Structural Changes of Precipitates by Aging of an Mg-4 at%dy Solid Solution Studied by Atomic-Scaled Transmission Electron Microscopy

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1 Materials Transactions, Vol. 52, No. 5 (2011) pp to 1015 #2011 The Japan Institute of Light Metals Structural Changes of Precipitates by Aging of an Mg-4 at%dy Solid Solution Studied by Atomic-Scaled Transmission Electron Microscopy Kaichi Saito 1; *, Akira Yasuhara 2, Masahiko Nishijima 3 and Kenji Hiraga 3 1 Department of Materials Science and Engineering, Akita University, Akita , Japan 2 EM Application Group, EM Business Unit, JEOL Ltd., Tokyo , Japan 3 Institute for Materials Research, Tohoku University, Sendai , Japan Phase transformation of solid solution decomposition occurring in a 96 at%mg-4 at%dy alloy, which was solution-treated at 540 C and subsequently aged at 250 C for various lengths of time, has been investigated by conventional transmission electron microscopy (TEM) in combination with high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). The atomic-scaled observations based on both techniques provide the evidence that the first appreciable change in microstructure caused by aging is the occurrence of a short-range ordered state in Dy-segregated regions and that the short-range ordered state allows full of the nuclei of 0 phase associated with an Mg 7 Dy-type structure to occur in the domains, just as in cases of Mg-Gd and Mg-Y systems. With an increase of age-hardening effect, the 0 precipitates become larger and increasingly anisotropic in morphology, accompanying three orientation variants in coherent with the Mg-matrix. When reaching at the stage of hardness maximum (as-aged at 250 C for 100 h), the 0 precipitates, which have an orthorhombic structure with lattice parameters of a ¼ 0:659 nm, b ¼ 2:231 nm, c ¼ 0:523 nm, take the form of a thin disk-shape with a thickness of nm and a diameter of nm. With an advance of over-aging effect, the 0 precipitates are gradually reduced in volumes and replaced by precipitates of cubic structure. [doi: /matertrans.l-m ] (Received September 29, 2010; Accepted January 29, 2011; Published April 13, 2011) Keywords: magnesium alloys, precipitation, crystal structure, microstructure, transmission electron microscopy (TEM), high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) 1. Introduction In recent years, studies of Mg alloys with rare earth metals (RE) have become greatly stimulated with the pressing demand for developing new light structural materials. It is well known that many of the Mg alloys with RE surpass the other Mg alloys without RE in high strength properties at elevated temperatures, which makes them attractive for automobile as well as aircraft applications in both, engine and body frame components. Decomposition of supersaturated solid solutions in Mg-RE alloys are of vital importance for understanding various physical behaviour of Mg-RE alloys. Transmission electron microscopy (TEM) studies of various Mg-RE alloys have, thus far, contributed a lot to reveal quite complicated phase transformations in the structures of the alloys during solid solution decomposition and their effect on the properties. 1) It is generally accepted that the solid solution decomposition in Mg-RE alloys proceeds in the following sequence: Mg supersaturated solution! (GP-zone)! 00! 0!, where GP-zone, 00, 0 are the metastable phases and is only the stable one. 1 5) Above all, it had never been much doubted that most of the Mg-RE alloys in common allow the 00 -phase characterized by a D0 19 -type structure to occur at the very early stage of aging. Recently, Nishijima et al. made thorough and detailed examinations on the precipitation behaviours of Mg-Gd alloys 6,7) and Mg-Y alloys 8) under a test temperature of 200 C, by making full use of high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) technique, and they finally drew rather sensational conclusions, contradicting the previous interpretation addressed above: the first aging product *Corresponding author, ksaito@ipc.akita-u.ac.jp is not the D0 19 -type structure phase but a certain short-range ordered state in the respective RE-enriched regions, which is structurally related to the subsequent product of the 0 - precipitate. Moreover, they presented a new structure model for the 0 -phase, i.e., an orthorhombic Mg 7 RE-type structure, which is essentially different from the one previously proposed (Mg 15 RE-type). 5) Ever since, a controversy has continued as to whether this new interpretation is really true of all the other groups of Mg-RE alloys than Mg-Gd and Mg- Y systems. Careful microstructural investigations and verifications of those alloys by means of atomic-scaled TEM in combination with HAADF-STEM are therefore an essential prerequisite for resolving the issue addressed above. The present study focuses on an age-hardened Mg-Dy alloy in order to verify its real precipitation behaviour occurring during the solid solution decomposition. 