University, Akita , Japan b Akita Prefecture Industrial Technology Center, Akita , Japan Published online: 21 Feb 2007.

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1 This article was downloaded by: [ECU Libraries] On: 10 October 2014, At: 01:21 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Philosophical Magazine Publication details, including instructions for authors and subscription information: Dissolution patterns caused by chemical etching of Al Co Cu and Al Co Ni decagonal quasicrystals with a HF HNO 3 H 2 O solution Kaichi Saito a, Yûsuke Saito a, Shigeo Sugawara a, Ryoetsu Shindo b, Jung Qung Guo c & An Pang Tsai c a Department of Materials Science and Engineering, Akita University, Akita , Japan b Akita Prefecture Industrial Technology Center, Akita , Japan c National Institute for Materials Science, Tsukuba , Japan Published online: 21 Feb To cite this article: Kaichi Saito, Yûsuke Saito, Shigeo Sugawara, Ryoetsu Shindo, Jung Qung Guo & An Pang Tsai (2004) Dissolution patterns caused by chemical etching of Al Co Cu and Al Co Ni decagonal quasicrystals with a HF HNO 3 H 2 O solution, Philosophical Magazine, 84:10, , DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.

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3 Philosophical Magazine, 1 April 2004 Vol. 84, No. 10, Dissolution patterns caused by chemical etching of Al Co Cu and Al Co Ni decagonal quasicrystals with a HF HNO 3 H 2 O solution Kaichi Saitoy, Yu suke Saito, Shigeo Sugawara Department of Materials Science and Engineering, Akita University, Akita , Japan Ryoetsu Shindo Akita Prefecture Industrial Technology Center, Akita , Japan Jung Qung Guo and An PangTsai National Institute for Materials Science, Tsukuba , Japan [Received 13 May 2003 and accepted in revised form 28 July 2003] Abstract Dissolution patterns essential for Al Co Cu and Al Co Ni decagonal quasicrystals (d-qcs) have been investigated in detail by chemical etching using a HF HNO 3 H 2 O solution followed by scanning electron microscopy (SEM) observations. The chemical etching of definite surface areas of the samples, which are either normal or parallel to the tenfold axes, using a solution with HF HNO 3 H 2 O (1 : 5 : 4 in volume ratio; 0 C; 5 10 min), produces a large number of microfacet pits of decagonal prismatic shape. In addition, the same etching test in combination with SEM observations was carried out on a deformed sample of the Al Co Ni d-qc, which had been subjected to concentrated mechanical stress at an elevated temperature of 850 C by means of the Vickers indentation technique. The morphological features of the resulting micropits and their possible origins are discussed on the basis of results obtained by SEM observations. } 1. Introduction Over the past few decades, various chemical or electrochemical etching methods have been commonly utilized to inspect lattice defects as well as inhomogeneities present in crystalline solids. The related techniques are all based on the essential phenomenon that various defect sites that emerge on a crystalline surface dissolve more rapidly than the surrounding matrix in an appropriately prepared etching solution, thus allowing the formation of crystallographic microfacet pits. Morphological features of the resulting pits strongly depend on the following factors: the etching procedure employed, crystallographic orientation of the solid matter and geometry of the existing lattice defects. Although there are large amounts of experimental data available on the dissolution behaviours of crystalline materials, the data for y Author for correspondence. ksaito@ipc.akita-u.ac.jp. Philosophical Magazine ISSN print/issn online # 2004 Taylor & Francis Ltd DOI: /

