Effect of intermetallic phases on the creep behavior of AZ91 magnesium alloy

Intermetallic phases play a major role in the creep behavior of Mg alloys. The effects of intermetallic phases on the creep behavior of AZ91 alloy have been studied in the present investigation. Thermally stable and finer intermetallic phases such as Mg2Si and Mg3Sb2 are introduced deliberately in AZ91 alloy by the addition of Si and Sb (0.5% Si. 0.5% Sb and 0.5% Si + 0.2% Sb, all are in wt%). Creep tests were carried out at 150 degrees C and 200 degrees C with an initial stress of 50 MPa on the as cast samples. The detailed microstructural analyses carried out on the crept samples indicate that the beta-Mg17Al12 intermetallic phase in the AZ91 alloy suffers sever cracking and facilities cavity formation due to its low melting point and incoherency with Mg matrix, which results in poor creep resistance. On the other hand, the creep behavior of AZ91 alloy at 150 degrees C is greatly improved with the presence of Mg2Si or Mg3Sb2 intermetallic phases because of their better thermal stability than Mg17Al12. These intermetallic phases strengthen the grain boundary against sliding and hence, reduce the possibility of void formation during creep. Furthermore, it also promotes more number of continuous Mg17Al12 dynamic precipitates near the grain boundary during creep, which in turn restricts the creep deformation. (C) 2009 Elsevier B.V. All rights reserved.