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Selected Area Electron Diffraction, a technique for determination of crystallographic texture in nanocrystalline powder particle of Alloy 617 ODS and comparison with Precession Electron Diffraction
Materials Characterization 157 (2019) 109883
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Selected Area Electron Diffraction, a technique for determination of crystallographic texture in nanocrystalline powder particle of Alloy 617 ODS and comparison with Precession Electron Diffraction
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M. Sivakumar, Arup Dasgupta
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Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam, Tamil Nadu, India
ARTICLE INFO
ABSTRACT
Keywords: Local texture analysis Nanocrystalline Individual powder particle
Texture analysis in nanocrystalline materials is a challenge. A simple technique as Selected Area Electron Diffraction (SAED) has been used to evaluate texture in a single nanocrystalline Alloy 617 ODS powder particle by carrying out Rietveld texture refinement using Material Analysis Using Diffraction (MAUD) software package and the results were compared with the ASTAR™/Precision Electron Diffraction (ASTAR™/PED) results. Extraction of texture from the SAED patterns involved image analysis and Rietveld refinement of radial plots divided into judiciously chosen angular spans. Inverse pole figure (IPF) from SAED pattern using MAUD revealed 〈110〉 texture parallel to the normal direction (electron beam direction) for unmilled and 6 h milled powders. IPF from ASTAR™/PED technique also shown 〈110〉 lying parallel to the normal direction (beam direction) in both unmilled and the 6 h milled powder. A one is to one correspondence of IPF obtained from MAUD to that of ASTAR™/PED was observed. Texture in unmilled powder could be attributed to the compressive force during rapid solidification in water atomisation process. During the milling process, inhomogeneous rolling and compression of the trapped powder particle in between the balls and between the balls and walls of the jar resulted in shear texture in 6 h milled powder. ASTAR™/PED technique also showed the presence of 〈100〉 //ND texture which was attributed to solidification in unmilled powder and 〈112〉 //ND type texture in milled powder attributed to twin mode of deformation during milling.
1. Introduction Crystallographic texture in bulk materials is well reported in the literature, but crystallographic texture in nanocrystalline materials is relatively scarce. This is mainly because the methodology to analyse the texture at such length scale is quite complex since the volume available for diffraction in locally textured particle is very small, extending only up to few micrometres. With the advancement of nanostructured materials in almost all fields of application from electronics [1–3] to structural materials [4,5] the role of texture can no longer be ignored [6–8] and simpler techniques are required to quickly assess the texture property. Transmission electron diffraction pattern using selected area apertures measuring few micrometer is a simple but promising technique [9]. Characterization of texture in nanometre scale can be carried out by various techniques such as Transmission Kikuchi Diffraction (TKD) [10], ASTAR™/Precision Electron Diffraction (ASTAR™/PED) [7], and Selected Area Electron Diffraction (SAED) [11]. Transmission Kikuchi
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diffraction (TKD), also known as transmission-EBSD (t-EBSD) can achieve spatial resolution of ~2 nm [12]. But small step size and long dwell time would result in very long acquisition time and possible specimen drift. On the other hand, ASTAR™/PED in a Transmission Electron Microscope (TEM), offers spatial resolution ~2 nm or even better, depending on the resolution of the TEM itself in the diffraction contrast mode [13]. This technique involves in automatic collection of spot diffraction pattern and cross correlating the experimental diffraction patterns with the pre- calculated diffraction patterns known as templates. Though it is similar to EBSD technique, the major difference lies in the elimination of the dynamical effect of the TEM diffraction by precessing the electron beam and approaching a quasi-kinematical condition [7,14]. Texture characterization in ultrafine-grained commercially pure titanium alloys have been reported by Iman Ghamerian et.al [6] and Dasgupta et al. [15] using the PED technique. On the other hand, Selected Area Electron Diffraction technique (SAED) may also be used to extract local texture in materials. Zghal et al. reported deformation texture in Cu-Ag ball milled powders by
Corresponding author. E-mail address: [email protected] (A. Dasgupta).
https://doi.org/10.1016/j.matchar.2019.109883 Received 25 July 2019; Received in revised form 22 August 2019; Accepted 22 August 2019 Available online 23 August 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 157 (2019) 109883
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analyzing the SAED pattern of powders [16,17]. Boulay et.al used SAED pattern of TiO2, and Mn3O4 nanocrystal aggregates and platinum films and carried out Rietveld refinement using MAUD (Material Analysis Using Diffraction) in which textural refinement was incorporated [9]. MAUD is free but powerful multipurpose Rietveld analysis program, easy to use with entire actions controlled by GUI and works with X-ray, synchrotron, Neutron, and electron diffraction data. Quantitative analysis, Texture analysis, Residual stress analysis can be carried out with MAUD. Boulay et al. [9] used MAUD for the illustration of quantitative texture analysis from the SAED pattern. However, it needs to be mentioned here that SAED employs a parallel beam of electrons in line with the optic axis of the TEM. This is in contrast to the convergent electron beam diffraction (CBED) and the PED techniques. CBED technique uses high angle of convergence (α) for the electron beam, which is usually of the order of the Bragg diffraction angle. The conical beam results in the formation of diffraction disks [18,19]. The CBED technique yields higher spatial resolution as well as much higher angular accuracy. The technique is used to extract detailed structural information of a crystal, viz., point and space-group symmetry details, lattice parameters, defects in crystals and three-dimensional information about the reciprocal lattice. In comparison, the PED technique, discussed earlier, employs a near parallel beam using a very
Fig. 1. Mechanically milled powder particles comprising several crystallites with different orientation.
Fig. 2. (a–d) TEM micrograph showing SAED of 0, 2, 4 and 6 h ball milled Alloy 617-0.6Wt.%Y2O3 powders respectively, at 1000 rpm with variation of intensity along the ring indicating preferred orientation. 2
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small condenser aperture (~50 μm) (very small α). This beam is tilted and rocked (precessed) and then also derocked for analysis. The quasikenimatical condition achieved in PED technique allows structure determination with good accuracy [20]. In this paper we have studied texture analysis from SAED patterns of milled Alloy 617 ODS [21] and also compared the results with those obtained by using PED technique. Alloy 617 ODS is a variant of Nickel based superalloy, a candidate structural material for gas turbines and heat exchange applications for Generation IV nuclear reactors [22] and in the boilers, super heaters and rotors of advanced ultra-supercritical (AUSC) thermal power plants [23]. In addition to its intermediate temperature strengthening by gamma prime precipitates and solid solution strengthening at room temperature [24–26] in the Alloy 617, the ODS variant offers excellent high-temperature mechanical properties imparted by hard and stable oxide particles dispersed in the 617 matrix and the material may be called Alloy 617 ODS [27,28]. In the Alloy 617, dissolution of the gamma prime precipitates occurs at temperatures above 900 °C [29], whereas the Alloy 617 ODS is expected to retain its mechanical strength even up to 1000 °C [22]. However, the Alloy 617 ODS must be synthesized by high energy ball milling process in which nano-sized Y2O3 particles are mechanically milled together with the prealloyed Alloy 617 powder [21]. During the high energy ball milling process, severe plastic deformation processes occur in the material which could result in dislocations, lattice rotation, local lattice parameter variations, grain fragmentation as well as local texture (shear compression type) development in the powder particles [16]. An individual powder particle may be comprised of a large number of nanocrystallites (schematically shown in Fig. 1) which could have a preference of orientation. Texture in individual Ti powder particles is reported by Jiří Kozlík et al. [31]. Such local texture development could affect the consolidation characteristics as well as physical properties of the powder material significantly. Gueth et al. reported the dependence of magnetic property on local texture of Nd–Fe–B powder particle [32]. Satoshi Motozukha et al. reported double-fiber texture ({001} + {111}) in conventional ball milled iron powder particle due to uniaxial compression and multidirectional rolling which affect its magnetic properties [33]. Ravi Sankar at.al characterized the texture on the high energy ball milled Ni powder particle and reported a {001} type of texture in the material with slip and twinning influencing the texture development [34]. It may be noted here that it is essential to limit the study to a single nanocrystalline powder particle, as every particle is independent. In fact, XRD study on a large number of such particles would actually reveal a typical polycrystalline powder pattern. As indicated above, this study mainly concentrates on texture development on a single particle, an example of typical nanocrystalline material. TEM thin lamella is picked up by FIB (Focussed Ion Beam in SEM) from a single representative powder particle and SAED patterns have been used for extracting texture information. The reliability of the texture data evaluated this way is verified by comparing with those obtained from ASTAR™/PED technique.
