Glass—liquid transition in hyperquenched metal alloys

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Abstract
Differential scanning calorimetry data of hyperquenched metal and shear amorphized alloys show pronounced exotherms, due to enthalpy or structural relaxation, which mask their glass—liquid transition endotherms. Thermal treatment of the alloys removes the exothermic effect, which reveals the amorphous nature of the alloys. Calorimetric effects associated with the approach of an amorphous structure towards a lower-energy, but still amorphous, state are indistinguishable from those associated with grain-growth in a microcrystalline solid. It is concluded that this exothermic calorimetric behaviour of structurally disordered alloys masks the characteristic endothermic step from their glass—liquid transition. A distinction between the microcrystalline and amorphous nature of a structurally disordered solid can be more satisfactorily made after the removal of this effect, and a two-step test by calorimetry, given here, alone can characterize its amorphous structure. Our conclusions contrast with those of Chen and Spaepen who proposed that the isothermal calorimetric effect or its signal is characteristic of grain growth and thus unambiguously distinguishes a microcrystalline from an amorphous solid.