Precipitation structures of Mg-Y alloys

Precipitation process and microstructures of both Mg-5%Y and Mg-10%Y (in mass%) alloys were investigated using hardness measurement, optical microscopy and transmission electron microscopy. In a Mg-5%Y alloy hardness increased slightly by about HV 10 with aging time at temperatures between 423 and 523 K. In a Mg-10%Y alloy hardness increased markedly after certain incubation periods from 4340 to 38.7 ks at aging temperatures between 423 and 498 K. The TTT diagram for the precipitation of the β″ phase was determined from the hardness curves of a Mg-10% Y alloy, the nose temperature being 498 K. The hardness increase in a Mg-10%Y alloy at 423 to 498 K was found to be caused by the homogeneous precipitation of the fine coherent metastable β″ phase. The metastable β′ and stable β phases precipitated and coarsened in a Mg-10%Y alloy at 523 to 573 K. Both the β″ and β′ phases were found to have a base centered orthorhombic (bco) structure. The atom configurations in their unit cells, however, were assumed to be different from each other in the β″ and β′ phases because of the existence of weak reflection spots at the forbidden positions.