Enhanced Plasticity of Cu-Zr-Ti Bulk Metallic Glass and Its Correlation with Fragility

Inadequate room-temperature plasticity limits the application of bulk metallic glasses (BMGs). This study focuses on enhancing the plasticity of BMGs for widening their applications. Composition modification of Cu₆₀Zr₂₅Ti₁₅ through the minor addition of Ni is the approach adopted in this study. A systematic increase in mechanical properties is observed with increasing Ni content (up to 5 at. pct), followed by a decrease thereafter. Values of yield stress (2425 MPa), fracture stress (2513 MPa), maximum stress (2725 MPa), and plastic strain (16 pct) exhibited by (Cu₆₀Zr₂₅Ti₁₅)₉₅Ni₅ BMGs are higher than those reported in the literature for Cu-BMGs (high Cu content). Analysis of the glass transition region indicated that the enthalpy values of the exothermic heat flow prior to glass transition (ΔH_bg) and the fragility parameter (m) are well correlated with the plasticity of the alloys. The increasing fragility parameter and exothermic enthalpy correspond to higher free volume. Therefore, high plasticity values can be attributed to the free volume modification (creation and distribution) caused by Ni addition. This free volume modification initiates shear bands and promotes their branching; consequent interactions among them increase the resistance to shear band propagation and, thereby, delay the fracture.