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

Chapter 2: Literature Review
solidification the liquidus <strong>and</strong> solidus temperatures <strong>of</strong> AZ91 is ~600°C <strong>and</strong> ~500°C
respectively. But, in most <strong>of</strong> the casting practice the solidification never approach the
equilibrium. Luo [99] reports the change in liquidus <strong>and</strong> solidus temperature <strong>of</strong> AZ91
at wide range <strong>of</strong> cooling rate <strong>and</strong> the data is presented in Table 2.5. The eutectic [Mg
(0) +Mg17Al12 (B)] <strong>of</strong> AZ91 is a divorced eutectic. In a slow cooled alloy, when
casting cools from eutectic to the room temperature, the solid deformation takes place,
in which the highly super saturated eutectic Mg decompose into altemative layers <strong>of</strong>
solute deployed Mg <strong>and</strong> M811Al12 phase.
2.8.2.1 Eutectic solidification
The eutectic solidification controls the size, shape <strong>and</strong> distribution <strong>of</strong> the more
brittle Mg17Al1; phase in the final microstructure, which, in turn, is likely to influence
mechanical properties particularly ductility <strong>and</strong> creep strength <strong>of</strong> the alloy. Moreover
eutectic growth affects feedability at a crucial stage when feeding is interdendritic <strong>and</strong>
large pressure differentials are required to draw liquid through the dendritic network.
[96]. The Mg-Mg17Al12 eutectic in Mg-Al alloy, exhibits a wide range <strong>of</strong>
morphologies depending on the alloy composition <strong>and</strong> cooling conditions. The Mgl
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33% Al (eutectic composition) alloy, exhibits a lamellar morphology <strong>and</strong> at low
growth rates <strong>and</strong> a fibrous morphology at higher growth rates. However, in low
aluminum content Mg-Al alloys such as AZ91 <strong>and</strong> AM6O, the eutectic has different
morphologies, described as either completely or partially divorced [96].
Table 2.5: Liquidus <strong>and</strong> solidus temperature <strong>of</strong> AZ91 alloy at various
cooling rates [99]
C0Olillg (dT/dt, R3“? l E "c/S) 0.03 0.06 0.4 if 7.8 if 20.6 , W . 41.1 9 , , _____ >
' ~—*’—— — ‘i ————’ |
Liquidus (TL, °C) 600.2 600 5 599.5 " 598 ; 595.5 593.8
Solidus (TS,°C) 435 435 430 2 328 - , ­
Both fully <strong>and</strong> partial divorced eutectic morphology is shown in Figure 2.6. ln
fully divorced morphology, as shown in Figure 2.6 (a), the two eutectic phases are
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