Microscopy (TEM) specimen from the milled powder by FIB lift-out technique with Ga + ions. The FIB lamellae were thinned down to 100 nm. In order to remove any FIB induced amorphization layer and Ga implantations during FIB process, lamellae were cleaned at ion acceleration voltages from 5 kV to 1 kV. Philips make CM-200 Analytical TEM operated at 200 kV attached with TVIPS make 2 K × 2 K CCD camera was used for characterization of the FIB lamellae. ImageJ software was used for the analysis of the TEM results. SAED patterns from single powder particles were analyzed by carrying out Rietveld refinement using the Java based MAUD application software [36] using the following methodology. Concentric rings of the SAED pattern were integrated over the full radial span of 360° using ImageJ plug-in within MAUD. Two- dimensional diffraction data were thus extracted. The original image coordinate positions for each data point were preserved in the process. In this technique, it is essential to
2. Experimental methods In the current study, Inconel 617 prealloyed water atomized powders procured from M/S Padmasree enterprises of particle size ± 100 μm were used. High energy ball milling of prealloyed powder with 0.6Wt% of cubic Y2O3 nano powder was conducted in Emax system from Retsch, using milling speed 2000 rpm fitted with water-cooled chiller. Ball to powder weight ratio was maintained at 10:1 and stainless steel balls of 5 mm diameter were used for the alloying process. Stearic acid was used as a process controlling agent (PCA) in order to avoid cold welding of powders to the balls and walls of the jars during the high energy ball milling [35]. FEI make Helios NanoLab-600i dual beam Field Emission Scanning Electron Microscope (SEM) was used to prepare Transmission Electron
Fig. 3. (a) TEM micrograph showing SAED of 6 h ball milled Alloy 617-0.6Wt.% Y2O3 powder at 1000 rpm, (b) Dark Field images from circled arc of (111) plane showing continuous elongated crystallites. 3
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designate the beam center, the azimuth range and the increment of integration during the processing of the image in MAUD. A file with .esg extension was generated at the completion of integration which was the required input file for Rietveld refinement together with the suitable CIF (Crystallographic information file). The CIF may either be generated from MAUD using crystal structure, and space group obtained from XRD data or alternatively accessed from the online databases using fullprofile search match-method [37], in which phase is identified from integrated intensities along with electron diffraction pattern. After loading the .esg file and the CIF along with proper instrumental parameters such as intensity calibration, angular calibration, the geometry of measurement, source and detector distances, the Rietveld refinement can finally be carried out. Refinement includes parameters such as scale factor, background factor, microstructure parameter, profile parameter and so on. After the refinement of above mentioned parameters, a texture model such as Extended Williams-Imhof-Matthies-Vinel (EWIMV) incorporated within MAUD can be used for extraction of texture data viz., pole figure and inverse pole figure (IPF) [38]. In addition, Orientation Microscopy in TEM was also carried out using ASTAR™/PED from Nanomegas. The Precision angle of the electron beam was 1° with respect to the optic axis. Acquired diffraction patterns were indexed with the simulated diffraction pattern corresponding to Nickel with the space group Fm-3 m and texture data were extracted using the ASTAR software. For absolute orientations of grains, sample reference frame calibration is usually carried out by finding the following: (a) rotation between an image and its diffraction pattern, and (b) aligning ASTAR reference frame to TSL-OIM (wherein the texture analysis is carried out) reference frame [39–41]. The rotation correction arising out of mismatch between image and its diffraction pattern is corrected by means of aligning the bright field image and the virtual bright field image from PED. The Correction with regard to the
frames of reference is insignificant for ball milled powders, wherein the powder particles undergo compressive shear deformation in random directions. Hence, RD (rolling direction) and TD (transverse direction) are inconsequential for the milled powder particles. Only texture along the ND (parallel to beam direction) is analyzed. The IPF obtained from the MAUD is compared with the IPF of ASTAR™/PED technique in order to check the reliability of the texture. 3. Results and discussion 3.1. Texture analysis by SAED pattern SAED patterns of ball milled powders of Alloy 617 ODS milled for durations of 0, 2, 4, and 6 h are shown in Fig. 2(a–d), respectively. The SAED pattern was indexed with reference to the ICDD data card No. 00004-0850 corresponding to Ni. Notations i, ii, iii and iv were used to denote the (111), (200), (220) and (311) planes of Ni, respectively, in Fig. 2(a), which corresponds to the 0 h milled (unmilled) powder. A careful observation of the SAED pattern reveals an intensity variation along the (111) plane. The strongest intensity positions along the (111) ring of Ni are indicated by arrows in the figure. Assuming that significantly large numbers of crystallites are probed under the SAED aperture, a polycrystalline material would exhibit a Debye ring pattern with uniform intensity. A variation in the intensity of the ring (arcing) signifies orientation preferences for the crystallites or texture. During the water atomization process, a high-pressure water jet impinges on the melt stream for rapid break up and solidification of the powders. Molten metal experiences a compressive force when it is rapidly solidified by pressurized water jet and hence the shear compression texture in the water atomized powder. Texture during rapid solidification of AlSi during melt spin is reported by Delhez et al. [42]. The result is
Fig. 4. Rietveld texture analysis of 0 h (unmilled) Alloy 617-0.6Wt.%Y2O3 - powder using MAUD (a) SAED pattern of unmilled Alloy 617-0.6Wt.%Y2O3 (b) Spectra with 72 images obtained by caking of SAED (c) Corresponding one dimensional pattern (Black-Experimental, Red-Calculated) (d) Two dimensional plot for 72 one dimensional plot with experimental data (bottom) and Fit data (Top).ngated crystallites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4
Materials Characterization 157 (2019) 109883
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Fig. 5. Rietveld texture analysis of 6 h milled Alloy 617-0.6Wt.%Y2O3 - powder using MAUD (a) SAED pattern of 6 h Alloy 617-0.6Wt.%Y2O3 (b) Spectra with 72 images obtained by caking of SAED (c) Corresponding one dimensional pattern (Black-Experimental, Red-Calculated) (d) Two dimensional plot for 72 one dimensional plot with experimental data (bottom) and Fit data (Top). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
interesting and the orientation of a multicrystalline single powder particle needs to be probed in depth since the texture between the crystallites within the particle is likely to affect its consolidation and other physical properties [31,32]. It is seen from Fig. 2(b) that the arcing along the (111) plane increases after 2 h milling. The phenomena get stronger as the milling duration is further increased to 4 and 6 h (Fig. 2(c) and (d)). In fact, the texturing effect along (220) and (311) planes are also seen here along with strong arcing along the (111) planes. Evolution of texture in a single powder particle of milled Alloy 617 ODS is thus clear from the SAED patterns. This warrants a detailed investigation on type of texture and mode of deformation, which is discussed in the subsequent sections. Fig. 3(a) shows the SAED of 6 h milled powder and, dark-field (DF) from (111) plane is shown in Fig. 3(b). DF is obtained by carefully selecting the arcing of (111) plane alone as encircled in Fig. 3(a) and it reveals the presence of elongated and interconnected regions. The elongated crystallites could be signature of texture in the milled powder particles. Similar kind of elongated and interconnected microstructure in the textured Cu-Ag powders has earlier been reported by Guet et.al [3]. Most importantly, the DF micrograph reveals that the material is nanocrystalline having width ~20 to 30 nm and length ~100 nm. In this study, the SAED aperture was always 10 μm ensuring that large numbers of nano-crystallites were enclosed within its field of view.
