Influence of Heat Treatment on Defect Structures in Single-Crystalline Blade Roots Studied by X-ray Topography and Positron Annihilation Lifetime Spectroscopy

Influence of Heat Treatment on Defect Structures in Single-Crystalline Blade Roots Studied by... TOPICAL COLLECTION: SUPERALLOYS AND THEIR APPLICATIONS Influence of Heat Treatment on Defect Structures in Single-Crystalline Blade Roots Studied by X-ray Topography and Positron Annihilation Lifetime Spectroscopy JACEK KRAWCZYK, WŁODZIMIERZ BOGDANOWICZ, ANETA HANC-KUCZKOWSKA, ANNA TONDOS, and JAN SIENIAWSKI Single-crystalline superalloy CMSX-4 is studied in the as-cast state and after heat treatment, with material being taken from turbine blade castings. The effect of the heat treatment on the defect structure of the root area near the selector/root connection is emphasized. Multiscale analysis is performed to correlate results obtained by X-ray topography and positron annihilation lifetime spectroscopy (PALS). Electron microscopy observations were also carried out to characterize the inhomogeneity in dendritic structure. The X-ray topography was used to compare defects of the misorientation nature, occurring in as-cast and treated states. The type and concentration of defects before and after heat treatment in different root areas were determined using the PALS method, which enables voids, mono-vacancies, and dislocations to be taken into account. In this way, differences in the concentration of defects caused by heat treatment are rationalized. https://doi.org/10.1007/s11661-018-4704-2 The Author(s) 2018 I. INTRODUCTION may be related to the inhomogeneity of the chemical composition, morphology and size of c¢ particles, and THE single-crystalline, Ni-based superalloys are the inhomogeneity of crystal orientation. Therefore, widely used for production of high-pressure and blades are subjected to complex heat treatment after high-temperature turbine components in aerospace and casting, among others, to eliminate the chemical hetero- energy industry sectors. Due to the extreme work [4] geneity caused by directional dendritic crystallization. conditions of blades, especially high mechanical and The heat treatment parameters are appropriately thermal stresses, the specific properties with low con- [4] selected to create new c/c¢ array (reprecipitation ) with [1–3] centration of structural defects are needed. Nowa- optimized properties, caused by more homogeneous days, the CMSX-4 single-crystalline superalloy is morphology, size, and chemical composition. Some commonly used by industry; castings produced in this defects of macro-, micro-, and nano-scale are recreated way are suitable candidates for defect characterization in the treated blades, but some of them may be inherited studies. from the as-cast state. All of them to some degree or Directional dendritic solidification by the Bridgman other will influence the mechanical properties of blades technique is widely used for production of single-crys- [5–9] and may cause damage during operation. The talline blades made from superalloys. Production tech- macroscopic inhomogeneity in crystal orientation (e.g., nology and the complex shape of blade castings produce subgrain boundaries or misoriented bands of dendrites, dendritic arrays that allow for the possibility of many bended during crystallization) may be one of the defects to be produced during solidification. The defects inherited defects. These defects are generally related to [4,10–12] the crystallization of complex shape casts. The inhomogeneity of crystal orientation may be characterized by X-ray diffraction topography in reflec- [13,14] tive geometry. This imaging technique—which JACEK KRAWCZYK, WŁODZIMIERZ BOGDANOWICZ, represents two-dimensional mapping of diffracted ANETA HANC-KUCZKOWSKA, and ANNA TONDOS are with the Institute of Materials Science, University of Silesia in Katowice, X-rays local intensity—is capable of providing informa- 41-500 Chorzow, Poland. Contact e-mail: jacek.krawczyk@us.edu.pl tion on the distribution and nature of structural defects JAN SIENIAWSKI is with the Department of Materials Science, in single-crystalline materials. The technique is sensitive Rzeszo´ w University of Technology, 35-959 Rzeszow, Poland. to changes in local orientations of diffraction (atomic) Manuscript submitted March 14, 2018. [15,16] planes and their spacing. The method allows the Article published online June 1, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4353 visualization of the differences in crystal orientation (misorientation) of neighboring areas accurate to arc minutes. However, the analyzed areas must have a size of 1 mm or higher. The boundaries between such areas (macroscopic low-angle boundaries, LABs) are created by microscopic defects like dislocations, and addition- ally, they may be related to the vacancies. This means that the general description of defect structure of blades must refer to defects over a wide spatial size ranging from nanometer up to millimeter scale, which are mutually related to each other. Determining the rela- tionship of different-scale defects, which is the basis of the multiscale analysis, helps to determine the reasons for their creation. Additionally, the macroscopic inhomogeneity of crys- tal orientation may be related to sub-micrometer-scale defects, which are difficult or impossible to reveal by SEM or TEM methods due to their small analyzed area. The positron annihilation lifetime spectroscopy (PALS), using a macroscopic area, may enable it. PALS is a promising nondestructive method of quality control of technologically important materials employed in various fields of science and technology. Positrons interact for example with foreign atoms, points, or linear defects of structure. Positrons are trapped preferentially in atomic defects leading to an extended positron lifetime. The advantage of this method is the detection of small concentrations of defects that could not be detected by [17] other methods. Positron techniques have been used [18] Fig. 1—(a) Illustration of blade parts and (b) samples for PALS for studies of defect behavior in the Ni Al system and [19] preparation; P —section of part C. polycrystalline Ni-based superalloys, but there are no results as yet for investigation of defects inhomogeneity P (Figure 1) was located near the surface P in the single-crystalline turbine blades. h a (Figure 1(b)) of the selector-root connection It is emphasized that the structural defects are mainly (u = 3.5 mm). The ST surface was a mirror-reflection created in the so-called critical areas of the blade. The image of the SC surface. The SC surface of C-parts single-crystalline blades possess critical areas with (Figure 1(a)) was studied in the as-cast state, and the ST regard to the crystallization process (selector-root and surface was analyzed after heat treatment of T-parts. root-airfoil connection areas, thin-walled areas) and to Section P in Figure 1(b) reveals the geometry of the the operation loads (areas near the root-airfoil connec- v [20,21] selector-root connection with the use of a selector tion, thin-walled areas of trailing edge, tip area). Regarding to the growth structure of dendrites set, the continuer higher in diameter on the plane P in relation most affected area is situated in the root part of the to the diameter of selector channel (D > d). The PSC in Figure 1 is the projection of a selector continuer on all blade, near the connection with the selector, where the [22] transverse sections of the root. step-change geometry of the castings occurs. The The heat treatment of part T was performed by several above justifies the selection of these regions for attention steps, consisting of convection heating to 950 C (in a in this article. Its overarching aim of the study is to helium-protective atmosphere), radiation heating to analyze and compare the defects structure of CMSX-4 1350 C (in vacuum), solution annealing, and finally single-crystalline blade root areas, located near the aging. The temperature-time settings for annealing were: selector-root connection, in an as-cast state and after 1277 C/4 h fi 1287 C/2 h fi 1296 C/3 h fi 1304 C/ heat treatment using X-ray topography and the PALS 3h fi 1313 C/2 h fi 1316 C/5 h fi gas furnace method. quench, and for aging: 1140 C/6 h (step 1) and 871 C/ 20 h (step 2). The blade production and heat treatment process were performed in the Research and Develop- II. EXPERIMENTAL ment Laboratory for Aerospace Materials, Rzeszo´ w University of Technology, Poland, using an industrial The investigated