“Influence of Si, Sb and Sr Additions on the Microstructure ...

Z g _ U Chapter 2: Literature Review
compositions such as HK3l (Mg-3Th-0.7Zr) is developed for high temperature
applications. However, in spite <strong>of</strong> their application in missiles <strong>and</strong> spacecraft, the
alloy usage is reduced because <strong>of</strong> environmental considerations. <strong>Si</strong>lver is also added
to magnesium <strong>and</strong> M g-Ag-RE-Zr alloys are developed with improved room <strong>and</strong> high
temperature mechanical properties [83]. The alloy QE22 (Mg-2.5Ag-ZRE-0.7Zr) has
been used for a number <strong>of</strong> aerospace applications including l<strong>and</strong>ing wheels, gear box
housings <strong>and</strong> rotor heads for helicopters.
2.7.2 Zirconium Free Alloys (Mg-Al Alloys)
Aluminium is the principal alloying element to magnesium. The binary Mg-Al
system is the basis for the first magnesium casting alloys but most current
compositions also contain small amounts <strong>of</strong> zinc <strong>and</strong> manganese. The most widely
used alloys in this group are AZ9l <strong>and</strong> AM50 [78]. These alloys have a wide range <strong>of</strong>
mechanical properties <strong>and</strong> good castability <strong>and</strong> mostly used in die casting application.
However, the draw back <strong>of</strong> using these alloys is its poor elevated temperature tensile
<strong>and</strong> creep properties above 150°C. [l5].
Alloys like AS2l, AS41 (Mg-Al-<strong>Si</strong>), which contain <strong>Si</strong> in it are developed for
better creep properties compared to AZ9l [66]. Later on, solubility <strong>of</strong> rare earth (RE)
in magnesium led to the development <strong>of</strong> AE alloys systems, which contain less
amount <strong>of</strong> aluminium <strong>and</strong> small percent <strong>of</strong> RE [84]. One such alloy system, AE42
(Mg-4Al-3RE-0.3Mn)) has a good combination <strong>of</strong> properties including creep strength,
which is superior to the Mg-Al-<strong>Si</strong> alloys [85]. The major drawback <strong>of</strong> these alloys is
addition <strong>of</strong> RE increase the alloy cost drastically [66]. Recently, it is also found that
the thermal stability <strong>of</strong> Al4RE intermetallic in RE containing Mg alloys does not
extend beyond 150°C <strong>and</strong> hence the AE42 alloy loses creep strength above this
temperature.
The addition <strong>of</strong> calcium to Mg-Al based alloys for improved creep resistance
is reported in a British patent [86]. This patent disclosed that calcium additions <strong>of</strong> 0.5­
3% provide creep resistance to magnesium alloys containing up to l0% Al. However,
the patent also revealed that calcium containing alloys prone to hot cracking.
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