Density, thermal stability and mechanical properties of Zr-Ti-Al-Cu-Ni bulk amorphous alloys with high Al plus Ti concentrations

Bulk amorphous Zr_70-x-yTi_xAl_yCu₂₀Ni₁₀ alloys with a thickness of 2.5 mm were formed in a wide composition range of 0 to 10 at%Ti and 7.5 to 15 at%Al by a squeeze casting process. The bulk amorphous alloys containing 0 to 5%Ti and 12.5 to 15%Al exhibit good combined properties of high tensile fracture strength above 1850 MPa, high specific strength of 28.4 × 10⁵ N·m/kg and high T_g/T_m above 0.60 which have not been obtained for the Zr-based amorphous alloys reported hitherto. These bulk amorphous alloys have small difference in density of 0.3 to 0.6% between ascast amorphous and crystallized states. The high dense random packing fraction in the multicomponent amorphous structure seems to cause the high glass-forming ability and the high thermal stability of the supercooled liquid region by the decrease in atomic mobility. Furthermore, the crystallized phases change from NiZr₂+CuZr+Zr₃Al in the range of 5 to 15%Al and 0 to 2.5%Ti to NiZr+CuZr₂+Zr₃Al in the range of 5 to 15%Al and 2.5 to 7.5%Ti. The melting temperatures (T_m) of these crystallized compounds increase in the order of ZrNi>Zr₂Ni>Zr₂Cu>ZrCu and hence the crystallized structure of the 5%Ti-containing alloys is composed of ZrNi and Zr₂Cu with higher T_m. The change in the amorphous structure leading to the precipitation of the compounds with higher T_m may be the reason for the higher mechanical strength in the higher Al and Ti contents.