unmilled powder. The SAED image was digitally caked with an integration step of 5° thus resulting in 72 spectra. The cake #1, Cake #2….. Cake #72, each with an angular span of 5° is shown in Fig. 4(a). Each of these spectra represents the observed intensity for all (hkl) planes within the angular span of the cake. The integration step should be judiciously chosen so as to reflect any variations in SAED pattern. Extremely small step sizes (~1°) increases processing time and 5° appear optimum in picking up all variations in intensity. Relatively, higher angular spans are likely to omit intensity information. Fig. 4(b) shows all of the 72 spectra. In this figure, intensity is plotted as a function of electron diffraction angle (2θ). Since each spectrum spans only 5° out of the full angular span 360°, an increase in intensity will reflect radial intensity variations in the SAED pattern and the location of such a peak would be characteristic of a (hkl). It may be pointed out here that in the absence of texture, all 72 profiles would be flat. Moreover, in the presence of texture, only some of the 72 spectra can show increased intensities. With this understanding, the intensity peaks observed for some of the spectra at around 0.7° imply that (111) plane exhibits texture. All of the 72 spectra over the full 2θ range are Rietveld refined with the calculated FCC-Ni pattern and then summed up to obtain the diffractogram. The summed experimental pattern, its Rietveld fit, and residuals are shown in Fig. 4(c). It is observed that the intensities of experimental one dimensional plot matches fairly well with the Rietveld fit (Rw = 3.5%). Each of the 72 spectrum shown in Fig. 4(b) are Rietveld refined and used to generate experimental and refined 2D plots. Fig. 4(d) shows the 2D intensity distribution for the 72 spectra are plotted as a function of 2θ and the intensity scale is shown at the bottom of the figure. In this figure, the bottom half represents the experimental 2D plot, while the top half represents the Rietveld fitted 2D plot. The broad blue area at the bottom of both the plots arises because of the physical stopper used while imaging the SAED. The 2D
3.2. Texture determination from SAED pattern Rietveld refinement of the SAED patterns for the unmilled (0 h) and 6 h milled powders were carried out using the MAUD application for the analysis of texture during ball milling and discussed here. Fig. 4(a) shows the same SAED pattern as in Fig. 2(a) for the 5
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plot is more reliable to assess the quality of the fit. As discussed in the case of Fig. 4(b), strong intensity concentrations for some of the spectrum numbers imply preferential orientation or texture in the material. As in the case of Fig. 4(b), here too, the intensity maxima appear at 2θ value of 0.7° corresponding to the (111) plane of Ni. It may also be pointed out that only 2 maxima positions are observed at 2θ value of 0.7° as was observed for Fig. 2(a) and indicated thereby arrows. Thus the texture information along the (111) plane could be successfully extracted by this technique. In order to study the development of texture, the Alloy 617-0.6Wt.% Y2O3 powders milled at 1000 rpm for the maximum duration of 6 h was studied using identical methodology as described above for the unmilled powders. Fig. 5(a) shows the SAED pattern of the 6 h milled powder. Fig. 5(b) shows all of the 72 spectra obtained by caking of the SAED image using 5° angular span. Intensity variation in all the planes of the SAED pattern is clearly reflected in the spectra. Fig. 5(c) shows the experimental one dimensional experimental radial distribution profile and Rietveld fit. The fit is considered good (Rw = 8.1%). The 2D plot extracted after carrying out the Rietveld refinement is shown in Fig. 4(d). The bottom half shows the experimental plot while the top half shows the fitted plot. It is interesting to note that there are as many as 5 intensity concentrations for the (111) plane as against 2 in case of the unmilled powder. From the symmetry point of view, there should have been 6 intensity concentrations, but 1 is blocked by the beam stopper and hence could not be recorded. Intensity concentrations are also observed for the (220) and (311) reflections indicating the development of texture during a high energy milling process. With evidence of the growing intensity of texture in the Alloy 617 ODS powders during milling, it is necessary to further explore the nature of texture. MAUD was further used to extract the Pole values from each of the Rietveld refined 72 spectra, discussed earlier, using tuned Le Bail algorithm during every refinement cycle [38]. From the pole values obtained, the orientation distribution function (ODF) can be computed using the Extended Williams-Imhof-Matthies-Vinel EWIMV model [38]. The EWIMV model can then be utilized to compute the texture intensities. It may be mentioned here that EWIMV does not require either harmonic function or matrix operators; it is one of the discrete texture methods. In EWIMV method, the ODF space is divided into small cells and the volume of each cell defines the value of ODF. Pole values are weighted proportionally to the square root of intensity in the algorithm, which makes it different from the Williams-Imhof-Matthies-Vinel (WIMV) method. After the texture refinement , the IPF is generated as a measure of the texture property of the material. Fig. 6(a) and (b) show the SAED generated IPF of 0 h and 6 h milled powders. Texture along 〈110〉//ND (electron beam direction) is observed for both 0 h and 6 h milled powders with strength being higher for the later. 〈110〉 type of texture is a shear compression texture in FCC materials [43]. Thus it is a clear indication of texture development during the milling process. However, a comparison with a standard technique as the PED is required to establish the methodology.
