Preparation of Ti-Cu-Ni-Si-B amorphous alloys with a large supercooled liquid region

A large supercooled liquid region exceeding 60 K before crystallization was found in a wide composition range of 15 to 20 at%Cu and 25 to 30 at%Ni for the Ti_94-x-yCu_xNi_ySi₄B₂ amorphous alloys which satisfy the three criteria of multi-components, significant atomic size ratios and negative heats of mixing. The glass transition temperature (T_g) and the reduced glass transition temperature (T_g/T_m) of the Ti₅₀Cu₂₀Ni₂₄Si₄B₂ alloy are 742 K and 0.63, respectively. The high thermal stability of the supercooled liquid as well as the high reduced glass transition temperature enabled us to produce bulk amorphous alloys with a diameter of 1 mm by copper mold casting. The crystallization of the Ti₅₀Cu₂₀Ni₂₄Si₄B₂ amorphous alloy takes place through a single exothermic reaction and the four phases of Cu₄Ti₃, CuTi, NiTi and TiB precipitate almost simultaneously. The crystallization mode requires the long-range atomic rearrangements of the constituent elements and the necessity seems to cause the increase in thermal stability of the super-cooled liquid. The success of forming the bulk amorphous alloys in the Ti-Cu-Ni-Si-B system with lower densities is important for the future development of bulk amorphous materials with a higher specific strength.