on the surface of turbine blades3,4,11,14–21). Of these defects, it is well known that stray grains are formed through nucleation from the extremity of a platform region due to undercooling15–18), remelting of dendrite fragments19,20), and heterogeneous nucleation of grains from TiN particles ahead of the solidification front21). However, our previous study showed another facet of stray grains; an intermediate layer containing Re-rich particles22). The intermediate layer, which is composed of many elongated γ′ phase, exists between the matrix and a stray grain in a CSMX-10 single crystal turbine blade alloy and contains Re-rich particles. The previous study also showed that when two solidifying fronts collide, forming a grain boundary, compositional irregularities form ahead of the dendrites and are frozen along the grain boundary, leading to the formation of these Re-rich phases. Based on these findings22), other types of defects with grain boundaries have been observed in this study, such as, stray grains in other region, polycrystalline grains, freckle chain grains, low angle grain boundaries, and recrystallized grains to identify similarities in microstructure which may occur.
Schematic diagram of a turbine blade.
Table 1 shows the composition of a nickel base single crystal superalloy CMSX-10 used in this study22,23), which is a third generation superalloy and one of the highest Re contents of any of these alloys. During manufacture as-cast turbine blades were removed from the runner system and underwent solution heat treatment to develop the required microstructure which is composed of cuboids of γ′ phase in a matrix of γ phase (see Fig. 2(g)). Subsequently the blades were etched to enable a visual inspection of any grain defects which might be present in the component. The inspection for secondary grains which can be exposed by the preferential attack into the boundary by the etchant is a key to product integrity. The detailed sample preparation and inspection explanation is described elsewhere3,4,22,24). Several types of grain defects exposed by the etching process were selected for high resolution analysis. Once identified, the samples were then cut out by wire electro-discharge machining (WEDM).
| Ni | Cr | Co | Mo | W | Ta | Re | Nb | Al | Ti | Hf | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| CMSX-10 | Bal. | 2 | 3 | 0.4 | 5 | 8 | 6 | 0.1 | 5.7 | 0.2 | 0.03 |
| Re-rich | Bal. | 0.7 | 1.5 | 2.0 | 4.8 | 0.8 | 80.3 | 0.0 | 0.5 | 0.1 | 0.0 |
A photograph (a) and SEM images (b-h) showing a stray grain in the shroud: b) low magnification BSE image showing a whole stray grain, c) BSE image of an interface of matrix/stray grain, d) magnified BSE image of the marked region in panel (c), e) BSE image of another interface which was deeply etched, f) magnified BSE image of the region in (e), g-h) high magnification SE images of matrix (g) and the stray grain (h). MG and SG are the matrix grain and the stray grain, respectively. The arrows in panels (d) and (f) indicate bright Re-rich particles.
Microstructural investigation was performed by using a field emission scanning electron microscope (FE-SEM, FEI Quant 3D dual beam FIB-SEM) capable of observing samples at ultra high magnifications of ×100,000～×200,000 as well as performing chemical analysis by energy dispersive X-ray spectroscopy (EDX). Two different SEM imaging modes, i.e. secondary electron (SE) and back-scattered electron (BSE) were used in this study. For TEM analysis, thin lamellae were fabricated by a focused ion beam (FIB) lift-out technique25,26) and then observed by high resolution transmission electron microscopy (FE-TEM, FEI Tecnai F20) equipped with a scanning mode (STEM) and an EDX system.
The close proximity of characteristic X-rays (Mα, keV) of the alloying elements in CMSX-10, i.e. Hf (1.644), Ta (1.709), W (1.774), and Re (1.842), made it difficult to clearly distinguish each element by EDX analysis6,27). Therefore, the existence of heavier elements was checked by SEM-EDX and subsequently by STEM-EDX with a nominal probe size of about 2 nm. STEM element mapping was performed using the TrumapTM qualitative analysis software (Oxford Instruments) integrated into to the EDX system, which resolved differences of more than 0.03 keV and allowed for the deconvolution of overlapping elements and background removal28). SEM- and STEM-EDX point analysis was carried out on more than ten analysis areas to give sufficient statistical confidence. During TEM observation the samples were cooled with liquid nitrogen to avoid or minimize microstructural changes in the irradiated area26).
In nickel-base single crystal superalloy turbine blades, several different types of surface defects can be formed from different sources. Stray grains, equiax grains, and freckle chains are formed during casting, while recrystallized grains are formed during subsequent heat treatment. Therefore, this section will be divided mainly into two parts: during casting and during subsequent heat treatment.