roots of single-crystalline turbine ALD Vacuum Technologies furnace. blades, made of CMSX-4 superalloy, were obtained by The metallographic sections of SC and ST surfaces directional crystallization via Bridgman technique at a were studied by scanning electron microscopy (SEM), withdrawal rate of 5 mm/min. with the use of the spiral the X-ray diffraction topography, and Laue diffraction. selector with [001] orientation (Figure 1). The blade A JEOL JSM-6480 microscope was applied for SEM roots were cut into two parts: C and T, perpendicular to observations using a backscattered electron (BSE) the withdrawal direction G (Figure 1). The cutting plane 4354—VOLUME 49A, SEPTEMBER 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A technique. A divergent beam of characteristic Cu Ka radiation, generated by a microfocus X-ray tube of the PANalytical system, was used for X-ray topography studies. The topograms were recorded on the AGFA Structurix D7 X-ray film in reflective geometry. The divergent beam of the characteristic radiation coming from a quasi-point (40 9 40 lm ) X-ray source illumi- [13] nates the whole surface of the SC or ST surfaces. The sample (part C or T) coupled with the film oscillates about the axis located on the studied surface. The individual parts of the tested surface, meeting the Bragg condition, are successively recorded on the film. After over a dozen minutes of exposure, the diffraction image (topogram) of the whole analyzed surface was obtained on the film. If for areas close to surface SC of part C or ST for part T, the crystal lattices are rotated relative to each other, then their diffraction images will be mutually shifted in the topogram, creating different types of contrast. Similar displacement in the topograms was created by changes of d-spacing. Specific contrast is also created for other defects that occurred in the single-crys- [13,23] talline materials. The misorientation angle may be [13] calculated using the shift value in the topograms. The minimal misorientation angle, determined by X-ray topography method, is on the order of arc minutes, which are much lower than for the EBSD method. Additionally, the PALS technique was applied to the study defect structure, revealed by X-ray topography on the surfaces SC and ST. PALS relies on the propensity of positrons to become localized at open-volume regions of a solid and the emission of annihilation gamma rays that escape the test system without any final state interaction. The scheme used for sample sandwich Fig. 2—(a) Scheme of samples sandwich arrangement in PALS packing in the PALS measurements and scheme of measurements, (b) scheme of the positron annihilation processes according to the three-state-trapping model: positron trapping in positron trapping are illustrated in Figure 2. The complex defects: positron trapping in a vacancy and dislocation. sources of positrons are generated by a pair of gamma rays that hold information about the electronic envi- spectra was recorded. Then, the spectra were added ronment around the annihilation site. PALS measures together by means of a special procedure accounting for the elapsed time between the implantation of the the drift of the zero-time channel. In this way, a positron into the material and the emission of annihi- resultant spectrum of very high statistics (at least 107 lation radiation. Positrons are trapped preferentially in counts) was obtained. The data were analyzed using a atomic defects that in turn have a locally smaller [24] least-square fitting procedure, which, contrary to the electron density leading to an extended positron lifetime. other existing programs, enables fitting not only a single The PALS technique therefore is a sensitive method to spectrum but also a series of spectra. The simultaneous derive sizes and concentration of vacancy-type defects fitting leads to reduction of the number of the free fitting like nano-cavities. The characteristics of annihilation parameters because some of the parameters can have trapping positron are different for various configura- common values for all of the spectra of the series. In the tions defects. Knowledge of the location of positrons in calculations, all the measured spectra were fitted simul- defect of lattice gives the possibility of the concentration taneously with a model, directly implemented within the of defects from proportion of trapping positron to free. [24] software code. All spectra were analyzed using the Here, it was applied in determining the energy of [24] three-state trapping model describing the positron vacancy formation. annihilations in the bulk material and two types of Two samples in a sandwich arrangement (Figures 2(a) defects (Figure 2(b)). The three-state trapping model and 1(b)) for PALS measurements were prepared was described by three parameters, i.e., the positron separately from each C-part and each T-part of the lifetime in bulk material s , the positron lifetime trapped blades. The PALS measurements were performed in two b by defects s, and the positron trapping rate j. areas (A1, A2 of Figure 2(b)) of each sandwich at room In Figure 2(b), the horizontal bars represent the free temperature with a conventional fast–fast spectrometer state of positrons, i.e., the delocalized state of each of 270-ps time resolution. The area A1 relates to the positron in bulk material and its localized states PSC area and A2 to the area outside it. The positron (dislocations and vacancy) in two types of defects. k source with the activity of about 370 kBq, covered with b is the annihilation rate of the delocalized (free) positron a5-lm Ni foil, was used. For each sandwich, a series of METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4355 from the bulk, k and k and k are the annihilation rates Figure 4 presents typical topograms obtained for the st t v from different type of defects, respectively. j , j , and j SC surface of C-type part (Figure 4(a)) and for the ST v t st are the trapping rates into these defects, which are surface of T-type part (Figure 4(b)). The ST surface was proportional to the defect concentrations. 0 are the a mirror-reflection image of the SC surface; therefore, trapping rate into dislocation-bound vacancy. The term the contrast details in the topograms for the same d is the detrapping (escape) rate. corresponding locations on the analyzed surfaces All the measured spectra were fitted simultaneously at (Figures 4(a) and (b)) are mirror-reflected, e.g., the each parameter relating to the resolution or the contri- L contrast band (Figure 4(a)) relate to the L band 1C 1T bution of source. This means that such a parameter was (Figure 4(b)). Additionally, the contrast inversion is constrained to have the same value for each spectrum of visible. It can be observed during a hypothetical move the series of spectra analyzed together. The positron from the corner K (Figure 4(a)) to the center of the A2 lifetimes relating to positron annihilation in the source area: In Figure 4(a), the dark-contrast band is visible were fixed. The ‘‘source’’ lifetimes were determined with first, and in Figure 4(b), the bright-contrast band is help of a Si lifetime spectrum of very high statistics. The visible first. source contribution of 36.7 pct consisted of three In both topograms, the R-rings (R , R ) of contrast C T components with lifetimes 125 ps, 386 ps, and 1.97 ns with change in intensity of the rim of the PSC are visible. in the proportion 94.6:4.7:0.7, respectively. The R ring (Figure 4(a)) consists of two sub-rings: the external one of S width with higher contrast intensity RC1 (Figure 4(e)) and the internal one of S width with RC2 lower contrast intensity (Figure 4(e)). The R ring III. RESULTS (Figure 4(b)) consists of two sub-rings: the external Metallographic observations of SC surfaces of C-type one of S width with lower contrast intensity RT1 parts revealed a dendritic structure (Figure 3(a)) typical (Figure 4(f)) and the internal one of S width with RT2 of the as-cast state. The macro-SEM images, created by higher contrast intensity (Figure 4(f)). The contrast merging multiple separate SEM images, allow us to inversion can be observed during a hypothetical move observe the whole section and compare the morphology from the center of the rings RC and RT toward the and distribution of dendrites in different areas of the SC outside: In Figure 4(a), the bright contrast ring is visible surface. A computer processing was applied for better first and the dark one is visible first in Figure 4(b). visibility of dendrites arms direction and