2018 Volume 59 Issue 8 Pages 1259-1266
The crystal structures of Long-Period Stacking-Ordered (LPS) Phases formed in Mg97−xLixY2Zn1 (x = 3, 6, 10, 17 at%) alloys have been thoroughly investigated by means of High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy. All the quaternary alloys with different Li contents, especially when subjected to annealing at 500°C for 50 hrs (hereafter called ‘as-annealed alloy’), allow certain polytypes of LPS-phases to form with α-Mg solid solutions. The as-annealed alloy with x = 3 at%Li has 14H-type LPS-phase with disordered arrangements of Y and Zn in the solute-enriched layers, as in the case of the ternary counterparts without Li. It is evidenced that an increase of Li additions has substantial effects of changing the LPS structure, progressively inducing both the phase transformation from 14H- to 24R-type and the in-plane ordering of Y/Zn atoms in their enriched layers. The as-annealed alloys with x = 6 at%Li, in fact, contain LPS-crystal grains made up of the two polytypes of 14H and 24R, both of which have the long-range in-plane ordered structures. Among all, the as-annealed alloy with x = 10 at%Li has the 24R-type LPS-phase dominant over the LPS-grains combined with the in-plane ordering. The crystal structures of such highly-ordered LPS-polytypes are found to be well characterized by the concept of order-disorder (OD) intermetallic phases.
Magnesium (Mg) alloys containing lithium (Li) have long been in the spotlight of research and development, since they can combine a few of the superior properties on which no other Mg-based systems can compete. Li-additions to Mg as well as to its alloys, in fact, can serve the double purpose of substantial improvements in weight reduction and plastic workability. It was in the early 40’s when the practical possibilities of such attractive alloys as a basis for ultralight, malleable and ductile structural materials were first suggested.1) There were, however, some critical problems to be solved, one of which was the fact that alloying Mg with Li causes a great loss of mechanical strength, which was detrimental to any conceivable structural applications. Historically, NASA pioneered in developing superlight structural materials based on Mg–Li system and applied the LA (Mg–Li–Al), LZ (Mg–Li–Zn) and LAZ (Mg–Li–Al–Zn) series to aerospace applications such as instrument boards set up in the interiors of rockets.2,3) However, this specific success has never been led to the expanding demand for their structural applications due to a few issues unsettled, especially their insufficient mechanical strengths. Indeed, combined additions of Li and the other elements such as Al, Zn and rare earth (RE) metals can cause certain strengthening effects, but those are rather limited and neither sufficiently high nor adequate for the Mg–Li alloys to replace traditional structural materials. As of today, the development of the methods to produce these materials having appropriate properties for a wide range of structural applications, which are based on sufficiently high strength, is still a long-standing issue.
Since the early 2000’s, the development of Mg–Y–Zn alloys containing fine intermetallic precipitates with special stacking-ordered structure has created a great sensation, since they can yield an outstanding mechanical property of extraordinary high strength combined with good ductility.4–8) The responsible microstructure for the property is a phase of Long-Period Stacking-ordered (LPS) structure. There have been, so far, four distinct polytypes of LPS structures found in several different Mg–RE–TM (Transition metal) systems, i.e. 10H-, 18R-, 14H- and 24R-types, which are assumed to be constructed of stacking structural blocks with 5-, 6-, 7- and 8-close packed atomic layers, respectively.4–9) In recent works, it has also become apparent that LPS-phases have a tendency to make their solute atoms orderly arranged in particular sets of four consecutive close-packed atomic planes containing a stacking fault, then leading to the formation of a two-dimensional hexagonal superstructure taking in TM6RE8 atom cluster with an L12-type ordered structure as a basis.10–15) Thus, a rather variable nature of LPS-phases has become generally recognized with different degrees and/or extents depending on the alloy system, the alloy composition and the thermal history. Meanwhile, a controversy exists as to whether the introduction of LPS-phases into Mg–Li alloys would be an effective way to develop new structural alloys with a combined property of ultralight weight, high strength and good ductility. In fact, there are increasing experimental results to be taken as evidence that LPS-phases can actually have considerable strengthening effects for Mg–Li alloys as well.16–19) However, the real alloying effects with Li on the LPS-structures and on the alloy’s physical properties are very little known.
