Mechanical properties and devitrification behavior of Cu-Zr-Ti-NM (NM-noble metals) bulk glass-forming alloys

The present work is devoted to an investigation of the formation kinetics, stability and homogeneity area of the nanoscale icosahedral phase formed on heating in the Cu-Zr-Ti-NM system alloys. The mechanical properties, glass-forming ability, supercooled liquid region and devitrification behaviour of the Cu-Zr-Ti-NM (NM= Pd, Ag, Au and Pt) bulk glass formers were studied. The bulk glassy alloys of diameter 2 mm were formed in the Cu₅₅Zr ₃₀Ti₁₀Pd₅ and Cu₅₅Zr ₃₀Ti₁₀Ag₅ alloys while Cu₅₅Zr ₃₀Ti₁₀Au₅ bulk alloy showed mixed glassy and crystalline structure. No glassy phase was formed in the Cu₅₅Zr ₃₀Ti₁₀Pt₅ bulk alloy whereas the glassy phase was formed in all of the ribbon samples prepared by rapid solidification. The studied alloys except for the Pt-bearing one have slightly increased ultimate compressive fracture strength and yield strength values compared to ternary Cu₆₀Zr₃₀Ti₁₀ glassy alloy. At the same time Pd and Au addition significantly expand the supercooled liquid region of Cu-Zr-Ti glassy alloy and increase Young's modulus. A nanoicosahedral phase is primarily formed in the Cu₅₅Zr₃₀Ti₁₀(Pd,Au)₅ glassy alloys in the initial stage of the devitrification process by nucleation and 3-dimensional diffusion controlled growth. Nearly the same quasilattice constant obtained in the Cu₅₅Zr₃₀Ti₁₀(Pd,Au) ₅ alloys illustrates the same type of the icosahedral phase in these alloys. However, no icosahedral phase was found in the Cu₅₅Zr ₃₀Ti₁₀(Ag,Pt)₅ alloys. The Cu-Zr-Ti-Pd icosahedral phase is not a Cu-rich part of the compositional homogeneity area of the Zr-Cu-Pd one. Nanoscale icosahedral phase has chemical composition close to that of the alloy composition. The possibility for the Cu-Zr-Ti-Pd icosahedral phase of being a Cu-rich part of the compositional homogeneity area of the Zr-Cu-Pd one is ruled out. Moreover, Ti is found to be important element stabilizing quasicrystalline phase in the Cu-Zr-Ti-Pd alloys. Formation criteria for Cu- and Zr/Hf-based icosahedral phases are discussed based on quasilattice constant to average atomic diameter ratio. Deviation from this criterion leads to destabilization of the icosahedral phase.