Kinetics of crystallization process of Mg-Cu-Gd based bulk metallic glasses

The crystallization behavior of Mg61Cu28Gd11 and (Mg61Cu28Gd11)(98)Cd-2 bulk metallic glasses was studied using DSC in the mode of continuous and isothermal heating, and its crystallization process and microstructure were confirmed by XRD and TEM. In continuous heating, the activation energies of glass transition, onset and peak crystallization were determined by the Kissinger method, which yields 110 +/- 12, 77 +/- 9 and 79 +/- 10 kJ/mol, respectively, for Mg61Cu28Gd11 glassy alloy, and 144 +/- 10, 126 +/- 6 and 131 +/- 5 kJ/mol, respectively, for (Mg61Cu28Gd11)(98)Cd-2 glassy alloy. The isothermal kinetics was modeled by the Johnson-Mehl-Avrami equation. The Avrami exponent of the base alloy was in the range from 1.98 to 2.56 (+/- 0.01), which indicated a decreasing nucleation rate and a diffusion-controlled growth. For Cd-added glassy alloy, the Avrami exponent was in the range from 3.26 to 4.08, which indicated an increasing nucleation rate. The activation energies in isothermal process were calculated to be 88 +/- 2 and 132 +/- 2 kJ/mol, respectively, for the base and Cd-added glassy alloys. It was found that Mg2Cu phase was the primary phase in the initial crystallization and the strong affinity between Cd and Mg/Gd tended to impose resistance to the formation of Mg2Cu phase and thus improves the thermal stability. (C) 2012 Elsevier BM. All rights reserved.