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

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Chapter 4: Results <strong>and</strong> Discussion Figure 4.62: Fractograph <strong>of</strong> creep ruptured AZ91+0.5% <strong>Si</strong> +0.2% <strong>Sb</strong> alloy at 200°C (a) SE image lower mag. (b) SE image higher mag. (c) BSE image
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 153
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iié»a'5 INFLUENCE OF Si</
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DECLARATION I hereby declare that t
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_ _ Acknowledgements I am very much
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ABSTRACT _ Abstract The use <strong
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A Abstract The microstructure <stro
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2.7.2 Page No Zirconium Free Alloys
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Page No 3.4.4 Hardness ------------
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J 1 1 J N0 No LIST OF TABLES Table
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LIST OF FIGURES Figure . Page l N0.
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1.1 MAGNESIUM lllllI"l'IlI 1: IIITI
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H Chapter 1: Introduction AZ9l allo
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