3.1 Surface defects formed during casting 3.1.1 Detection of Re-rich particles in the shroud region: at the boundary between a stray grain and a matrix grainFigure 1 shows a schematic diagram of a turbine blade showing the representative areas where the surface defects were located. Several different types of surface defects, such as, stray grains, recrystallized grains, equiax grains, and freckle chains were found near the shroud region. Firstly, in order to compare the findings to the previous results, which showed an intermediate layer of 3～4 μm containing a Re-rich particle at the interface of stray and matrix grains in the platform region of a turbine blade22), the interface of a stray grain at the shroud region and matrix was chosen and observed at high resolution. Figure 2 is a photograph and SEM images showing a stray grain formed in the shroud region of a CMSX-10 single crystal turbine blade. It is clear that there is a stray grain giving different (post etch) optical contrast (Fig. 2(a)). An SEM image (Fig. 2(b)) shows that the grain can be distinguishable from the matrix due to a boundary line surrounding it. At this magnification, however, it was difficult to observe the detailed microstructure of the interface of the stray grain and matrix. High magnification images acquired at ×6,500 (Fig. 2(c)) and at ×80,000 (Fig. 2(d)) indicate that there is clearly an intermediate layer between the stray grain and matrix, and most of all, a bright particle was detected in the BSE mode, which indicates that it is composed of heavier elements than nickel. The previous study showed that the bright particles are inherent to the CMSX-10 blade and they are not contamination adhered to the sample surface during sample preparation or observation. Figure 2(e) and 2(f) were acquired from the opposite side of the stray grain, thereby comparing both the upper and lower cast surfaces. The morphology of the lower side was almost identical to that of the top surface stray grain; there are also several bright particles as well as an intermediate layer. When considering the composition of CMSX-10, the particles were probably rich in Mo, Hf, Ta, W and Re. Figure 2(g) and 2(h) show high resolution images of the matrix grain and the stray grain respectively. The shape and size of γ′ phase are equivalent, however, the {100} direction, indicated by the cube edges of the γ′ phase, highlighted the orientation change across the intermediate layer.
In order to analyse the bright particles, an intermediate region was selected for TEM sampling and a platinum layer was deposited onto the region to protect it during TEM sampling (as shown in Fig 3(a)). Figure 3(b) is an SEM tilting view acquired after taking the cross-section. During through-thickness examination, several bright particles were detected and these were all detected along the boundary of the matrix grain and the intermediate layer. An SEM-EDX point analysis indicated that the particles are composed of high amounts of Re and Ni. This result is in good agreement with the previous result showing the bright particles which were detected at the interface of a stray grain at the platform region and matrix, and rich in Re22). Therefore, it was concluded that regardless of the turbine blade region, such as the platform or shroud, if a stray grain exists, Re-rich particles can be detected at the boundary of a stray grain and a matrix grain.
SEM images (a-b), SEM-EDX point analysis (c), and STEM element maps (d-g) near the intermediate layer formed between matrix and a stray grain: a) top view of a selected layer, b) SEM tilting view after making the cross section on the Pt deposited region in panel (a), c) SEM-EDX spectrum on one bright particle in panel (b), d-g) STEM maps of Re (d, f) and Al (e, g) near the interface of matrix and the intermediate layer (d, e) and at the interface of stray grain/intermediate layer (f, g). MG, SG, and IL are the matrix grain, the stray grain, and the intermediate layer, respectively. Arrows in panel (b) and an arrow in (d) indicate Re-rich particles, and the boundary of MG and IL, respectively.
After the SEM observation, a TEM sample was taken from the cross section in Fig. 3(b) using FIB and observed by TEM. Figures 3(d)–3(g) are STEM element maps of two representative elements in CMSX-10; aluminum and rhenium, and show their partitioning behavior. Aluminum and rhenium are seen to strongly partition to the γ′ phase and γ phase, respectively, which is in good agreement with previous results10,11,29). It should be emphasized that the maps of aluminium and rhenium are not continuous across the boundary of the matrix and the intermediate layer, as shown in Fig. 3(d) and 3(e). On the contrary, the element maps are continuous across the boundary of the intermediate layer and the stray grain (Figs. 3(f) and 3(g)).
