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

_ pg Chapter 4: Results <strong>and</strong> Discussion
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Figure 4.14: Binary phase diagram <strong>of</strong> Mg-<strong>Sr</strong> alloy system [253]
Figure 4.15 shows the SEM photograph <strong>of</strong> <strong>Sr</strong> added AZ91 alloys. With 0.2%
<strong>Sr</strong> addition, there is no intermetallic, other then Mg11Al|2, is found in the
microstructure. However, this <strong>Sr</strong> addition slightly refines the grain size. With little
higher percentage <strong>of</strong> addition i.e., 0.5%, few white-colored needle like precipitates are
seen along with Mg17Al,2 intermetallic at the grain boundary. On the other h<strong>and</strong>, the
microstructure <strong>of</strong> 0.7% <strong>Sr</strong> added alloy shows more number <strong>of</strong> white needle shaped
precipitates. The grain size <strong>of</strong> both 0.5 <strong>and</strong> 0.7% <strong>Sr</strong> added alloys are not differed
much (around 50 um). A closer inspection with 0.7% <strong>Sr</strong> added alloy, shown in
Figure 4.16, indicate that there are two different kinds <strong>of</strong> precipitates appear at the
grain boundary: one is a long needle shaped particle, which is presented in large
number (marked as 1) <strong>and</strong> another one is a bulky precipitate (marked as 2) which
appears rarely. The EDS analysis on the bulky precipitate indicate that it is a temary
precipitate <strong>of</strong> Al, Mg <strong>and</strong> <strong>Sr</strong>. Moreover, it is found that the bulky precipitate is not a
general feature. No such intermetallic is found in 0.2% <strong>Sr</strong> <strong>and</strong> 0.5% <strong>Sr</strong> additions,
which might be due to very low content <strong>of</strong> <strong>Sr</strong> to fomi temary intermetallic.
In the present work, according to <strong>Sr</strong>/Al ratio, only Al4<strong>Sr</strong> intermetallic is
observed in the microstructure, which is confirmed by XRD studies. The XRD
spectrum <strong>of</strong> solution treated 0.7% <strong>Sr</strong> added alloy is given in Figure 4.17, which shows
peaks for both Mg <strong>and</strong> Al4<strong>Sr</strong> intemetallic. But no peaks are observed for any temary
phases since the amount <strong>of</strong> ternary intermetallic present is very much low.
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