THE CRYSTALLIZATION OF A PD60TI20SI20 METALLIC-GLASS

The kinetics and mechanism of crystallization have been followed in a Pd60Ti20Si20 metallic glass by means of a combination of experimental techniques. As quenched alloys contain a variable population of crystals and/or nuclei. Oxide islands were found mostly on the dull (wheel-side) surface of the ribbon, favouring partial crystallization around 710 K. The glass transition temperature, T(g), is manifest in DSC and low-frequency internal friction measurements from heating rates of 1 K/min upwards, prior to the main crystallization event (761 and 772 K at 20 K/min heating rate, respectively). Isothermal crystallization in the temperature range from 739.0 to 758.7 K occurred in two steps because of a different population of quenched-in nuclei in various parts of the ribbon. A Johnson-Mehl-Avrami analysis of the DSC isothermal output gave an Avrami exponent of 3, indicating growth on preexisting nuclei, as shown by microscopy observations. The activation energy and the preexponential factor for growth are very high: 566 +/- 20 kJ/mol and 10(36) ms-1 respectively. This finding is discussed with reference to analogous data for diffusion in metallic glasses.