Reversible α′ ↔ β′ transformation in preferentially oriented sialon ceramics

A preferential orientation was observed in a Nd-(α + β)-sialon ceramic, of the composition Nd0.4Si9.6Al2.4O1.2N14.8, after hot pressing at 1800 °C for 2 h. Post-sintering heat treatment was performed at 1450 °C for 72 h, resulting in the α′ → β′ transformation, accompanied by an increase in the M ss ′ and 21R phases. Subsequent heat treatment at 1800 °C for 4 h resulted in a reverse transformation from β′ to α′, with a corresponding decrease in M ss ′ and 21R. This indicates that the α′ + liquid ↔ β′ + M ss ′ + 21R transformation reaction is thermodynamically reversible. The microstructure was also found to be reversible, except for a certain degree of grain growth. The mechanical properties were determined from indentation tests, and were found to cycle with the transformation cycling. In addition, the preferred orientation introduced during hot pressing was observed in both the forward and reverse transformations, indicating that there is a crystallographic relationship between the transformation phases.