The aim of the present investigation is to clarify the essential effects of Li additions on the Mg–Y–Zn-based LPS-structures and also to characterize atomic structures of Li-containing LPS-phases. The present study focuses on the description of structural features of the LPS-phases formed in four different Mg97−xLixY2Zn1 (x = 3, 6, 10 and 17) alloys observable by scanning transmission electron microscopy. The structural characteristics are detailed on the basis of Kishida’s findings for the Mg–TM–RE LPS/OD phases.10,12–15)
As for mother alloys in target, we selected the Li-adding amounts within the solid solubility range of α-Mg assumed for the binary alloy.20) The quaternary alloys with nominal compositions of Mg97−xLixY2Zn1 (x = 3, 6, 10, 17) were, thus, prepared from a mixture of high-purity metals of Mg (99.99%), Zn (99.99%) and master alloys of Mg–16 mass%Li and Mg–15 mass%Y by induction heating under an Ar gas in a graphite crucible (As-solidified). These alloys differing in compositions are, hereafter, designated as Li3, Li6 and so on. All the as-solidified alloys were subsequently subjected to annealing at 500°C for 50 hrs (As-annealed). The microstructures of the as-annealed alloys were examined by means of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM). Specimens for TEM and STEM were cut from the as-annealed alloys and thinned by mechanical polishing, and finally completed by ion-milling. Selected Area electron Diffraction (SAD) patterns and High-Angle Annular Dark-Field STEM (HAADF-STEM) were taken by using a 200 kV electron microscope (JEOL JEM-2100F) as well as a 300 kV electron microscope (FEI Titan3 G2 60-300 Probe Corrector). It should be noted here that there was no definite evidence obtained as to the distribution or the location of the solute Li atoms in the alloys from any electron microscopic analyses executed in this study.
Figure 1 shows SEM back-scattered electron images showing microstructures of the four different as-annealed alloys of (a) Li3, (b) Li6, (c) Li10 and (d) Li17. All the images reveal a rather similar contrast feature associated with a cellular structure, comprising grains of α-Mg matrix displayed in dark and secondary-phase precipitates with bright contrast in the surrounding. There are also many precipitates accompanied with layered-contrasts, typical for LPS-phases. Some of the corresponding precipitates are enclosed by circles in the images. Although the precipitates with layered-contrasts are less noticeable in the as-annealed alloy of Li17, the presence of LPS-phases was actually confirmed in all by the HAADF-STEM observations subsequently made. The EDS analysis made additionally indicated that the precipitates with layered-contrasts are rich in Y and Zn. As will be later addressed, the layered-contrasts are due to the segregation of Y/Zn atoms in four consecutive close-packed atomic planes including a stacking fault in the LPS-precipitates.
SEM back-scattered electron images of the microstructures of four as-annealed alloys of (a) Li3, (b) Li6, (c) Li10 and (d) Li17. Precipitates accompanied with some layered-contrasts are of LPS-phases, and a few of the corresponding precipitates are enclosed by circles.