Misorientation of the grain boundary is an important factor influencing on the precipitate formation and grain boundary migration because the grain boundary energy and migration rate are related to the misorientation angle of grain boundary30). Therefore, in high angle grain boundaries, the discontinuous precipitation (DP) reaction can easily occur in order to reduce the energy of the grain boundary. As a result, the intermediate layer as well as the Re-rich particles in the boundary of matrix and a stray grain observed in this study might be formed by the DP reaction. The reaction has been intensively and widely observed in over 500 binary and multicomponent systems even though it still remains a controversial issue31–33). The DP reaction can be written as following34–37):
| \[(\gamma + \gamma ')_p \to \gamma_{I} + \gamma '_I + R\] |
| \[\Delta G_{T} = \Delta G_{Chem} + \Delta G_{C} + \Delta G_{S} + \Delta G_{D}\] |
There is a possibility that Re-rich particles might have precipitated during cooling from the melt40), and they were surrounded by a corresponding Re-depleted region, such as the intermediate γ′ phase observed in this study. This may have encouraged the growth of the Re-depleted γ′ phase into rods with a higher volume fraction than in the base matrix. Finally, there is another possibility for the formation of the intermediate layer containing Re-rich particles. The stray grain in this study was probably formed through either a thermal or compositional cause, such as nucleation from the corner of the platform region15–18) or the remelting of dendrite fragments19,20). During casting, the Re-rich particles formed early in the solidification process and were pushed ahead of each solidification front present; those of the stray grain and the matrix grain, and were piled up at the boundary of the matrix/stray grain. Then, the intermediate layer containing large amount of Re solidified, as observed in this study. It is likely that the Re-rich particles are pushed ahead of all solidification fronts but in the majority of castings, which are single crystals, they settle at the external surface of the component and are removed through subsequent processing. However, it was observed that TCP phases formed in the dendrite cores of these cast components and could co-exist with the Re-rich phase in the same component but spatially separated. As the TCP phases are also rich in Re, although to a lower level than the Re-rich phase identified here, it is interesting that they form in the dendrite core regions as these solidify first and contain the highest melting point elements, whereas the Re-rich phase seems to form in the interdendritic regions.
In summary, if a stray grain exists, many particles with large amounts of rhenium can be detected at the boundary of the stray grain and matrix grain regardless of the turbine blade region. Furthermore, the Re-rich particles form early in the solidification process and not through the usual DP reaction, but it requires further study on the exact formation mechanism of Re-rich particles.
3.1.2 Detection of Re-rich particles in the shroud region: at the boundary of equiax grains and a matrix grainFigure 4 shows another surface defect which was composed of several different grains in the shroud region of a CMSX-10 single crystal turbine blade. These defects were formed through the parallel nucleation of multiple grains due to the poor thermal gradient in the casting. As observed previously at the boundary of a stray grain and a matrix grain, an intermediate layer was detected along the boundary between each of the grains as shown in Figs. 4(b) and 4(c). In order to confirm whether the grains are orientationally independent, the angle between each equiax grain and matrix grain, or between one grain and another grain, was measured using the {100} orientation of γ′ phase in each case. The measured angle of equiax/matrix grain boundary was between 30° and 60°, and the angle of equiax grain/grain boundary was between 35° and 65°. It was important to identify the misorientation of the grain boundary because the aforementioned DP reaction could only occur along boundaries with a misorientation greater than 10°39).
A photograph (a) and SEM images (b-d) of equiax grains: b) SEM top view of equiax grains, c) magnified image near the intermediate layer marked at panel (b), d) SEM tilting view after making a cross section. MG and PC are the matrix grain and the equiax grain, respectively. The arrows in panel (d) indicate Re-rich particles.
In order to detect any precipitation and the through-thickness morphology of the intermediate layer, a region was selected and cross-sectioned as shown in Figs. 4(c) and 4(d). Fine particles were detected regularly along the boundary of the equiax grains and matrix. In addition, the particles were rich in Re (not shown here) as already seen for the particles identified along the boundary of a stray grain and a matrix grain. It should be emphasized that the intermediate layer is not wavy along the boundary exposed on the surface (Fig. 4(c)) and through-thickness (Fig. 4(d)). It has been suggested that the DP reaction initiates at high angle incoherent boundaries39), however, it should be noted that high energy (rather than just high angle) boundaries are likely to initiate DP33). A sufficiently high rate in the decrease in the energy of a grain boundary as well as the increase in solute segregation in front of a grain boundary would initiate the boundary migration and sustain the growth of the precipitation colony33,41). As a result of the DP reaction, the cellular morphology of precipitates and matrix has been observed and reported many times31–37,41). However, in this study, the morphology of the intermediate and Re-rich particles was not cellular. Also the boundary was straight and Re-rich particles existed only one side along the boundary, therefore the intermediate layer containing Re-rich particles was probably formed through another mechanism rather than the DP reaction. It is known that the equiax grains are formed during casting so it seems reasonable that, similar to the formation of the stray grain in the shroud region, the Re-rich particles were pushed ahead of the solidification front and trapped at the boundary of the matrix and equiax grains and subsequently the intermediate layer containing the Re-rich particles was formed.
