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

_ H Chapter 2: Literature Review
bismuth refines the Mg|7Al12 precipitate in the as cast alloys <strong>and</strong> effectively
suppresses the discontinuous [5-Mg17Al12 precipitation during the aging process [43].
2.9.4 Rare earth
Rear earths are important alloying elements to magnesium alloys, which can
improve casting characteristics, high temperature properties <strong>and</strong> corrosion resistance
[227-230]. With addition <strong>of</strong> rare earth elements a rod-like All IRE; intermetallic forms
at the grain boundary. Due to the formation <strong>of</strong> AIHRE3, the Al available for the
formation <strong>of</strong> MgnAl|2 phase reduces. The effect <strong>of</strong> RE addition on the tensile
properties <strong>of</strong> AZ9l alloy is scatter in the literature. Y. Lu [45] has reported that RE
has little effect on the UTS <strong>of</strong> AZ9l at ambient temperature but greatly increase the
high temperature tensile strength <strong>and</strong> elongation. Fan [46, 47] has reported a sharp
increase in both UTS <strong>and</strong> ductility with the addition <strong>of</strong> La <strong>and</strong> Ce to AZ9l alloy. On
the other h<strong>and</strong>, G. H. Wu et al [48] have found that with increasing RE additions, the
UTS increases but the elongation decreases. S. Lee et al [231] have reported that
addition <strong>of</strong> Y <strong>and</strong> Nd improves hardness <strong>and</strong> fracture toughness <strong>of</strong> AZ9l. He observes
planer slip b<strong>and</strong>s <strong>and</strong> transgranular crack propagation along these slip b<strong>and</strong>s. When
planer slip b<strong>and</strong>s are forms, a heavier load is requires for fracture than in the case <strong>of</strong>
intergranular fracture, thereby yielding an overall improvement <strong>of</strong> mechanical
properties [231 ].
G. Pettersen [232] indicates that RE could reduce the range <strong>of</strong> crystallization
temperature <strong>of</strong> magnesium alloy, <strong>and</strong> hence the Mg-RE eutectic has good fluidity.
However, Y. Lu [45] has reported a decrease in fluidity up to 2% RE addition <strong>and</strong>
then it increase with higher percentage addition to AZ9l alloy. Qudong et al [233]
observed the fluidity <strong>of</strong> RE added alloy is depending on the mould section thickness
also. The fluidity decreases with increase in RE addition in mould section thickness
under 2 mm whereas it decrease first with section thickness above 2 mm when RE
content increases from l to 2% <strong>and</strong> increase when RE content increases from 2 to 3%
to AZ91 alloy.
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