Electronic structures and impurity cluster features in Mg-Zn-Y alloys with a synchronized long-period stacking ordered phase
Graphical abstract
Introduction
In the 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. However, Mg alloys with a few amounts of Zn and Y impurities reveal superior mechanical properties, such as the tensile yield strength of MPa and the elongation of % at room temperature [[1], [2], [3]]. Moreover, the flammable pure Mg turns to non-flammable nature and high thermal stability by adding the impurities [3]. Because of such excellent properties together with the ease of recycling, these Mg alloys are expected as 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 Mg97Zn1Y2 and Mg85Zn6Y9 alloys have 18R type LPSO structures and the Mg75Zn10Y15 alloy has a 10H type after the Ramsdell notation [6] with the volume fractions of the LPSO phase up to %, % [7], and % [8], respectively, depending on the thermal history [4].
According to an atomic-resolution angle annular dark field (HAADF) STEM observations performed by Abe et al. [9], 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. [10] and for Mg-Zn-Y alloys by Egusa and Abe [11]. The L12-type clusters are composed of Zn6Y8, and embedded in the stacking faults of the LPSO phase of the Mg alloys. From the structural point of view, the overall features of the L12 clusters are well described in the host Mg as cited in the above papers [10,11].
From the viewpoint of mechanical properties, the LPSO phase shows unique plastic deformation behavior. In-plane ordering of L12 clusters within the close-packed planes in the LPSO structure restricts slip system to the basal slip at room temperature [12]. Furthermore, twin deformation hardly occurs. Instead, Kink deformation occurs under relatively high stress through basal slips [13,14]. Critical resolved shear stress (CRSS) for the basal slip in the LPSO phase was estimated to be MPa [12,15,16]. Large CRSS for basal slip in the LPSO phase was considered to be due to in-plane ordering of L12 clusters. Therefore, formation mechanisms of L12 cluster and LPSO structure were carefully investigated by in situ synchrotron radiation small-angle X-ray scattering [17], an ab initio based Monte Carlo simulation [18], and so on.
However, the electronic structures of LPSO structure have not been clarified yet, although they are important for the understandings of the electronic properties and the chemical natures of the impurities, which may highly be influenced on the stability of the L12 clusters in the Mg LPSO alloys. We have recently investigated the valence- and conduction-band electronic structures of Mg97Zn1Y2, Mg85Zn6Y9 (18R), and Mg75Zn10Y15 (10H) LPSO polycrystal alloys together with pure α-Mg by measuring photoemission and inverse-photoemission spectroscopies (PES and IPES), respectively. The obtained spectra are compared with the results of a density functional theory (DFT) calculation. The core levels of the constituent elements were also studied using core-level PES to examine the chemical natures of the elements. From these results, we found a clear heterogeneous indication of the difference in the electronic structures concerning the L12 clusters.
Section snippets
Experimental and theoretical procedures
The Mg97Zn1Y2, Mg85Zn6Y9, and Mg75Zn10Y15 cast ingots were prepared using high frequency induction melting of pure Mg (99.99 wt.%), Zn (99.9 wt.%), and Y (99.9 wt.%) metals in a cylindrical carbon crucible in a pure Ar atmosphere. The molten alloys were kept at 1023 K and solidified at a cooling rate of about 0.03 K/s. The pure α-Mg ingot was manufactured in the similar way. The ingots were cut to be a disk with the size of mm3 for the PES and IPES experiments.
The PES spectra were
Results
Fig. 2 shows the valence-band PES and conduction-band IPES spectra of (a) the reference pure α-Mg, (b) Mg97Zn1Y2, (c) Mg85Zn6Y9, (d) Mg75Zn10Y15 polycrystal 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
Discussion
Firstly, we discuss the valence- and conduction-band spectral features of pure Mg. Fig. 6 shows the calculated total DOS (solid curve) of pure Mg together with the partial DOSs of the s (dashed curve), p (dotted curve), and d (chain curve) contributions. Although the calculated DOSs have some peak structures, the overall feature seems to be a free-electron-like parabola.
To compare the theoretical results with the experimental data, information on the partial contributions of orbital angular
Conclusions
The PES and IPES measurements were carried out on polycrystalline Mg97Zn1Y2, Mg85Zn6Y9, and Mg75Zn10Y15 alloys with a synchronized LPSO phase together with non-doped Mg to investigate the valence- and conduction-band electronic structures, respectively. It was found from the valence-band PES spectra that the non-doped Mg includes at most 25% of an intrinsic oxidation portion. With increasing impurity concentration, the valence-band DOS very slightly changes, while the conduction-band DOS
Acknowledgements
The PES and IPES spectra were measured at the beamline BL-7 and the RIPES station, respectively, 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). The authors thank the Supercomputer Center, the Institute for Solid State Physics, The University of Tokyo for the use of the facilities. The computation in this work has also been done using the facilities of the Research
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Present address: Department of Chemistry, Physical Chemistry, Philipps University of Marburg, 35032 Marburg, Germany.