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

Chapter 4: Results <strong>and</strong> Discussion
The creep fracture surfaces <strong>of</strong> AZ9l+0.5% <strong>Si</strong> alloy tested at both 150 <strong>and</strong>
200°C is presented in Figure 4.61. The fractograph exhibits ductile features with large
number dimples. The size <strong>of</strong> the dimples is similar with AZ9l alloy
(ref figure 4.54 (a)). However, still there are some cleavages observed in the fracture
surface (marked in the figure). The cleavage facets are clearly shown in the Figure
4.61 (b), which is mainly due to the presence <strong>of</strong> brittle coarse Chinese script Mg;<strong>Si</strong>
precipitates. Figure 4.61 (c) present the fracture surface <strong>of</strong> AZ9l+O.5% <strong>Si</strong> alloy failed
at 200°C. Coarse dimples are evident from the figure along with cleavage facets. The
size <strong>of</strong> the dimples is bigger than the dimples observed at 150°C but smaller than the
O
dimples observed in the base alloy failed at 200 C.
The fracture surface <strong>of</strong> AZ9l+0.5% <strong>Si</strong>+0.2% <strong>Sb</strong> alloy creep ruptured at 200°C
is presented in Figure 4.62. The effect <strong>of</strong> modification <strong>of</strong> coarse Chinese script Mg;<strong>Si</strong>
morphology is reflected in the fracture surface, which exhibits lot dimples <strong>of</strong> different
sizes. Mainly two sizes <strong>of</strong> dimples are present: fine dimples observed in matrix <strong>and</strong>
relatively coarse dimples initiated at the grain boundary. Figure 4.62 (b) <strong>and</strong> (c)
shows the magnified secondary <strong>and</strong> back-scattered SEM image <strong>of</strong> the same area <strong>of</strong>
the fractured surface. It clearly shows that most <strong>of</strong> the dimples are originated at Mg2<strong>Si</strong>
precipitates. There are some fine eutectic Mg11Al12 particles are also seen under
dimples with or without fracture. Cracking <strong>and</strong> debonding <strong>of</strong> Mg2<strong>Si</strong> precipitates are
observed in some areas. Nevertheless, most <strong>of</strong> the particles are still firmly bounded in
the matrix. Several matrix cracks around the particles are also seen.
Figure 4.60: Fractographs <strong>of</strong> creep ruptured AZ9l alloy at (a) 150°C (b) 200°C
150