Application of EBSD technique to investigation of modern materials for aero engines turbine blades

bartosz chmiela, maria sozańska, JAN CWAJNA Application of EBSD technique to investigation of modern materials for aero engines turbine blades INTRODUCTION Turbine blades, vanes, and other parts of aero engines exposed to high temperature are produced of nickel-based superalloys via investment casting. Directional solidification (DS) allows obtaining excellent mechanical properties of blades [1]. Because turbine blades are flight safety parts, they must be free of any defects. However, directional solidification is very complex and many casting defects may appear during this process [1]. The commonly defects in DS and single crystal (SX) castings are freckles [1 6] and stray grains [7 9]. Freckles are casting defects that appear on the surface of DS or SX castings in the form of long chains of equiaxed grains aligned parallel to gravity [1 6]. Freckle dimensions depend on casting dimensions: the length is usually the same as the casting, but the width is from one to several millimeters. Freckles are enriched with elements segregated to the liquid phase during solidification [2 4]. Compared with the freckle-free part of a casting, freckled areas are characterized by an increased carbide content, γ + γ eutectics, and porosity [4]. Freckle formation is attributed to thermosolutal convection and buoyancy forces in the mushy zone, which are in turn caused by density inversion in the interdendritic liquid [5]. The tendency for various superalloys to freckle is characterized by the freckling index F, which depends on the chemical composition [1]: F C + 15. C + 05. = C 05. C C + 12C Ta Hf Mo Ti W. Re where C Ta, C Hf, C Mo, C Ti, C W, and C Re are the concentration (wt %) of tantalum, hafnium, molybdenum, titanium, tungsten, and rhenium, respectively. In case of F > 1, the freckling tendency is low. The density inversion of interdendritic liquid in the mushy zone is caused by strong segregation of some elements (i.e., W, Re) into the solid phase. Therefore, during directional solidification, these elements are depleted in the liquid alloy in interdendritic areas and its density becomes lower than that of the molten alloy at the liquidus temperature. This phenomenon leads to plume flow of interdendritic liquid [5], wherein the interdendritic liquid flowing through the mushy zone causes dissolution of secondary-dendrite-arm tips, which become nuclei for new equiaxed grains (i.e. freckles). The resistance to interdendritic-liquid flow increases for small primary dendrite arm spacing (PDAS), which depends on the thermal gradient and withdrawal rate during directional solidification as per the Bridgman technique: greater thermal gradient and withdrawal rates lead to smaller PDAS and freckling tendency. Some investigations [6] suggest that freckling tendency strongly depends on orientation of the SX: casting with a [001] orientation relatively aligned to the solidification direction should be much more prone to freckles than casting with a [001] direction highly misaligned from solidification Dr Bartosz Chmiela (bartosz.chmiela@polsl.pl), Prof. dr hab. inż. Maria Sozańska, Prof. zw. dr hab. inż. Jan Cwajna Wydział Inżynierii Materiałowej i Metalurgii, Politechnika Śląska, Katowice (1) direction. In the present study, we use EBSD to determine the crystallographic orientation of freckles with respect to SX casting on the longitudinal section. Freckles appeared in the casting investigated, although the angle between the [001] direction and the solidification direction was about 40. Some freckle grains are oriented very close to the SX axis, but in many cases their orientation is quite different. These results indicate that nuclei growing in the plume of the interdendritic liquid have different capacities to rotate [4]. Modern SX turbine blades are produced by the Bridgman method. The preferred crystallographic orientation of the seed is usually [001] [1]. In practice, a deviation between the solidification direction and the [001] direction of up to 15 is acceptable. To obtain single crystal structure, the starter block and the helical selector are used during directional solidification [7 9]. Optimization of the grain orientation is controlled by the starter block, and single crystal selection is determined by geometric factors of the helical selector [9]. However, in some cases, grain selection in the helical part is not efficient and stray grains can grow in the selector. In this case, the geometrical blocking mechanism is usually ineffective and stray grain can grow farther in the casting. MAterial and experimental procedure In this study we used the single-crystal rods (13 mm in diameter) made of CMSX-4 superalloy. The castings were made in the ALD Vacuum Technologies laboratory in Hanau, Germany. The chemical composition of the alloy is shown in Table 1. Withdrawal rate were 3 and 4 mm min 1. We identified freckles on the surface of the SX rod and investigated the longitudinal section near the surface and in the crosssection. The analysis of stray grain formation was performed on 13 cross-sections of the starter and selector. The specimens were prepared according to procedure described earlier [4]. To reveal freckles, the rod surface was etched in a solution of 7 cm 3 HNO 3, 145 cm 3 HCl, 86,5 g FeCl 3 (anhydrous), and 55 cm 3 H 2 O. The microstructure, chemical composition, and crystallographic orientation were characterized with a scanning electron microscope (SEM, Hitachi S-3400N) equipped with an energy-dispersive spectrometer (EDS) (Thermo NORAN, System Six) and an electron backscatter diffraction (EBSD) detector INCA HKL Nordlys II (Channel 5 software). The energy of primary electron beam for imaging and EDS analyses was 15 kev. EBSD analyses were done with a 20 kev primary electron beam and a 15 μm step size. Because of the large area of freckles on the casting surface, we used the stage scanning mode to obtain the orientation map. After acquisition of the maps, they were merged together. Because the indexa- Table 1. The chemical composition of CMSX-4 superalloy (wt %) Tabela 1. Skład chemiczny nadstopu CMSX-4 (% mas.) Ni Cr Co Mo W Ta Al Ti Hf Re balance 6.5 9 0.6 6 6.5 5.6 1 0.1 3 140 INŻYNIERIA MATERIAŁOWA ROK XXXIV