4 1012 K. Saito et al. quasicrystals (QCs) are still lacking. In the course of the present study, we developed effective etching methods to produce essential dissolution patterns of Al Pd Mn icosahedral quasicrystals (i-qcs) and Al Co Cu decagonal quasicrystals (d-qcs) (Saito et al. 2000a, b, c, 2002). For instance, electrochemical etching of the Al Pd Mn i-qc with a CH 3 OH HNO 3 solution has been found to cause a quasicrystallographic dissolution pattern on the etched surface, namely a number of definitely shaped microfacet pits, showing various sections of a rhombic triacontahedron. More recently, the same type of dissolution pattern was reported in a Zn Mg Y i-qc elsewhere (Suchodolskis et al. 2002). Detailed studies of dissolution patterns for a broad range of other crystalline materials, indeed, have never before resulted in such a unique morphology as found for the Al Pd Mn and Zn Mg Y i-qcs (Saito et al. 2000a, b, c, 2002, Suchodolskis et al. 2002). However, a controversy still exists as to what the origins for these microfacet pits are and, further, which types of structural inhomogeneity present in the quasicrystalline materials could become active nucleation centres for pitting. One of the most probable origins for pitting is the intersection of existing dislocation with the surface, as commonly observed for conventional crystalline solids. However, this has not yet been experimentally evidenced in quasicrystalline materials. Careful investigations of dissolution behaviours for different types of quasicrystalline sample, that have the same alloy composition but markedly different dislocation densities, is therefore an essential prerequisite for resolving the problem addressed above. To date, the dissolution behaviour of d-qcs has not been fully examined (Saito et al. 2002). In the present investigation, two different single grains of Al Co Cu and Al Co Ni d-qcs were selected for the following etching study. In addition, the etching test was carried out on a deformed Al Co Ni d-qc, which had been subjected to indentation testing at the elevated temperature of 850 C. The dissolution patterns resulting from the as-grown and as-indented samples were carefully examined by means of scanning electron microscopy (SEM), in order to explore the morphological features and also to find whether there existed any relations between indentation-induced defects and the resultant etch figures. } 2. Experimental details Large single grains of Al 66 Co 17 Cu 17 and Al 72 Co 16 Ni 12 d-qcs were grown by the Bridgman or floating-zone method. The growth methods of these quasicrystalline alloys have been detailed elsewhere (Guo et al. 1999, Sato et al. 1998). Their structural qualities were checked by selected-area electron diffraction (SAD) and high-resolution electron microscopy (HREM) using a 200 kv transmission electron microscopy (TEM) instrument (JEOL JEM-2010). Prior to etching experiments, some definite surface areas of interest were finished by mechanical polishing with alumina abrasives finer to a 0.1 mm size. Then, chemical etching was made by using a solution consisting of 47% HF 61% HNO 3 H 2 O (4 : 5 : 1 in volume ratio) at 0 C for various lengths of times between 5 and 10 min (Saito et al. 2002). During etching, special attention was paid constantly to stirring the etchant containing the test sample. Without this operation, the etching did not always yield a reproducible dissolution pattern. After the etching experiment, the sample was washed with distilled water and dried in a N 2 gas blast. The etch figures formed on the definite surface areas of the samples, which are either normal or parallel to the tenfold axes, were mainly targeted for observations by scanning electron microscope (JEOL JSM-5900LV). The SEM instrument was