1 4) 2. Experimental Procedures An alloy with a nominal composition of 96 at%mg- 4 at%dy (Mg-4 at%dy) was prepared by melting Mg (99.9%), Dy (99.9%) metals by induction heating under an Ar gas in a carbon crucible. The alloy was homogenized at 540 C for 2 h and then quenched immediately in water. Subsequently, the quenched alloy was divided into several parts, and the parts were subjected to aging treatments at 250 C for various lengths of time. The hardening responses of these alloys upon aging at a temperature of 250 C were determined by measuring Micro-Vickers hardness number at a load of 0.1 kgf (MATSUZAWA MMT-X3). The hardness number was taken as the average of ten fairly equal values. Specimens for TEM were cut from the as-aged alloys and thinned by mechanical polishing, and finally completed by ion-milling. A transmission electron microscope, JEM-2010

2 1010 K. Saito, A. Yasuhara, M. Nishijima and K. Hiraga 130 Vickers Hardness, HV Aging Time, t /hours 100 Fig. 1 Age-hardening response of Mg-4 at%dy alloy obtained at 250 C with aging time. (200 kv), was used for making TEM images together with selected area electron diffraction patterns. A JEM-2100F (200 kv) instrument was additionally used for making HAADF-STEM images. 3. Results and Discussion Figure 1 shows an age-hardening response of the Mg4 at%dy alloy which was solution-treated at 540 C and subsequently aged at 250 C. In this result, the alloy sample is found to have the hardness maximum recorded around 100 h and subsequently decreased with prolonged aging. On the basis of this result, four differently-aged alloy samples, i.e., an as-aged alloy at 250 C for 10 h, for 100 h, for 200 h, and an excessively over-aged alloy, which was subjected to a combined annealing of 250 C for 200 h and 320 C for 60 h, were selected for the following microscopic examinations. Among all, the as-aged alloy at 250 C for 10 h has been found to contain various microstructural regions characteristic of different levels of under-aging stage. We will, therefore, make the following discussion on the basis of the data mainly obtained from the as-aged alloy at 250 C for 10 h. Figure 2 shows TEM micrographs obtained from the four alloys taken with the incident beam parallel to the [001] direction of the Mg-matrix: the as-aged alloys at 250 C for 10 h (a), for 100 h (b), for 200 h (c), and 250 C for 200 h plus 320 C for 20 h (d). The index of this viewing direction as well as all the others will, hereafter, be designated as ½001 m and so on, where the subscript letter m represents the Mgmatrix. At the early stage of aging, there are a number of fine precipitates having approximately a round shape with a few tenth nanometers in diameter distributed in the matrix (Fig. 2(a)). With an advance of aging effect, the precipitates become larger and increasingly anisotropic in morphology (Figs. 2(b), (c)), extending longer along one of the three particular crystallographic directions in the Mg-matrix, as indicated by arrows in Fig. 2(b). These microstructural features are typical for 0 phase. We have confirmed by separate experiments that the 0 precipitates could remain survived even after undergoing annealing for long-hours at a higher temperature than 300 C, but that then they would gradually disappear and instead the precipitates of the Fig. 2 TEM micrographs obtained from four differently-aged alloys, taken with the ½001 m incidence of the Mg-matrix: (a) 250 C for 10 h, (b) 250 C for 100 h, (c) 250 C for 200 h, (d) 250 C for 200 h and 320 C for 20 h. following sequence become dominant. Figure 2(d) presents the microstructure resulting after a combined annealing of 250 C for 200 h and 320 C for 60 h. It has turned out that the precipitates, which are certainly identified as Mg24 Dy5 type cubic structure phase,1,2) become grown and more dominated (recognized as dark particles in the figure) in place of the 0 precipitates with the advance of over-aging effect. In fact, small volumes of the 0 precipitates could only be found in limited narrow regions distributed like a net in the matrix, as indicated by arrows in Fig. 2(d). This will be discussed later again with Fig. 9. Figure 3 shows electron diffraction patterns obtained from two differently-aged alloys together with a few of their schematic illustrations. The patterns of Figs. 3(a), (c) were recorded at different regions in the as-aged alloys at 250 C for 10 h with the ½001 m incidence, and their corresponding schematic figures are respectively shown in Figs. 3(b), (d). The