Nanoscale icosahedral phase formation in Cu-based bulk glass formers

Quaternary Cu₅₅Zr₃₀Ti₁₀NM₅ glassy alloys (NM= noble metal) prepared by rapid solidification of the melt and by copper mold casting are studied in the present work. Alloys ingots were prepared by arc-melting mixtures of pure metals in an argon atmosphere. Glass-forming ability, supercooled liquid region and devitrification behaviour of Cu-Zr-Ti-(Pd, Ag, Pt and Au) glassy alloys were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and isothermal calorimetry. A nanoicosahedral phase of about 3-10 nm in size is primarily formed in the Cu₅₅Zr₃₀Ti ₁₀(Pd,Au)₅ glassy alloys in the initial stage of the devitrification process. A multi-peak fitting by the Lorentzian function was used to separate the overlapped X-ray diffraction peaks. A direct evidence of the formation of the icosahedral phase was obtained using nanobeam diffraction in TEM. The results of the isothermal calorimetry data analysis indicate steady-state nucleation and 3-dimensional diffusion-controlled growth of nuclei. The quasilattice constant values of 0.495 nm and 0.496 nm obtained in Cu ₅₅Zr₃₀Ti₁₀Pd₅ and Cu ₅₅Zr₃₀Ti₁₀Au₅ glassy alloys, respectively, in the relationship with the average atomic diameter indicate that the icosahedral particles consist of rhombic triacontahedra. The icosahedral particles have a metastable nature and transform to the equilibrium phases through the intermediate crystalline phases. At the same time Pd and Au additions significantly expand the supercooled liquid region of the Cu-Zr-Ti glassy alloy. However, no icosahedral phase but the crystalline intermetallic compounds were primarily formed in the Cu₅₅Zr₃₀Ti ₁₀(Ag,Pt)₅ glassy alloys.