Noncontact measurement of crystallization behavior, specific volume, and viscosity of bulk glass-forming Zr-Al-Co-(Cu) alloys

Crystallization behavior, specific volume, and viscosity are investigated using the noncontact electrostatic levitation technique for metallic glass-forming alloys in the Zr-Al-Co-(Cu) system. The compositions investigated are Zr55Al20Co25, Zr55Al22.5Co22.5, and Zr55Al19Co19Cu7. Free radiative cooling in the electrostatic levitator could vitrify all the three alloys. This allowed, for the first time, the determination of time-temperature-transformation (TTT) curves for ternary metallic alloys by isothermal annealing over a wide temperature range between the liquidus and glass transition temperatures. While the TTT curve for Zr55Al22.5Co22.5 shows the expected "C" shape with a single "nose," the TTT curves for the other two alloys show two noses. X-ray diffraction study shows that the double-nose structure for Zr55Al20Co25 and Zr55Al19Co19Cu7 is caused by the overlap of TTT curves for two different crystalline phases. For Zr55Al22.5Co22.5, however, the same crystalline phases are formed over the temperature range of the entire TTT curve, which is supported by the single nose. The average critical cooling rate for glass formation of the Zr-Al-Co-(Cu) alloys estimated from the measured TTT curves is about 17 K/s. This value of critical cooling rate is supported by the trends in specific volume and dynamic viscosity of these alloys compared to other bulk metallic glass-forming alloys.