Phase separation and enhancement of plasticity in Cu-Zr-Al-Y bulk metallic glasses

Enhancement of plasticity and the occurrence of liquid-state phase separation have been investigated by partially substituting Zr with Y in Cu₄₆Zr_47-xY_xAl₇ (x = 0, 2, 5, 10, 15, 20, 25, 30, 35) alloys. Since Y has a positive enthalpy of mixing with Zr (+35 kJ/mol) in the liquid state, the alloy composition moves to the metastable miscibility gap of the two amorphous phases with increasing Y content. Phase separation into Y-rich and Zr-rich amorphous phases occurs during cooling from the liquid state when the Y content is above 15 at.%. The bulk amorphous alloys consisting of two phase-separated amorphous phases exhibit extreme brittleness, while the single-phase amorphous alloys containing 2-5 at.% of Y exhibit enhanced plasticity because of the introduction of chemical inhomogeneity in the amorphous matrix. The results indicate that the addition of an element having a positive enthalpy of mixing with the constitutive element in bulk amorphous alloys can increase the plasticity as well as the glass-forming ability, but for a limited composition range.