Abstract: (Ca,Mg)-α-Sialon for the compositions M_xSi_3-xAl_xO_xN_16-x with x ranging from 0.3 to 1.4 were prepared by hot-pressing technique. The result showed that the
crystalline phase in the sintered material consists of α-Sialon and Mg-containing AlN-polytypoid when x is equal to or greater than
1.0 and the AlN-polytypoid content increases as the x-value increases. By the use of duplex metal oxide CaO/MgO, instead of CaO, as additives
to form α-Sialon, the material possesses a better sintering ability. TEM and EDAX results further revealed that the most of added Mg⁺⁺ can be incorporated into the Sialon to form magnesium AlN-polytypoid
phase, whereas the most added Ca⁺⁺ enters α-Sialon structure. The formation of magnesium-containing AlN-polytypoid phase in the composite
materials can provide a new approach to reduce the amount of glassy phase at the grain boundary.

Key words: Ca, Mg, Sialon, AlN-polytypoid