The as-annealed alloys, especially with 6 at%Li and more, have been found to have an inhomogeneous mixture of two or more LPS-polytypes, and therefore the resulting SAD patterns can be variable in their small details depending on the local areas to select. In other words, there are some areas where the SAD patterns show the character of a single polytype only, and there are also other areas where the mixed character of a few polytypes appear in the patterns. Nonetheless, a number of separate observations have allowed us identify that there are one or two predominant polytypes for each alloy. Figure 2 shows typical SAD patterns obtained from the LPS-phases formed in the as-annealed alloys of (a, b) Li3, (c, d, e, f) Li6, and (g, h) Li10, taken in (a, c, e, g) the $[2\bar{1}\bar{1}0]$ incidences and (b, d, f, h) the $[1\bar{1}00]$ incidences, in which the indices indicated are based on the hcp-Mg structure. Note that the SAD patterns regarding Li17 are not included here, but they were found to have almost the same features as those of Li10. The diffraction spots discernible in these patterns arise from three different causes: fundamental spots due to the α-Mg matrices, superlattice spots due to the LPS-phases and also double reflections involved with the causes above. When considering this fact and closely examining a particular set of superlattice spots, for instance, those aligned in between the transmitted beam and 0002hcp, this leads us to identify polytypes of LPS-phases formed in the alloy concerned. The superlattice spots present in (a, b) as well as in (c, d) are, in fact, located at positions specified by m/7(0002)hcp (m is an integer), while those in (e, f) as well as in (g, h) are located at m/8(0002)hcp. These features indicate that the number of close-packed atomic planes comprising each of the basic structural blocks is either 7 or 8. The SAD patterns in (a∼d) and (e∼h) can, hereby, be attributed to 14H-type and 24R-type LPS-phases, respectively. The as-annealed alloy of Li6 is, in particular, found to have two LPS-polytypes with 14H and 24R coexisting with the α-Mg matrix. It is also remarkable that the SAD patterns in (c∼h) obtained from the alloys of higher Li contents than 3% have more distinct and discrete superlattice spots together with diffuse streaks in the positions of $1/2[01\bar{1}l]^{*}$ for their $[2\bar{1}\bar{1}0]$ patterns and in the positions of $n/6[11\bar{2}l]^{*}$ for their $[1\bar{1}00]$ patterns (n = 1, 2, 3, 4, 5; l is an integer), while the patterns (a, b) for Li3 have diffuse streaks only in the slightest degree (see the diffuse spots and/or streaks along the c*-axis, which are indicated by upward arrows). The occurrence of these superlattice spots and diffuse streaks is the evidence that the LPS-phases have certain long-range ordering of the constituent solute atoms. In other words, the clearness of those spots and streaks can be regarded as a measure of the ordering states in the basal planes of the hcp-Mg structure as well as of the stacking-order along the c-axis. These characteristic features of the SAD patterns recognized in the quaternary Mg–Li–Y–Zn alloys, except for small details, are essentially the same as those reported for the ternary counterparts without Li.6–15)
SAD patterns obtained from the LPS-phases formed in (a, b) Li3, (c, d, e, f) Li6, and (g, h) Li10, taken in the (a, c, e, g) $[2\bar{1}\bar{1}0]$ incidences and the (b, d, f, h) $[1\bar{1}00]$ incidences. The indices indicated are based on the hcp-Mg structure. Weak reflections of discrete and/or streaked characters appearing along the c*-axis, which are indicated by upward arrows, are due to in-plane ordering of Y and Zn atoms in the enriched layers.
Figure 3 shows HAADF-STEM images of the LPS-phases present in the as-annealed alloys of (a) Li3, (b) Li6 and (c) Li10, taken at low magnification in the $[1\bar{1}00]$ incidences. In these images, there are contrasts of striped-pattern extending along the basal planes of the hcp-Mg structure, where each bright line in the striped patterns corresponds to a set of atomic layers enriched by Y and Zn in the corresponding LPS-phases. Apparently, the LPS-phase in (a) Li3 has the striped-pattern regularly distanced apart from top to bottom in the photograph, while the striped-patterns in (b) Li6 as well as in (c) Li10 look rather irregular and variable in the interval distance depending on the locations. It is evident from these observations that the as-annealed alloy of Li3 allows single-phased grains of 14H-type to grow larger, while those of Li6 and Li10 (as well as of Li17) have their LPS-grains consisting of a fine mixture of different polytypes, particularly 14H- and 24R-types. With the images of (b) and (c), one can approximately identify the locations with different LPS-polytypes by looking at interval variations of the striped-patterns: the 24R-type LPS-phase is present in the regions with the larger interval, and the 14H-type and to a lesser extent the 18R-type are in the regions with the smaller and the smallest intervals, respectively. It is worth noting that the 24R-type LPS-structure occurs most frequently in the as-annealed alloy of Li10.
HAADF-STEM images of the LPS-phases present in the as-annealed alloys of (a) Li3, (b) Li6 and (c) Li10, taken at a low magnification in the $[1\bar{1}00]$ incidences.
