Ab Initio Molecular Dynamics Simulation of the Amorphous Structure of Ca-Mg-Cu and Ca-Mg-Zn Alloys
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INTRODUCTION
CA-MG-BASED bulk metallic glasses (BMGs) have unique properties. They are built on two simple metals, Ca and Mg, while all other BMGs are transitionmetal based alloys.[1,2] Many Ca-Mg-based alloys have very good glass forming ability (GFA), and even their ternary alloys, e.g., Ca-Mg-Zn or Ca-Mg-Cu, can be made fully amorphous when they are cast in ~8 to 10 mm thick plates[3–8] or up to 15 mm diameter rods.[9,10] These glasses have very low Young’s and shear moduli that are comparable with the moduli of human bones,[11–13] low density (1.6 to 2.8 g/cm3),[1] and strong relaxation dynamics of the super-cooled liquid.[14] GFA, as well as physical and mechanical properties of metallic glasses, is believed to depend on the type of short range order (SRO) and medium range order (MRO) of alloying elements in the amorphous structure. It is therefore interesting to analyze the amorphous structure of Ca-Mg-based BMGs. Structural analysis of metal-metal BMGs is mainly focused on transition metal glasses such as Zr-Cu,[15–18] Zr-Pt,[19] and Zr-Cu-Al.[20,21] The results indicate that the packing of atoms in these materials is not random, but is strongly influenced by chemical interactions. In particular, icosahedral SRO has been identified and correlated to their good GFA.[22] On the other hand, the atomic structure of Ca-Mg-Zn BMGs, identified with the use of X-ray and neutron diffraction and Reverse Monte Carlo O.N. SENKOV, Senior Scientist, is with the UES, Inc., Dayton, OH, and also with the Air Force Research Laboratory, Wright Patterson Air Force Base, OH 45433. Contact email: oleg.senkov@ wpafb.af.mil Y.Q. CHENG, R&D Associate, is with the Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, and also with the Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218. Manuscript submitted May 29, 2012. Article published online September 26, 2012 1980—VOLUME 44A, MAY 2013
(RMC) simulation, shows no icosahedral SRO.[23] Instead, five-fold bonds in the form of pentagonal bi-pyramids have been found to be the most populous structural units in these BMGs.[23] A large fraction of fivefold bonds and the lack of icosahedral SRO has also been found in the molecular dynamic (MD) simulated amorphous structure of Mg-Cu alloys.[24] In the present work, the atomic structures of several Ca-Mg-Cu and Ca-Mg-Zn BMGs were simulated with the use of ab initio (quantum) molecular dynamics (QMD) simulation. The simulated structures were then used to calculate partial (PRDF) and total (RDF) radial distribution functions, pair bond distances and partial and total coordination numbers. The QMD-simulated results were validated through the experimentally determined RDFs.
II.
EXPERIMENTAL PROCEDURES
QMD simulation of the amorphous structures of the Ca60Mg15Zn25, Ca60Mg25Zn15, Ca60Mg15Cu25, Ca60Mg25 Cu15, Ca50Mg25Cu25, and Ca40Mg25Cu35 alloys was conducted using the Vienna ab initio Simulation Package (VASP).[25] The Projector Augmented-Wave (PAW) method[26,27] a
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