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

Chapter 4: Results <strong>and</strong> Discussion
Figure 4.28: Microstructure <strong>of</strong> solution treated AZ9l showing no Mg17Al1;
intermetallic
Even though the intermetallic is stable during solution treatment, refinement in
Mg;<strong>Si</strong> Chinese script morphology is observed in AZ9l+0.5% <strong>Si</strong> alloy. Figure 4.30
shows a higher magnified view <strong>of</strong> Chinese script Mg;<strong>Si</strong> intermetallic after solution
treatment, which illustrate the linear branches <strong>of</strong> the Chinese script phase become
disconnected <strong>and</strong> spheriodized due to the prolonged treatment at 410°C for 48 h.
Refinement <strong>of</strong> Chinese script Mg2<strong>Si</strong> precipitates during heat treatment in Mg-6Al-X<strong>Si</strong>
alloy at 420°C is observed in earlier study [28l]. Solution treatment principally
influences the alloys in two ways (1) improving the vibration energy <strong>and</strong> the diffusion
coefficient <strong>of</strong> the atoms, <strong>and</strong> (2) increasing the vacancies in the alloys. Due to this, the
solutes will dissolve into the matrix, resulting in the decomposition <strong>of</strong> the
intermetallic compounds. This mechanism is valid for Mg|1Al|; because the solubility
<strong>of</strong> Al increases at the high temperature. From the Mg-<strong>Si</strong> phase diagram (ref Figure
4.5), it could be seen that <strong>Si</strong> does not have much solubility in Mg matrix <strong>and</strong> hence
the refinement mechanism must be different.
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