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

Chapter 5: Conclusions
However, the addition <strong>of</strong> 0.5%<strong>Sb</strong> with O.5%<strong>Sr</strong> increases the properties due to
the presence <strong>of</strong> intermetallics <strong>and</strong> refinement <strong>of</strong> grains.
Higher percentage <strong>of</strong> <strong>Sb</strong> <strong>and</strong> <strong>Sr</strong> additions (0.7%) reduces the properties due to
the presence <strong>of</strong> more volume fraction <strong>of</strong> needle shaped Mg3<strong>Sb</strong>2 <strong>and</strong> A14<strong>Sr</strong>
intemietallics respectively, which acted as a stress raisers during loading.
With 0.2% <strong>Sb</strong> <strong>and</strong> 0.1% <strong>Sr</strong>, the AZ9l+X<strong>Si</strong> (X= 0.2% <strong>and</strong> 0.5%) alloys exhibit
improved tensile properties particularly the ductility due to the favorable
morphological change <strong>of</strong> Mg2<strong>Si</strong> intermetallic.
Improved high temperature (150°C) tensile properties are also noticed with
alloying additions. As like at room temperature, higher percentage <strong>of</strong> additions
leads to the reduction in properties.
The alloying additions greatly improve the creep properties <strong>of</strong> AZ91 alloy at
150°C whereas such improvement is not observed at 200°C. Addition <strong>of</strong> <strong>Si</strong>
(0.2%, 0.5%), <strong>Sb</strong> (0.5%) <strong>and</strong> <strong>Sr</strong> (0.5%, 0.7%) improves the creep resistance <strong>of</strong>
AZ91 alloy considerably at 150°C whereas 0.7% <strong>Sb</strong> additions reduce the creep
properties. Among various additions, <strong>Sb</strong> addition has yielded higher creep
properties.
Both AZ9l+0.5% <strong>Si</strong> <strong>and</strong> AZ9l+0.5% <strong>Si</strong>+0.2% <strong>Sb</strong> alloys exhibit similar creep
behavior suggesting morphology <strong>of</strong> the Mg2<strong>Si</strong> intermetallic (Chinese script
<strong>and</strong> polygonal shape) does not have much effect on the creep properties.
Dynamic continuous precipitates from the supersaturated eutectic or-Mg solid
solution occur during creep exposure. However, the additional intermetallics
presented due to the alloying additions leads to the fomiation <strong>of</strong> more amounts
<strong>of</strong> precipitates.
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