Structural study of amorphous and nanocrystalline Mg-based metallic glass examined by neutron diffraction, X-ray photoelectron spectroscopy, Reverse Monte Carlo calculations and high-resolution electron microscopy

Abstract

The microstructure of amorphous and nanocrystalline Mg65Cu20Y10Zn5 alloy in as-quenched and post-annealed state was examined using neutron diffraction (ND), Reverse Monte Carlo modeling (RMC) and high-resolution transmission electron microscopy (HRTEM). The surface characteristic of Mg-based glass was investigated by X-ray photoelectron spectroscopy (XPS) method. The corrosion activity of alloy in glassy and nanocrystalline state was determined by electrochemical polarization measurements conducted in 3.5% NaCl solution. It was found that the structure of the melt-spun glass is homogeneous, but some medium-range order (MRO) regions as small as 1–2 nm were observed. The RMC analysis of ND data was used for local atomic structure and coordination numbers of the Mg65Cu20Y10Zn5 alloy in as-quenched state. The three-dimensional model representing a simulation box with atomic arrangements was also proposed. An average coordination number for Mg-Mg and Mg-Cu atoms is 8.1 and 2.5, adequately. The formation of nanocrystalline structure after annealing was confirmed by X-ray diffraction (XRD) and HRTEM observations. The TEM and HRTEM techniques allowed to observe regions containing very small particles in the amorphous matrix with size of 4–10 nm. The selected area electron diffraction (SAED) patterns allowed to identified hexagonal Mg and orthorhombic Mg2Cu phases. XPS results indicated the co-existence of MgO and Mg(OH)2 on the surface film formed on glassy ribbon in as-quenched state. The electrochemical polarization measurements revealed that the glassy ribbon exhibited lower corrosion current density than nanocrystalline sample.