other set of diffraction patterns Figs. 3(e), (f) were obtained from the peak-hardened alloy, i.e., the as-aged alloy at 250 C for 100 h taken with the ½001 m and the ½110 m incidence, respectively. In Fig. 3(a) as well as 3(f), some of main diffraction spots resulting from the Mg-matrix are indexed with a subscript letter of m. It is found in Fig. 3(a) that there are diffuse reflections at ½1=2 0 0 m -typed positions, which had been generally considered as a result of the occurrence of fine crystalline domains characterized by the D019 -type structure.1 5) Close inspection of this pattern, however, allows us to notice that the diffuse reflections have a characteristic feature in shape forming an oval, as depicted by Fig. 3(b). By contrast, the ½001 m pattern of Fig. 3(e) obtained from the peak-hardened alloy shows many sets of extra sharp reflections around the ½1=2 0 0 m -typed positions, and each set takes a face-centred rectangle formation. These extra reflections are considered to be due to the 0 precipitates

3 Structural Changes of Precipitates by Aging of an Mg-4 at%dy Solid Solution Studied which have a coherent orientation relationship to the Mgmatrix, accompanying the three orientation variants defined around the ½001 m axis. The diffraction pattern of Fig. 3(e) as well as Fig. 3(f) can similarly be observed in the other types of as-aged alloys such as Mg-Gd5 7) and Mg-Y systems,8) especially when they reach at peak-hardness and contain a substantial volume of the 0 precipitates. On the other hand, Fig. 3(c) shows both the diffuse and the superlattice reflections at the ½1=2 0 0 m -typed positions, as is depicted by Fig. 3(d). More significantly, the extension of each diffuse reflection appears to fit approximately the intensity distribution of one set of superlattice reflections forming a basecentred rectangle. This indicates that the pattern represented by Fig. 3(c) has resulted from a local region containing the microstructures of two different aging stages, i.e., a very Fig. 3 Electron diffraction patterns obtained from the as-aged alloy at 250 C for 10 h taken with the ½001 m incidence of the Mg-matrix (a), (c) and their corresponding schematic figures (b), (d). The patterns obtained from the as-aged alloy at 250 C for 100 h in the ½001 m incidence of the Mg-matrix (e) and the ½110 m incidence (f) early stage and the little more advanced stage. It is, therefore, reasonable to deduce that the diffuse reflections observed in Fig. 3(a) as well as Fig. 3(c) originate in a structural order common to the 0 phase. Typical microstructural features observable at a very early stage of aging are presented by Fig. 4. Figure 4(a) shows an atomic-scaled TEM micrograph viewed along ½001 m obtained from the as-aged alloy at 250 C for 10 h, together with its corresponding Fourier diffractogram in an inset of Fig. 4(b). Among all rows of bright dots present in Fig. 4(a), there exist particular sets of dot-rows exhibiting a brighter contrast, indicating the formation of a certain ordered structure. Accordingly, the Fourier diffractogram shown in Fig. 4(b) reveals the presence of diffuse spots located at the ½1=2 0 0 m -typed positions, which agrees well with the diffraction pattern presented in Fig. 3(a). Details of the ordered structure can best be inspected by means of HAADF-STEM technique. Figure 4(c) is an atomic-scaled ½001 m HAADF-STEM image, which was obtained from another area found in the same sample, taken in the same magnification. In this image, there appear many bright dots corresponding to the columns of heavier constituent elements, which are identified as Dy-atoms in the present case, aligned along the ½001 m direction of the Mg-hexagonal lattice. The bright dots form characteristic local arrangements such as hexagonal arrays and zigzag lines, some of which are marked by arrows and arrowheads, respectively. In fact, these bright dots are distributed with a constant interval of about 0.37 nm, as will be detailed below. Figure 5 shows three different structure models of precipitates in the [001] projections, supposedly appearing in various underaged Mg-RE alloys. Figure 5(a) is the model for the D019 type structure designated as 00, while Figs. 5(b) and 5(c) are two of the recently-proposed models for the 0 phase, and especially Fig. 5(c) is the latest one.6 8) In each model, the corresponding structure unit is specified by a thick solid line. Fig. 4 Atomic-scaled TEM (a) and HAADF-STEM (c) images of Mg matrix regions formed in the as-aged alloy 250 C for 10 h, taken with the ½001 m incidence of the matrix. A Fourier diffractogram obtained for (a) is shown in (b). Arrows and arrowheads appearing in (c) indicate characteristic contrasts of hexagonal arrays and zigzag lines, respectively.

