| A study was made of the effects of alloying elements on the microstructure and microfracture mechanism of squeeze cast AZ91-X(X=Y, Sr, Nd) magnesium alloys. Particular emphasis was placed on the role of Y and Nd, which are important alloying elements in magnesium alloys fabricated by rapid solidified process. Microstructural observation, in situ fracture test, and fractographic observation were conducted on the alloys to clarify the microfracture process. Microstructural analyses indicated that the grain refinement could be achieved by small addition of alloying elements, although Mg_(17)Al_(12) phases were still present along grain boundary. From in situ observation of the AZ-Sr alloy, coarse needle-shaped particles and Mg_17)Al_(12) phases located along grain boundary provided easily intergranular fracture sites under low stress intensity factor levels, resulting in the drop in toughness. In the AZ-Y and the AZ-Nd alloys, on the other hand, a number of planar slip bands and twinnings were well developed inside grains, thereby leading to the enhancement of overall mechanical properties. These findings indicated that small addition of Y or Nd was very effective in improving mechanical properties on the basis of the well-developed twinnings and slip bands. |
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