tion was better than 95%, raw maps were submitted to the standard cleaning procedure as per the instructions for users (Oxford Instruments HKL Technology, 2006). results and discussion Based on the chemical composition of the CMSX-4 superalloy, the freckling index F = 0.67 has been calculated, which is much less than 1. This value indicates that the fraction of elements segregated within the liquid is too small to effectively counteract the density inversion. Therefore, the CMSX-4 superalloy is prone to freckles due to its chemical composition. Figure 1a shows the etched surface of a SX rod (withdrawal rate 3 mm min 1 ) on which the freckle chain is clearly visible. The PDAS was about 620 μm, as determined from the cross section of the rod. The characteristic shape of dendrites indicates high angular deviation from the preferred [001] orientation (Fig. 1b). The longitudinal section has been analyzed by EDS in two stages: (i) by point analysis of dendrite cores and interdendritic areas of SX and freckles and (ii) by area analysis of SX and freckles. In both cases 10 measurements were made and then calculated the average and standard deviation. The results confirm the characteristic changes in chemical composition that can be attributed to segregation during directional solidification. Calculations of the relative changes in chemical composition reveal that compared with the SX part of the casting, freckles are enriched with tantalum and depleted of tungsten and rhenium (Fig. 2). EBSD analysis provides a detailed description of freckles. Figures 3a and 3b present freckles on the longitudinal section of a casting (about 1 mm under the casting surface). Grain boundaries with misorientation larger than 5 are highlighted by white lines. Freckles are generally equiaxed grains, but with different sizes. The orientation map shows that of some freckles are oriented very similarly to the single crystal, but the orientation of other freckles is quite different (Fig. 3c). The misorientation distribution in freckled areas reveals low-angle boundaries and high angle boundaries (Fig. 4), confirming that freckles are not only a result of macrosegregation. Pole figures and inverse pole figures of the freckled area confirm the generally random orientation of freckles (Fig. 5). EDS results suggest that the segregation of tungsten and rhenium into dendrite cores decreases the density of the interdendritic liquid in the mushy zone, which is less than the density of the molten alloy (density inversion). Simultaneously, tantalum segregates to interdendritic liquid, but this occurs in too-low quantities to prevent the density inversion. Next, the buoyancy force begins to act and causes the plume flow. Because of the large PDAS, there is low resistance to the interdendritic-liquid flow. Even if the primary dendrite Fig. 2. Relative changes in concentration of Ta, W and Re in freckles Rys. 2. Względne zmiany zawartości Ta, W i Re we freklach Fig. 3. Grain boundaries of freckles (a), band contrast image of part of freckled area (b), and orientation map of freckled area (c) Rys. 3. Granice ziaren frekli (a), obraz z kontrastem dyfrakcyjnym fragmentu obszaru frekli (b), mapa orientacji frekli (c) Fig. 1. Freckles in the rod made of CMSX-4 superalloy Rys. 1. Frekle w pręcie z nadstopu CMSX-4 arms deviate highly from the preferred [001] orientation about 40 (Fig. 6) the plume flow is not suppressed. The large PDAS probably plays an essential role in this phenomenon. Note that these results are inconsistent with published results [6]. It indicates that plume flow is not prevented even by highly deviated dendrites. It seems that the probability of plume flow depends on the combination of many factors, including density inversion, PDAS (thermal gradient and withdrawal velocity), and dendrites deviation. Plume flow causes dissolution and remelting of the tips of secondary dendrite arms, which become homogenous nuclei for new grains (i.e. freckles). These results indicate that freckles form during the flow of interdendritic liquid between dendrites in the mushy zone. Large differences in freckle orientation suggest that these nuclei have different capacities to rotate. The orientation of some freckles is similar to the orientation of the SX NR 3/2013 INŻYNIERIA MATERIAŁOWA 141

