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

4.5.7 Post Creep Microstructural Analysis
4.5.7.1 AZ91 alloy
Chapter 4: Results <strong>and</strong> Discussion
The microstructure <strong>of</strong> as cast AZ9l alloy is unstable at high temperature due
to the supersaturation <strong>of</strong> aluminum in the solid solution [28, 30, 31, 184, 186]. In the
present study also many high aluminum concentrated areas are found in the as cast
AZ9l microstructure (ref. Table 4.1). In the initial stage <strong>of</strong> creep the aluminum in
solid solution provides solid solution strengthening against moving dislocations. Then
secondary precipitates forms from the solid solution during creep exposure [l84,
186]. Many literatures reported that the dynamic precipitation (continuous <strong>and</strong>/or
discontinuous Mg11Al1;) takes places during creep but it is still inconclusive that how
these precipitates influence the creep behavior <strong>of</strong> this alloy. Dargusch et al [186] have
reported that the dynamic discontinuous precipitation occurred at the grain boundary
in the die cast AZ9l alloy leads to the grain boundary sliding whereas Regev et al
[29] have reported that the continuous secondary precipitates occurred from the
supersaturate eutectic matrix strengthens the ingot cast AZ9l alloy against creep
deformation. The result <strong>of</strong> Blum et al [184] also suggest that continuous precipitates
dominate during creep exposure <strong>of</strong> die cast AZ9l alloy <strong>and</strong> provide strengthening
against creep.
In the present study, a detailed post creep test microstructural analyses were
carried out to underst<strong>and</strong> the structural changes due to the high temperature exposure.
Figure 4.63 shows the microstructure <strong>of</strong> longitudinal cross section near the fracture
surface <strong>of</strong> AZ9l alloy creep ruptured at 150°C, which contain lot <strong>of</strong> coarse continuous
precipitates. Higher magnified view <strong>of</strong> the same sample (Figure 4.63) clearly shows
the presence <strong>of</strong> continuous precipitates with r<strong>and</strong>om orientation, which occurred
during creep. <strong>Si</strong>milar kind <strong>of</strong> dynamic continuous precipitates are observed in all the
tested alloys. These continuous precipitates would have formed at the initial stage <strong>of</strong>
creep <strong>and</strong> become coarsen during the continuous exposure to high temperature. The
interrupted microstructure <strong>of</strong> AZ9l creep tested at 150°C for 2000 h (secondary creep
region) shown in Figure 4.64 contains finer continuous precipitates. These
precipitates, in earlier stage <strong>of</strong> creep, block dislocation motion <strong>and</strong> encourage cross
slip leading to strengthening [3l5]. Hence, hardening mechanism is changed from
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