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

-- ChaPt°l-2LLil°'3t"'° Review
much, <strong>and</strong> the grain size is usually determined at an early stage <strong>of</strong> solidification by
nucleation <strong>of</strong> dendrites.
2.8.3.1 Eflect <strong>of</strong> aluminum
Figure 2.9 shows the grain size change with increasing aluminum content in
Mg-Al alloys [96]. A small addition <strong>of</strong> aluminum to pure magnesium leads to a
morphological change <strong>of</strong> the primary phase from a cellular to a dendritic structure.
Rosette-like globular equiaxed grains form with aluminum-rich solid solution
between the dendrite arms. As the aluminum content increases further to 5%,
dendrites with pools <strong>of</strong> eutectic phase between the dendrite arms start to develop <strong>and</strong>,
when the aluminum content is further increase, a fully developed dendritic structure
with sharp tips is observed.
The addition <strong>of</strong> small amounts <strong>of</strong> alloying elements such as zinc, manganese
<strong>and</strong> rare-earths to Mg-Al alloys has little effect on nucleation <strong>of</strong> the primary phase
since these elements are mostly segregated to form secondary phases well after the
primary phase has nucleated [99].
2.8.3.2 Super healing treatment
Several methods <strong>of</strong> grain refinement have been developed. The first method <strong>of</strong>
grain refinement is by a simple thermal treatment prior to casting, known as
‘superheating treatment’. This method involves rapid cooling <strong>of</strong> the melt to the
desired casting temperature after short holding time at an elevated temperature,
generally between 150°C <strong>and</strong> 260°C above the equilibrium liquidus temperature <strong>of</strong> the
alloy [I02]. This method is only useful for Mg-Al alloys like AZ9l, where as in other
magnesium alloys refinement by the means described is marginal [I 03, 104]. Usually,
superheating treatment is not much effective, owing to the small number <strong>of</strong> nucleation
sites forms during this treatment [I05]. It is assumed that superheating <strong>of</strong> Mg-Al
alloys facilitate the formation <strong>of</strong> many intermetallic compounds, which serves as the
nucleation site for or-Mg. One hypothesis is that hexagonal Al4Mn forms as a result <strong>of</strong>
the manganese content in the commercial alloys <strong>of</strong> the AZ series [I06]. According to
another theory the compound is <strong>of</strong> the type Al4C3_, or iron Aluminide, both <strong>of</strong> which
28