type of There is also a visible dark band L in the topogram 1C arrangement (Figure 3(b)). The processing combines of the SC surface of the C-type part (Figures 4(a), (c), creation of skeletal images and modification of contrast. and (e)). Bright band L (Figures 4(b), (d), and (f)) may 2T The procedure gives results possessing certain common be observed in topograms obtained for T-type parts. features with the skeletonization procedure referred This band appears after heat treatment. Fine parallel to in Reference 25. Skeletonization is a technique contrast bands (fine arrows, Figures 4(a), (c), and (e)), whereby a binary image of dendrites is eroded step by arranged along the m direction, appear in the topograms step away until the skeleton of the image is obtained. of the C-type part. However, in the topograms of T-type The skeletal image is created as a thin line equidistant parts, the bands mentioned above are not visible. The m [25] from the original edges of the binary dendrites shape. and n directions from Figure 4(a) are parallel to the Analyzing images from Figure 3, two different types of secondary dendrite arms and to the [100] and [010] dendrite arms arrangement can be distinguished: crystallographic directions. The blurred, wider bands H short-range alternate (inside the center of PSC) and visible in the topograms from Figures 4(b), (d), and (f) long-range continuous (L -type). The center part of the are parallel to the h direction, which divides the angle sections consists of dendrites with a homogenous, between m and n in half. The G fragment (Figure 4(b)) statistical arrangement. A small disorder in dendrite with lack of contrast suggests that this area does not arm directing (shifts of linear fragments) is noticeable satisfy the Bragg’s condition and fragments of contrast around the PSC area (Figure 3). The mutual arrange- are shifted creating an overlapped high contrast G near ment of short-range and long-range arms (depending on the area G of the topogram. the m and n directions) allows visualization of a ring that On the basis of the PALS spectra analysis, the defect overlaps the edge of the PSC area and is marked by R concentration values were determined for both the A1 (Figure 3(b)). The diameter of the ring is equal to the and A2 areas of as-cast and treated samples. The diameter D of the PSC area (Figure 1(b)). The R ring obtained values of defect concentration in the A1 and may be better visible when the figure is reduced A2 areas (Figures 2(a) and 4(a)) of as-cast parts of the (Figure 3(c)). The largest difference in dendrites mor- roots (C-type) and the defect concentration estimated phology occurs in narrow corner areas (K , K ). The for the analogous areas of the heat-treated parts 1 2 dendrite arms are elongated along the n direction and (T-type) were averaged and summarized in Figure 5. fine ternary arms appear. No clear dendritic structure For the purposes of analysis, it was assumed that the was observed on the metallographic sections of the ST chemical composition of the undefected material (bulk) surface of T-type parts. By Laue diffraction, it was does not radically change qualitatively during heat stated that the n and m directions correspond to the treatment and changes occur in the distribution of [010] and [100] crystallographic directions. matrix chemical components only, so the positron 4356—VOLUME 49A, SEPTEMBER 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 3—(a) Typical macro-SEM image obtained for SC surface of C-type part, (b) dendritic structure presented in part (a) after computer processing, and (c) part (b) reduced in size. annihilation lifetime in bulk is an invariant value, Two types of defects were found in the T-type parts in determined in the first step of analysis. The positron both studied areas. Based on the positron lifetime annihilation lifetime s was 134 ps, and this value is calculations, the occurrences of mono-vacancies and [17] close to the literature values given for alloys based on dislocations were established. The defects concentra- Ni Al. tion in the T-type parts (A1 and A2 areas) indicates that The results obtained by the PALS method suggest the dominant type of defects in both areas is vacancies. that in the as-cast parts of the root (C-type), there is However, it should be noted that the concentration of such a high concentration of defects that it is not each defect type in the discussed areas is different. It was possible to indicate differences in the defect structure observed that the vacancies concentration in the A1 area between two selected areas A1 and A2. The determined is about six times higher in relation to the dislocation values of positron lifetime (s = 1.5 ns), obtained as a concentration. The dislocation concentration in the A2 result of a numerical analysis of the experimental area is higher than in the A1 area, and additionally, the spectrum, allow us to identify the type of dominant dislocation concentration in the A2 area is twice lower defects. The determined lifetime corresponds to volu- than the determined concentration of vacancies. The metric defects of the micro-voids type also called the free positron lifetimes for the vacancy in the area A1 [26] volumes. (s = 212 ps) and in the area A2 (s = 220 ps) were v v METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4357 determined. Similar calculations of positron lifetime were performed for dislocations in the area A1 (s = 380 ps) and in the area A2 (s = 410 ps). v v IV. DISCUSSION The difference in the dendrites’ arrangement and arms’ length inside and outside the PSC area (Figure 3) is related to the difference in growth kinetics. The growth of dendrites is not disturbed inside the PSC area on the P plane (Figure 1(b)). Outside the PSC, the dendrite array in a whole root is conditioned by the rapid growth of the secondary dendrite arms on the P [4,20] plane (Figure 1(b)). The dendritic structure disorder in the K and K areas may be caused by local changes in 1 2 the heat dissipation and by curvature of the solidifica- [27] tion front in the narrow corner areas. It may also be the reason for the formation of contrast bands in topograms, separating the K and K areas 1 2 (Figures 4(e), (c), and (a)), and this suggests a somewhat different crystal orientation of these areas in relation to other ones. Additionally, the secondary dendrite arms, growing in a transverse plane from the selector exten- sion, do not reach into the K and K area, so 1 2 the dendritic structure in these areas is different (Figures 3(a) and (b)) and often affected by the deflec- tion process described in Reference 28. Structure alter- ation in the R -ring region (Figure 3) similar to the specific shape of contrast of the PSC circle in topograms Fig. 4—Example of original topograms, obtained for (a) the SC (Figures 4(a), (c), and (e)) is related to the circular shape surface of C-type part and (b) the ST surface of T-type part; 113 of a selector continuer–root connection. This suggests reflection, Cu Ka radiation; (c) and (d) the same topograms after that changes in the crystal orientation occur in this computer processing; (e) and (f) schemes of contrast in topograms from parts (a) and (b). circle. The microstructure alteration visible on the Fig. 5—Estimated defects concentrations with error bars and the determined values of positron lifetime in different types of defects in areas A1 and A2 of as-cast and treated parts of the root. 4358—VOLUME 49A, SEPTEMBER 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 6—(a) Scheme of contrast formation in the R ring of topograms (Figs. 4(a), (c), and (e)); (b) and (c) scheme of contrast in stripes d for C y topogram obtained from SC and ST surface; and (d) scheme of contrast formation in the R ring of topograms (Figs. 4(b), (d), and (f)). Small arrows indicate [001] direction, PB, DB—primary and diffracted X-ray beams, and the B and B bend is enlarged for figure clarity. c¢ I II macro-SEM images (e.g., K , K areas, R ring) and process does not homogenize the defect structure in the 1 2 C misoriented areas separated by boundaries similar to the sub-microscopic scale. This requires further studies on a LAB, called the ‘‘LAB-like’’ (L , L , R-type rings) mechanism of c¢ re-precipitating and its atomic ordering 1 2 visible in topograms (Figure 4), suggest crystal misori- during heat treatment. entation. As a result of heat treatment, some defects Elimination of inhomogeneity, created during den- disappear, e.g., the LAB-like, visualized in topogram as dritic crystallization of a complex-shape cast, is one