3.1.3 Detection of Re-rich particles at freckle chainsFigure 5 shows freckle chain grains formed within the shroud of a CMSX-10 single crystal turbine blade. The freckle chain grains are clearly composed of a number of fine grains about 200～400 μm in diameter (Figs. 5(a) and 5(b)). The boundary of a freckle grain and the matrix grain is easily distinguishable due to the deep etching as shown in Fig. 5(c). An SEM image at a high magnification (×25,000) shows clearly that there is an intermediate layer between a freckle grain and a matrix grain (Fig. 5(d)). Figure 5(d) also shows that the shape and size of γ′ phase in the freckle grain and matrix are almost identical and they form high angle grain boundary. After locating the intermediate layer, the region was cross-sectioned as shown in Fig. 5(e). Even though the top surface was not flat, a platinum layer could protect the region during FIB cross-sectioning using a strong gallium ion beam and as a result, the top surface of interest was preserved without any critical damage. Figure 5(e) shows clearly that Re-rich particles also exist in the intermediate layer.
A photograph (a) and SEM images (b-e) of freckle chains: b) low magnification image, c) at the interface of matrix and a freckle grain, d) magnified image at the interface of panel (c) after rotation, e) SEM tilting view after cross-sectioning. MG and FG are the matrix grain and the freckle chain grain, respectively. The arrows in panel (e) indicate Re-rich particles.
It is well known that the freckle chains are caused by double diffusive convection of solute-enriched interdendritic liquid in the mushy zone during solidification42), and the resultant defect arises from dendrite fragmentation through remelting which is driven by convection in the interdendritic channels43). In addition, it is also known that there is a strong correlation between the level of refractory alloying additions, such as Mo, Hf, Ta, W and Re in CMSX-10 in this study, and the occurrence of the freckle chains44). Therefore, solute segregation probably induces the buoyancy driven convective fluid flow and result in the formation of freckle defects during dendritic solidification44). However, despite much research on the nature of freckle chains, there is little literature showing the boundary of freckle chains and matrix. Figure 5 shows clearly the straight interface as well as the detection of Re-rich particles. As already explained in the previous sections, the intermediate layer containing Re-rich particles was also probably formed by the pushing of Re-rich particles into the boundary of the matrix/freckle chain grains.
3.1.4 Detection of Re-rich particles at a low angle boundaryThe surface defects observed in the previous sections have high angle grain boundaries. Therefore, it was necessary to observe a sample with a low angle grain boundary because it is known that the development of DP reaction can only occur along boundaries with a mis-orientation angle of greater than 10°34–37). Figure 6 shows a low angle (6.1°) grain boundary found in a CMSX-10 single crystal turbine blade. A reflected Laue X-ray system was used to determine the orientation of the adjacent grains and hence the arising mis-orientation angle. The angle of 6.1° between the two grains can be also measured using the {100} orientation of γ′ phase in each grain. As shown in Figs. 6(c) and 6(d), the orientation was only slightly different when compared to the higher mis-orientation boundaries reported earlier in this paper. It should be emphasized that, depending on the location, there were two different interface morphologies along the boundary of the two grains: one was the sharp interface without any intermediate layer as shown in Figs. 6(c) and 6(d), and the other had an intermediate layer as shown in Fig. 6(e). The shape and morphology of the interface (Fig. 6(e)) were similar to the interfaces reported in the previous sections, such as the stray grain, the equiax grain, and the freckle chain grains. In order to detect any particles, a cross-section was made from the interface. Figure 6(f) shows an interesting result that several particles are clearly visible along the low angle boundary. To enable compositional analysis a TEM sample was made from the cross-section. Figure 6(g) is a STEM-EDX spectrum acquired on a particle which was detected at the interface. The composition of the particle measured by STEM-EDX with a nominal probe size of about 2 nm was summarized in Table 1 and it is clear that the particles were also Re-rich particles. It should be emphasized that the composition of the Re-rich particles is different from any TCP phases which have been discovered in Ni-base superalloys, such as, σ, μ, P, and laves phases. The detection of these Re-rich particles at the low angle (6.1°) grain boundary supports the formation mechanism of the intermediate layer containing Re-rich particles suggested in the previous sections because the development of DP reaction can only occur at the boundary with mis-orientations greater than 10°.