4 1012 K. Saito, A. Yasuhara, M. Nishijima and K. Hiraga The D019 structure has an ordered arrangement in which Dy atoms occupy every second nearest neighbour position present in the Mg-lattice. As a result, hexagonal arrangements pffiffiof ffi the atom columns of Dy with a side length of 2a0 = 3 ¼ 0:37 nm can be recognized especially when viewed along the [001] projection, and at the same time Fig. 5 Atomic arrangements of D019 (a) and Mg15 Dy (b) and Mg7 Dy (c) structures illustrated in the ½001 m projection of Mg-matrix.5 8) Unit cell profiles for the corresponding structures are indicated by thick solid lines. pffiffiffi their zigzag arrangements with an interval of 3a0 ¼ 0:55 nm may also be observable as indicated by dotted lines in Fig. 5(a), where a0 is a lattice constant ( nm) of an Mg hexagonal lattice. Assuming that the occurrence of the diffuse reflections appearing in Fig. 3(a) as well as Fig. 4(b) originates from the D019 -type structure, it is to be expected that the periodic arrays of the hexagonal bright dots are present anywhere in the corresponding microscopic regions. However, this is not the case: the hexagonal dots as well as the zigzag dots are found to be only irregularly or sporadically distributed in Fig. 4(b) without forming any long-range ordered domains. Besides, some of the zigzag dots arranged in parallel rows have a much longer interval than 0.55 nm, reaching approximately to 1.1 nm. Thus, the diffuse reflections resulting at the early stage of aging in the ½001 m diffraction pattern of the Mg-Dy alloy cannot be attributed to the growth of the D019 structure phase but to some other type of short-range ordered state in the Dyenriched regions. Figure 6 shows atomic-scaled TEM and HAADF-STEM images taken with the ½001 m incidence, both of which were recorded at local regions present in the as-aged alloy at 250 C for 10 h, where number of well-defined precipitates are present. Both images in Fig. 6 along with some other observations made normal to the ½001 m direction have indicated that the precipitates have an isotropic extension ranging between 5 and 10 nanometers in diameter, each of which is based on an ordered structure characterized by a set of the zigzag arrangements of bright dots distanced apart by 1.1 nm, extending along one of the ½210 m -typed directions in the Mg-matrix. A set of zigzag dots constituting one individual domain of the precipitate is indicated by a set of arrowheads in Fig. 6. These observations in addition to diffraction experiments separately executed have made us convinced that these precipitates are the 0 phase including Fig. 6 Atomic-scaled TEM (a) and HAADF-STEM (b) images of the 0 precipitates formed in the as-aged alloy at 250 C for 10 h, taken with the ½001 m incidence of the Mg-matrix. Arrowheads in the figures indicate a set of zigzag lines constituting one of the individual 0 precipitates.

5 Structural Changes of Precipitates by Aging of an Mg-4 at%dy Solid Solution Studied 1013 Fig. 7 HAADF-STEM images showing the 0 precipitates precipitated in the as-aged alloy at 250 C for 10 h, taken with the ½001Š m incidence of the Mg-matrix. The image in intermediate enlargement is shown in (a) and an enlarged portion of (a) marked by an arrowhead is in (b). A rectangular outline in (b) indicates the unit cell profile of the 0 precipitate. its three orientation variants. The microstructure of a further advanced stage of aging than that of Fig. 6 is exemplified by Fig. 7. Figures 7(a), (b) were HAADF-STEM ½001Š m -images recorded with different magnifications, obtained again from a microscopic region present in the as-aged alloy at 250 C for 10 h: the image in a lower magnification is shown in Fig. 7(a), while the one in a higher is Fig. 7(b), which was targeted at an area indicated by an arrowhead in Fig. 7(a). In Fig. 7(a), there appear a number of as-grown 0 precipitates having an oval shape, in some of which the crystallographic axes are marked with arrows. By contrast, the atomic-scaled image presented by Fig. 7(b) reveals the presence of many parallel rows of bright dots with an interval of 1.1 nm, each of which has a zigzag pattern continuing along the a-axis. A rectangle line indicated with letters a and b in Fig. 7(b) corresponds to a structure unit of the 0 precipitate. Previously, the structure model illustrated in Fig. 5(b) was appreciated as one of the most likely models for the 0 - precipitate. 5) This model has an ordered arrangement of Dy atoms with face-centred symmetry in z ¼ 0 plane, based on a base-centred orthorhombic structure expressed by Mg 15 Dy p with lattice constants of a ¼ 2a 0 ¼ 0:64 nm, b ¼ 4 ffiffi 3 a0 ¼ 2:28 nm and c ¼ c 0 ¼ 0:52 nm, where a 0 and c 0 are lattice constants of a hexagonal unit of the Mg-matrix. In fact, this structure model could reproduce well the diffraction patterns in Figs. 3(e), (f) which are characterized by a reflection condition of h þ k ¼ 2n, but not the zigzag arrays of bright dots with an interval of 1.1 nm recognized in the atomic-scaled HAADF-STEM image. On the other hand, Fig. 5(c) is a revised version of Fig. 5(b), based on an orthorhombic structure of the same lattice constants but having a different ordered state of Dy with face-centred symmetry in both successive planes of z ¼ 0 and z ¼ 1=2. The resulting structure has an atomic ratio of Mg 7 Dy, which has turned out to show the best correspondence to all the data presently observed, just as in cases of Mg-Gd and Mg-Y systems. 