KODETOVÁ Veronika Charles University in Prague

Spoluautoři VLACH Martin, STULÍKOVÁ Ivana, SMOLA Bohumil, HORNÁT Bohumil, KEKULE Tomáš

The MgYNdScMn alloy was squeeze-cast under a protective gas atmosphere (Ar + 1 % SF6). Precipitation reactions were studied by differential scanning calorimetry measurements. Additionally the electrical resistometry and hardness (HV3) measurements were performed. The samples of the alloy were annealed from room temperature up to 510 °C. Transmission electron microscopy and electron diffraction of specimens quenched from temperatures of significant resistivity and/or thermal changes were used to identify microstructural processes responsible for these changes. Differential scanning calorimetry revealed three exothermic effects connected to precipitation processes. No direct evidence for a precipitation process was observed in the temperature range of the first effect. As this early stage is already associated with a small resistivity decrease and small hardness increase, we suppose a submicroscopic spherical particle formation of a transient β‘‘phase with D019 structure there. Precipitation of the transient β‘ phase (cbco structure) in the form of fine plates was observed after annealing up to 270 °C. A weak thermal reaction corresponding to this process was detected only for heating rates of 5 and 10 K/min. The β‘ phase transforms into the stable β phase of the Mg5Gd-type which was observed after annealing up to 390 °C. Small discs of the Mn2Sc phase parallel to basal planes and tiny hexagonal basal Mn-RE plates of a transient hexagonal phase were observed simultaneously at this temperature. The main resistivity decrease in the temperature range 270 – 390 °C is caused by precipitation of these three phases. The apparent activation energy for the early stage, Q = (119 +/- 10) kJ.mol-1, and for β phase formation, Q = (147 +/- 10) kJ.mol-1, corresponds excellently to the results obtained in our previous study for pre-precipitation process and the stable phase in ternary MgYNd and WE43 alloys, respectively. No phases containing Y and/or Nd were detected above 420 °C. The aim of the conference paper is to report phase transformations during heat treatment with constant heating rates and the effect of precipitated phases on thermal properties.

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