Isothermal and non-isothermal crystallization kinetics of ultraviscous melt of Mg65Cu25Tb10 glass

Crystallization of ultraviscous melt of Mg65Cu25Tb10 glass has been studied by two procedures, (i) by keeping the samples at several fixed temperatures, and (ii) by heating the samples at different rates. One prominent exotherm appeared on crystallization in procedure (i), and two appeared on crystallization in procedure (ii). The Kolmogorov–Johnson–Mehl–Avrami relation for the fraction crystallized, x(t) = 1 − exp(−ktm), and the corresponding relation for rate-heating, fitted the results from both studies until x began to approach 1 and the measured crystallization rate became slower than the calculated rate. This is attributed to the slower crystallization of inter-granular melt of the same composition as the crystals. The rate coefficient value in ln k [min−m] increased from −8.25 at 448 K to −1.90 at 468 K with an activation energy of 169 kJ/mol, and m increased from 3.11 to 3.25. On rate-heating at 5 K/min, ln k [min−m] of the first exothermic peak increased from −21.3 at 458 K to −14.5 at 473 K with an activation energy of 246 kJ/mol and m decreased from 3.30 to 3.20. On annealing the ultraviscous melt, the first peak vanished but the second persisted. On reheating it also vanished. The slower kinetics of the second exothermic peak indicates that the melt crystallized partially to a metastable phase which transformed on heating.