Effects of Lanthanum and Zirconium on Cast Structure and Room Temperature Mechanical Properties of Mg-La-Zr Alloys

Abstract

Cast Mg-La and Mg-La-Zr alloy ingots were prepared from 99.96% magnesium and 99.9% lanthanum with a zirconium addition made using a Mg-33Zr master alloy. The microstructure was examined and tensile tests performed for the cast alloys. Lanthanum showed a mild grain refinement effect on magnesium, generating coarse equiaxed grains in the casting. The microstructure within the equiaxed grain contained the primary Mg dendrites and degenerated lamellar eutectic in the interdendritic regions. An addition of zirconium to the Mg-La alloys transformed coarse primary α-Mg dendrites into fine globular grains surrounded by eutectic regions. With this change tensile properties improved significantly in comparison with the binary Mg-La alloys of comparative lanthanum content. The hardness value increased linearly with lanthanum content due to an increase in the eutectic Mg12La phase. Fracture occurred owing to the decohesion between the primary Mg grains and eutectic Mg12La phases. An increase in the eutectic regions thus leads to a reduction in ductility by means of crack propagation through the regions.