of L (Figures 4(a), (c), and (e)) but some remain the aims of heat treatment. A transition of the crystal- 3C (L  L , Figures 4(a) and (b) and 4(c) and (d)). A lization front through the areas of the step-change 1C 1T new defects formation after heat treatment, e.g., the geometry causes momentary changes in the crystalliza- LAB-like, marked as L (Figures 4(b), (d), and (f)), was tion kinetics due to the rapid growth of the secondary 2T also observed. dendrite arms, perpendicular to the withdrawal direc- For the treated parts of the roots, the positron lifetime tion. This growth takes place in a thin layer of the root for vacancy in the area A1 (s = 212 ps) and in the area (near P —Figure 1) and occurs at a much higher growth v a A2 (s = 220 ps) was determined. Similar calculations rate than the withdrawal rate. As a result, a local of positron lifetime were performed for dislocations in disorder in crystal orientation and bending of dendrites, [22] the area A1 (s = 380 ps) and in the area A2 (s = 410 as well as the LAB-like formation, occur. The angle v v ps). Differences in the given positron lifetime values can of misorientation of these boundaries can be estimated be related to different chemical environments of the basing on the width of areas with increased or decreased [13] defects. The differences in the concentration of defects contrast in the topogram. The areas of topograms of estimated by PALS in the area A1 and A2 of T-type the increased contrast, derived from the SC surface, parts are probably related to different kinetics of the should be compared with the areas of the decreased crystallization process in these areas. Electron micro- contrast, derived from the ST surface, due to a contrast scopy studies of a dendritic structure (Figure 3) indicate inversion resulting from the mirror-reflected SC and ST the differences in the dendritic array of the A1 area surfaces. In the case of the LAB L (Figures 4(a), (c), 1C (included in the PSC) and the A2 area. It may be and (e)), the width S is significantly smaller than the L1c observed in Figure 3(b). Probably heat treatment width S (Figures 4(b), (d), and (f)), which means that L1T implies changes in the structure of point and linear the angle of misorientation is significantly smaller. defects; however, areas with a relatively homogeneous Similarly, as in a previous case, the width of the S RC2 distribution of dendrites related to undisturbed crystal- sub-ring is significantly smaller than the width of the lization of the PSC area (contained the A1 area) are S sub-ring (Figures 4(e) and (f)). It follows that heat RT2 characterized by a different concentration of point and treatment caused an increase of the misorientation angle linear defects, related to the area (outside the PSC) at the R and L boundaries. These macroscopic C 1C where the crystallization process was disturbed for a heterogeneities involve a defect generation at the micro- moment by the step-change geometry of the mold (near and nano-metric scale. Perfect heat treatment should, the P plane—Figure 1). The differentiated density of among others, eliminate dendritic segregation. Dissolu- point defects in the studied areas may also be related to tion of the c¢ phase after heat treatment and then its the inhomogeneity of the chemical composition in the re-precipitation eliminates some defects. The basic defects environment, which suggests a different positron physical process controlling homogenization is diffu- lifetime of the same-type defects. Thus, the treatment sion. Macroscopic inhomogeneity in the crystal METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4359 orientation of the millimeters- or tenths of millime- 2. In the region near the rim of the selector continuer ters-sized areas cannot be eliminated by diffusion due to projection (PSC), a specific macroscopic distribu- limited processing time. In addition, changes in the tion of crystal misorientation is present in both crystal orientation do not directly correlate with the as-cast and heat-treated roots. This distribution has concentration gradient of the components. Therefore, the character of a flexure mirror-like effect and does the defects related to the crystal misorientation may not disappear after heat treatment. remain after treatment. 3. The as-cast regions of the blade roots, located near The effect visible in topograms as the R and R rings the selector continuer, contain high concentrations C T is related to the local crystal misorientation and can be of micro-voids and pores; these make it impossible explained on the basis of a scheme shown in Figure 6 to determine by PALS the differences in vacancies and called the ‘‘flexure mirror-like effect.’’ When the and dislocations concentration. crystal orientation does not change throughout the 4. It has been found by PALS that for heat-treated analyzed areas (areas 1 and 3 of Figure 6(a)), the parts of the roots, there are differences in the orientation contrast obtained in topograms is homoge- concentration and type of defects between the area neous and the intensity of diffracted beam is the same. of PSC and beyond it. Inside it the concentration of In the case of the orientation changes in a certain area dislocations is found to be lower than outside it. (area 2, Figure 6(a)), the diffracted beam overlaps The concentration of vacancies inside the PSC area neighboring areas increasing the intensity (area IN, is higher. Figure 6(b)); meanwhile, in the expected place, there is a 5. Additionally, it was found that both inside and decreased contrast (area DE, Figure 6(b)). Two types of outside the PSC area after treatment, the concen- contrast shifts can be created in topograms, outside tration of vacancies is higher than the concentration (Figures 6(a) and (b)) and inside (Figures 6(c) and (d)), of dislocations. This may be caused by migration of in relation to the PSC area, and two types of misorien- dislocations during treatment to the like-LAB and tation can be determined from topograms: the convex their annihilation. This assumption is consistent (B –B , Figure 6(b)) and the concave (B –B , with the fact that the LAB-like angle of misorien- I I II II Figure 6(c)) mirror-like effect, respectively. The B –B tation increases after heat treatment. I I and B –B curves describe a distribution of [001] 6. In this article, it has been demonstrated that the II II c¢ crystallographic direction, which is perpendicular to combination of X-ray topography and positron these curves along the X axis in the PSC region. This is annihilation spectroscopy allows for a multiscale extremely important when blades are obtained by structural examination for the determination of [29] seeding. It was assumed that topograms are created defect structure, including the low-angle boundary only by diffraction from the c¢ phase (volume content (LAB) areas and nano- and micro-scale defects. about 70 pct). The example shown in Figure 6 occurs for a symmetrical deviation from the growth direction Z*, but in other cases, the contrast rings are not centrosymmetric. The presented mirror-like effect may be related to the crystallization front bend in a selector OPEN ACCESS continuer or in a seed. This article is distributed under the terms of the A high concentration of volumetric defects revealed Creative Commons Attribution 4.0 International by PALS method in the as-cast parts of the roots may be [30] License (http://creativecommons.org/licenses/by/4.0/), related to a porosity. Nucleation of pores depending which permits unrestricted use, distribution, and on the local stress level in the interdendritic melt may be [31] reproduction in any medium, provided you give driven by stress relaxation after pore nucleation that appropriate credit to the original author(s) and the appeared near areas of step-changes in the mold source, provide a link to the Creative Commons geometry. The signal of positron annihilation from license, and indicate if changes were made. volumetric defects is high and prevents detecting vacan- cies and dislocations in an as-cast state. After heat treatment, the volumetric defects disappear and the vacancies with the dislocations may be detected by PALS. 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Purushothaman: Electron 31. A.J. Torroba, O. Koeser, L. Calba, L. Maestro, E. and Positron Spectroscopies in Materials Science and Engineering, Carren˜ o-Morelli, R. Mehdi, S. Milenkovic, I. Sabirov, and J. Academic Press, New York, 1979, pp. 73–118. Lorca: Integr. Mater. Manuf. Innov., 2014, vol. 3 (1), pp. 1–25. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4361 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Metallurgical and Materials Transactions A Springer Journals

