Background

A major step forward in the development of magnesium alloys was the discovery in 1937 of the intense grain refining effect of zirconium. The presence of zirconium gives a fine, equiaxed grain structure with typical grain sizes of 30-50 microns in sand castings compared to the relatively coarse dendritic structure of Mg-Al-Zn alloys. As zirconium forms stable compounds with aluminium and manganese, it could not be used to grain refine Mg-Al-Zn alloys and consequently a totally new series of Mg-Zr alloys was required.

The first Mg-Zr alloys contained zinc as strengthening element and subsequent alloys contained thorium, silver and most recently yttrium. Most Mg-Zr alloys contain rare earth elements such as cerium, neodymium and lanthanum, which form simple eutectic systems with magnesium and enhance castability due to formation of grain boundary networks of relatively low melting point eutectics.

Continuous alloy development has led to major improvements in ambient temperature mechanical properties of Mg-Zr alloys. However, the significant improvement in high temperature properties allows the most recent alloys to be used up to 300°C compared to 150°C for earlier Mg-Zr alloys. Figure 1 shows the variation of tensile strength and yield strength with temperature for various magnesium casting alloys.

Mg-Zn-rare earth (RE)-Zr alloys

For many years, Mg-Zn-RE-Zr alloys containing cerium or lanthanum rare earth elements have been the most widely used alloys for sand and gravity casting. The single most used alloy for gearbox casings is RZ5 (ZE41) which retains mechanical properties up to about 150°C. A second alloy ZRE1 (EZ33) has been used extensively in jet engines where good creep properties are required at around 150°C. Both alloys have excellent castability and complex castings can be made with virtually no microporosity.

Some Mg-Zn-Zr alloys without rare earth elements, including ZK51 (Mg 4.5%Zn 0.7%Zr), were developed but were not widely used due to poor castability and weldability.
Thorium-containing alloys

Magnesium thorium alloys, ZT1 (HZ32) and TZ6 (ZH62), were developed for high temperature creep resistance up to 350°C. They are used primarily in aero engines, where good creep properties are essential, as lightweight alternatives to high temperature aluminium alloys or titanium.

However, concerns over manufacture and foundry handling of Mg-Th alloys have led to complex and costly production operations, while environmental factors have increased disposal costs significantly. Mg-Th alloys are not being considered for new projects and yttrium containing alloys are likely to be used for similar future applications.
Silver-containing alloys

These alloys were developed for improved ambient and high temperature properties compared to Mg-Zn-RE-Zr alloys. Silver is the main strengthening element and neodymium or praseodymium rare earth elements are used. In the T6 heat treated condition, good tensile properties are maintained at temperatures above 200°C although creep properties are inferior to those of thorium-containing alloys.

The most common alloy is MSR (QE22) containing 2.5% silver. In a later development, a cheaper, low silver alloy, EQ21, was produced which had similar ambient temperature and slightly better high temperature mechanical properties. These alloys are generally used in aerospace gearbox and engine casings where improved high temperature performance is required, such as the Rolls Royce Tay intermediate casing.
Yttrium-containing alloys (WE Alloys)

The Mg-Y-Nd-RE-Zr alloys, WE43 and WE54, were developed primarily for improved mechanical properties compared to existing magnesium alloys, particularly at elevated temperatures and as an alternative to thorium-containing alloys. WE43 contains less yttrium and rare earth elements than WE54 and so has lower strength but higher ductility. A major feature of WE alloys is their inherently superior corrosion resistance compared to other magnesium alloys. Corrosion rates of WE alloys in salt fog tests are similar to high purity AZ91 alloy and many aluminium casting alloys.

The aerospace industry has shown considerable interest in WE43 due to its combination of strength, ductility and corrosion resistance. Figure 1 shows that WE43 has superior strength to MSR at elevated temperatures and RZ5 at all temperatures. Properties of WE43 also compare very well with those of common aerospace aluminium alloys, A356 and A203, especially above 200°C, figure 2. In addition, the strength of WE43 is not reduced significantly after long term elevated temperature exposure whereas some aluminium casting alloys show substantial loss of properties.