Figure 4 shows atomic-resolution HAADF-STEM images obtained from two regions with different LPS-polytypes formed in the as-annealed alloy of Li6, both of which were taken in the $[2\bar{1}\bar{1}0]$ incidences. HAADF-STEM imaging allows us to interpret that contrast spots of higher brightness in the resulting image correspond to atomic columns containing larger amounts of heavier solute atoms like Y and Zn. A set of four horizontal arrows in the images denote one set of four consecutive close-packed atomic planes displayed with brighter dots, among which the inner two are particularly conspicuous. To be precise, the inner two layers are found respectively to consist of two alternating dots with different degrees of brightness (see the set of bright dots indicated by downward-arrows). A similar contrast feature can also be recognized in bright dots belonging to the upper layers but to a much slighter degree (see the set of dots indicated by upward-arrows). These observations are consistent with the presence of periodic arrangement of Zn6Y8 atom clusters with an L12-type ordered structure formed in the corresponding four consecutive atomic planes.14)
Atomic-resolution HAADF-STEM images showing stacking sequences of two LPS-phases of (a) 14H-type and (b) 24R-type formed in different regions of the as-annealed alloy of Li6. The images were both taken in the $[2\bar{1}\bar{1}0]$ incidences.
According to early studies,6–9) the LPS structures can be constructed by stacking structural blocks, each of which is based on a set of 5, 6, 7 or 8 close-packed atomic planes, accordingly falling into the four LPS-polytypes designated as 10H, 18R, 14H and 24R. Each structural block is assumed to have two distinct parts, namely ‘the inner 4 layers’ with an fcc-stacking order and ‘the outer layers’. The inner 4 layers particularly have the solute atoms such as Y and Zn enriched and further, when conditions are compatible, they can have those solutes orderly arranged. In accordance with this knowledge, when labelling the three types of stacking layers A, B and C to specify their stacking sequences present in both images, the LPS-phase in (a) is found to be identified as 14H-type with a periodic stacking sequence of ABABCACACACBAB, which is based on two 7-layer structural blocks of ABABCAC and ACACBAB, while that in (b) is as 24R-type with a periodic sequence of ABABCACACACABCBCBCBCABAB based on an 8-layer structural block of ABABCACA and so on, where the sets of layers underlined correspond to the inner layers addressed above, each consisting of an fcc-stacking layers enriched with Y and Zn atoms.
It has been pointed out for the ternary Mg–TM–RE LPS/OD phases10,12–15) that the occurrence of the in-plane ordering of solute atoms is, in principle, not compatible with crystal growth of a three-dimensional ordered structure, because of a multiplying effect of the in-plane ordering on the stacking manner of adjacent structural blocks. More specifically, the in-plain ordering possible for the OD phases makes each of the three original stacking positions denoted as A, B and C divided into as many as twelve different stacking positions which fall into fewer types with different symmetry characteristics. There is, however, increasing evidence that real crystal structures of the OD phases exhibit tendencies to have their structural blocks stacked selectively on a limited part of the twelve positions.10,12–15) Information regarding this characteristic stacking manner can efficiently be obtained from the HAADF-STEM images recorded in the incidence of $\langle 2\bar{1}\bar{1}0\rangle $ as well as of $\langle 1\bar{1}00\rangle $. Atomic-resolution HAADF-STEM images of the 14H-type LPS-phase present in the as-annealed alloy of Li6, having a relatively long-range stacking order, are shown in Fig. 5(a) and (b), taken with the incident beam directions $[2\bar{1}\bar{1}0]$ and $[1\bar{1}00]$, respectively. Characteristic contrast features with bright dots, exhibiting either a ‘dual-rectangular’ or a ‘double-dagger’ pattern14) in the images, each correspond to Zn6Y8 cluster with an L12-type ordered structure, resulting from the in-plane ordering of Y and Zn atoms. It is worth emphasizing here that, in addition to the long-range in-plane ordering, certain long-range ordering appears along the stacking direction of the structural blocks. In the images, each graphic symbol represented by two circles connected by a short line indicates an fcc-stacking sequence of the four consecutive close-packed atomic planes enriched by Y and Zn atoms. When looking at the selected series of the symbols lined up along the c-axis in each image, we come to realize that the symbols seen in (a) are aligned in right above with their diagonally-forward inclinations alternately changed in direction, while those in (b) are aligned upright but with relative shifts of ±1/3 along the horizontal alternately one after another, where the centre-distance between two double-daggers (two Zn6Y8 atom clusters) along the horizontal, which corresponds to 3aMg, is assumed to be a unity. Note that the symbols with two differently-forwarded inclinations recognized in (a) correspond to different stacking sequences of either ABCA or ACBA. These observations enable us to determine a unit cell of the three-dimensional ordered structure assumed for the 14H-type LPS-phase present in the images, as outlined by the rectangle in each view.