Influence of Heat Treatment on Defect Structures in Single-Crystalline Blade Roots Studied by X-ray Topography and Positron Annihilation Lifetime Spectroscopy

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Materials Science; Metallic Materials; Characterization and Evaluation of Materials; Structural Materials; Surfaces and Interfaces, Thin Films; Nanotechnology
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TOPICAL COLLECTION: SUPERALLOYS AND THEIR APPLICATIONS Influence of Heat Treatment on Defect Structures in Single-Crystalline Blade Roots Studied by X-ray Topography and Positron Annihilation Lifetime Spectroscopy JACEK KRAWCZYK, WŁODZIMIERZ BOGDANOWICZ, ANETA HANC-KUCZKOWSKA, ANNA TONDOS, and JAN SIENIAWSKI Single-crystalline superalloy CMSX-4 is studied in the as-cast state and after heat treatment, with material being taken from turbine blade castings. The effect of the heat treatment on the defect structure of the root area near the selector/root connection is emphasized. Multiscale analysis is performed to correlate results obtained by X-ray topography and positron annihilation lifetime spectroscopy (PALS). Electron microscopy observations were also carried out to characterize the inhomogeneity in dendritic structure. The X-ray topography was used to compare defects of the misorientation nature, occurring in as-cast and treated states. The type and concentration of defects before and after heat treatment in different root areas were determined using the PALS method, which enables voids, mono-vacancies, and dislocations to be taken into account. In this way, differences in the concentration of defects caused by heat treatment are rationalized. https://doi.org/10.1007/s11661-018-4704-2 The Author(s) 2018 I. INTRODUCTION may be related to the inhomogeneity of the chemical composition, morphology and size of c¢ particles, and THE single-crystalline, Ni-based superalloys are the inhomogeneity of crystal orientation. Therefore, widely used for production of high-pressure and blades are subjected to complex heat treatment after high-temperature turbine components in aerospace and casting, among others, to eliminate the chemical hetero- energy industry sectors. Due to the extreme work [4] geneity caused by directional dendritic crystallization. conditions of blades, especially high mechanical and The heat treatment parameters are appropriately thermal stresses, the specific properties with low con- [4] selected to create new c/c¢ array (reprecipitation ) with [1–3] centration of structural defects are needed. Nowa- optimized properties, caused by more homogeneous days, the CMSX-4 single-crystalline superalloy is morphology, size, and chemical composition. Some commonly used by industry; castings produced in this defects of macro-, micro-, and nano-scale are recreated way are suitable candidates for defect characterization in the treated blades, but some of them may be inherited studies. from the as-cast state. All of them to some degree or Directional dendritic solidification by the Bridgman other will influence the mechanical properties of blades technique is widely used for production of single-crys- [5–9] and may cause damage during operation. The talline blades made from superalloys. Production tech- macroscopic inhomogeneity in crystal orientation (e.g., nology and the complex shape of blade castings produce subgrain boundaries or misoriented bands of dendrites, dendritic arrays that allow for the possibility of many bended during crystallization) may be one of the defects to be produced during solidification. The defects inherited defects. These defects are generally related to [4,10–12] the crystallization of complex shape casts. The inhomogeneity of crystal orientation may be characterized by X-ray diffraction topography in reflec- [13,14] tive geometry. This imaging technique—which JACEK KRAWCZYK, WŁODZIMIERZ BOGDANOWICZ, represents two-dimensional mapping of diffracted ANETA HANC-KUCZKOWSKA, and ANNA TONDOS are with the Institute of Materials Science, University of Silesia in Katowice, X-rays local intensity—is capable of providing informa- 41-500 Chorzow, Poland. Contact e-mail: jacek.krawczyk@us.edu.pl tion on the distribution and nature of structural defects JAN SIENIAWSKI is with the Department of Materials Science, in single-crystalline materials. The technique is sensitive Rzeszo´ w University of Technology, 35-959 Rzeszow, Poland. to changes in local orientations of diffraction (atomic) Manuscript submitted March 14, 2018. [15,16] planes and their spacing. The method allows the Article published online June 1, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4353 visualization of the differences in crystal orientation (misorientation) of neighboring areas accurate to arc minutes. However, the analyzed areas must have a size of 1 mm or higher. The boundaries between such areas (macroscopic low-angle boundaries, LABs) are created by microscopic defects like dislocations, and addition- ally, they may be related to the vacancies. This means that the general description of defect structure of blades must refer to defects over a wide spatial size ranging from nanometer up to millimeter scale, which are mutually related to each other. Determining the rela- tionship of different-scale defects, which is the basis of the multiscale analysis, helps to determine the reasons for their creation. Additionally, the macroscopic inhomogeneity of crys- tal orientation may be related to sub-micrometer-scale defects, which are difficult or impossible to reveal by SEM or TEM methods due to their small analyzed area. The positron annihilation lifetime spectroscopy (PALS), using a macroscopic area, may enable it. PALS is a promising nondestructive method of quality control of technologically important materials employed in various fields of science and technology. Positrons interact for example with foreign atoms, points, or linear defects of structure. Positrons are trapped preferentially in atomic defects leading to an extended positron lifetime. The advantage of this method is the detection of small concentrations of defects that could not be detected by [17] other methods. Positron techniques have been used [18] Fig. 1—(a) Illustration of blade parts and (b) samples for PALS for studies of defect behavior in the Ni Al system and [19] preparation; P —section of part C. polycrystalline Ni-based superalloys, but there are no results as yet for investigation of defects inhomogeneity P (Figure 1) was located near the surface P in the single-crystalline turbine blades. h a (Figure 1(b)) of the selector-root connection It is emphasized that the structural defects are mainly (u = 3.5 mm). The ST surface was a mirror-reflection created in the so-called critical areas of the blade. The image of the SC surface. The SC surface of C-parts single-crystalline blades possess critical areas with (Figure 1(a)) was studied in the as-cast state, and the ST regard to the crystallization process (selector-root and surface was analyzed after heat treatment of T-parts. root-airfoil connection areas, thin-walled areas) and to Section P in Figure 1(b) reveals the geometry of the the operation loads (areas near the root-airfoil connec- v [20,21] selector-root connection with the use of a selector tion, thin-walled areas of trailing edge, tip area). Regarding to the growth structure of dendrites set, the continuer higher in diameter on the plane P in relation most affected area is situated in the root part of the to the diameter of selector channel (D > d). The PSC in Figure 1 is the projection of a selector continuer on all blade, near the connection with the selector, where the [22] transverse sections of the root. step-change geometry of the castings occurs. The The heat treatment of part T was performed by several above justifies the selection of these regions for attention steps, consisting of convection heating to 950 C (in a in this article. Its overarching aim of the study is to helium-protective atmosphere), radiation heating to analyze and compare the defects structure of CMSX-4 1350 C (in vacuum), solution annealing, and finally single-crystalline blade root areas, located near the aging. The temperature-time settings for annealing were: selector-root connection, in an as-cast state and after 1277 C/4 h fi 1287 C/2 h fi 1296 C/3 h fi 1304 C/ heat treatment using X-ray topography and the PALS 3h fi 1313 C/2 h fi 1316 C/5 