observations from Fig. 6(a), the 0 h milled sample shows a strong texture component of 〈110〉 //ND. Thus these results confirm the reliability of the texture data obtained from SAED analysis. Shear texture in FCC metals may have single or mixture of components {001}〈110〉, {111}〈112〉, and {111}〈110〉 [44]. In the current scenario, the 〈110〉 texture is predominant. 〈110〉 texture during shear compression of oxygen free high conductive copper is reported in the literature by Bhattacharya et.al [43]. Upon milling to 6 h, Fig. 6(b) (SAED) revealed intensification of the 〈110〉 //ND texture while Fig. 8(b) (PED) also shows this type of texture without significant intensification. PED is a more sensitive technique as the PED spot patterns are matched with pre-calculated ED templates generated every 1° (orientation resolution) [20] via cross-correlation matching techniques. In this process of indexing, care is taken in using precise image processing parameters, like spot detection radius, centering pattern and setting noise threshold. It may be mentioned here that if the spot detection radius happens to be greater than half of the shortest reciprocal vector, then the procedure of enhancing the image contrast might delete the relatively weaker spots surrounding the stronger ones, resulting in artefact. Higher sensitivity of the PED technique allowed us to obtain additional minor texture components of 〈100〉 //ND for the 0 h milled sample and 〈112〉 //ND for the 6 h milled sample, are also observed in PED. Presence of the 〈100〉 //ND texture in the unmilled sample, is attributed to solidification texture acquired during its production
3.3. Texture determination by ASTAR™/Precession Electron Diffraction For the detailed analysis of texture, PED studies were carried out on FIB lamellae of the 0 h and 6 h milled powders and the analysis of the results are discussed in this section. Fig. 7(a and b) shows the ASTAR™/PED orientation distribution maps of 0 h and 6 h respectively. Virtual bright field of the mapped region is shown in Fig. 7(c) and (d), respectively. The figures show nanaocrystalline grains oriented along with various directions. Fig. 7(e) shows the IPF colour legend for the ASTAR™/PED. It is clear that {110} (green colour) texture is the most dominant one for the Ni alloy grains, in both h and 6 h milled powders. The corresponding IPF of 0 and 6 h milled powders are shown in Fig. 8(a) and (b), respectively. A one is to one comparison of these figures with IPF obtained from SAED and shown in Figs. 6(a & b) reveal a very good match. Similar to the
Fig. 6. (a–b) Inverse pole figure obtained from MAUD after Texture Refinement of Alloy 617-0.6Wt.%Y2O3 powders of 0 h and 6 h respectively showing 〈110〉 parallel to normal direction (beam direction). 6
Materials Characterization 157 (2019) 109883
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Fig. 7. (a) and (b) represents ASTAR™/PED orientation distribution maps 0 h and 6 h respectively, (c) and (d) virtual bright field of 0 h and 6 h Alloy 617-0.6Wt.% Y2O3, respectively (e) Legend of the IPF map. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
process. Such solidification texture along 〈100〉 direction is reported in the literature for nickel based super alloy during additive manufacturing [45]. However, upon milling for 6 h, the 〈100〉 //ND type of texture is no longer observed. This is attributed to the rotation of crystallites oriented along 〈100〉 to the 〈110〉 fiber texture. Such type of rotation of 〈100〉 component during compression (as in the case of milling) has been reported by Y. Y Zhang et.al in copper [46]. The 〈112〉 //ND present in the 6 h milled sample is attributed to twinning
deformation. After 6 h of high energy ball milling, the powder particles are severely sheared when they are trapped in between the balls and walls of the jars. The powder particles are flattened and also convoluted during the course of the milling by multiple impacts. In this process, the powder undergoes multidirectional inhomogeneous rolling giving rise to shear texture. Twinning is reported to be the most significant mode of deformation for Ni [47] owing to its high stacking fault energy. We have earlier reported [21] the existence of (111) twin during the 7
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be shared at this time as the data also forms part of an ongoing study. Acknowledgement The authors thank Dr. A. K. Bhaduri, Director IGCAR, Dr. G Amarendra, Director MMG and Dr. S. Raju for their constant support and encouragement during the course of this work. Mr. Sivakumar sincerely acknowledges Dr. Mythili and Mr. Pradyumna Kumar Parida for their help in PED experiments and also thanks Dr. Chanchal Ghosh for his valuable suggestions. Authors gratefully acknowledge the experimental support provided by UGC-DAE CSR Kalpakkam node for SEM facility one of the authors (Sivakumar) acknowledges DAE, India for the fellowship. References [1] D. Jariwala, V.K. Sangwan, L.J. Lauhon, T.J. Marks, M.C. Hersam, Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing, Chem. Soc. Rev. 42 (2013) 2824–2860. [2] A. Kamyshny, S. Magdassi, Conductive nanomaterials for printed electronics, Small 10 (2014) 3515–3535. [3] A.D. 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Fig. 8. (a-b) Inverse pole figure obtained from ASTAR™/PED orientation distribution maps of 0 h and 6 h of Alloy 617-0.6Wt.%Y2O3 respectively showing 〈110〉 parallel to normal direction (beam direction).
optimisation of high energy ball parameter in Alloy 617 ODS. 4. Summary and conclusions Local texture analysis on a single nanocrystalline Alloy 617 ODS powder particle, synthesized by high energy ball milling was carried out by Rietveld refinement of SAED pattern from MAUD and also from ASTAR™/PED technique. The above mentioned techniques confirm the existence of 〈110〉 texture in the milled powder and in turn establish the reliability of texture determination from SAED patterns of nanocrystalline materials. Following important conclusions were drawn from these analyses. ➢ From the Rietveld refinement of SAED pattern using MAUD, the local texture in the unmilled and the milled powder was identified as 〈110〉 texture parallel to the normal direction (electron beam direction). ➢ ASTAR™/PED and its IPF confirmed the presence of 〈110〉 texture in both unmilled and milled powder and there is good one to one correspondence to the IPF obtained from SAED Rietveld refinement using MAUD. ➢ Texture in unmilled powder could be attributed to the compressive force during rapid solidification in water atomisation process. ➢ During milling process, inhomogeneous rolling and compression of the trapped powder particle in between the balls and between the balls and walls of the jar resulted in shear compression texture in 6 h milled powder. ➢ ASTAR™/PED technique also confirms the presence of 〈100〉 solidification texture in unmilled powder and 〈112〉 type texture in milled powder due to twin mode of deformation during milling. Data availability The raw/processed data required to reproduce these findings cannot 8
