Calibration Of Differential Thermal Analysis And Its Application In The Crystallization Process Of Mg-based Amorphous Alloy

Magnesium and Mg-based alloys were widely used as the lightest weight material for engineering, and further improvement of the strength and toughness of Mg-based alloys was required to meet the requirements due to the rapid development of aero-and auto-industries. Controlled crystallization from bulk amorphous Mg-based alloys is proved to be an effective route to achieve this goal. Thus, investigations on kinetics and thermodynamics of the controllable crystallization process became more and more popular. Differential Thermal Analysis (DTA), as an important kinetic method for the research of crystallization process, possesses systematical error that mostly comes from the asymmetrical distribution of the working parts and the smearing effects due to the interface thermal resistance, which will also introduce distortion of experimental information and then bring non-determinacy in the fields of engineering application and scientific research. So, thermal flow model of DTA process is designed for calibration. Here a thermal flow model of DTA measurement process is proposed, and the (heating-and cooling-) rate independent characteristic of the ferro-to paramagnetic transition (Curie temperature) is adopted for the calibration of DTA apparatus. Three pure metals with apparent Curie transition, nickel, iron and cobalt, are selected for calibration of temperature in the corresponding temperature ranges. The heat capacity of three materials after calibration is more accurate compared with the apparent ones. Following the above procedures, more accurate expression of heat capacities of nickel, iron and cobalt were obtained. Amorphous Mg82.0Ni18.0 alloy was successfully prepared by means of melt spinning, and its crystallization process was systematically explored by adopting Differential Thermal Analysis combined with X-ray diffraction analysis and Transmission Electron Microscopy. It was found out: (1) The higher the heating rates, the higher the onset crystallization temperature and the wider the temperature range of crystallization, but the less the time of crystallization process. (2) The crystallization of amorphous Mg82.0Ni18.0 alloy takes place in a primary fashion as defined: Intermetallics Mg2Ni and metastable Mg6Ni phase firstly separated out, and then the remaining amorphous matrix transformed into the stable phases (i.e. Mg-rich solid solution phase and Mg2Ni intermetallics), and the last step is the decomposition of the metastable phase (i.e. Mg6Ni) into stable ones. (3) Metastable phase precipitates and exist in a temperature range which overlaps the whole crystallization peak, and the corresponsing precipitation and decomposition are temperature dependent.