EFFECT OF Y, SR, AND ND ADDITIONS ON THE MICROSTRUCTURE AND MICROFRACTURE MECHANISM OF SQUEEZE-CAST AZ91-X MAGNESIUM ALLOYS

This study aims to investigate the effects of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of the four squeeze- cast magnesium alloys based on the commercial AZ91 alloy. Microstructu ral observation, in situ fracture tests, and fractographic observation were conducted on the alloys to clarify the microfracture process. Mi crostructural analyses indicated that grain refinement could be achiev ed by small additions of alloying elements, although the discontinuous ly precipitated Mg17Al12 phases still existed on grain boundaries. Fro m in situ fracture observation of an AZ91-Sr alloy, it was seen that c oarse needle-shaped compound particles and Mg17Al12 phases located on the grain boundary provided easy intergranular fracture sites under lo w stress intensity factor levels, resulting in the drop in toughness. On the other hand, the AZ91-Y and AZ91-Nd alloys showed improved fract ure toughness, since deformation and fracture paths proceeded into gra ins rather than to grain boundaries, as the planar slip bands and twin nings actively developed inside the grains. These findings suggested, on the basis of the well-developed planar slip bands and twinnings, th at the small addition of Y or Nd was very effective in improving fract ure toughness.