h fi gas furnace method. quench, and for aging: 1140 C/6 h (step 1) and 871 C/ 20 h (step 2). The blade production and heat treatment process were performed in the Research and Develop- II. EXPERIMENTAL ment Laboratory for Aerospace Materials, Rzeszo´ w University of Technology, Poland, using an industrial The investigated roots of single-crystalline turbine ALD Vacuum Technologies furnace. blades, made of CMSX-4 superalloy, were obtained by The metallographic sections of SC and ST surfaces directional crystallization via Bridgman technique at a were studied by scanning electron microscopy (SEM), withdrawal rate of 5 mm/min. with the use of the spiral the X-ray diffraction topography, and Laue diffraction. selector with [001] orientation (Figure 1). The blade A JEOL JSM-6480 microscope was applied for SEM roots were cut into two parts: C and T, perpendicular to observations using a backscattered electron (BSE) the withdrawal direction G (Figure 1). The cutting plane 4354—VOLUME 49A, SEPTEMBER 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A technique. A divergent beam of characteristic Cu Ka radiation, generated by a microfocus X-ray tube of the PANalytical system, was used for X-ray topography studies. The topograms were recorded on the AGFA Structurix D7 X-ray film in reflective geometry. The divergent beam of the characteristic radiation coming from a quasi-point (40 9 40 lm ) X-ray source illumi- [13] nates the whole surface of the SC or ST surfaces. The sample (part C or T) coupled with the film oscillates about the axis located on the studied surface. The individual parts of the tested surface, meeting the Bragg condition, are successively recorded on the film. After over a dozen minutes of exposure, the diffraction image (topogram) of the whole analyzed surface was obtained on the film. If for areas close to surface SC of part C or ST for part T, the crystal lattices are rotated relative to each other, then their diffraction images will be mutually shifted in the topogram, creating different types of contrast. Similar displacement in the topograms was created by changes of d-spacing. Specific contrast is also created for other defects that occurred in the single-crys- [13,23] talline materials. The misorientation angle may be [13] calculated using the shift value in the topograms. The minimal misorientation angle, determined by X-ray topography method, is on the order of arc minutes, which are much lower than for the EBSD method. Additionally, the PALS technique was applied to the study defect structure, revealed by X-ray topography on the surfaces SC and ST. PALS relies on the propensity of positrons to become localized at open-volume regions of a solid and the emission of annihilation gamma rays that escape the test system without any final state interaction. The scheme used for sample sandwich Fig. 2—(a) Scheme of samples sandwich arrangement in PALS packing in the PALS measurements and scheme of measurements, (b) scheme of the positron annihilation processes according to the three-state-trapping model: positron trapping in positron trapping are illustrated in Figure 2. The complex defects: positron trapping in a vacancy and dislocation. sources of positrons are generated by a pair of gamma rays that hold information about the electronic envi- spectra was recorded. Then, the spectra were added ronment around the annihilation site. PALS measures together by means of a special procedure accounting for the elapsed time between the implantation of the the drift of the zero-time channel. In this way, a positron into the material and the emission of annihi- resultant spectrum of very high statistics (at least 107 lation radiation. Positrons are trapped preferentially in counts) was obtained. The data were analyzed using a atomic defects that in turn have a locally smaller [24] least-square fitting procedure, which, contrary to the electron density leading to an extended positron lifetime. other existing programs, enables fitting not only a single The PALS technique therefore is a sensitive method to spectrum but also a series of spectra. The simultaneous derive sizes and concentration of vacancy-type defects fitting leads to reduction of the number of the free fitting like nano-cavities. The characteristics of annihilation parameters because some of the parameters can have trapping positron are different for various configura- common values for all of the spectra of the series. In the tions defects. Knowledge of the location of positrons in calculations, all the measured spectra were fitted simul- defect of lattice gives the possibility of the concentration taneously with a model, directly implemented within the of defects from proportion of trapping positron to free. [24] software code. All spectra were analyzed using the Here, it was applied in determining the energy of [24] three-state trapping model describing the positron vacancy formation. annihilations in the bulk material and two types of Two samples in a sandwich arrangement (Figures 2(a) defects (Figure 2(b)). The three-state trapping model and 1(b)) for PALS measurements were prepared was described by three parameters, i.e., the positron separately from each C-part and each T-part of the lifetime in bulk material s , the positron lifetime trapped blades. The PALS measurements were performed in two b by defects s, and the positron trapping rate j. areas (A1, A2 of Figure 2(b)) of each sandwich at room In Figure 2(b), the horizontal bars represent the free temperature with a conventional fast–fast spectrometer state of positrons, i.e., the delocalized state of each of 270-ps time resolution. The area A1 relates to the positron in bulk material and its localized states PSC area and A2 to the area outside it. The positron (dislocations and vacancy) in two types of defects. k source with the activity of about 370 kBq, covered with b is the annihilation rate of the delocalized (free) positron a5-lm Ni foil, was used. For each sandwich, a series of METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4355 from the bulk, k and k and k are the annihilation rates Figure 4 presents typical topograms obtained for the st t v from different type of defects, respectively. j , j , and j SC surface of C-type part (Figure 4(a)) and for the ST v t st are the trapping rates into these defects, which are surface of T-type part (Figure 4(b)). The ST surface was proportional to the defect concentrations. 0 are the a mirror-reflection image of the SC surface; therefore, trapping rate into dislocation-bound vacancy. The term the contrast details in the topograms for the same d is the detrapping (escape) rate. corresponding locations on the analyzed surfaces All the measured spectra were fitted simultaneously at (Figures 4(a) and (b)) are mirror-reflected, e.g., the each parameter relating to the resolution or the contri- L contrast band (Figure 4(a)) relate to the L band 1C 1T bution of source. This means that such a parameter was (Figure 4(b)). Additionally, the contrast inversion is constrained to have the same value for each spectrum of visible. It can be observed during a hypothetical move the series of spectra analyzed together. The positron from the corner K (Figure 4(a)) to the center of the A2 lifetimes relating to positron annihilation in the source area: In Figure 4(a), the dark-contrast band is visible were fixed. The ‘‘source’’ lifetimes were determined with first, and in Figure 4(b), the bright-contrast band is help of a Si lifetime spectrum of very high statistics. The visible first. source contribution of 36.7 pct consisted of three In both topograms, the R-rings (R , R ) of contrast C T components with lifetimes 125 ps, 386 ps, and 1.97 ns with change in intensity of the rim of the PSC are visible. in the proportion 94.6:4.7:0.7, respectively. The R ring (Figure 4(a)) consists of two sub-rings: the external one of S width with higher contrast intensity RC1 (Figure 4(e)) and the internal one of S width with RC2 lower contrast intensity (Figure 4(e)). The R ring III. RESULTS (Figure 4(b)) consists of two sub-rings: the external Metallographic observations of SC surfaces of C-type one of S width with lower contrast intensity RT1 parts revealed a dendritic structure (Figure 3(a)) typical (Figure 4(f)) and the internal one of S width with RT2 of the as-cast state. The macro-SEM images, created by higher contrast intensity (Figure 4(f)). The contrast merging multiple separate SEM images, allow us to inversion can be observed during a hypothetical move observe the whole section and compare the morphology from