Glass Forming Ability And Mechanical Behaviors Of ZrCuNiAl Bulk Metallic Glasses

In this dissertation, Zr-Cu-Ni-Al quaternary system of bulk metallic glass was studied. The chemical composition of Zr₅₁Cu_24.70Ni_14.34Al_9.96 bulk metallic glass was designed by a method of proportionally mixing the deep-euectic compositions of relevant binary alloys. Along with Zr₅₁Cu_20.7Ni₁₂Al_16.3 and Zr₅₁Cu_24.22Ni_14.06Al_10.72 bulk metallic glasses which were obtained in the design process, the glass forming ability and mechanical behavior of the three BMGs were investigated.The results show that Zr₅₁Cu_24.70Ni_14.34Al_9.96 bulk metallic glass has high glass forming ability (GFA) of nearly 12 mm critical diameter with the glass transition temperature T_g = 728 K and the reduced glass transition temperature Trg (T_g / T_m) = 0.707. But the 12 mm sample was found to contain farthing crystalline phases, including a hexagonal crystalline phase Zr₅Al₄ (a = 8.437, c = 5.8) and a cubic crystalline phase Cu9Al4 (a = 8.702). Accordingly, reducing the content of Al can further improve the glass forming ability of the bulk metallic glass.Mechanical behaviors of Zr₅₁Cu_20.7Ni₁₂Al_16.3, Zr₅₁Cu_24.22Ni_14.06Al_10.72 and Zr₅₁Cu_24.70Ni_14.34Al_9.96 bulk metallic glasses were evaluated at room temperature, including compressive fracture, hardness and nanoindentation. Experimental results show that the fracture strength of Zr₅₁Cu_24.70Ni_14.34Al_9.96 bulk metallic glass is about 2481 MPa. Due to the linearity relation between the fracture strength and the glass transition temperature, Zr₅₁Cu_24.70Ni_14.34Al_9.96 bulk metallic glass has simultaneously excellent mechanical properties and high glass forming ability.The mechanical behaviors of Zr₅₁Cu_24.70Ni_14.34Al_9.96 bulk metallic glass under different test conditions were investigated. It is shown that the fracture strength and plastic strain of the metallic glass increased as the strain rate decreased, while the fracture strength and plastic strain of the metallic glass increased as the sample diameter decreased. The reason is that the smaller sample with a high cooling rate reserved more free volume, which initiated shearing bands multiplication. As a result, the plasticity of the bulk metallic glass was improved. The low-temperature compressive tests of Zr₅₁Cu_20.7Ni₁₂Al_16.3 bulk metallic glass were detected. The results show that the strength and plasticity of the alloy both increase at lower temperature. The fracture strength is about 2361 MPa and plastic strain is about 14 %. Higher strength of the alloy is due to lower concentration of free volume, a higher load is needed to promote shear bands. Low temperature restrains the adiabatic heating, reducing shear bands motion, and consequently enhancing the plastic.