PSI - Issue 23

Jaroslav Čapek et al. / Procedia Structural Integrity 23 (2019) 3 –8 Jaroslav Čapek et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results and discussions

3.1. Chemical composition

The exact chemical composition of the prepared alloy obtained by the AAS analysis is listed in Table 1. In Table 1, it is clearly visible that the chemical composition fitted very well the desired one and only small inaccuracies in the chemical composition were detected.

Table 1. Chemical composition of the prepared alloys (AAS). Alloy Mg ( wt.% )

Ca/Sr ( wt.% )

Zn ( wt.% )

ZnMg0.8Ca0.2 ZnMg0.8Sr0.2

0.82 0.81

0.18 0.17

balance balance

3.2. Phase composition

Phase composition was measured throughout the whole diameter of the ingots. It was found that the phase composition differed throughout the cross-section of the as-cast materials. In the center of the samples, Zn, MgZn 2 and CaZn 13 , or SrZn 13 were found. In contrast to that, Mg 2 Zn 11 was found instead of the MgZn 2 phase in the outer regions of the ingots. In the middle regions, both of those two Mg-based phases were detected. Those results show that the formation of the Mg-based intermetallic phases depends on the cooling rate. According to the binary phase Zn-Mg phase diagram, the Mg 2 Zn 11 phase is thermodynamically stable, while the MgZn 2 phase is metastable at the studied chemical compositions. Therefore, it seems to be surprising that the MgZn 2 phase (metastable) was preferentially formed at a low cooling rate and the stable Mg 2 Zn 11 phase at the high cooling rates. This phenomenon, however, was also observed in some other studies and was explained by a competitive growth of the Zn-MgZn 2 and Zn-Mg 2 Zn 11 eutectics and by the difficulties in the formation of a seed of the Mg 2 Zn 11 phase, which possesses a complex and large crystallographic structure Liu and Jones (1992), Akdeniz and Wood (1996). Compared to the as cast materials, the phase composition of the annealed materials was homogenous in the whole volume. The annealed materials consisted of Zn, Mg 2 Zn 11 and CaZn 13 or SrZn 13 phases. The microstructures of the as-cast alloys are shown in Fig. 1. Both alloys possessed similar microstructures consisting of several phases and some casting defects such as pores and shrinkages. The alloys were formed by α -Zn dendrites, an interdendritic eutectic network consisting of an eutectic mixture of Zn and MgZn 2 , or Mg 2 Zn 11. The last components were particles of CaZn 13 /SrZn 13 intermetallic phases. The majority of those intermetallic phases was embedded in the eutectic network (Fig. 1). Although both CaZn 13 and SrZn 13 phases possessed the same crystallographic structure, they differed significantly in their morphology and size. The CaZn 13 particles were sharp edged and relatively coarse with a size ranging between units and several tens of micrometers. On the other hand, the SrZn 13 phase formed round and very fine particles with submicron dimensions (see Fig. 1d). Microstructures of the selected annealed materials are shown in Fig. 2. Annealing for short times (4 and 8 h) led to changes in both phase composition and microstructure. The metastable MgZn 2 phase transformed in the Mg 2 Zn 11 phase and the eutectic structure disappeared and transformed to an interdendritic network consisting of the Mg 2 Zn 11 and CaZn 13 /SrZn 13 intermetallic phases. After longer annealing times (16 and 24 h), the intermetallic network started to disrupt (Figs. 2b and d). Those disruptions took place in order to decrease the interfacial energy of the phases. 3.3. Microstructure