DSC and FIB/TEM investigation of calcium and yttrium additions in the sintering of magnesium powder

Powder metallurgy can be used to produce near-net-shape magnesium parts and also allows unique chemical compositions to be achieved that can lead to new alloys with novel properties. However, the surface layer formed on the magnesium powders during processing acts as a barrier to diffusion and sintering is less effective. The layer, formed by reactions with the atmosphere, contains oxides, hydroxides and carbonate hydrates of magnesium. To overcome this barrier, small additions of calcium or yttrium metal powders were made to the Mg matrix. The oxides of Ca and Y are thermodynamically more stable than magnesium oxide, and will disrupt the surface layer. The present work reports the results of investigation by differential scanning calorimetry, and focussed ion beam (FIB) microscopy and transmission electron microscopy (TEM), on the effect of calcium and yttrium additions during the sintering of magnesium powder. FIB examination of sintered Mg–1 wt-% Ca show that secondary phases are located along grain boundaries as well as inside porosity, which confirms the characteristic mechanism of magnesium sintering. A TEM specimen made by a FIB liftout process was also examined by TEM and EDS and indicates that the sintering process was successful, with the elemental distribution being as predicted by sintering.