Local Electronic Structures and Chemical Bonds in Zr-Based Metallic Glasses

Local electronic structures of Zr-based metallic glasses have been calculated using the discrete variational Xcluster molecular orbital method. The cluster models are constructed for Zr-Cu, Zr-Ni and Zr-Pd metallic glasses on the base of local structure parameters determined by extended X-ray absorption fine structure analysis. The valence-band X-ray photoelectron spectra computed from the local electronic structures agree well with experimental spectra. Based on the feature of chemical bonds in those glasses evaluated from the electronic structures, we discuss the relationship between the chemical bond and the stability of supercooled liquid state. The strength of the Zr-Ni bond is larger than that of Zr-Cu bond, while the Cu-Cu bond strength is nearly equal to the Ni-Ni bond. Moreover, the Zr-Zr bond strength is larger in Zr-Cu glass than in Zr-Ni glass. This indicates that the formation of primary crystalline phase is more difficult in Zr-Cu glass than in Zr-Ni glass. The difficulty contributes to the relative stabilization of supercooled liquid state in Zr-Cu glass compared with that in Zr-Ni glass. It is also shown that local atomic structures of Zr-Pd glass are well described by the icosahedral atomic configuration. Recently, bulk metallic glasses have attracted much attention because of the importance in materials science and the extension of their application fields. 1) Bulk metallic glasses are characterized by a wide supercooled liquid region, � Tx which is defined as the temperature interval between glass transition temperature, Tg and crystallization temper- ature, Tx. The appearance of the wide supercooled liquid region implies the high resistance against crystallization, leading to large glass-forming ability. The stable supercooled liquid state enables the production of bulk metallic glasses. It is therefore quite important to understand the reason for the stabilization of the supercooled liquid state from the view- point of interests in materials science as well as development of new bulk metallic glasses. The three empirical rules have been proposed by Inoue 2) in order to explain the origin of the wide supercooled liquid region and good glass-forming ability in bulk metallic glasses. It is also suggested that the stability of the supercooled liquid state is strongly correlated with their local structure. 3-6) Alamgir et al. 7,8) have inves- tigated the relation between the electronic structure and the glass-forming ability in Pd-Ni-P glassy alloys, and pointed out that the existing electron models are not fully adequate to explain the high glass stability of the Pd40Ni40P20 alloy. However, little is known about the relation of the chemical bond among constituent atoms with the stability of super- cooled liquid state in bulk metallic glasses. The present study is intended to investigate the local electronic structures and the chemical bonds among constit- uent atoms in Zr-based metallic glasses, and to clarify the relation between the chemical bond and the stability of supercooled liquid state. The local electronic structures are calculated for Zr-Cu, Zr-Ni and Zr-Pd metallic glasses using the discrete variational (DV) Xcluster molecular orbital