Atomic-resolution HAADF-STEM images of the 14H-type LPS-phase present in the as-annealed alloy of Li6. The images were taken from local regions with a relatively long-range stacking order in the incidences of (a) $[2\bar{1}\bar{1}0]$ and (b) $[1\bar{1}00]$, respectively. Each graphic symbol represented by two circles connected by a short line indicates an fcc-stacking sequence of the four consecutive close-packed atomic planes enriched by Y and Zn atoms.
The structure model of the 14H-type LPS-phase deduced from observations made in Fig. 5 is illustrated in Fig. 6. Here, we assume that the 14H-type LPS-phase is constructed by alternate stacking of two different 7-layer structural blocks of ABABCAC and ACACBAB. Figure 6(a, b) illustrate the atomic arrangements for the inner 4 layers enriched with Y and Zn atoms in the [0001]hcp projection, corresponding to the first- and the second structural blocks, respectively (hereinafter abbreviated as the 1st block and the 2nd block and so on). Note that the constituent atoms and the atomic stacking layers they belong to are distinguished by different paint-colours and sizes: the circles of large, medium and small sizes represent Y, Zn and Mg atoms, ignoring Li atoms, and the circles painted in white, grey and black are different atoms lying in three separate atomic layers they belong to, namely A, B, and C, respectively. The Zn6Y8 clusters present in the 1st block, each of which consists of the ABCA stacking sequence as represented by (c), are located on the two-dimensional primitive hexagonal superlattice with a cell parameter of $2\sqrt{3} a_{\text{Mg}}$, where aMg refers to the unit cell length in the basal plane of hcp-Mg. By contrast, Fig. 6(b) indicates that the 2nd block stacks on top of the 1st block in such a way that the atom clusters in the 2nd block, each of which consists of the ACBA sequence as represented by (d), have their centres displaced at the position indicated in the figure (corresponding to one of the A2 positions in Kishida’s notation12,14)). The adjacent structural blocks can, then, be relatively related with each other by such translation vectors as $\boldsymbol{{t}} = \pm \frac{1}{3}(\boldsymbol{{a}}_{\text{s}} - \boldsymbol{{b}}_{\text{s}}) + \boldsymbol{{h}}$, where as and bs refer to the two basic vectors in the basal plane of the superlattice resulting from the in-plane ordering, and h is a unit vector of the 7-layer structural block along the stacking direction. After all, the crystal structure of the 14H-type LPS-phase observable in Fig. 5 is illustrated in (e): it belongs to the hexagonal system (space group: P6322)14) with a supercell volume of $2\sqrt{3} a_{\text{Mg}} \times 2\sqrt{3} a_{\text{Mg}} \times 7c_{\text{Mg}}$, where cMg refers to the unit cell length along the stacking direction of the hcp-Mg structure.
Atomic structure model of the 14H-type LPS-phase formed in the as-annealed alloy of Li6: (a) Atomic arrangement of Zn6Y8 clusters present in the inner 4 layers of the first structural block in the [0001]hcp projection, in which each cluster is made up of ABCA-stacking shown in (c); (b) Atomic arrangement of Zn6Y8 clusters present in the inner 4 layers of the second structural block, in which the cluster is made up of ACBA-stacking in (d); (e) Overview of the unitcell structure of the 14H-type LPS-phase, in which two dotted-line circles painted in white and grey correspond to the Zn6Y8 clusters with ABCA-stacking and ACBA-stacking, respectively.
