The efficiency of magnesium is not only result of its physical properties but added constituents.
The consequence of the constituent added is mostly directly pro rata to its amount. The processing and property effects of the individual alloying elements are more important in most structural applications than the physical properties.
Here you can read about important magnesium alloy components. First place takes aluminum.
Aluminum has the most favorable effect on magnesium of any of the alloying elements. It improves strength and hardness, and it widens the freezing range, and makes the alloy easier to cast. When exceeding 6 wt%, the alloy becomes heat treatable. Beryllium is only slightly soluble in magnesium. Nevertheless adding up to about 0.001 wt% beryllium decreases the tendency for the surface of the molten metal to oxidize during melting, casting and welding. It can be used successfully in die-cast and wrought alloys, but must be used sensibly in sand-casting alloys because of its ingrain-coarsening effect. Calcium is added in very small amounts, being a special alloying component. It has a dual purpose:
when added to casting alloys immediately prior to pouring, it reduces oxidation in the molten condition as well as during subsequent heat treatment of the casting, and it improves the roll ability of magnesium sheet. Its addition is under 0.3 wt%. Copper adversely affects the corrosion resistance of magnesium if present in amounts exceeding 0.05 wt%. However, it improves high-temperature strength. Iron is one of the most harmful impurities in magnesium alloys due to considerable reduction of corrosion resistance even in present in small amounts. In ordinary commercial-grade alloys, the iron content can average as high as 0.01-0.03 wt%. Lithium has relatively high solid solubility in magnesium (5.5 wt%, 17.0 at. %) and low relative density (only 0.54). It has attracted interest as an alloying element in magnesium alloys to lower the density to values even lower than that of unalloyed magnesium. Mg-Li alloys are also amenable to age hardening, although they tend to overage at only slightly elevated temperatures but they have limited application. Manganese does not affect tensile strength considerably, yet it slightly increases the yield strength.
Its most important function is to improve saltwater resistance of Mg-Al and Mg-Al-Zn alloys by removing iron and other heavy-metal elements into relatively harmless intermetallic compounds, some of which separate out during melting. The amount of manganese that can be added is limited by its relatively low solubility in magnesium. The Nickel , just like iron, is another harmful impurity in magnesium alloys because it also reduces the corrosion resistance if present, even in small amounts. In ordinary commercial-grade alloys, the nickel content can average as high as 0.01-0.03 wt%. Rare earth metals are added to magnesium alloys either as mischmetal or as didymium. Mischmetal is a natural mixture of rare earths containing about 50 wt% cerium, the remainder being mainly lanthanum and neodymium; didymium is a natural mixture of approximately 85% neodymium and 15% praseodymium. Additions of rare earths increase the strength of magnesium alloys at elevated temperatures. Silicon increases fluidity of the metal in the molten state. However, it decreases corrosion resistance of magnesium alloys in case of iron presence in the alloy. Silver additions improve the mechanical properties of magnesium alloys by increasing response to age hardening. Thorium increases the creep strength of magnesium alloys at temperatures up to 370o C (700o F) The most common alloys contain 2-3 wt% thorium in combination with zinc, zirconium, or manganese. Tin is useful when alloyed with magnesium in combination with small amounts of aluminum. It increases the ductility of the alloy and makes it better for hammer forging, because it reduces the tendency for the alloy to crack while being hot-worked. Yttrium has a relatively high solid solubility in magnesium (12.4 wt %) and is added with other rare earths to promote creep resistance at temperatures up to 300o C (570o F). About 4-5% Zr is added to magnesium to form commercial alloys such as WE54 and WE43. Zinc is next to aluminum in effectiveness, as an alloying ingredient in magnesium. It is often used in combination with aluminum to produce improvement in room-temperature strength. However, it increases hot shortness when added in amounts greater than 1 wt% to magnesium alloys containing 7-10 wt% aluminum. Zinc also helps overcome the harmful corrosive effect of iron and nickel impurities that might be present in the magnesium alloy. Zirconiumhas a powerful grain-refining effect on magnesium alloys. It is added to alloys containing zinc, rare earths, thorium, or a combination of these elements, where it serves as a grain refiner (up to its limit of solid solubility). It also forms stable compounds with any iron, silicon, carbon, nitrogen, oxygen, and hydrogen present in the melt.
Source Magnesium . com