A photograph (a), SEM images (b-f),and STEM-EDX point analysis (g) near a low angle grain boundary: b) low magnification image, c) interface showing sharp interface, d) magnified image at the sharp interface, e) another interface showing an intermediate layer, f) SEM tilting view of the interface after cross-sectioning, g) STEM-EDX spectrum on a bright particle in panel (f) after TEM sampling. LG and RG are the left grain and the right grain, respectively. The arrows in panel (f) indicate Re-rich particles.
In summary, there were many Re-rich particles, which were precipitated from the melt, along grain boundaries formed during casting, such as stray grains, equiax grains, freckle chains, and low angle boundaries.
3.2 Surface defects formed during subsequent heat treatment: Detection of a Re-rich particle at the boundary of a recrystallized grain and matrixSuperalloy turbine blades are subjected to multiple steps of melting, casting and heat treatment during manufacturing. All of the surface defects which were observed and reported in the previous sections, such as stray grains, equiax grains, freckle chain grains, and low angle grain boundaries, are formed during casting. Therefore, it is necessary to observe any grain defect which can be formed during subsequent heat treatment. Figure 7 shows a recrystallized grain on the component surface near the shroud region of a CMSX-10 single crystal turbine blade, which is a typical location to find such grains. The boundary was easily identifiable due to the boundary region as shown in Figs. 7(b) and 7(c). In Fig. 7(c), it seems that there was an intermediate layer. However, a high magnification image (×100,000) in Fig. 7(d) shows clearly that the layer was just a slightly elongated γ′ phase and in addition, the interface of the recrystallized grain and matrix is sharp without the larger intermediate layer previously observed. In order to confirm the sharp interface with no intermediate layer, the region was cross-sectioned by FIB. Figure 7(e) is a representative SEM tilting view at the interface and shows that, as expected from the top view of Fig. 7(d), there was no large scale intermediate layer. However, it was interesting to find thin gaps or voids along the boundary, which is detrimental to the mechanical properties of the component. In addition to this, just one Re-rich particle was detected at the boundary. In order to confirm the detection of the Re-rich particle and the lack of any intermediate layer, another five regions in the sample were cross-sectioned and observed. However, except for the region shown in Fig. 7(e), no other particle or intermediate layer was detected. To observe the boundary at high resolution, a TEM sample was made from the cross section of Fig. 7(e). Figure 7(f) is a STEM image along the boundary. A slightly larger γ′ phase (when compared to the matrix) as well as the sharp nature of the interface was visible along the boundary.
A photograph (a), SEM (b-e), and STEM bright field image (f) of a recrystallized grain: b) low magnification image, c) near the interface of a recrystallized grain/matrix, d) magnified image at the marked region of panel (c), e) SEM tilting view after cross-sectioning, and f) STEM image at the interface. MG and RG are the matrix grain and the recrystallized grain, respectively. The dotted arrow in panel (d) and the arrow in (e) indicate the interface of MG/RG and a Re-rich particle, respectively.
The turbine blades used in this study are manufactured through a number of shaping processes, shot-peening, and grit-blasting, which can be either concentrated at the component surface, or extend into the bulk material45,46). Other strains can be induced in the base metal due to the microscale plasticity which is caused by differential thermal contraction of metal, mould, and core47–49). It was estimated that when the mould and core do not crush or deform sufficiently or crack the metallic components can impose a strain value of about 2～3%46), which is over the critical plastic strain (0.2%) for the recrystallization of turbine blades47). These processes can lead to extensive plastic deformation being imparted to an as-cast turbine blade at high temperature and this can be responsible for strain relaxation through recrystallization during subsequent heat treatments. In addition, as the recrystallized grains are initiated from the as-cast surface, the surface condition of turbine blades can play an important role in the amount of recrystallization observed48,49). Since the recrystallized grains introduce disadvantageous orientations and high-angle grain boundaries, which can dramatically reduce the creep rupture strength and fatigue life of the turbine blades38,50–52), there are a number of studies on the recrystallized grain which have been exposed on the surface of the turbine blades. In order to enhance the susceptibility of the blades to recrystallization during solution heat treatment, non-uniform surface deformation, such as hardness indentations, have been applied after manufacturing to allow the quantification of the relationship between deformation/recrystallization behavior and for the determination of the appearance of recrystallization when the bulk material is deformed38,45,46,50–55). However, in this study, an as-manufactured sample with a recrystallized grain has been observed, and thin gap or voids as well as a Re-rich particle have been detected.
