Effects of bismuth and antimony additions on the microstructure and mechanical properties of AZ91 magnesium alloy

The effects of bismuth and antimony additions on the microstructure and mec hanical properties of AZ91 alloy have been studied. Results show that a sma ll amount of bismuth or antimony additions to AZ91 increases the yield stre ngth and creep resistance significantly at elevated temperatures up to 200 degreesC. The highest creep resistance has been obtained from the alloy wit h combined additions of bismuth and antimony. The activation energies of th e steady-state creep for AZ91-based alloys studied were close to that of pu re magnesium self-diffusion, indicating that dislocation climb is responsib le for the creep mechanism under the present conditions. Microstructural ob servations reveal that the additions of bismuth or antimony have the effect of refining the beta (Mg17Al12) precipitates in as-cast alloys and suppres sing discontinuous precipitation of the beta phase effectively during the a ging process. Some rod-shaped Mg3Bi2 or Mg3Sb2 particles distributed mainly at grain boundaries have been observed in the alloys with bismuth or antim ony additions. Both Mg3Bi2 and Mg3Sb2 have a high thermal stability and pla y important roles in improving creep resistance of the alloys at elevated t emperatures. (C) 2001 Elsevier Science B.V. All rights reserved.