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M. Sivakumar and A. Dasgupta [24] W.-G. Kim, J.-Y. Park, I. Ekaputra, M.-H. Kim, Y.-W. Kim, Analysis of creep behavior of Alloy 617 for use of VHTR system, Procedia Mater. Sci. 3 (2014) 1285–1290. [25] P. Ganesan, G. Smith, D. Yates, Performance of Inconel Alloy 617 in actual and simulated gas turbine environments, Material and manufacturing process 10 (1995) 925–938. [26] K. Murty, I. Charit, Structural materials for Gen-IV nuclear reactors: challenges and opportunities, J. Nucl. Mater. 383 (2008) 189–195. [27] M. Wang, H.N. Han, C.H. Han, W.-G. Kim, J. Jang, Microstructural investigation of oxide dispersion strengthened Alloy 617 after creep rupture at high temperature, Mater. Charact. 139 (2018) 11–18. [28] J. Jang, T.K. Kim, C.H. Han, H.-K. Min, S.-H. Jeong, D.H. Kim, A preliminary development and characterization of Ni-based ODS alloys, Procedia Engineering 55 (2013) 284–288. [29] F. Masoumi, M. Jahazi, D. Shahriari, J. Cormier, Coarsening and dissolution of γ′ precipitates during solution treatment of AD730TM Ni-based superalloy: mechanisms and kinetics models, J. Alloys Compd. 658 (2016) 981–995. [31] J. Kozlík, P. Harcuba, J. Stráský, H. Becker, J. Šmilauerová, M. Janeček, Microstructure and texture formation in commercially pure titanium prepared by cryogenic milling and spark plasma sintering, Mater. Charact. 151 (2019) 1–5. [32] K. Güth, T. Woodcock, L. Schultz, O. Gutfleisch, Comparison of local and global texture in HDDR processed Nd–Fe–B magnets, Acta Mater. 59 (2011) 2029–2034. [33] S. Motozuka, T. Ikeda, T. Miyagawa, H. Sato, M. Morinaga, Formation process of the {001} fiber texture on iron particles using simple ball milling, Powder Technol. 321 (2017) 9–12. [34] N. Chawake, R.S. Varanasi, B. Jaswanth, L. Pinto, S. Kashyap, N. Koundinya, A.K. Srivastav, A. Jain, M. Sundararaman, R.S. Kottada, Evolution of morphology and texture during high energy ball milling of Ni and Ni-5 wt% Cu powders, Mater. Charact. 120 (2016) 90–96. [35] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1–184. [36] L. Lutterotti, S. Matthies, H.-R. Wenk, A. Schultz, J. Richardson Jr., Combined
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Selected Area Electron Diffraction, a technique for determination of crystallographic texture in nanocrystalline powder particle of Alloy 617 ODS and comparison with Precession Electron Diffraction
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M. Sivakumar, Arup Dasgupta
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Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam, Tamil Nadu, India
ARTICLE INFO
ABSTRACT
Keywords: Local texture analysis Nanocrystalline Individual powder particle
Texture analysis in nanocrystalline materials is a challenge. A simple technique as Selected Area Electron Diffraction (SAED) has been used to evaluate texture in a single nanocrystalline Alloy 617 ODS powder particle by carrying out Rietveld texture refinement using Material Analysis Using Diffraction (MAUD) software package and the results were compared with the ASTAR™/Precision Electron Diffraction (ASTAR™/PED) results. Extraction of texture from the SAED patterns involved image analysis and Rietveld refinement of radial plots divided into judiciously chosen angular spans. Inverse pole figure (IPF) from SAED pattern using MAUD revealed 〈110〉 texture parallel to the normal direction (electron beam direction) for unmilled and 6 h milled powders. IPF from ASTAR™/PED technique also shown 〈110〉 lying parallel to the normal direction (beam direction) in both unmilled and the 6 h milled powder. A one is to one correspondence of IPF obtained from MAUD to that of ASTAR™/PED was observed. Texture in unmilled powder could be attributed to the compressive force during rapid solidification in water atomisation process. During the milling process, inhomogeneous rolling and compression of the trapped powder particle in between the balls and between the balls and walls of the jar resulted in shear texture in 6 h milled powder. ASTAR™/PED technique also showed the presence of 〈100〉 //ND texture which was attributed to solidification in unmilled powder and 〈112〉 //ND type texture in milled powder attributed to twin mode of deformation during milling.
1. Introduction Crystallographic texture in bulk materials is well reported in the literature, but crystallographic texture in nanocrystalline materials is relatively scarce. This is mainly because the methodology to analyse the texture at such length scale is quite complex since the volume available for diffraction in locally textured particle is very small, extending only up to few micrometres. With the advancement of nanostructured materials in almost all fields of application from electronics [1–3] to structural materials [4,5] the role of texture can no longer be ignored [6–8] and simpler techniques are required to quickly assess the texture property. Transmission electron diffraction pattern using selected area apertures measuring few micrometer is a simple but promising technique [9]. Characterization of texture in nanometre scale can be carried out by various techniques such as Transmission Kikuchi Diffraction (TKD) [10], ASTAR™/Precision Electron Diffraction (ASTAR™/PED) [7], and Selected Area Electron Diffraction (SAED) [11]. Transmission Kikuchi
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diffraction (TKD), also known as transmission-EBSD (t-EBSD) can achieve spatial resolution of ~2 nm [12]. But small step size and long dwell time would result in very long acquisition time and possible specimen drift. On the other hand, ASTAR™/PED in a Transmission Electron Microscope (TEM), offers spatial resolution ~2 nm or even better, depending on the resolution of the TEM itself in the diffraction contrast mode [13]. This technique involves in automatic collection of spot diffraction pattern and cross correlating the experimental diffraction patterns with the pre- calculated diffraction patterns known as templates. Though it is similar to EBSD technique, the major difference lies in the elimination of the dynamical effect of the TEM diffraction by precessing the electron beam and approaching a quasi-kinematical condition [7,14]. Texture characterization in ultrafine-grained commercially pure titanium alloys have been reported by Iman Ghamerian et.al [6] and Dasgupta et al. [15] using the PED technique. On the other hand, Selected Area Electron Diffraction technique (SAED) may also be used to extract local texture in materials. Zghal et al. reported deformation texture in Cu-Ag ball milled powders by
Corresponding author. E-mail address: [email protected] (A. Dasgupta).
https://doi.org/10.1016/j.matchar.2019.109883 Received 25 July 2019; Received in revised form 22 August 2019; Accepted 22 August 2019 Available online 23 August 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 157 (2019) 109883
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analyzing the SAED pattern of powders [16,17]. Boulay et.al used SAED pattern of TiO2, and Mn3O4 nanocrystal aggregates and platinum films and carried out Rietveld refinement using MAUD (Material Analysis Using Diffraction) in which textural refinement was incorporated [9]. MAUD is free but powerful multipurpose Rietveld analysis program, easy to use with entire actions controlled by GUI and works with X-ray, synchrotron, Neutron, and electron diffraction data. Quantitative analysis, Texture analysis, Residual stress analysis can be carried out with MAUD. Boulay et al. [9] used MAUD for the illustration of quantitative texture analysis from the SAED pattern. However, it needs to be mentioned here that SAED employs a parallel beam of electrons in line with the optic axis of the TEM. This is in contrast to the convergent electron beam diffraction (CBED) and the PED techniques. CBED technique uses high angle of convergence (α) for the electron beam, which is usually of the order of the Bragg diffraction angle. The conical beam results in the formation of diffraction disks [18,19]. The CBED technique yields higher spatial resolution as well as much higher angular accuracy. The technique is used to extract detailed structural information of a crystal, viz., point and space-group symmetry details, lattice parameters, defects in crystals and three-dimensional information about the reciprocal lattice. In comparison, the PED technique, discussed earlier, employs a near parallel beam using a very
Fig. 1. Mechanically milled powder particles comprising several crystallites with different orientation.
