Crystallization kinetics of Pd-based amorphous alloy

Abstract

Crystallization kinetics of Pd40Ni10Cu30P20 amorphous alloy has been studied by using differential scanning calorimetry (DSC) methods in both isothermal and non-isothermal modes. One prominent exotherm appeared on crystallization in isothermal mode, and two appeared on crystallization in non-isothermal mode. 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. The Kolmogorov–Johnson–Mehl–Avrami relation for the fraction crystallized, x(t)=1−exp(−ktm), and the corresponding relation for rate-heating as in non-isothermal mode, 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 k [s−m] increased from 1.8 x 10−14 at 593 K to 1.7 x 10−9 at 633 K with an activation energy of 252 kJ/mol, and m increased from 4.04 to 4.41. On rate-heating, m of the first exothermic peak decreased from 4.10 at 5 K/min to 4.02 at 80 K/min with an activation energy of 249 kJ/mol. The resulting values of the exponent m of both isothermal and non-isothermal crystallizations, suggests that crystal growth of the Pd40Ni10Cu30P20 amorphous alloy is interface-controlled with increasing nucleation rate.