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

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.. /‘__‘\ - A/ \‘ .--“' ‘ D -1599 1 i ' A/I ‘T \ 7 77$“ -1000 — .»" 10000 _// A 1 \ \ - s - — A291 - — AZ91+0.5<strong>Si</strong> ~ — AZ91+0.5<strong>Sr</strong> ° — — - AZ91+0.5<strong>Sb</strong> Chapter 4: Results <strong>and</strong> Discussion '|'|'|'|'|'|'|' _ — — AZ91+0.5<strong>Si</strong>+0.2<strong>Sb</strong> 0100020003000400050006000 Zrelohm.cm' Q‘ — _ ~ \\ . \__ K u \ \\> _>\1~-__7 x" \_ '\‘_* -_ \‘ ' A — - AZ91+0.5<strong>Si</strong> I m " A291 ‘§‘:»\ AZ91+0.5<strong>Sb</strong> 100 1 — —AZ91+O.5<strong>Sr</strong> \ \\ AZ91+0.5<strong>Si</strong>+0.2<strong>Sb</strong> \ *: 10 100 Frequency Hz Figure 4.71: Electrochemical impedance spectroscopy measurements <strong>of</strong> different alloys (a) Nyquist plot (b) Bode plot I66
_ _ K Chapter 4; Results <strong>and</strong>liiiscussion Table 4.15: Corrosion parameters obtain from EIS measurement for different alloys H Alloy |z| ohm cm’ RP ohm em’ “ c...x10‘ |1F/cm AZ91 A 2910 30159 1.25 AZ9l+0.5% <strong>Si</strong> 5 3450 3581 7.4 AZ91+0.5% <strong>Sr</strong> 3556 3950 7.2 TiAZ91+0.5% <strong>Sb</strong> 7 ; AZ9l+0.5% l si+o,g% <strong>Sb</strong> 2425 2502 8 87.4 4.6.4 Corrosion Morphology 4987 4518 7 06 I Figure 4.72 shows the macrograph <strong>of</strong> 100 h immersion tested samples in 3.5 NaCl. From the macrostructure it is seen that almost all the samples are similar due to the longer immersion time in aggressive Cl environment. However, it is noticed that the extent <strong>of</strong> corrosion in the <strong>Sb</strong> added alloy is more severe. The entire surface is attacked by corrosion. Large uncorroded areas are observed with combined addition <strong>of</strong> <strong>Si</strong> <strong>and</strong> <strong>Sb</strong>. Figure 4.73 exhibits the corroded microstructure <strong>of</strong> AZ91. From Figure 4.73 (a), which shows 2 h immersion sample, it can be seen that the alloy exhibits pitting kind <strong>of</strong> corrosion. These pits are mainly initiated at the centre <strong>of</strong> ot Mg matrix <strong>of</strong> AZ91 alloy <strong>and</strong> spread towards the boundary. No corrosion is observed on the [3 —Mg|7Al12 phase. Even under the pits many discontinuous Mgl7Al12 intermetallics are seen. These results COI1fiI'I11 that during corrosion process, both massive <strong>and</strong> lamellar Mg|1Al12 phases are stable. When immersion time increases, the corrosion spread throughout the sample as seen from the figure 4.73 (b) which shows the corroded surface <strong>of</strong> 100 h immersed sample. Many massive eutectic <strong>and</strong> lamellar [3-—Mg17Al1; phases at the grain boundary are still appears intact at grain boundary. It is observed in the present study that all other intermetallics like Mg2<strong>Si</strong> (both in Chinese script <strong>and</strong> polygonal), Mg3<strong>Sb</strong>2 <strong>and</strong> Al4<strong>Sr</strong> are also stable even after 100 h exposure in the NaCl solution (ref. Figure 4.74). 167
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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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_ _ 1 Abstract intermetallic are al
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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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5 3.7 it . — 7 ‘ 7 ———
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; Intermetallic ‘ 4.23 Microstruc
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at 200°C 4.59 Creep curves <strong
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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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