Fig. 2. (a–d) TEM micrograph showing SAED of 0, 2, 4 and 6 h ball milled Alloy 617-0.6Wt.%Y2O3 powders respectively, at 1000 rpm with variation of intensity along the ring indicating preferred orientation. 2
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small condenser aperture (~50 μm) (very small α). This beam is tilted and rocked (precessed) and then also derocked for analysis. The quasikenimatical condition achieved in PED technique allows structure determination with good accuracy [20]. In this paper we have studied texture analysis from SAED patterns of milled Alloy 617 ODS [21] and also compared the results with those obtained by using PED technique. Alloy 617 ODS is a variant of Nickel based superalloy, a candidate structural material for gas turbines and heat exchange applications for Generation IV nuclear reactors [22] and in the boilers, super heaters and rotors of advanced ultra-supercritical (AUSC) thermal power plants [23]. In addition to its intermediate temperature strengthening by gamma prime precipitates and solid solution strengthening at room temperature [24–26] in the Alloy 617, the ODS variant offers excellent high-temperature mechanical properties imparted by hard and stable oxide particles dispersed in the 617 matrix and the material may be called Alloy 617 ODS [27,28]. In the Alloy 617, dissolution of the gamma prime precipitates occurs at temperatures above 900 °C [29], whereas the Alloy 617 ODS is expected to retain its mechanical strength even up to 1000 °C [22]. However, the Alloy 617 ODS must be synthesized by high energy ball milling process in which nano-sized Y2O3 particles are mechanically milled together with the prealloyed Alloy 617 powder [21]. During the high energy ball milling process, severe plastic deformation processes occur in the material which could result in dislocations, lattice rotation, local lattice parameter variations, grain fragmentation as well as local texture (shear compression type) development in the powder particles [16]. An individual powder particle may be comprised of a large number of nanocrystallites (schematically shown in Fig. 1) which could have a preference of orientation. Texture in individual Ti powder particles is reported by Jiří Kozlík et al. [31]. Such local texture development could affect the consolidation characteristics as well as physical properties of the powder material significantly. Gueth et al. reported the dependence of magnetic property on local texture of Nd–Fe–B powder particle [32]. Satoshi Motozukha et al. reported double-fiber texture ({001} + {111}) in conventional ball milled iron powder particle due to uniaxial compression and multidirectional rolling which affect its magnetic properties [33]. Ravi Sankar at.al characterized the texture on the high energy ball milled Ni powder particle and reported a {001} type of texture in the material with slip and twinning influencing the texture development [34]. It may be noted here that it is essential to limit the study to a single nanocrystalline powder particle, as every particle is independent. In fact, XRD study on a large number of such particles would actually reveal a typical polycrystalline powder pattern. As indicated above, this study mainly concentrates on texture development on a single particle, an example of typical nanocrystalline material. TEM thin lamella is picked up by FIB (Focussed Ion Beam in SEM) from a single representative powder particle and SAED patterns have been used for extracting texture information. The reliability of the texture data evaluated this way is verified by comparing with those obtained from ASTAR™/PED technique.
Microscopy (TEM) specimen from the milled powder by FIB lift-out technique with Ga + ions. The FIB lamellae were thinned down to 100 nm. In order to remove any FIB induced amorphization layer and Ga implantations during FIB process, lamellae were cleaned at ion acceleration voltages from 5 kV to 1 kV. Philips make CM-200 Analytical TEM operated at 200 kV attached with TVIPS make 2 K × 2 K CCD camera was used for characterization of the FIB lamellae. ImageJ software was used for the analysis of the TEM results. SAED patterns from single powder particles were analyzed by carrying out Rietveld refinement using the Java based MAUD application software [36] using the following methodology. Concentric rings of the SAED pattern were integrated over the full radial span of 360° using ImageJ plug-in within MAUD. Two- dimensional diffraction data were thus extracted. The original image coordinate positions for each data point were preserved in the process. In this technique, it is essential to
2. Experimental methods In the current study, Inconel 617 prealloyed water atomized powders procured from M/S Padmasree enterprises of particle size ± 100 μm were used. High energy ball milling of prealloyed powder with 0.6Wt% of cubic Y2O3 nano powder was conducted in Emax system from Retsch, using milling speed 2000 rpm fitted with water-cooled chiller. Ball to powder weight ratio was maintained at 10:1 and stainless steel balls of 5 mm diameter were used for the alloying process. Stearic acid was used as a process controlling agent (PCA) in order to avoid cold welding of powders to the balls and walls of the jars during the high energy ball milling [35]. FEI make Helios NanoLab-600i dual beam Field Emission Scanning Electron Microscope (SEM) was used to prepare Transmission Electron
Fig. 3. (a) TEM micrograph showing SAED of 6 h ball milled Alloy 617-0.6Wt.% Y2O3 powder at 1000 rpm, (b) Dark Field images from circled arc of (111) plane showing continuous elongated crystallites. 3
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designate the beam center, the azimuth range and the increment of integration during the processing of the image in MAUD. A file with .esg extension was generated at the completion of integration which was the required input file for Rietveld refinement together with the suitable CIF (Crystallographic information file). The CIF may either be generated from MAUD using crystal structure, and space group obtained from XRD data or alternatively accessed from the online databases using fullprofile search match-method [37], in which phase is identified from integrated intensities along with electron diffraction pattern. After loading the .esg file and the CIF along with proper instrumental parameters such as intensity calibration, angular calibration, the geometry of measurement, source and detector distances, the Rietveld refinement can finally be carried out. Refinement includes parameters such as scale factor, background factor, microstructure parameter, profile parameter and so on. After the refinement of above mentioned parameters, a texture model such as Extended Williams-Imhof-Matthies-Vinel (EWIMV) incorporated within MAUD can be used for extraction of texture data viz., pole figure and inverse pole figure (IPF) [38]. In addition, Orientation Microscopy in TEM was also carried out using ASTAR™/PED from Nanomegas. The Precision angle of the electron beam was 1° with respect to the optic axis. Acquired diffraction patterns were indexed with the simulated diffraction pattern corresponding to Nickel with the space group Fm-3 m and texture data were extracted using the ASTAR software. For absolute orientations of grains, sample reference frame calibration is usually carried out by finding the following: (a) rotation between an image and its diffraction pattern, and (b) aligning ASTAR reference frame to TSL-OIM (wherein the texture analysis is carried out) reference frame [39–41]. The rotation correction arising out of mismatch between image and its diffraction pattern is corrected by means of aligning the bright field image and the virtual bright field image from PED. The Correction with regard to the
frames of reference is insignificant for ball milled powders, wherein the powder particles undergo compressive shear deformation in random directions. Hence, RD (rolling direction) and TD (transverse direction) are inconsequential for the milled powder particles. Only texture along the ND (parallel to beam direction) is analyzed. The IPF obtained from the MAUD is compared with the IPF of ASTAR™/PED technique in order to check the reliability of the texture. 3. Results and discussion 3.1. Texture analysis by SAED pattern SAED patterns of ball milled powders of Alloy 617 ODS milled for durations of 0, 2, 4, and 6 h are shown in Fig. 2(a–d), respectively. The SAED pattern was indexed with reference to the ICDD data card No. 00004-0850 corresponding to Ni. Notations i, ii, iii and iv were used to denote the (111), (200), (220) and (311) planes of Ni, respectively, in Fig. 2(a), which corresponds to the 0 h milled (unmilled) powder. A careful observation of the SAED pattern reveals an intensity variation along the (111) plane. The strongest intensity positions along the (111) ring of Ni are indicated by arrows in the figure. Assuming that significantly large numbers of crystallites are probed under the SAED aperture, a polycrystalline material would exhibit a Debye ring pattern with uniform intensity. A variation in the intensity of the ring (arcing) signifies orientation preferences for the crystallites or texture. During the water atomization process, a high-pressure water jet impinges on the melt stream for rapid break up and solidification of the powders. Molten metal experiences a compressive force when it is rapidly solidified by pressurized water jet and hence the shear compression texture in the water atomized powder. Texture during rapid solidification of AlSi during melt spin is reported by Delhez et al. [42]. The result is
Fig. 4. Rietveld texture analysis of 0 h (unmilled) Alloy 617-0.6Wt.%Y2O3 - powder using MAUD (a) SAED pattern of unmilled Alloy 617-0.6Wt.%Y2O3 (b) Spectra with 72 images obtained by caking of SAED (c) Corresponding one dimensional pattern (Black-Experimental, Red-Calculated) (d) Two dimensional plot for 72 one dimensional plot with experimental data (bottom) and Fit data (Top).ngated crystallites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4
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Fig. 5. Rietveld texture analysis of 6 h milled Alloy 617-0.6Wt.%Y2O3 - powder using MAUD (a) SAED pattern of 6 h Alloy 617-0.6Wt.%Y2O3 (b) Spectra with 72 images obtained by caking of SAED (c) Corresponding one dimensional pattern (Black-Experimental, Red-Calculated) (d) Two dimensional plot for 72 one dimensional plot with experimental data (bottom) and Fit data (Top). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
interesting and the orientation of a multicrystalline single powder particle needs to be probed in depth since the texture between the crystallites within the particle is likely to affect its consolidation and other physical properties [31,32]. It is seen from Fig. 2(b) that the arcing along the (111) plane increases after 2 h milling. The phenomena get stronger as the milling duration is further increased to 4 and 6 h (Fig. 2(c) and (d)). In fact, the texturing effect along (220) and (311) planes are also seen here along with strong arcing along the (111) planes. Evolution of texture in a single powder particle of milled Alloy 617 ODS is thus clear from the SAED patterns. This warrants a detailed investigation on type of texture and mode of deformation, which is discussed in the subsequent sections. Fig. 3(a) shows the SAED of 6 h milled powder and, dark-field (DF) from (111) plane is shown in Fig. 3(b). DF is obtained by carefully selecting the arcing of (111) plane alone as encircled in Fig. 3(a) and it reveals the presence of elongated and interconnected regions. The elongated crystallites could be signature of texture in the milled powder particles. Similar kind of elongated and interconnected microstructure in the textured Cu-Ag powders has earlier been reported by Guet et.al [3]. Most importantly, the DF micrograph reveals that the material is nanocrystalline having width ~20 to 30 nm and length ~100 nm. In this study, the SAED aperture was always 10 μm ensuring that large numbers of nano-crystallites were enclosed within its field of view.
