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

Chapter 2: Literature Review
Figure 2.16: Micrograph showing grain boundary sliding in die cast
AZ91 alloy [28]
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1:-1 /4 » 8
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12-10 | 20 40 I0 80 100
. Etna (MP1)
Figure 2.17: Stress dependency on secondary creep rates <strong>of</strong> different Mg-Al
based alloys [186]
The superior creep resistance by other alloys over AZ91 alloy is explained by
the amount <strong>of</strong> dynamic B precipitates form during high temperature exposure
[28, 186]. The cast microstructure <strong>of</strong> AS2l <strong>and</strong> AE42 contain less amount <strong>of</strong>
supersaturated a Mg because <strong>of</strong> less aluminum content compared to AZ91 alloy <strong>and</strong>
as a result, their microstructures are much more stable <strong>and</strong> considerably less
discontinuous precipitates occurs during exposure at elevated temperatures. Moreover
these alloys also contain strong stable intergranular phases, Mg2<strong>Si</strong>, in the case <strong>of</strong>
A821 <strong>and</strong> Al4RE in the case <strong>of</strong> AE42, which can be expected to pin grain boundaries
<strong>and</strong> hinder both grain boundary movement.
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