Elsevier

Journal of Non-Crystalline Solids

Volume 498, 15 October 2018, Pages 281-287
Journal of Non-Crystalline Solids

Electronic structures and heterogeneity of Zr-Cu-Ag metallic glasses

https://doi.org/10.1016/j.jnoncrysol.2018.06.036Get rights and content

Abstract

Photoemission and inverse-photoemission spectroscopy (PES and IPES) measurements were carried out on Zr50Cu50, Zr45Cu45Ag10, and Zr40Cu40Ag20 metallic glasses to investigate the valence- and conduction-band electronic structures, respectively. From the incident photon energy, , dependence of the PES spectra, partial DOSs of s and d electrons for the constituent elements were estimated, and it was found that the DOS near EF is mainly composed of Zr 4d electrons and the Cu 3d electrons are localized in the valence band. Core-level PES measurements were also performed at the Zr 3d, Cu 3p, and Ag 3d levels. The Zr 3d core spectra indicate three chemical states in all of these glasses, suggesting a chemical heterogeneity. Energy shifts are observed in the Zr 3d core levels, indicating the charge transfer in the Zr-Cu-Ag metallic glass system.

Introduction

In the recent two decades, bulk metallic glasses (BMGs) with distinct glass transitions have been discovered in various multi-component metallic alloys. They show extremely excellent glass-forming abilities (GFAs), where even a very slow cooling rate such as 1 Ks−1 can avoid crystallization. Physical and technological properties of these BMGs were well investigated, including glass transitions, structural changes, phase stabilities, elastic constants, magnetic properties, etc. [1]. Among these BMGs, the Zr-Cu alloys strongly improve the GFAs by adding Al, having a critical cooling rate of some Ks−1 and allowing to form a massive BMG with a diameter of about 10 mm [2].

Zhang et al. [3] reported that by adding Ag to Zr50Cu50 alloy, the GFA greatly improves as Al, and the critical sample diameter (dc) manufactured with a Cu mold tilt casting increases with increasing Ag content up to dc = 6 mm at 10 at. % Ag. The further addition of Ag, however, degrades the GFA, e.g., dc = 2 mm at 16 at.% Ag [3].

Louzguine-Luzgin et al. [4] measured high-energy X-ray diffraction on Zr45Cu55, Zr45Cu45Ag10, and Zr45Cu35Ag20 glassy alloys to examine the total structural effect of the Ag addition on these alloys, and it was suggested that the addition of Ag causes a formation of a more homogeneous local atomic structure compared with that of the binary Zr-Cu alloy, which could be considered as a reason for the improved GFA in this alloy. On the other hand, Fujita et al. [5] concluded from an ab initio molecular dynamics (MD) simulations and Zr, Cu, and Ag K x-ray absorption fine structure (XAFS) measurements that an atomic-scale heterogeneity may play an important role in improving the GFA.

Kawamata et al. [6] performed anomalous X-ray scattering (AXS) in combination with reverse Monte Carlo (RMC) modeling to investigate intermediate-range local structure experimentally, and concluded that the improvement in the GFA appears to be associated with the local coordinations around the Ag and Cu atoms. Hosokawa et al. [7] also carried out AXS combined with the complementary neutron diffraction (ND). The analysis using RMC revealed that only the Zr-Ag partial structure factor indicates an intermediate-range atomic correlation, and the Cu and Ag slightly exhibit the phase separation tendencies.

Although the above structural studies gave information about the structure-property relationship in the Zr-Cu-Ag BMG, critical structural reason on the GFA was not yet clarified, or even the existence of the structural heterogeneity is still controversial as mentioned above. Thus, a different view is necessary to clarify the structure of BMGs. We have measured electronic structures on several Pd-based BMGs, such as Pd42.5Ni7.5Cu30P20 [[8], [9]], Pd40Ni40P20 [[8], [9]], Pd40Cu40P20 [9], and Pd30Pt17.5Cu32.5P20 [10], by measuring photoemission and inverse photoemission spectroscopies (PES and IPES). We found that the Pd 4d electrons in the valence band are highly localized, compared with those in pure Pd metal. It was also concluded that the P 2p core levels in these Pd-based BMGs clearly separate into two states, indicating that the P atoms have two different chemical sites, which is a strong proof for the existence of an elastic inhomogeneity [[9], [10]]. Furthermore, the Pd 3d5/2 core level in Pd30Pt17.5Cu32.5P20 exhibits a band broader than those in Pd42.5Ni7.5Cu30P20 and Pd40NiCu40P20, suggesting that the local structures around the Pd atoms become heterogeneous by replacing Ni with Pt [10].

As shown in our previous works, electronic structures in BMGs are highly feasible to investigate local environments with element-selective. Recently, we have performed valence-band and core-level PES and conduction-band IPES measurements on Zr45Cu45Ag10, Zr40Cu40Ag20, and the reference Zr50Cu50 metallic glasses with dc = 6, 2 [3], and 2 mm [11], respectively, to investigate local structures from the viewpoint of electronic structures. In this paper, we report results of the PES and IPES measurements, and discuss the features of element-selective heterogeneity in these metallic glasses in detail.

Section snippets

Experimental procedure

The Zr-Cu-Ag alloy ingots with nominal compositions were prepared by arc-melting the mixtures of Zr, Cu, and Ag metals with purities of 99.5, 99.99, and 99.99%, respectively, in a high-purity Ar atmosphere. For the Zr45Cu45Ag10 alloy, a cylindrical rod of ∼3 mm in diameter and ∼10 mm in length was manufactured by a tilt casting method with a Cu mold. The Zr40Cu40Ag20 and the reference Zr50Cu50 amorphous ribbons of ∼0.03 mm thick and ∼2 mm width were prepared by a single roller-spinning

Results

Fig. 2 shows the structure factors S(Q) of Zr50Cu50, Zr45Cu45Ag10, and Zr40Cu40Ag20 metallic glasses measured by neutron diffraction (ND), the first two of which were already published in Figs. 1(a) and 2(a) of Ref. [7]. The samples are the same as those used for the present PES and IPES measurements to examine the glass phase of the samples. The ND experiment was carried out using the High Intensity Total Diffractometer (NOVA) installed at the beamline BL21 of Materials and Life Science

Discussion

The electron configurations of Zr, Cu, and Ag in the outmost shells are 4d25s2, 3d104s1, and 4d105s1, respectively. Thus, the vicinity of EF are mainly composed of highly localized Zr 4d, Cu 3d, and Ag 4d electrons, and extended Zr 5s, Cu4s, and Ag 5s electrons.

An ultraviolet PES experiment was performed by Oelhafen et al. [15] on Zr1−xCux metallic glasses (x = 0.30, 0.40, and 0.60) using a He I discharged lamp light source ( = 21.2 eV). Fig. 7(a) shows the results of Zr40Cu60 (red solid

Conclusions

The PES and IPES measurements were carried out on Zr50Cu50, Zr45Cu45Ag10, and Zr40Cu40Ag20 metallic glasses to investigate the valence- and conduction-band electronic structures, respectively. From the dependence of the PES spectra, partial DOSs of s and d electrons for the constituent elements were estimated, and it was found that the DOS near EF is mainly composed of Zr 4d electrons and the Cu 4d electrons are localized in the valence band. Core-level PES measurements were also performed

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 Nos. 13-B-23 and 13-B-22), respectively. The ND experiments were performed at BL21/MLF/J-PARC (Proposal No. 2013A0032). This work was performed under the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (Proposal Nos. 14 K0033, 15K0003, and 16K0076).

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