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 togeth er with small particles of a dispersed second phase. The yttria-alumina sys tem, 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 (Y(3)A(5) i.e. YAG) or Y4Al2O9 (Y(2)A ) were studied. After milling with steel tools for times up to 8 h, the pow ders were subjected to thermal cycles up to 1500 degrees C during which the phase evolution was monitored using X-ray diffractometry (including high-t emperature XRD) and differential thermal analysis. During milling the origi nal crystal structures were quickly broken down, in some cases partially re placed by an intermediate structure after milling. Upon subsequent heating the milled mixtures crystallized to give the expected phases, YA in Y(3)A(5 ) and YA in Y(2)A respectively, but the reaction route was seen to be diffe rent depending on the amount of amorphization of the yttria. Contamination by iron was seen to affect the phase distribution and the lattice parameter s. (C) 2000 Elsevier Science Ltd. All rights reserved.