CRITICAL COOLING RATES FOR GLASS FORMATION OF ZR-AL-CU-NI AND MG-NI-ND ALLOYS

Abstract

The critical cooling rates Rc for glass formation in four Zr-based Zr-Al-Cu-Ni alloys were determined using three methods, namely, from Time-Temperature-Transformation (TTT) curves, from Continuous Cooling Transformation (CCT) curves and from thermal analytical measurements. These alloys were rapidly solidified by chill-block melt-spinning to produce ribbons between 40 to 100 µm in thickness. They exhibit a wide supercooled liquid region before crystallisation. The temperature spans between onset glass transition temperature, Tg, and crystallisation temperature, Tx, ?Tx (=Tx-Tg) are more than 50 K and the largest ?Tx reaches 99 K. In addition, the Re of four Mg-based Mg-Ni-Nd alloys, which also exhibit a wide supercooled liquid region, were determined from thermal analytical measurements. Determination of Rc from TTT curves is an approach developed by Uhlmann [6]. His analysis is based on the theories of homogeneous nucleation, crystal growth and transformation kinetics. This method involves the construction of a TTT curve which defines the time required at any temperature to produce a volume fraction of crystallisation, x = 10-6. The calculation of the TTT curve requires the knowledge of the viscosity-temperature curve of each alloy, in addition to other physical properties. The dependence of viscosity on temperature for the alloys were determined from thermal measurements based on the method developed by Chen [67].Factors such as viscosity and nucleation barrier which affect Uhlmann's analysis were also examined and discussed. The second method to determine Rc is an extension of the first method. Instead of the isothermal TTT curves, continuous cooling transformation (CCT) curves are considered. The CCT curves were constructed from TTT curves following the approach suggested by Grange and Keifer [56].The critical cooling rates for glass formation in the Zr-AI-Cu-Ni alloys calculated from CCT curves were found to be half of that determined from TTT curves. The method of determining Rc from thermal analytical measurements is developed by Barandarian and Colmenero [19]. The equation used to determine Rc is derived by considering liquid-solid transformation under non-isothermal conditions. This approach involves the measurement of the offset temperature for fusion, TL, and the onset temperature of solidification, TXC, upon cooling at different rates, R, using thermal analytical techniques, such as Differential Thermal Analysis (OTA). Lastly, parameters that are used to reflect glass forming ability (GFA) in metallic glasses were calculated and discussed for these easy glass forming Zr-AI-Cu-Ni and Mg-Ni-Nd alloys.