As was evident from Fig. 3, the LPS-phases formed in the as-annealed alloys of Li-contents with 6 at% and more have considerable amounts of stacking irregularities, but the majority of their constituent structural blocks is based on 8-layer. Figure 7 is an atomic-resolution HAADF-STEM image of the 24R-type LPS-phase found in the as-annealed alloy of Li10, taken in the $[1\bar{1}00]$ incidence. Apparently, a number of double-daggers of bright dots are aligned along the horizontal in parallel having a regular interval along the vertical, comparable with the height of 8-layer structural block. There are, again, relative shifts of the centre positions of the double-daggers recognized along the horizontal, but the shifts are, in this case, found to be either 1/6, 1/3 or 1/2. This observation agrees well with the case found in the 18R-type LPS/OD phase in the Mg–Y–Zn system: the present 24R-type LPS-phase is constructed by stacking 8-layer structural blocks preferably on positions labelled C3 in the Kishida’s notation.14) There is no regularity or periodicity in the appearance pattern of shifts of 1/6, 1/3 or 1/2, which is characteristic of the OD structure with one-dimensional disorder along the stacking direction.
Atomic-resolution HAADF-STEM images of 24R-type LPS-phase present in the as-annealed alloy of Li10. The images was taken in the $[1\bar{1}00]$ incidence. Each graphic symbol represented by two circles connected by a short line indicates an fcc-stacking sequence of the four consecutive close-packed atomic planes enriched by Y and Zn atoms.
On the basis of those observations above, the atomic structure model of the 24R-type is illustrated in Fig. 8, where Li atoms actually existing as solute are omitted again. In this model, the 24R-type LPS-phase is assumed to consist of ‘triple-stacked’ 8-layer structural blocks with such stacking sequence as ABABCACA. Figure 8(a) is the same illustration as Fig. 6(a), likewise showing the atomic arrangement for the inner 4 layers present in the 1st block, where the Zn6Y8 clusters with the stacking sequence of ABCA as represented in (d) are located on the two-dimensional primitive hexagonal lattice with a cell parameter of $2\sqrt{3} a_{\text{Mg}}$. There are, then, three possible stacking manners allowed for the 2nd block. More specifically, Fig. 8(b) indicates that the 2nd block stack on top of the 1st block in such a way that the atom clusters in the 2nd block have their centres displaced at either of the three C-positions as labelled 1, 2 and 3 (corresponding to a part of the C3 positions in Kishida’s notation10,14)), through which the atom clusters in the 2nd block have their fcc-stacking sequences changed into CABC as represented in (e). Accordingly there are three translation vectors defined for this stacking manner, one of which can be represented as $\boldsymbol{{t}}_{1} = \frac{3\boldsymbol{{a}}_{\boldsymbol{{s}}} + 2\boldsymbol{{b}}_{\boldsymbol{{s}}}}{6} + \boldsymbol{{h}}$, where as and bs refer to the two basic vectors in the basal plane of the superlattice resulting from the in-plane ordering, and h is a unit vector of the 8-layer structural block along the stacking direction. Figure 8(c) further indicates that, if the 3rd block stacks on top of the 2nd block in exactly the same stacking manner as the 2nd block does on the 1st block, the Zn6Y8 clusters present in the 3rd block have their centres further displaced at the other C-positions specified by the corresponding translation vectors, respectively, through which the atom clusters have their fcc-stacking sequences changed into BCAB as represented in (f). The stacking manner repeated that way can bring about a three-dimensional ordered structure and the simplest one can be characterized by the monoclinic system, although this is not the case for the present 24R-type LPS-phase. In fact, the random appearance pattern of the shifts of 1/6, 1/3 and 1/2 recognized in Fig. 7 is due to the existence of such a set of three equivalent positions as labelled 1, 2 and 3 for the structural blocks to take. Structural characteristics of the LPS-phases revealed in this study are summarized in Table 1. It has, thus, turned out that an increase of Li additions has substantial effects of changing the LPS structure, progressively inducing both the phase transformation from 14H- to 24R-type and the in-plane ordering of Y/Zn atoms in their enriched layers.
Atomic structure model of the 24R-type LPS-phase formed in the as-annealed alloy of Li10: (a) Atomic arrangement of Zn6Y8 clusters present in the inner 4 layers of the first structural block in the [0001]hcp projection, in which each cluster is made up of (d) ABCA-stacking; (b) Three possible translations of Zn6Y8 clusters from the original positions in the first block, with which the stacking sequence is replaced by (e) CABC; (c) Three possible translations of Zn6Y8 clusters from the original positions in the second block, with which the stacking sequence is replaced by (f) BCAB.