5 Chemically etched dissolution patterns 1013 operated at an accelerating voltage of 20 kv and then the topographic contrast images of the etched surfaces were obtained using either secondary electrons (SEs) or back-scattered electrons (BEs). Additionally, the same etching test in combination with SEM observations was arranged for a deformed sample of the Al Co Ni d-qc. For this purpose, using a Vickers hardness testing instrument (Akashi AVK-HF), a concentrated load of 5 kgf was applied on surface areas normal to the tenfold axis of the Al Co Ni d-qc (designated as a tenfold surface ) at an elevated temperature of 850 C in an argon atmosphere. The load was kept at the maximum value for 15 s. It is widely accepted now that many quasicrystalline materials are hard and brittle at room temperature but become plastic at elevated temperatures above approximately ( )T m (T m is the melting point) (for example Wolf et al. (2001)). Evidently, dislocation glide is a responsible cause for the plastic deformation in QCs (see for example Wollgarten et al. (1995)), as generally explained for ordinary crystals. Yan and Wang (1993) have proven by TEM observations that a mechanical stress applied for an Al 70 Co 15 Ni 15 d-qc at approximately 800 C causes a large increase in dislocations as well as stacking faults. In consideration of these findings, we set the test temperature at 850 C, which is apparently high enough to introduce substantial amounts of dislocations as well as other defects in the quasicrystalline sample. In reality, the highest temperature attained by the sample may have been somewhat lower than 850 C, since a thermocouple attached to the instrument did not directly monitor temperatures of the sample but those of a sample stage to mount the sample on its top. } 3. Results and discussion 3.1. General etch figures of Al Co Cu and Al Co Ni decagonal quasicrystals Figure 1 shows two pairs each consisting of a SAD pattern and corresponding HREM image, which were obtained from an Al Co Cu d-qc (figures 1 (a) and (b)) and an Al Co Ni d-qc (figures 1 (c) and (d)). The SAD patterns in both figure 1 (a) and figure 1 (c), which resemble each other in their intensity distributions, have a set of strong reflection spots present at almost exact tenfold positions, indicating good long-range quasiperiodic order of their atomic arrangements. The SAD patterns of this type, which has scaling in the intensity distribution without any superlattice reflections, represent, as generally explained, the so-called basic d-qc structure characterized by a standard pentagonal Penrose lattice (for example, Hiraga et al. (2001)). The corresponding HREM images shown in figures 1 (b) and (d) were recorded in such conditions that unique ring contrasts, each of which represents a fundamental columnar atom cluster, were particularly enhanced. It is, therefore, found that the HREM images are both characterized by aperiodic arrangements of these atom clusters. Curiously, the clusters often take the formation of pentagon, as indicated by the solid lines in the figures. Such a contrast feature observable by HREM imaging is typical of the basic d-qc structure mentioned above. Figure 2 shows two SE images of etch figures obtained from different etched surfaces of the Al Co Cu d-qc sample, that is in figure 2 (a) a surface area parallel to the tenfold axis and in figure 2 (b) a surface area inclined considerably from the tenfold axis. It is found that the chemical etching made for 5 min caused a number of microfacet pits showing different sections of a definite prismatic shape on the surfaces. Careful SEM observations of various etched surfaces have convinced us

6 1014 K. Saito et al. Figure 1. Two pairs of TEM results each consisting of a tenfold SAD pattern and corresponding HREM image obtained from single-grained samples of (a), (b) Al Co Cu and (c), (d) Al Co Ni d-qcs. Figure 2. SEM SE images of etch figures obtained from (a) a definite surface area parallel to the tenfold axis and (b) a surface area inclined from the tenfold axis of the Al Co Cu d-qc.

7 Chemically etched dissolution patterns 1015 Figure 3. (a) SEM BE image of the etch figure appearing on the tenfold surface of the Al Co Ni d-qc. The arrows drawn in the figure are visual guides to aid recognition of micropits having pointed apices at the bottoms. (b) Enlarged SE image of a microfacet pit having a decagonal facet at the bottom and (c) a corresponding BE image. that the complete image of these microfacet pits is closely related to a decagonal prism, although the top and bottom faces of the pits in reality are not exactly flat but considerably curved. The microfacet pits revealing such a faceted shape have resulted from an anisotropic dissolution effect due to the decagonal structure. The results of the etching test in the (as-grown) Al Co Ni d-qc are presented in figures 3 and 4. Figure 3 (a) is a BE image showing a certain area on the tenfold surface after chemical etching for 5 min. The etching resulted in the occurrence of a number of indefinitely shaped pits instead of the well-defined decagonal pits expected. This is, however, not contradictory. It should be realized that the dissolution caused by chemical etching often obscures a clear-cut crystallographic figure, which was clearly illustrated in the result of the Al Co Cu d-qc shown in figure 2; the top and bottom faces of the resulting prismatic pits are not exactly flat but considerably curved (see figure 2 (a)). Then, it follows that the prismatic pits have their top (or bottom) faces only exposed on the tenfold surface, showing a dimple shape without clear facets. Apparently, the same etching effect took place in case of the Al Co Ni d-qc. Despite these circumstances, we had a few chances to succeed in recording the expected facet-etching behaviour on the tenfold surface. An example of this observation is presented in two alternate SEM images, namely a SE image in figure 3 (b) and a BE image in figure 3 (c). Dramatically, the micropit imaged in both figures has a well-defined decagonal facet at the bottom enclosed by a curved side wall. This is a symbolic example representing a unique dissolution effect of d-qcs.