Fig. 4. Grain boundaries misorientation distribution of freckles Rys. 4. Rozkład dezorientacji granic ziaren frekli These grains form high angle boundaries with the casting and with other grains. Experimental investigations of stray grains reveal that a welldesigned selector is sometimes inefficient, and that more than one grain can survive the route through the selector (Fig. 7). Grain density determined by EBSD decreased monotonically in the starter block, but it is quite high (Fig. 8a). In case of selector, there were two grains at a height of 30 mm in the selector. Hence, one of these grains is a stray grain. It is likely that very poor efficiency of initial part of the selector (Fig. 8b) can cause stray grain formation. The grain orientation distribution determined by experiment is shown in Figure 9. At a height of 30 mm in the starter, 77.5% of Fig. 7. View of starter and selector of SX casting: a) casting without stray grains, b) casting with stray grain Rys. 7. Widok startera i selektora odlewu SX: a) odlew bez obcego ziarna, b) odlew z obcym ziarnem Fig. 5. Pole figures (a) and inverse pole figures (b) of freckles Rys. 5. Figury biegunowe (a) i odwrotne figury biegunowe (b) dla frekli Fig. 6. Pole figures (a) and inverse pole figures (b) of SX Rys. 6. Figury biegunowe (a) i odwrotne figury biegunowe (b) dla SX in areas where secondary dendrite arms are likely to be very close to each other. In this case, resistance to flow should increase and new nuclei would have only a negligible probability to rotate. The remaining freckles are randomly orientated, which could indicate that the nuclei could easily rotate during interdendritic liquid flow. Fig. 8. Changes of grain density on the cross section: a) starter, b) helical selector Rys. 8. Zmiana liczby ziaren na przekrojach poprzecznych startera (a) i selektora śrubowego (b) 142 INŻYNIERIA MATERIAŁOWA ROK XXXIV

Fig. 11. SX bar with the stray grain (a), starter and the selector of SX bar (b), longitudinal section of SX bar with the stray grain (c) and cross-section of SX bar with the stray grain (d) Rys. 11. Pręt SX z obcym ziarnem(a), starter i selektor odlewu(b), przekrój wzdłużny pręta SX z obcym ziarnem(c) oraz przekrój poprzeczny pręta SX z obcym ziarnem (d) Fig. 9. Experimental distribution of grain orientation: a) starter block, 30 mm from the chill plate, b) helical selector, 30 mm height Rys. 9. Rozkład orientacji ziaren: a) starter, 30 mm od ochładzalnika, b) selektor, 30 mm wysokości the grains deviated less than 10 from the [001] orientation, and 22.5% of the grains are characterized by higher deviation (Fig. 9a). The presence of highly deviated grains is the main reason for the observed orientation distribution. At a height of 30 mm in the selector, there are two grains with deviations from the [001] orientation of 7.4 and 12 (Fig. 9b and Fig. 10). The grain orientation in the starter and the selector is also clearly visible on the inverse pole figures (Fig. 10). Figure 11 shows the view of the single crystal bar obtained in the experiment. One stray grain runs through the whole length of the bar, which is visible on the bar s surface. The longitudinal section reveals the dendritic structure of a single crystal and stray grain (Fig. 11c). The [001] direction of both grains deviated from the solidification direction. Moreover, the [100] directions of the SX and stray grain are shown rotated relative to each other, and the angle between them is 38.2 as determined by EBSD. This indicates that there is a high angle boundary between the SX and stray grain (Fig. 11d). This is a very interesting case of the competitive grain growth mechanism [7 9]. Although the SX and stray grain deviate from the preferred [001] orientation, one is not overgrown by the other. The deviation of the [001] direction from the solidification direction in the upper part of the bar was 9.2 for the SX and 7.4 for the stray grain. The difference between orientation deviations is less than 2. It seems that this is too small a value to cause the overgrowth process due to the difference in heat flow or branching of secondary dendrite arms of either grain. Our experimental results show that during grain growth in the helical selector, the decreasing rate of grain density plays a very important role. An extremely small decrease in grain number for the first 5 mm of selector height is characteristic. Then, too many grains survive the route through the selector, which can cause stray grain formation. CONCLUSIONS By combining SEM, EDS, and EBSD, a detailed description of casting defects has been obtained. Especially, it is possible to evaluate many aspects of freckles. Analysis by EDS reveals changes in the chemical composition of freckles and SX casting. Investigation by EBSD shows the size, shape, and crystallographic orientation of freckles on the longitudinal section of the casting, which has heretofore not been reported. Freckles form low-angle and high-angle boundaries within the SX casting, which indicates that causes other than the segregation of elements during directional solidification contribute to the appearance of freckles. These results suggest that nuclei have different capacities to rotate during plume flow in the mushy zone. In case of stray grains, poor selector efficiency exhibited in the experiment suggests that a strong decrease in the thermal gradient of the initial part of the selector could disturb grain growth. This appears to be the most likely reason for stray grain formation. ACKNOWLEDGEMENTS Fig. 10. Experimental inverse pole figures of the cross section: a) starter block, 30 mm from the chill plate, b) selector, 30 mm height Rys. 10. Odwrotne figury biegunowe dla przekrojów poprzecznych: a) starter, 30 mm od ochładzalnika, b) selektor, 30 mm wysokości Financial support of Structural Funds in the Operational Programme Innovative Economy (IE OP) financed from the European Regional Development Fund, Project Modern material technologies in aerospace industry, No. POIG.01.01.02-00-015/08-00 is gratefully acknowledged. The authors also wish to thank Mr. J. Jarczyk from ALD Vacuum Technologies GmbH for carrying out the directional solidification and provision of castings. NR 3/2013 INŻYNIERIA MATERIAŁOWA 143

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