6 8) The Mg 7 Dy model has some structural properties common to the Mg 15 Dy, as was addressed above, and it also has other properties common to the D0 19. Both models of D0 19 and Mg 7 Dy, indeed, have ordered structures in which second nearest neighbour elements existing in the Mg-lattice are substituted by Dy atoms. The latter structure, however, keeps a particular set of the second nearest neighbours unsubstituted by Dy atoms, so that the pjigzag arrangements of Dy atom distanced apart by 2 ffiffi 3 a0 ¼ 1:1 nm are recognizable in the [001] projection. 6,7) In consideration of all the results presented above, the real precipitation behaviour during decomposition of the supersaturated Mg- Dy solid solution is summarized as follows: at the very early stage, a short-range ordered state occurs in Dy-enriched domain regions together with full of the nuclei of 0 phase. The 0 precipitates, which are based on an Mg 7 Dy-type orthorhombic structure, become larger and increasingly anisotropic with an advance of aging, making a substantial contribution to the age-hardening effect. After the peak stage, the 0 phase is gradually replaced by the thermodynamically stable phase. The D0 19 -type structure phase, which was supposed to occur at the very early stage of aging, is not the real product in any aging stage of the Mg- Dy alloy. Figure 8 shows HAADF-STEM images obtained from the peak-hardened alloy, i.e., the as-aged alloy at 250 C for 100 h, taken with the incident beam parallel to the ½001Š m (a) and the ½110Š m (b), in both of which a number of the 0 precipitates can be recognized as bright contrasts. These images reveal that the 0 precipitates have approximately a disk-shape with a thickness of nm along the a-axis and a diameter of nm along both, the b- and c-axes. Remarkably, a considerable part of the precipitates, but not

6 1014 K. Saito, A. Yasuhara, M. Nishijima and K. Hiraga Fig. 8 HAADF-STEM images obtained from the as-aged alloy at 250 C for 100 h, taken with the incident beam parallel to the ½001Š m (a) and the ½100Š m (b) directions of the Mg-matrix. all, have a unique morphology showing a wavy continuation along the c-axis, although its origin remains uncertain. In addition to the 0 precipitates, there exists another type of planar precipitates in the basal plane of the Mg-matrix. The example is found in some limited regions enclosed by circles in Fig. 8(b), where they are populously distributed. Separate experiments have evidenced that they are stacking faults due to a Dy-enriched layer. It is worthwhile pointing out that many of the 0 precipitates form rows along the stacking faults, frequently standing in both upright and downright positions, which has an analogy to a tree structure composed by branches and leaves. Probably, these stacking faults function well as an effective diffusion path for Dy atoms to activate the nucleation and growth of the 0 precipitates. Recent works on Mg-Gd 6,7) and Mg-Y 8) systems have demonstrated that a lattice misfit defined between the 0 precipitate and the surrounding Mg-matrix is a determining factor for the morphology of the 0 precipitate. In general, a larger misfit results in the formation of a smaller interface between the corresponding two phases. As is evident from the present results, the 0 precipitates, which initially start to grow isotropically forming approximately a sphere, become larger and increasingly anisotropic taking the form of a diskplate with an advance of aging. This can be assumed to be due to considerable variations in one or two of the lattice constants of the 0 precipitate caused by a substantial aging effect. In fact, the lattice constants of the 0 precipitates resulting at the peak stage have been estimated from separate atomic-scaled TEM observations as a ¼ 0:659 nm, b ¼ 2:231 nm and c ¼ 0:523 nm. Comparing these values with those of the Mg 7 Dy structure defined in the p ideally coherent case, i.e., a 1 2a 0 ¼ 0:6418 nm, b 1 4 ffiffiffi 3 a0 ¼ 2:2232 nm and c 1 c 0 ¼ 0:5210 nm, gives the misfit values (¼ ja 1 aj=a 100, and so on), i.e., 2.6% along the a-axis (the ½110Š m direction), 0.35% for the b-axis (the ½210Š m Fig. 9 HAADF-STEM images obtained from the as-aged alloy at 250 C for 100 h plus 320 C for 60 h, taken with the incident beam parallel to the ½001Š m direction of the Mg-matrix. direction) and 0.38% for c-axis (the ½001Š m direction), respectively. This estimation agrees well with the morphological feature of the 0 precipitates presently observed, that is, they have relatively larger extensions in morphology along both the b- and the c-axes. It is fair to summarize that the 0 precipitates of an Mg-Dy alloy have a similar growth morphology to that of the Mg-Y system especially at the early stage of aging, but with an advance of aging it becomes closer to that of the Mg-Gd system. 6 8) Figure 9 shows an HAADF-STEM image obtained from the as-aged alloy at 250 C for 100 h plus 320 C for 60 h, taken with the incident beam parallel to the ½001Š m direction.

