The seeds of Zn6Y8 L12-type clusters in amorphous Mg85Zn6Y9 alloy investigated by photoemission spectroscopy
Graphical abstract
Introduction
In recent fifteen years, novel Mg-based alloys containing Zn and rare-earth metal impurities [1] have achieved much attention owing to the potential for widespread applications as structural materials because these alloys are light-weighted rather than Al. The strength and ductility of pure Mg are very poor for the applications. The Mg alloys with a few amounts of Zn and Y impurities, however, reveal superior mechanical properties, such as the tensile yield strength of ∼600 MPa and the elongation of ∼8% at room temperature [[1], [2], [3]]. Moreover, the flammable and chemically active pure Mg turns to non-flammable nature and high thermal stability [3] by adding the impurities. Because of such excellent properties together with the ease of recycling, these Mg alloys are expected as a next-generation structural materials for, e.g., bodies of subways or even aircrafts.
To clarify the origin of these remarkable properties from the structural point of view, extensive studies were performed using scanning transmission electron microscope (STEM) and electron diffraction [4,5] According to these studies, a long-period stacking ordered (LPSO) phase is formed in the Mg alloys. The Mg85Zn6Y9 alloy has a 18 R type LPSO structure after the Ramsdell notation [6] with the volume fraction of up to ∼100% [7] depending on the thermal history [4].
According to a high-angle annular dark-field (HAADF) STEM observations performed by Abe et al. [8], which can reveal chemical sensitive Z-contrast, it was found that the Zn and Y impurities are enriched around the stacking faults. In other words, the concentration of the impurity elements is synchronized with the stacking faults in the LPSO structure. From this reason, this curious structure is referred to as the synchronized LPSO phase.
The existence of the L12-type clusters formed by the impurities in the LPSO alloys was proposed for Mg-Al-Gd alloys by Yokobayashi et al. [9] and for Mg-Zn-Y alloys by Egusa and Abe [10]. Fig. 1 shows the local structures around the Zn6Y8 L12 clusters embedded in stacking faults of the LPSO phase [10]. The figures are taken from Ref. [11]. The figures are drawn using the (a) [0001]hcp and (b) [100]hcp projections. The small and large balls indicate the Zn and Y atoms, respectively, and dashed lines exhibit the Mg layers.
Recently, Okuda et al. investigated the transformation process of amorphous-crystal phases in the Mg85Zn6Y9 LPSO alloy using small angle X-ray scattering (SAXS) [12]. They found a hierarchical transformation that the clustering of the impurity atoms occurs first, and the spatial rearrangement of the clusters induces a secondary transformation leading to two-dimensional ordering of the L12-type Zn6Y8 impurity clusters. The process was clearly displayed in their manuscript from the isolated impurities to the Zn6Y8 L12 clusters through the fragments of the clusters for the first process, and then the L12 clusters are aligned to form the LPSO phase for the second one.
More recently, we have measured photoemission and inverse-photoemission spectroscopy (PES and IPES) on polycrystalline Mg-Zn-Y LPSO alloys to investigate the valence- and conduction-band density of states (DOS) as well as chemical natures by the shift of the core levels of the constituent elements using core-level PES experiments [13]. The most important results from this study is that the Y 3d core spectrum exhibits three doublets in the lightly doped Mg97Zn1Y2 alloy, while only single doublet in the heavily doped Mg85Zn6Y9 and Mg75Zn10Y15 alloys. Since the volume fractions of the LPSO phase in the lightly and heavily doped alloys are [7] and [7,14], respectively, it is reasonable to speculate that the Mg97Zn1Y2 alloy show threefold chemical natures for the Y atoms, one located in the Zn6Y8 L12 clusters of about 67%, one in their fragments of about 20%, and one in the isolated atoms of about 13%. For the heavily doped alloys, almost all of the Y atoms belong to the Zn6Y8 L12 clusters.
From these interesting results on the electronic structures of the Mg-Zn-Y LPSO alloys, we expect that the seeds of the clusters may be observed in the amorphous phase by investigating the electronic structures using PES and IPES technique, and Okuda et al.’s scenario on the amorphous-to-crystalline phase transformation can be enriched with the viewpoint of the electronic structure. For this purpose, we have carried out PES and IPES measurements on amorphous Mg85Zn6Y9 alloy and compared with the results of crystalline Mg85Zn6Y9 alloy.
Section snippets
Experimental procedure
An amorphous sample was manufactured at Magnesium Research Center, Kumamoto University. Pure Mg (99.99 wt%), Zn (99.9 wt%), and Y (99.9 wt%) metals were mixed and melted using high frequency induction a cylindrical carbon crucible in a pure Ar atmosphere, and then rapidly quenched using a normal melt-spinning technique. The obtained amorphous ribbon was about 1 mm in width and about 0.02 mm in thickness. The amorphous phase of the ribbon was examined by X-ray diffraction and transmission
Results
Fig. 3 shows the valence-band PES and conduction-band IPES spectra of (a) amorphous and (b) polycrystalline [13] Mg85Zn6Y9 alloys indicated by the solid and dashed curves, respectively. The PES spectra are normalized to the corresponding maximum intensities of the valence bands at about −6 eV. The value for the PES measurements varies from 40 to 150 eV as indicated upper-left of each spectrum. The intensity of the actual PES spectra decreases with increasing due to the decrease of the
Discussion
As discussed in Ref. [13], the valence-band PES spectra reveal that about 25% of the Mg atoms are intrinsically oxidized. Due to the large value of the photo-ionization cross-sections, , of O 2p electrons in the Mg oxide by 10–30 times larger than that of Mg 3s electrons [16] in the present range, the metallic Mg signals of the 3s electrons hide under the large O 2p partial PES signals. Thus, the effect of the crystallization in the valence-band PES spectra is hardly observed in the
Conclusions
PES and IPES measurements were carried out on amorphous Mg85Zn6Y9 alloy together with the polycrystal of the same concentrations with a synchronized LPSO phase to investigate the valence- and conduction-band electronic structures, respectively. The valence- and conduction-band density of states (DOS) seems to be mostly the same as that of the crystalline phase. The core-level PES measurements were also performed at the Mg 2s, 2p, Zn 3p, 3d, and Y 3d levels to study the chemical natures of the
Acknowledgements
The PES and IPES spectra were measured at the beamline BL-7 and the RIPES station in the HiSOR with the approval of the Hiroshima Synchrotron Radiation Center, Hiroshima University (Proposal No. 15-A-11, 15-A-12, 16AG012, 17AU004, 17AU005, and 17BG035). This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘Materials Science on Synchronized LPSO structure’ (No. 26109716) from the Japan Society for the Promotion of Science (JSPS). JRS gratefully acknowledges a
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Present address: Department of Chemistry, Physical Chemistry, Philipps University of Marburg, 35032 Marburg, Germany.