8 1016 K. Saito et al. It should be remarked here that in figure 3 (a) there appear two different types of micropit; one type is pits having sharply pointed apices at the bottom, several of which are indicated by arrows in the figure, and the other type is those without apices. We note that the formation of the pointed pits was rarely found in any other surfaces than the tenfold surface. The pit shown in figures 3 (b) and (c) belongs to the latter type, that is, pits without apices. In principle, any defects as well as inhomogeneities emerging on the surface, for example dislocations, phason strains, stacking faults and polygonized domains (Yan and Wang 1993; Wollgarten et al. 1999), could become preferred-etching centres. As for typical etching behaviour of crystalline solids, dislocations cause local variations in the rate of etching, thus resulting in the formation of the so-called etch pits having a downward-pointed shape. It can be likewise deduced here that the micropits with pointed apices, appearing in figure 3 (a) in particular, have their origins in existing dislocations. This will be discussed again later. Figure 4 (a) is a SE image taken at a low magnification, showing the etching result for the surface parallel to the tenfold axis (containing an accidentally cleaved part in the upper left), while figures 4 (b) and (c) are images enlarged from two regions enclosed by rectangles b and c respectively in figure 4 (a). Figure 4 (b) demonstrates that many microfacet pits of a prismatic shape with curved top and bottom faces exist. It is also interesting to find that some of the pits have an extremely prolonged shape along the tenfold axis, as shown in figure 4 (c). This may suggest that there is special defect geometry present. In summary, the chemical etching of the Al Co Cu and Al Co Ni d-qcs with a HF HNO 3 H 2 O solution essentially has the same dissolution effect, producing a large number of microfacet pits of decagonal prism shape. Figure 4. (a) SEM SE image of an etched surface parallel to the tenfold axis of the Al Co Ni d-qc sample. (b), (c) Enlarged SE images of specific regions enclosed by rectangles b and c, respectively, in (a).

9 Chemically etched dissolution patterns Indentation-related etch figures of Al Co Ni decagonal quasicrystal The Vickers indentation measurement was made on the tenfold surface of single grain of the Al Co Ni d-qc at a temperature of 850 C with a load of 5 kgf. The indentation testing was made, in total, at four different spots on the surface, and the Vickers hardness H V estimated from these tests was found to be 7.2 GPa on the average, which appears comparable with the result reported elsewhere (Wolf et al. 2001). Figure 5 (a) is a SEM BE image showing a typical example of indentation, which was taken after slight mechanical polishing. There is a distinct mark of indentation in the centre and Palmqvist-type microcracks arising radially from the indentation corners (Palmqvist 1957). On the other hand, figure 5 (b) shows a BE image of the same area taken after chemical etching for 5 min. Obviously, the etchant attacked severely the indented area as well as its vicinity. It should be additionally noted that there are distinct pitting marks formed at the tips of cracks, exhibiting dimple-like figures (see the three spots indicated by arrows in figure 5 (b)). Figures 5 (c) and (d) are enlarged SE images showing the two dimples indicated by arrows c and d respectively in figure 5 (b). Each of the images clearly shows that the dimple further contains an array of smaller pits, each of which has a unique structure tapered to a point at the bottom end, appearing in a dark spot, together with its sloping side decorated by concentric loops. In figure 5 (d), there appear a number of irregularly spaced terraces, some of which are outlined by a clear-cut decagon. In the case of crystalline materials, indeed, there have been many reports concerning the formation of looped or spiral etch figure (for example Ellis (1955) and Vogel and Lovell (1956)). Although a controversy has long existed as to where and how these etch figures are created, it is generally agreed that they result from preferred etching of edge- or screw-dislocation sites on the surface. Figure 5. (a) SEM BE image of an area containing the trace of indentation applied on the tenfold surface of the Al Co Ni d-qc. This was obtained before chemical etching. (b) SEM BE image of the same area obtained after chemical etching for 8 min. (c), (d) Enlarged SE images of specific areas indicated by arrows c and d in (b).

10 1018 K. Saito et al. Figure 6. (a) (c) One set of images which are the results concerning the same area as in figure 5; (d) (e) the other set of images which are for a different area that had been similarly subjected to indentation testing. (a), (d) SEM BE images of the two different indentation positions after additional mechanical polishing. (b), (e) BE images of the corresponding areas obtained after chemical etching for 8 min. (c), (f) Enlarged SE images of specific regions enclosed by squares c and f in (b) and (e) respectively. In order to investigate the extent of an indentation effect on the dissolution behaviour, we repeated the series of experiment for this sample once more: starting from mechanical polishing followed by chemical etching (for 8 min), and then finishing by SEM observations. Two examples of the test result are shown in figure 6. One set of images presented in figures 6 (a) (c) was obtained from the same indented area as in figure 5 (a), while the other set of images in figures 6 (d) (f) was from a different area that had been similarly subjected to indentation testing. Figures 6 (a) and (d) are SE images of the corresponding indented areas taken right after mechanical polishing, showing almost no specific contrast feature except small details of remaining cracks in the centre. It should be noted here that the thickness of surface layer removed by mechanical polishing then was in the order of several tens of micrometres. As is evident from both figure 6 (b) and figure 6 (e), the chemical

11 Chemically etched dissolution patterns 1019 etching attacked the surface selectively to reveal indentation-damaged regions, even after the topmost layer damaged by indentation was apparently removed by mechanical polishing (see figures 6 (b) and (e)). This indicates that the surface layer full of defects has a considerable extent of depth in the interior. Figures 6 (c) and (f) are enlarged SE images of areas enclosed by squares c and f in figures 6 (b) and (e), respectively. It is apparent here that many micropits having pointed apices with concentric loops, which appear similar in morphology to those discussed in figure 5, are arranged in a line along crack propagation directions approximately at a regular interval of a few micrometres. This is, indeed, a very significant effect, suggesting that there is such special geometry of active pitting centres. The microscopic deformation mechanisms in quasicrystalline materials, which vary essentially according to test temperatures, are still controversial issues to be dealt with. It is, however, generally explained that creation and/or movement of a variety of defects, for example dislocations, stacking faults and polygonized domains, are principal causes for deformation, as in the case of conventional crystals. Wollgarten et al. (1999) conducted indentation testing at various temperatures followed by TEM observations for an Al Pd Mn i-qc and proved that the resulting cracks had many accompanying dislocations in their vicinities, most of which originated from the crack tips. Evidently, indentation stress can cause plastic deformation in quasicrytalline materials owing to creation and motion of dislocations. Meanwhile, it has been established that dislocation motion occurring in QCs, which can be activated at elevated temperatures, necessarily leaves phason strains behind in its path (for example Trebin et al. (1993)). With these various pieces of information, we are convinced that the indentation test that we made at 850 C must have resulted in substantial plastic deformation in addition to crack generation, by introducing large amounts of dislocations, phason strains, stacking faults and polygonized domains, especially in the vicinities of cracks. Indeed, these defects had measurable effects on the resulting dissolution patterns, as discussed above. Then, we are tempted to speculate that the micropits having pointed apices are the products of preferred etching of dislocation sites. In particular, a special group of micropits, which are regularly arranged in a line, could have their origins in dislocations lying in activated slip planes present at the tips of cracks. Vogel et al. (1953) reported similar etch figures occurring in a germanium crystal and attributed them to edge dislocations lying in existing small-angle tilt boundaries. In practice, it is a common effect that a deformed crystalline material, especially when it is thermally annealed, allows many small-angle tilt boundaries to form in its interior. This effect could have happened to the present d-qc, resulting in the formation of subgrain structure during indentation testing at 850 C. This provides another possible explanation for the appearance of such linear arrays of micropits in figure 6. So far we have made a convincing argument supported by curious results, but we realize that there are still some contradictions and questions left. For instance, it is not certain whether all the dislocation sites existing in the surface have become the nucleation centres of pointed pits, since the pit density deduced from observations (approximately 10 5 cm 2 for each case, i.e. the surface parallel or perpendicular to tenfold axis) was not convincingly high. It also remains uncertain which type of defect (or something else) is the origin for the pits having no pointed apices. The conclusions and remaining questions mentioned above have yet to be examined more closely and directly, if possible, by means of TEM.

