PSI - Issue 23

Jan Pinc et al. / Procedia Structural Integrity 23 (2019) 21–26 Jan Pinc et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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detector OXFORD Instruments INCA 350 (EDX). The sizes of the Zn dendrites and intermetallic phases were evaluated using a ImageJ software. The micro- (HV 0.01) and macro-hardness (HV 30) was also measured in order to characterize the mechanical behavior of the materials.

2. Results and discussion

2.1. Microstructure of as-cast samples

It can be seen in Fig. 1 that the microstructure of the prepared material, with the diameter of 50 mm, consisted of three different phases. A major part of the structure was formed by zinc dendrites which were separated by an eutectic network. The majority of the eutectics was identified as a Zn-Mg 2 Zn 11 mixture by XRD; although, a minority of the eutectics consisted of a Zn-MgZn 2 mixture. In the lamellar eutectics, particles of the CaZn 13 intermetallic phase occurred. These particles were sharp-edged with a size ranging between 2 and 15 µm . Analysis of the material composition done by AAS confirmed ideal weight ratio (99:0.8:0.2) of added elements.

Fig. 1 Microstructure (SEM) and X-ray elemental maps (EDX) of the cast ZnMg0.8Ca0.2 with a F50 diameter

It is also obvious from Fig.1 that the calcium phases were embedded in the eutectics. It is caused by the solidification order of the particular phases. The CaZn 13 phase stay according to the Ca-Zn phase diagram in solid state, at the used melting temperature. During cooling, α -Zn dendrites formed first in the temperature interval of 420 – 364 °C. In this temperature interval, the “structure” consisted of α -Zn dendrites and a liquid in which the CaZn 13 particles were dispersed. At the temperature of 364 °C and lower, the remaining liquid underwent the eutectic reaction and the Zn-Mg 2 Zn 11 eutectic mixture formed. This rapid transformation from liquid to solid led to the embedding of the CaZn 13 particles in the eutectics. During the thermal treatment, the metastable MgZn 2 phase transformed into the stable Mg 2 Zn 11 phase. In addition, annealing led to the destruction of the lamellar structure of the eutectics and their transformation into a massive Mg 2 Zn 11 phase (Fig. 2).