Chapter 5: Magnesium Alloys

5.1 INTRODUCTION TO ALLOYING BEHAVIOUR

Magnesium is readily available commercially with purities exceeding 99.8% but it is rarely used for engineering applications without being alloyed with other metals. Key features that dominate the physical metallurgy of the alloys are the hexagonal lattice structure of magnesium (Fig. 5.1) and the fact that its atomic diameter (0.320 nm) is such that it enjoys favourable size factors with a diverse range of solute elements. Aluminium, zinc, cerium, silver, thorium, yttrium and zirconium are examples of widely different elements that are present in commercial alloys.

Fig. 5.1: Principal planes and directions in the magnesium unit cell.

Solubility data for binary magnesium alloys are shown in Table 5.1 with the first section containing elements used in commercial compositions. Apart from magnesium and cadmium which form a continuous series of solid solutions, the magnesium-rich sections of the phase diagrams show peritectic or, more commonly, eutectic systems. A wide range of intermetallic compounds may form, with the three most frequent types of structures being as follows:

1. AB. simple cubic CsCl structure. Examples are MgTl, MgAg, CeMg, SnMg and it will be seen that magnesium can be either the electropositive or the electronegative component.

2. AB ₂. Laves phases with ratio R _A/ R _B = 1.23 preferred. Three types exist namely:

   
   

3. CaF ₂. f.c.c. This group contains Group IV elements and examples are Mg ₂Si and Mg ₂Sn.