Influence of various Yb additions on microstructures of a casting Mg−8Gd−1.2Zn−0.5Zr alloy

Abstract

Microstructures of the Mg−8Gd−xYb−1.2Zn−0.5Zr (x = 0, 1, 2, 3, and 4, in wt.%) alloys prepared by gravity permanent mold casting were investigated. The results indicate that the components of intermetallic phases are closely depended on the content of Yb additions. Firstly, the eutectic phase in the alloy with free Yb addition is Mg3Gd. As the Yb addition increases, Mg5(Gd,Yb) and Mg12(Gd,Yb) eutectic phases successively formed. Furthermore, both Mg5(Gd,Yb) and Mg12(Gd,Yb) are coherent with Mg3(Gd,Yb) following orientation relationships (ORs) as: (200)Mg3(Gd,Yb)//(200)Mg5(Gd,Yb), [001]Mg3(Gd,Yb)//[001]Mg5(Gd,Yb), and (002)Mg3(Gd,Yb)//(006)Mg12(Gd,Yb), [010]Mg3(Gd,Yb)//[010]Mg12(Gd,Yb), respectively. Then, the volume fractions of Mg5(Gd,Yb) and Mg12(Gd,Yb) gradually increase as the Yb content increases. When the Yb addition is over 3 wt.%, the Mg5(Gd,Yb) phase becomes the dominant eutectic phase. Finally, the volume fraction of 14H-type long-period stacking ordered (LPSO) phase which mainly distributes in the regions near grain boundaries clearly decreases as the Yb content increases.