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

_ pg Chapter 4: Results <strong>and</strong> Discussion
The major reason for the poor high temperature properties <strong>of</strong> AZ91 alloy is the
microstructural degradation takes place at elevated temperature. As far as AZ91 alloy
is concemed, the principal room temperature strengthening element is Mg17Al;;;
intermetallics. But this intermetallic has low melting point (4370 C) <strong>and</strong> has higher
diffusivity in Mg matrix. Hence, this coarsen at a faster rate <strong>and</strong> unstable at higher
temperatures i.e. above l00°C <strong>and</strong> no longer acts as a barrier for dislocations [230,
78]. Moreover, Mg17Al12 has a cubic crystal structure due to which it is incoherent
with the hcp magnesium matrix [29]. Therefore, presence <strong>of</strong> this phase leads to poor
elevated temperature properties. In addition, the microstructure <strong>of</strong> AZ91 alloy is
found to be unstable during high temperature exposure due to the dynamic
precipitation [28, 30, 31, 184, 186]. As seen earlier, the as cast microstructure has
eutectic o-Mg which is highly supersaturated with Al. Even though, most <strong>of</strong> the
supersaturated Al in the matrix is precipitated out as discontinuous precipitates at
grain boundaries during solidification itself, due to the slow cooling in gravity casting,
still aluminum rich supersaturated areas are presented in the as cast microstructure
(ref. Table 4.1). It is said that this supersaturated matrix decomposes into
discontinuous precipitates during high temperature exposure [184, 186]. The nature <strong>of</strong>
these dynamic precipitates is that they forms at the grain boundary <strong>and</strong> grows in to the
adjacent grains [l34, 135]. it is believed that the formation <strong>of</strong> discontinuous
precipitate assists the movement <strong>of</strong> the grain boundary <strong>and</strong> hence reduces the high
temperature properties [96]. However, this problem is more severe during creep
deformation, where the material is exposed at high temperature for prolonged time
rather than in high temperature tensile testing, which is a short-term test.
4.4.2 AZ91+X<strong>Si</strong> Alloys (X=0.2%, 0.5%)
The effect <strong>of</strong> <strong>Si</strong> addition to the tensile properties <strong>of</strong> AZ91 alloy is shown in
Figure 4.38. The strength values are presented also in Table 4.6. At room temperature,
the <strong>Si</strong> addition reduces the tensile properties compared to the AZ91 base alloy.
Marginal reduction in both strength <strong>and</strong> elongation is observed with 0.2% <strong>Si</strong> addition.
However, considerable reduction is observed with 0.5% <strong>Si</strong>. The presence <strong>of</strong> coarse
Mg2<strong>Si</strong> intermetallic in the <strong>Si</strong> added AZ91 alloys is the reason for such reduction in
tensile properties, however substantial reduction obtained in 0.5% <strong>Si</strong> added alloy is
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