There is the possibility of recovery. However, as the stacking fault energy for most single crystal nickel-based superalloys is below 20 mJ/m2 52), the recovery of the turbine blade is very weak in the sample used in this study. The stored deformation energy can be expressed in terms of dislocation density52). In case dislocations cannot transmit across or be absorbed at a grain boundary, they will pile-up at the grain boundary. In CMSX-10 superalloys, there are three active slip systems, such as ($\bar{1}\bar{1}1$)[101], ($\bar{1}\bar{1}1$)[$\bar{1}10$], and ($\bar{1}\bar{1}1$)[011]51). As the transmission of dislocation across grain boundary requires rotation and/or climb into the new slip plane, the angle between intersections of slip plane and grain boundary plane is important51). If there is large mis-orientation angle between two grains, then dislocation transmission across a grain boundary is not easy. This stored deformation energy is released by recrystallization. In the case of a recrystallised grain boundary, when compared to the surface defects formed during casting, there is no reason for particles to form the intermediate layer or be concentrated there through the pushing of Re-rich particles into the boundary during melting. However, as shown in Fig. 7(e), there was a possibility that a few Re-rich particles can be found at the recrystallised grain boundary and from the above results it is proposed that these are present from solidification. Therefore, it seems that the Re-rich particle pre-existed the recrystallized grain boundary and may have acted to pin the boundary as it moved through the parent grain. This particle may have been trapped within a single crystal grain through the confluence of orientated dendrites, thus leading to no grain boundary but a trapped Re-rich particle, and may be due to a local increase in Re content in the alloy.
In summary, there were many Re-rich particles along an intermedate layer at a boundary formed during solidification due to the segregation of the particles to the grain boundary, while there were very few Re-rich particles (without any intermediate layer) at boundaries formed during solution heat treatment, for example the boundary of a recrystallized grain and matrix in this study.
Finally, however, it should be noted that compared other defect grains which were observed in this study, the recrystallized sample did not have an intermediate layer. Therefore, there is another possibility that the Re-rich particles were probably observed only in the sample containing the intermediate layer regardless of the solidification or the heat treatment, which means that the formation of the particles might have some relationship with the layer not the process, but it requires further study.
The grain boundary regions with different casting grain defects were investigated using high resolution electron microscopy. An intermediate layer containing fine particles was detected between a stray grain and a matrix grain. An SEM-EDX point analysis on the particles indicated that they are composed of high amounts of Re with Ni. These particles were detected regardless of the location on the component, such as the platform or shroud. In addition, the intermediate layer containing Re-rich particles was also detected at the boundaries of matrix grains and equiax grains or freckle chain grains, even along low angle grain boundaries. The boundary is straight and Re-rich particles exist only one side along the boundary. The results suggested that the Re-rich particles are pushed ahead of each solidification front and settle at the boundary of the matrix/defect grains during solidification. Subsequently, the intermediate layer containing the elongated γ′ morphology developed during solution heat treatment possibly due to grain boundary stresses affecting the growth. The observations of the low angle grain boundary supported this formation mechanism for the intermediate layer containing Re-rich particles. On the contrary, there were few Re-rich particles and no intermediate layer at the boundary of a recrystallized grain and matrix which formed during solution heat treatment.
It should be emphasized that even though this study does not suggest the formation mechanism of each surface defect grain, it is shown the effect of rhenium in Ni-base turbine blades containing large amounts of rhenium on the microstructural development. If there are any boundary regions formed by stray grains, freckle chain, low angle grain boundary, and equiax grains as in this study in turbine blades, Re-rich particles can be deposited from the melt along the boundary, and even along low angle grain boundaries, which is detrimental to the mechanical properties.
The financial support and provision of evaluation test pieces by Rolls-Royce is acknowledged.