the center of the rings RC and RT toward the and distribution of dendrites in different areas of the SC outside: In Figure 4(a), the bright contrast ring is visible surface. A computer processing was applied for better first and the dark one is visible first in Figure 4(b). visibility of dendrites arms direction and type of There is also a visible dark band L in the topogram 1C arrangement (Figure 3(b)). The processing combines of the SC surface of the C-type part (Figures 4(a), (c), creation of skeletal images and modification of contrast. and (e)). Bright band L (Figures 4(b), (d), and (f)) may 2T The procedure gives results possessing certain common be observed in topograms obtained for T-type parts. features with the skeletonization procedure referred This band appears after heat treatment. Fine parallel to in Reference 25. Skeletonization is a technique contrast bands (fine arrows, Figures 4(a), (c), and (e)), whereby a binary image of dendrites is eroded step by arranged along the m direction, appear in the topograms step away until the skeleton of the image is obtained. of the C-type part. However, in the topograms of T-type The skeletal image is created as a thin line equidistant parts, the bands mentioned above are not visible. The m [25] from the original edges of the binary dendrites shape. and n directions from Figure 4(a) are parallel to the Analyzing images from Figure 3, two different types of secondary dendrite arms and to the [100] and [010] dendrite arms arrangement can be distinguished: crystallographic directions. The blurred, wider bands H short-range alternate (inside the center of PSC) and visible in the topograms from Figures 4(b), (d), and (f) long-range continuous (L -type). The center part of the are parallel to the h direction, which divides the angle sections consists of dendrites with a homogenous, between m and n in half. The G fragment (Figure 4(b)) statistical arrangement. A small disorder in dendrite with lack of contrast suggests that this area does not arm directing (shifts of linear fragments) is noticeable satisfy the Bragg’s condition and fragments of contrast around the PSC area (Figure 3). The mutual arrange- are shifted creating an overlapped high contrast G near ment of short-range and long-range arms (depending on the area G of the topogram. the m and n directions) allows visualization of a ring that On the basis of the PALS spectra analysis, the defect overlaps the edge of the PSC area and is marked by R concentration values were determined for both the A1 (Figure 3(b)). The diameter of the ring is equal to the and A2 areas of as-cast and treated samples. The diameter D of the PSC area (Figure 1(b)). The R ring obtained values of defect concentration in the A1 and may be better visible when the figure is reduced A2 areas (Figures 2(a) and 4(a)) of as-cast parts of the (Figure 3(c)). The largest difference in dendrites mor- roots (C-type) and the defect concentration estimated phology occurs in narrow corner areas (K , K ). The for the analogous areas of the heat-treated parts 1 2 dendrite arms are elongated along the n direction and (T-type) were averaged and summarized in Figure 5. fine ternary arms appear. No clear dendritic structure For the purposes of analysis, it was assumed that the was observed on the metallographic sections of the ST chemical composition of the undefected material (bulk) surface of T-type parts. By Laue diffraction, it was does not radically change qualitatively during heat stated that the n and m directions correspond to the treatment and changes occur in the distribution of [010] and [100] crystallographic directions. matrix chemical components only, so the positron 4356—VOLUME 49A, SEPTEMBER 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 3—(a) Typical macro-SEM image obtained for SC surface of C-type part, (b) dendritic structure presented in part (a) after computer processing, and (c) part (b) reduced in size. annihilation lifetime in bulk is an invariant value, Two types of defects were found in the T-type parts in determined in the first step of analysis. The positron both studied areas. Based on the positron lifetime annihilation lifetime s was 134 ps, and this value is calculations, the occurrences of mono-vacancies and [17] close to the literature values given for alloys based on dislocations were established. The defects concentra- Ni Al. tion in the T-type parts (A1 and A2 areas) indicates that The results obtained by the PALS method suggest the dominant type of defects in both areas is vacancies. that in the as-cast parts of the root (C-type), there is However, it should be noted that the concentration of such a high concentration of defects that it is not each defect type in the discussed areas is different. It was possible to indicate differences in the defect structure observed that the vacancies concentration in the A1 area between two selected areas A1 and A2. The determined is about six times higher in relation to the dislocation values of positron lifetime (s = 1.5 ns), obtained as a concentration. The dislocation concentration in the A2 result of a numerical analysis of the experimental area is higher than in the A1 area, and additionally, the spectrum, allow us to identify the type of dominant dislocation concentration in the A2 area is twice lower defects. The determined lifetime corresponds to volu- than the determined concentration of vacancies. The metric defects of the micro-voids type also called the free positron lifetimes for the vacancy in the area A1 [26] volumes. (s = 212 ps) and in the area A2 (s = 220 ps) were v v METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4357 determined. Similar calculations of positron lifetime were performed for dislocations in the area A1 (s = 380 ps) and in the area A2 (s = 410 ps). v v IV. DISCUSSION The difference in the dendrites’ arrangement and arms’ length inside and outside the PSC area (Figure 3) is related to the difference in growth kinetics. The growth of dendrites is not disturbed inside the PSC area on the P plane (Figure 1(b)). Outside the PSC, the dendrite array in a whole root is conditioned by the rapid growth of the secondary dendrite arms on the P [4,20] plane (Figure 1(b)). The dendritic structure disorder in the K and K areas may be caused by local changes in 1 2 the heat dissipation and by curvature of the solidifica- [27] tion front in the narrow corner areas. It may also be the reason for the formation of contrast bands in topograms, separating the K and K areas 1 2 (Figures 4(e), (c), and (a)), and this suggests a somewhat different crystal orientation of these areas in relation to other ones. Additionally, the secondary dendrite arms, growing in a transverse plane from the selector exten- sion, do not reach into the K and K area, so 1 2 the dendritic structure in these areas is different (Figures 3(a) and (b)) and often affected by the deflec- tion process described in Reference 28. Structure alter- ation in the R -ring region (Figure 3) similar to the specific shape of contrast of the PSC circle in topograms Fig. 4—Example of original topograms, obtained for (a) the SC (Figures 4(a), (c), and (e)) is related to the circular shape surface of C-type part and (b) the ST surface of T-type part; 113 of a selector continuer–root connection. This suggests reflection, Cu Ka radiation; (c) and (d) the same topograms after that changes in the crystal orientation occur in this computer processing; (e) and (f) schemes of contrast in topograms from parts (a) and (b). circle. The microstructure alteration visible on the Fig. 5—Estimated defects concentrations with error bars and the determined values of positron lifetime in different types of defects in areas A1 and A2 of as-cast and treated parts of the root. 4358—VOLUME 49A, SEPTEMBER 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 6—(a) Scheme of contrast formation in the R ring of topograms (Figs. 4(a), (c), and (e)); (b) and (c) scheme of contrast in stripes d for C y topogram obtained from SC and ST surface; and (d) scheme of contrast formation in the R ring of topograms (Figs. 4(b), (d), and (f)). Small arrows indicate [001] direction, PB, DB—primary and diffracted X-ray beams, and the B and B bend is enlarged for figure clarity. c¢ I II macro-SEM images (e.g., K , K areas, R ring) and process does not homogenize the defect structure in the 1 2 C misoriented areas separated by boundaries similar to the sub-microscopic scale. This requires further studies on a LAB, called the ‘‘LAB-like’’ (L , L , R-type rings) mechanism of c¢ re-precipitating and