unmilled powder. The SAED image was digitally caked with an integration step of 5° thus resulting in 72 spectra. The cake #1, Cake #2….. Cake #72, each with an angular span of 5° is shown in Fig. 4(a). Each of these spectra represents the observed intensity for all (hkl) planes within the angular span of the cake. The integration step should be judiciously chosen so as to reflect any variations in SAED pattern. Extremely small step sizes (~1°) increases processing time and 5° appear optimum in picking up all variations in intensity. Relatively, higher angular spans are likely to omit intensity information. Fig. 4(b) shows all of the 72 spectra. In this figure, intensity is plotted as a function of electron diffraction angle (2θ). Since each spectrum spans only 5° out of the full angular span 360°, an increase in intensity will reflect radial intensity variations in the SAED pattern and the location of such a peak would be characteristic of a (hkl). It may be pointed out here that in the absence of texture, all 72 profiles would be flat. Moreover, in the presence of texture, only some of the 72 spectra can show increased intensities. With this understanding, the intensity peaks observed for some of the spectra at around 0.7° imply that (111) plane exhibits texture. All of the 72 spectra over the full 2θ range are Rietveld refined with the calculated FCC-Ni pattern and then summed up to obtain the diffractogram. The summed experimental pattern, its Rietveld fit, and residuals are shown in Fig. 4(c). It is observed that the intensities of experimental one dimensional plot matches fairly well with the Rietveld fit (Rw = 3.5%). Each of the 72 spectrum shown in Fig. 4(b) are Rietveld refined and used to generate experimental and refined 2D plots. Fig. 4(d) shows the 2D intensity distribution for the 72 spectra are plotted as a function of 2θ and the intensity scale is shown at the bottom of the figure. In this figure, the bottom half represents the experimental 2D plot, while the top half represents the Rietveld fitted 2D plot. The broad blue area at the bottom of both the plots arises because of the physical stopper used while imaging the SAED. The 2D
3.2. Texture determination from SAED pattern Rietveld refinement of the SAED patterns for the unmilled (0 h) and 6 h milled powders were carried out using the MAUD application for the analysis of texture during ball milling and discussed here. Fig. 4(a) shows the same SAED pattern as in Fig. 2(a) for the 5
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plot is more reliable to assess the quality of the fit. As discussed in the case of Fig. 4(b), strong intensity concentrations for some of the spectrum numbers imply preferential orientation or texture in the material. As in the case of Fig. 4(b), here too, the intensity maxima appear at 2θ value of 0.7° corresponding to the (111) plane of Ni. It may also be pointed out that only 2 maxima positions are observed at 2θ value of 0.7° as was observed for Fig. 2(a) and indicated thereby arrows. Thus the texture information along the (111) plane could be successfully extracted by this technique. In order to study the development of texture, the Alloy 617-0.6Wt.% Y2O3 powders milled at 1000 rpm for the maximum duration of 6 h was studied using identical methodology as described above for the unmilled powders. Fig. 5(a) shows the SAED pattern of the 6 h milled powder. Fig. 5(b) shows all of the 72 spectra obtained by caking of the SAED image using 5° angular span. Intensity variation in all the planes of the SAED pattern is clearly reflected in the spectra. Fig. 5(c) shows the experimental one dimensional experimental radial distribution profile and Rietveld fit. The fit is considered good (Rw = 8.1%). The 2D plot extracted after carrying out the Rietveld refinement is shown in Fig. 4(d). The bottom half shows the experimental plot while the top half shows the fitted plot. It is interesting to note that there are as many as 5 intensity concentrations for the (111) plane as against 2 in case of the unmilled powder. From the symmetry point of view, there should have been 6 intensity concentrations, but 1 is blocked by the beam stopper and hence could not be recorded. Intensity concentrations are also observed for the (220) and (311) reflections indicating the development of texture during a high energy milling process. With evidence of the growing intensity of texture in the Alloy 617 ODS powders during milling, it is necessary to further explore the nature of texture. MAUD was further used to extract the Pole values from each of the Rietveld refined 72 spectra, discussed earlier, using tuned Le Bail algorithm during every refinement cycle [38]. From the pole values obtained, the orientation distribution function (ODF) can be computed using the Extended Williams-Imhof-Matthies-Vinel EWIMV model [38]. The EWIMV model can then be utilized to compute the texture intensities. It may be mentioned here that EWIMV does not require either harmonic function or matrix operators; it is one of the discrete texture methods. In EWIMV method, the ODF space is divided into small cells and the volume of each cell defines the value of ODF. Pole values are weighted proportionally to the square root of intensity in the algorithm, which makes it different from the Williams-Imhof-Matthies-Vinel (WIMV) method. After the texture refinement , the IPF is generated as a measure of the texture property of the material. Fig. 6(a) and (b) show the SAED generated IPF of 0 h and 6 h milled powders. Texture along 〈110〉//ND (electron beam direction) is observed for both 0 h and 6 h milled powders with strength being higher for the later. 〈110〉 type of texture is a shear compression texture in FCC materials [43]. Thus it is a clear indication of texture development during the milling process. However, a comparison with a standard technique as the PED is required to establish the methodology.