It should be noted here that the Li-adding amount of 6 at% is likely in a critical range, where not only different LPS-polytypes but also different types of structural blocks with in-plane ordered- and disordered-layers appear simultaneously as an intergrowth structure in the LPS-grains. Figure 9 illustrates atomic-resolution HAADF-STEM images showing the evidence for the latter effect observed in the as-annealed alloy of Li6, which were taken in the $[2\bar{1}\bar{1}0]$ incidence for (a, b, c) and the $[1\bar{1}00]$ incidence for (d, e, f): two LPS-regions with either in-plane ordered- or disordered-layers are placed on top of another. In the images, the capital letters of O and D stand for stacking regions with in-plane ordered- and disordered-layers, respectively. The images (b) and (c) are enlarged from the regions labelled O and D in (a), while those of (e) and (f) are from the two counterparts in (d), respectively. Whether the long-range in-plane ordering exists or not in the location concerned can be immediately determined by appearance of the corresponding contrast features: in the surrounding regions labelled O, the contrast features of brighter dots typical for the ordered-arrangement of Zn6Y8 clusters as described above are noticeable (see (b) and (f)). Figure 10 shows another example of HAADF-STEM image evidencing the coexistence of structural blocks with in-plane ordered- and disordered-layers, which was obtained from the as-annealed alloy of Li6 in the $[1\bar{1}00]$ incidence. In this case, two LPS-regions with either in-plane ordered- or disordered-layers are arranged side by side with some transit regions in the middle. These results may reflect the fact that the solute Li atoms are not so homogeneously distributed over the LPS-grains that the solute concentration of Li is significantly varied with locations in a microscopic scale. In other words, it is likely that the LPS-grains sufficiently rich in Li have the in-plane ordering highly and extensively developed but that those lacking in Li do not.
HAADF-STEM images showing coexisting regions of in-plane ordered-and disordered-stacking layers of the LPS-phase present in the as-annealed alloy of Li6, taken in the $[2\bar{1}\bar{1}0]$ incidence for (a, b, c) and the $[1\bar{1}00]$ incidence for (d, e, f). In the images, the letters of O and D stand for local regions of either an ordered or a disordered layer, respectively. The images (b) and (c) are enlarged from the regions designated as O and D in (a), while those of (e) and (f) are from the counterparts in (d).
HAADF-STEM image showing the coexistence of structural blocks with in-plane ordered- and disordered layers, which was obtained from the as-annealed alloy of Li6 in the $[1\bar{1}00]$ incidence. Two LPS-stacking regions with either in-plane ordered- or disordered layers are arranged side by side with small regions of the intermediate character in the middle.
Although the present HAADF-STEM observations provided us the information as to how the Mg-sites are substituted by the solutes of Y and Zn in the LPS-phases, there was no evidence found regarding the locations or chemical substitution of Li. It is quite certain that the contrast features arising from the inner four layers enriched by Y and Zn atoms in the quaternary LPS-phases are almost the same as those observed in the ternary counterparts without Li. Besides, we know well that the long-range in-plane ordering of Y and Zn solute atoms do not usually occur in the ternary Mg97Y2Zn1 LPS-phases. Then, it leads us to speculate that the solute Li atoms do not substitute for Y or Zn atoms in Zn6Y8 clusters but partially replace Mg atoms distributed in the outsides of the clusters. Meanwhile, it is also common knowledge that the addition of Li to Mg can considerably decrease the lattice constants of a and c of the hcp-Mg structure and even the ratios of c/a, too. Therefore, it seems highly probable that the reduction of atomic distances in the hcp-Mg structure due to alloying with Li is the main cause for improving the correlation of 8-layer structural blocks as well as the long-range in-plane ordering of Y and Zn atoms, which are hardly found in the ternary systems without Li additions.
The crystal structures of LPS-phases formed in the quaternary Mg97−xLixY2Zn1 (x = 3, 6, 10, 17) alloys, which were subjected to annealing at 500°C for 50 hrs, have been investigated mainly by means of HAADF-STEM technique. Some of the new findings can be summarized as follows:
