Multistage devitrification of Mg-Ni-Mm and g-Ni-Y-Mm metallic glasses (Mm = misch metal)

Abstract

Devitrification behaviour of Mg-Ni-Mm and Mg-Ni-Y-Mm metallic glasses (Mm = misch metal) was studied by differential scanning calorimetry, isothermal calorimetry, X-ray diffractometry and transmission electron microscopy. These materials exhibited a multistage devitrification with a primary exothermic peak with an onset temperature of about 450 K. Local atomic order changed during primary devitrification. This change occurs without an incubation period and leads to embrittlement of the samples. The activation energy value for the first exothermic heat effect in the Mg 86 Ni 10 Y 2 Mm 2 alloy is higher than the activation energy for Mg self-diffusion, implying that redistribution of the other alloying elements also takes place. The strongest heat effects are connected with the devitrification of the amorphous phase (changes in the local atomic order, precipitation of hcp Mg and the metastable compounds) while the magnitude of the heat effect of the transformation of metastable crystalline phases to stable phases in the Mg 86 Ni 10 Y 2 Mm 2 alloy, for example, is 17 times weaker than the effect of devitrification of the amorphous phase. A continuous heating transformation diagram is derived from the results.