*4.1. The E*ff*ect of Solid-Solution Treatment*

The initial homogenization treatment of as-cast materials had the intention to re-solutionize primary precipitates, in all the materials considered in this study. Therefore, a homogenization treatment at 450 ◦C for 24 h was applied for Ca-containing alloys. An exception was made for Mg5Zn, for which a temperature of 350 ◦C for 12 h was chosen in order to avoid melting. Lower annealing times and temperatures did not remove the primary precipitates, and increasing the annealing time beyond 24 h did not change their density.

In order to stabilize at RT this well-homogenized state and to obtain supersaturated solid-solution condition (which is called the initial state (IS) in the forthcoming text), two types of cooling were used and investigated: furnace-cooling and quenching in ice water. The microhardness values of furnace-cooled alloys Mg5Zn0.3Ca, Mg5Zn, Mg5Zn0.15Ca and Mg5Zn0.15Ca0.15Zr were noticeably lower than those of as-cast samples (Table 3); the microhardness values of the quenched samples, however, were slightly higher.

Our explanation of the latter effect is as follows: As intended, the higher-temperature heating of the materials led to the dissolution of primary precipitates and annihilation of dislocations along with the reduction of the total area of grain boundaries. Nevertheless, the concentration of thermal vacancies was high at the high temperatures. Quenching froze the vacancies, and the higher the cooling rate, the higher was the number of quenched-in vacancies. However, in contrast to the mono-vacancies, di- and multi-vacancies and/or vacancy agglomerates form barriers to the dislocation motion, leading to the so-called quench-hardening [53]. The actually measured rise in microhardness after quenching in comparison with the as-cast state can be thus attributed to the quench-hardening effect.

In contrast, furnace-cooling leads to a decrease in hardness as a result of annealing of thermal vacancies during slow cooling. At the same time, grains start to coarsen and dislocations leave the lattice, both leading to the softening of materials.

SEM images (Figure 6) show that for the alloys Mg5Zn0.15Ca0.15Zr, Mg5Zn0.15Ca and Mg0.3Ca, no significant difference between the IS (furnace-cooled) and IS (quenched) exist in terms of primary precipitates. However, for Mg5Zn0.3Ca, it was not possible to completely dissolve Ca in the Mg matrix. Here, furnace-cooling was more effective than quenching for approaching the solid solution supersaturated state, and fewer residual primary precipitates were found.

However, the negative effect of furnace-cooling is reflected in the possible formation of various complex phases, which may not occur in the quenched samples. The nature and the composition of such phases may affect the microstructure and therefore the properties of the alloy. The densities of these precipitates may be difficult to measure, and the determination of parameters that control the formation of certain phases and/or types of precipitates, is not straight-forward [54].
