4.3.3. Be

Cao et al. [67] investigated the effect of trace addition of Be on the grain refinement of Mg-0.5Zr alloy, as shown in Figure 11, where a significant grain coarsening effect can be observed. However, since Be was added in the form of Al-5Be master alloy, the influence of the consumption of Zr by Al cannot be neglected. Thus, the alloys were designed in two groups, i.e., a low-Zr-content group (ZM1~ZM4) and a high-Zr-content group (ZM5). It can be seen that with increasing addition Be from 10 to 100 ppm, the grain size of ZM1~ZM4 increased gradually, which was mainly because of the continuous consumption of Zr by Al. Nevertheless, with the further addition of 0.5% Zr (ZM5), the grain size continued to coarsen, rather than recovering. Therefore, a poisoning effect of Be was deduced, whereby Be, as a surface-active 'surfactant', readily coats the surfaces of heterogeneous nucleant particles such as Zr particles or native particles (Fe-rich, carbon-rich nuclei) destroying the nucleating potency.

**Figure 11.** (**a**) Grain size as a function of Be addition. (**b**) Be, Al and Zr contents in alloys. ZM1: base alloy Mg-0.5% Zr; ZM2: 10 ppm Be addition; ZM3: 50 ppm Be addition; ZM4: 100 ppm Be addition; ZM5: 100 ppm Be + 0.5% Zr. [67]. Reprinted with permission from ref. [67]. Copyright 2004 Elsevier.
