Mechanical alloying of alumina–yttria powder mixtures

Mechanical alloying has been used to prepare powder mixtures of alumina and yttria as a means to create composites with a dominant matrix phase together with small particles of a dispersed second phase. The yttria–alumina system, containing five possible phases, has the potential for creating eight combinations of matrix and dispersed phases. Here compositions designed to give YAlO3 (YA) dispersed in Y3Al5O12 (Y3A5 i.e. YAG) or Y4Al2O9 (Y2A) were studied. After milling with steel tools for times up to 8 h, the powders were subjected to thermal cycles up to 1500°C during which the phase evolution was monitored using X-ray diffractometry (including high-temperature XRD) and differential thermal analysis. During milling the original crystal structures were quickly broken down, in some cases partially replaced by an intermediate structure after milling. Upon subsequent heating the milled mixtures crystallized to give the expected phases, YA in Y3A5 and YA in Y2A respectively, but the reaction route was seen to be different depending on the amount of amorphization of the yttria. Contamination by iron was seen to affect the phase distribution and the lattice parameters.