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

ii
A
r
I
{I
i
L 1
x Chapter 4: Results <strong>and</strong> Discussion
higher hardness obtained with <strong>Si</strong> <strong>and</strong> <strong>Sb</strong> addition at peak aged condition might also be
attributed to the presence <strong>of</strong> more number <strong>of</strong> continuous precipitates.
It is found from the aged microstructure <strong>of</strong> <strong>Sr</strong> added alloy that more amounts
<strong>of</strong> discontinuous precipitate forms in the initial stage <strong>of</strong> ageing. More number <strong>of</strong>
boundaries is seen with DP compared to other alloys as shown in Figure 4.36 (d). The
volume fraction <strong>of</strong> discontinuous precipitates calculated is slightly higher / equal
compared to the base alloy (ref. Table 4.4). It could be also seen from Table 4.3 that
<strong>Sr</strong> added alloy reaches the peak hardness little early than the base alloy. This is due to
low amount <strong>of</strong> Al available in the solid solution. Nussbaum et al [238] <strong>and</strong> Lee et al
[231] reported that <strong>Sr</strong> modifies the precipitation behavior <strong>of</strong> Mg-Al alloys by
promoting more precipitates along the grain boundaries. However, the reason behind
_ such occurrence is not known. On the other h<strong>and</strong> the formation <strong>of</strong> DP during ageing
<strong>of</strong> Mg-Al is a combined effect <strong>of</strong> Al in the solid solution, grain size <strong>and</strong> ageing
temperature [I46]. High amount <strong>of</strong> Al, finer grain size (high angle grain boundary)
<strong>and</strong> medium ageing temperature (I50-250°C) leads to more DP in Mg-Al alloys.
Hence, the boundary characterization <strong>of</strong> <strong>Sr</strong> added alloy is necessary to underst<strong>and</strong> the
precipitation mechanism.
Table 4.4: Volume fraction <strong>of</strong> discontinuous precipitates formed in 120 min aged
Samples
Volume 4 i W
fraction <strong>of</strong>
I
ll
.
X.
,.
­ Allo AZ91 4 . ' 4 ' A AZ91+0.5%<strong>Sr</strong> ­
L g Y ._ X %si _ . g <strong>Sb</strong> g i<strong>Si</strong>+0.2% <strong>Sb</strong>
discontinuous y 8.51:4 2.7i2.Z 4.l¢3.6 » l0.2¢5.2 3.5:l:2.8
precipitates,
% .
4 AZ91+05 Az91+o 5% it 4 P Az91+o.5°/
ll6