The nucleation mechanism and microstructural developments in M-Si-Al-O-N system(M=Y, Ca) have been investigated. In a study on α'→β' transformation, sintered Y-α'-sialon($Y_0.5Si_9.75Al_2.25O_0.75N_15.25$) containing large grains of about 5㎛ was resintered within a glass matrix(14.43 $Si_3N_4$-4.19 AlN-40.79 Al_2O_3-40.59 Y_2O_3(wt.%)) of which the composition was in equilibrium with β'-sialon. During resintering the glass matrix penetrated into sintered α'-sialon specimen and induced the transformation from α'- to β'-sialon. No trace of α' was found within β' grains formed. The nucleation of β' grains is thus believed to occur not on the preexisting α' grains but in the penetrated glass matrix. The concentration of Al in β' grains increased from their center towards their edge, implying that the glass matrix had penetrated continuously with resintering time. Continuous penetration of glass matrix and thus a concentration gradient in the penetrated glass were also confirmed by a directional growth of 12H($SiAl_5O_2N_5$) polytype grains formed between transformed β' and bulk glass matrix.
For a study on β'→α' transformation, $β'_{z=3}-sialon$($Si_3Al_3O_3N_5$) was resintered in a Ca-Si-Al-O-N glass matrix(58.43 $Si_3N_4+2.26$ AlN+39.31 CaO(wt.%)) of which the composition was in equilibrium with Ca-α'-sialon. A β'→α' transformation was induced by the infiltration of bulk glass matrix. The formed α' did not show any heterogeneity, such as the fragments of β', dislocation network, or secondary phases similar to the case of α'→β' transformation. These results show that the α'↔β' transformation occurs by nucleations of a new phase in a liquid matrix, not on the preexisting another sialon grains which have a crystal structure different from that of newly formed grains.
The microstructure of hot-pressed α'-sialon could be controlled by changing crystalline form of $Si_3N_4$ or ratio of the raw powder mixture. α'-sialon prepared using β-$Si_3N_4$ powder as the starting material showed a microstructure with some elongated grains, in contrast to more equiaxed structure when using α-$Si_3N_4$. The difference in microstructure with the phase of starting $Si_3N_4$ powder could be explained by the nucleation of α' grains on the preexisting α-$Si_3N_4$ particles with a similar crystal structure. Some elongated grains may enhance the mechanical properties of compacts, especially fracture toughness by crack bridging and deflection.