its atomic ordering 1 2 visible in topograms (Figure 4), suggest crystal misori- during heat treatment. entation. As a result of heat treatment, some defects Elimination of inhomogeneity, created during den- disappear, e.g., the LAB-like, visualized in topogram as dritic crystallization of a complex-shape cast, is one of L (Figures 4(a), (c), and (e)) but some remain the aims of heat treatment. A transition of the crystal- 3C (L  L , Figures 4(a) and (b) and 4(c) and (d)). A lization front through the areas of the step-change 1C 1T new defects formation after heat treatment, e.g., the geometry causes momentary changes in the crystalliza- LAB-like, marked as L (Figures 4(b), (d), and (f)), was tion kinetics due to the rapid growth of the secondary 2T also observed. dendrite arms, perpendicular to the withdrawal direc- For the treated parts of the roots, the positron lifetime tion. This growth takes place in a thin layer of the root for vacancy in the area A1 (s = 212 ps) and in the area (near P —Figure 1) and occurs at a much higher growth v a A2 (s = 220 ps) was determined. Similar calculations rate than the withdrawal rate. As a result, a local of positron lifetime were performed for dislocations in disorder in crystal orientation and bending of dendrites, [22] the area A1 (s = 380 ps) and in the area A2 (s = 410 as well as the LAB-like formation, occur. The angle v v ps). Differences in the given positron lifetime values can of misorientation of these boundaries can be estimated be related to different chemical environments of the basing on the width of areas with increased or decreased [13] defects. The differences in the concentration of defects contrast in the topogram. The areas of topograms of estimated by PALS in the area A1 and A2 of T-type the increased contrast, derived from the SC surface, parts are probably related to different kinetics of the should be compared with the areas of the decreased crystallization process in these areas. Electron micro- contrast, derived from the ST surface, due to a contrast scopy studies of a dendritic structure (Figure 3) indicate inversion resulting from the mirror-reflected SC and ST the differences in the dendritic array of the A1 area surfaces. In the case of the LAB L (Figures 4(a), (c), 1C (included in the PSC) and the A2 area. It may be and (e)), the width S is significantly smaller than the L1c observed in Figure 3(b). Probably heat treatment width S (Figures 4(b), (d), and (f)), which means that L1T implies changes in the structure of point and linear the angle of misorientation is significantly smaller. defects; however, areas with a relatively homogeneous Similarly, as in a previous case, the width of the S RC2 distribution of dendrites related to undisturbed crystal- sub-ring is significantly smaller than the width of the lization of the PSC area (contained the A1 area) are S sub-ring (Figures 4(e) and (f)). It follows that heat RT2 characterized by a different concentration of point and treatment caused an increase of the misorientation angle linear defects, related to the area (outside the PSC) at the R and L boundaries. These macroscopic C 1C where the crystallization process was disturbed for a heterogeneities involve a defect generation at the micro- moment by the step-change geometry of the mold (near and nano-metric scale. Perfect heat treatment should, the P plane—Figure 1). The differentiated density of among others, eliminate dendritic segregation. Dissolu- point defects in the studied areas may also be related to tion of the c¢ phase after heat treatment and then its the inhomogeneity of the chemical composition in the re-precipitation eliminates some defects. The basic defects environment, which suggests a different positron physical process controlling homogenization is diffu- lifetime of the same-type defects. Thus, the treatment sion. Macroscopic inhomogeneity in the crystal METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, SEPTEMBER 2018—4359 orientation of the millimeters- or tenths of millime- 2. In the region near the rim of the selector continuer ters-sized areas cannot be eliminated by diffusion due to projection (PSC), a specific macroscopic distribu- limited processing time. In addition, changes in the tion of crystal misorientation is present in both crystal orientation do not directly correlate with the as-cast and heat-treated roots. This distribution has concentration gradient of the components. Therefore, the character of a flexure mirror-like effect and does the defects related to the crystal misorientation may not disappear after heat treatment. remain after treatment. 3. The as-cast regions of the blade roots, located near The effect visible in topograms as the R and R rings the selector continuer, contain high concentrations C T is related to the local crystal misorientation and can be of micro-voids and pores; these make it impossible explained on the basis of a scheme shown in Figure 6 to determine by PALS the differences in vacancies and called the ‘‘flexure mirror-like effect.’’ When the and dislocations concentration. crystal orientation does not change throughout the 4. It has been found by PALS that for heat-treated analyzed areas (areas 1 and 3 of Figure 6(a)), the parts of the roots, there are differences in the orientation contrast obtained in topograms is homoge- concentration and type of defects between the area neous and the intensity of diffracted beam is the same. of PSC and beyond it. Inside it the concentration of In the case of the orientation changes in a certain area dislocations is found to be lower than outside it. (area 2, Figure 6(a)), the diffracted beam overlaps The concentration of vacancies inside the PSC area neighboring areas increasing the intensity (area IN, is higher. Figure 6(b)); meanwhile, in the expected place, there is a 5. Additionally, it was found that both inside and decreased contrast (area DE, Figure 6(b)). Two types of outside the PSC area after treatment, the concen- contrast shifts can be created in topograms, outside tration of vacancies is higher than the concentration (Figures 6(a) and (b)) and inside (Figures 6(c) and (d)), of dislocations. This may be caused by migration of in relation to the PSC area, and two types of misorien- dislocations during treatment to the like-LAB and tation can be determined from topograms: the convex their annihilation. This assumption is consistent (B –B , Figure 6(b)) and the concave (B –B , with the fact that the LAB-like angle of misorien- I I II II Figure 6(c)) mirror-like effect, respectively. The B –B tation increases after heat treatment. I I and B –B curves describe a distribution of [001] 6. In this article, it has been demonstrated that the II II c¢ crystallographic direction, which is perpendicular to combination of X-ray topography and positron these curves along the X axis in the PSC region. This is annihilation spectroscopy allows for a multiscale extremely important when blades are obtained by structural examination for the determination of [29] seeding. It was assumed that topograms are created defect structure, including the low-angle boundary only by diffraction from the c¢ phase (volume content (LAB) areas and nano- and micro-scale defects. about 70 pct). The example shown in Figure 6 occurs for a symmetrical deviation from the growth direction Z*, but in other cases, the contrast rings are not centrosymmetric. The presented mirror-like effect may be related to the crystallization front bend in a selector OPEN ACCESS continuer or in a seed. This article is distributed under the terms of the A high concentration of volumetric defects revealed Creative Commons Attribution 4.0 International by PALS method in the as-cast parts of the roots may be [30] License (http://creativecommons.org/licenses/by/4.0/), related to a porosity. Nucleation of pores depending which permits unrestricted use, distribution, and on the local stress level in the interdendritic melt may be [31] reproduction in any medium, provided you give driven by stress relaxation after pore nucleation that appropriate credit to the original author(s) and the appeared near areas of step-changes in the mold source, provide a link to the Creative Commons geometry. The signal of positron annihilation from license, and indicate if changes were made. volumetric defects is high and prevents detecting vacan- cies and dislocations in an as-cast state. After heat treatment, the volumetric defects disappear and the vacancies with the dislocations may be detected by PALS. 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