observations from Fig. 6(a), the 0 h milled sample shows a strong texture component of 〈110〉 //ND. Thus these results confirm the reliability of the texture data obtained from SAED analysis. Shear texture in FCC metals may have single or mixture of components {001}〈110〉, {111}〈112〉, and {111}〈110〉 [44]. In the current scenario, the 〈110〉 texture is predominant. 〈110〉 texture during shear compression of oxygen free high conductive copper is reported in the literature by Bhattacharya et.al [43]. Upon milling to 6 h, Fig. 6(b) (SAED) revealed intensification of the 〈110〉 //ND texture while Fig. 8(b) (PED) also shows this type of texture without significant intensification. PED is a more sensitive technique as the PED spot patterns are matched with pre-calculated ED templates generated every 1° (orientation resolution) [20] via cross-correlation matching techniques. In this process of indexing, care is taken in using precise image processing parameters, like spot detection radius, centering pattern and setting noise threshold. It may be mentioned here that if the spot detection radius happens to be greater than half of the shortest reciprocal vector, then the procedure of enhancing the image contrast might delete the relatively weaker spots surrounding the stronger ones, resulting in artefact. Higher sensitivity of the PED technique allowed us to obtain additional minor texture components of 〈100〉 //ND for the 0 h milled sample and 〈112〉 //ND for the 6 h milled sample, are also observed in PED. Presence of the 〈100〉 //ND texture in the unmilled sample, is attributed to solidification texture acquired during its production
3.3. Texture determination by ASTAR™/Precession Electron Diffraction For the detailed analysis of texture, PED studies were carried out on FIB lamellae of the 0 h and 6 h milled powders and the analysis of the results are discussed in this section. Fig. 7(a and b) shows the ASTAR™/PED orientation distribution maps of 0 h and 6 h respectively. Virtual bright field of the mapped region is shown in Fig. 7(c) and (d), respectively. The figures show nanaocrystalline grains oriented along with various directions. Fig. 7(e) shows the IPF colour legend for the ASTAR™/PED. It is clear that {110} (green colour) texture is the most dominant one for the Ni alloy grains, in both h and 6 h milled powders. The corresponding IPF of 0 and 6 h milled powders are shown in Fig. 8(a) and (b), respectively. A one is to one comparison of these figures with IPF obtained from SAED and shown in Figs. 6(a & b) reveal a very good match. Similar to the
Fig. 6. (a–b) Inverse pole figure obtained from MAUD after Texture Refinement of Alloy 617-0.6Wt.%Y2O3 powders of 0 h and 6 h respectively showing 〈110〉 parallel to normal direction (beam direction). 6
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Fig. 7. (a) and (b) represents ASTAR™/PED orientation distribution maps 0 h and 6 h respectively, (c) and (d) virtual bright field of 0 h and 6 h Alloy 617-0.6Wt.% Y2O3, respectively (e) Legend of the IPF map. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
process. Such solidification texture along 〈100〉 direction is reported in the literature for nickel based super alloy during additive manufacturing [45]. However, upon milling for 6 h, the 〈100〉 //ND type of texture is no longer observed. This is attributed to the rotation of crystallites oriented along 〈100〉 to the 〈110〉 fiber texture. Such type of rotation of 〈100〉 component during compression (as in the case of milling) has been reported by Y. Y Zhang et.al in copper [46]. The 〈112〉 //ND present in the 6 h milled sample is attributed to twinning
deformation. After 6 h of high energy ball milling, the powder particles are severely sheared when they are trapped in between the balls and walls of the jars. The powder particles are flattened and also convoluted during the course of the milling by multiple impacts. In this process, the powder undergoes multidirectional inhomogeneous rolling giving rise to shear texture. Twinning is reported to be the most significant mode of deformation for Ni [47] owing to its high stacking fault energy. We have earlier reported [21] the existence of (111) twin during the 7
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be shared at this time as the data also forms part of an ongoing study. Acknowledgement The authors thank Dr. A. K. Bhaduri, Director IGCAR, Dr. G Amarendra, Director MMG and Dr. S. Raju for their constant support and encouragement during the course of this work. Mr. Sivakumar sincerely acknowledges Dr. Mythili and Mr. Pradyumna Kumar Parida for their help in PED experiments and also thanks Dr. Chanchal Ghosh for his valuable suggestions. Authors gratefully acknowledge the experimental support provided by UGC-DAE CSR Kalpakkam node for SEM facility one of the authors (Sivakumar) acknowledges DAE, India for the fellowship. References [1] D. Jariwala, V.K. Sangwan, L.J. Lauhon, T.J. Marks, M.C. Hersam, Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing, Chem. Soc. Rev. 42 (2013) 2824–2860. [2] A. Kamyshny, S. Magdassi, Conductive nanomaterials for printed electronics, Small 10 (2014) 3515–3535. [3] A.D. Franklin, Nanomaterials in transistors: from high-performance to thin-film applications, Science 349 (2015) aab2750. [4] I. Beyerlein, A. Caro, M. Demkowicz, N. Mara, A. Misra, B. Uberuaga, Radiation damage tolerant nanomaterials, Mater. Today 16 (2013) 443–449. [5] K. Raghavendra, A. Dasgupta, C. Ghosh, K. Jayasankar, V. Srihari, S. Saroja, Development of a novel ZrO2 dispersion strengthened 9Cr ferritic steel: characterization of milled powder and subsequent annealing behavior, Powder Technol. 327 (2018) 267–274. [6] I. Ghamarian, P. Samimi, A. Telang, V. Vasudevan, P.C. Collins, Characterization of the near-surface nanocrystalline microstructure of ultrasonically treated Ti-6Al-4V using ASTARTM/precession electron diffraction technique, Mater. Sci. Eng. A 688 (2017) 524–531. [7] E.F. Rauch, J. Portillo, S. Nicolopoulos, D. Bultreys, S. Rouvimov, P. 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Fig. 8. (a-b) Inverse pole figure obtained from ASTAR™/PED orientation distribution maps of 0 h and 6 h of Alloy 617-0.6Wt.%Y2O3 respectively showing 〈110〉 parallel to normal direction (beam direction).
optimisation of high energy ball parameter in Alloy 617 ODS. 4. Summary and conclusions Local texture analysis on a single nanocrystalline Alloy 617 ODS powder particle, synthesized by high energy ball milling was carried out by Rietveld refinement of SAED pattern from MAUD and also from ASTAR™/PED technique. The above mentioned techniques confirm the existence of 〈110〉 texture in the milled powder and in turn establish the reliability of texture determination from SAED patterns of nanocrystalline materials. Following important conclusions were drawn from these analyses. ➢ From the Rietveld refinement of SAED pattern using MAUD, the local texture in the unmilled and the milled powder was identified as 〈110〉 texture parallel to the normal direction (electron beam direction). ➢ ASTAR™/PED and its IPF confirmed the presence of 〈110〉 texture in both unmilled and milled powder and there is good one to one correspondence to the IPF obtained from SAED Rietveld refinement using MAUD. ➢ Texture in unmilled powder could be attributed to the compressive force during rapid solidification in water atomisation process. ➢ During milling process, inhomogeneous rolling and compression of the trapped powder particle in between the balls and between the balls and walls of the jar resulted in shear compression texture in 6 h milled powder. ➢ ASTAR™/PED technique also confirms the presence of 〈100〉 solidification texture in unmilled powder and 〈112〉 type texture in milled powder due to twin mode of deformation during milling. Data availability The raw/processed data required to reproduce these findings cannot 8
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