12 1020 Chemically etched dissolution patterns } 4. Conclusion Dissolution patterns essential for Al Co Cu and Al Co Ni d-qcs have been investigated in detail by chemical etching using a HF HNO 3 H 2 O solution followed by SEM observations. The chemical etching of definite surface areas, which are either normal or parallel to the tenfold axes, in a solution composed of HF HNO 3 H 2 O (1 : 5 : 4 in volume ratio; 0 C; 5 8 min), produced a large number of microfacet pits exhibiting various sections of a definite polyhedral shape related to a decagonal prism. The chemical etching method employed has been found effective to cause an essential dissolution effect due to the decagonal structures. In addition, the etching test was made on the tenfold surface of a deformed Al Co Ni d-qc, which had been subjected to indentation testing at an elevated temperature of 850 C. The results have shown that the etchant severely attacks the indented area as well as its vicinity to produce a large number of micropits. Various types of defect induced by indentation, for example dislocations, phason strains, stacking faults, polygonized domains and small-angle tilt boundaries, are the conceivable types of origin for this pitting effect. Remarkably, at the tips of the cracks produced by indentation, there appear arrays of micropits regularly arranged along crack propagation directions, each of which has a unique tapered structure to a point at the bottom end, with its sloping side decorated by concentric terraces. Furthermore, some of these terraces have clear-cut decagonal facets differing in size. The micropits of this type, which are characterized by the presence of pointed apices, are assumed to have their origins in existing dislocations in the d-qcs. References Ellis, S. G., 1955, J. appl. Phys., 26, Guo, J. Q., Abe, E., Sato, T. J., and Tsai, A. P., 1999, Jap. J. appl. Phys., 38, L1049. Hiraga, K., Ohsuna, T., Sun, W., and Sugiyama, K., 2001, Mater. Trans., Japan Inst. Metals, 42, Palmqvist, S., 1957, Jernkontorets Ann., 141, 300. Saito, K., Saito, Y., Sugawara, S., Guo, J. Q., Tsai, A. P., Kamimura, Y., and Edagawa, K., 2002, J. Alloys Compounds, 342, 45. Saito, K., Sasaki, T., Sugawara, S., Guo. J. Q., and Tsai, A. P., 2000a, Phil. Mag. Lett., 80, 307. Saito, K., Sugawara, S., Guo, J. Q., and Tsai, A. P., 2000b, Jap. J. appl. Phys., 39, Saito, K., Sugawara, S., Sato, T., Guo, J. Q., and Tsai, A. P., 2000c, Mater. Trans. Japan Inst. Metals, 41, Sato, T.J., Hirano, T., and Tsai, A. P., 1998, J. Crystal Growth, 191, 545. Suchodolskis, A., Babonas, G.J., Jasutis, V., Karpus, V., R_eza, A., Sˇ imkien_e, I., Assmus, W., and Sterzel, R., 2002, Phil. Mag. Lett., 82, 157. Trebin, H.-R., Mikulla, R., and Roth, J., 1993, J. non-crystalline Solids, , 272. Vogel, F. L., Jr., and Lovell, L.C., 1956, J. appl. Phys., 27, Vogel, F. L., Pfann, W. G., Corey, H. E., and Thomas, E. E., 1953, Phys. Rev., 90, 489. Wolf, B., Bambauer, K.-O., and Paufler, P., 2001, Mater. Sci. Engng, A298, 284. Wollgarten, M., Bartsch, M., Messerschmidt, U., Feuerbacher, M., Rosenfeld, R., Beyss, M., and Urban, K., 1995, Phil. Mag. Lett., 71, 99. Wollgarten, M., Saka, H., and Inoue, A., 1999, Phil. Mag. A, 79, Yan, Y., and Wang, R., 1993, Phil. Mag. Lett., 67, 51.

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