7 Structural Changes of Precipitates by Aging of an Mg-4 at%dy Solid Solution Studied 1015 Many curved linear contrasts exhibiting a net appear brightly, which correspond to the dark linear contrasts indicated by arrows in Fig. 2(d). They have been identified as arrays of small 0 particles. The 0 precipitates, which had stayed dominant from the beginning to the aging stage of 250 C for 200 h (Fig. 2(c)), have dramatically reduced the volumes after the excessive over-aging effect, then replacing themselves with the precipitates. 4. Conclusions The present microstructural investigations by atomicscaled TEM in combination with HAADF-STEM have revealed the real precipitation behaviour and crystal structure of an Mg-4 at%dy solid solution aged at 250 C. The results are summarized as follows: (1) The Mg-4 at%dy solid solution shows a pronounced age-hardening effect under a test temperature of 250 C and has its hardness maximized (HV 115) at an aging time of 100 h. (2) At the very early stage of aging a certain short-range ordered state occurs in Dy-segregated regions together with full of the nuclei of 0 phase identified as an Mg 7 Dy-type structure, just as in cases of Mg-Gd and Mg-Y systems. The presence of the D0 19 -type structure phase, which had been supposed to be the first product at the early stage of aging in common for Mg-RE alloys, has not been recognized in the present Mg-Dy solid solution. (3) With an advance of aging, the 0 precipitates become larger in coherent with the Mg-matrix and increasingly anisotropic in morphology. They have a definite growth tendency extending longer along both, the b-axis (the ½210Š m -type directions) and the c-axis (the ½001Š m direction). When reaching at the stage of hardness maximum (as-aged at 250 C for 100 h), they take the form of a thin disk-shape with a thickness of nm and a diameter of nm, based on an orthorhombic structure unit with lattice parameters of a ¼ 0:659 nm, b ¼ 2:231 nm, and c ¼ 0:523 nm. (4) After the ending of substantial age-hardening effect, the 0 precipitates are gradually reduced in volumes and replaced by the precipitates probably identified as an Mg 24 Dy 5 -type cubic structure. Acknowledgements This work was partly supported by the Center for Integrated Nanotechnology Support at Tohoku University and also by Nanotechnology Network Project of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. REFERENCES 1) For instance L. L. Rokhlin: Magnesium Alloys Containing Rare Earth Metals, (Taylar & Francis, London & New York, 2003). 2) L. L. Rokhlin: Phys. Met. Metall. 55 (1983) ) S. Iwasawa, Y. Negishi, S. Kamado, Y. Kojima and R. Ninomiya: Keikinzoku (in Japanese) 44 (1994) ) P. J. Apps, H. Karimzadeh, J. F. King and G. W. Lorimer: Scr. Mater. 48 (2003) ) T. Honma, T. Ohkubo, K. Hono and S. Kamado: Mater. Sci. Eng. A 395 (2005) ) M. Nishijima, K. Hiraga, M. Yamasaki and Y. Kawamura: Mater. Trans. 47 (2006) ) M. Nishijima and K. Hiraga: Mater. Trans. 48 (2007) ) M. Nishijima, K. Yubuta and K. Hiraga: Mater. Trans. 48 (2007)