Microstructure and thermal expansion of Al2TiO5–MgTi2O5 solid solutions obtained by reaction sintering

Solid solutions Mg0.1Al1.8Ti1.1O5 and Mg0.5AlTi1.5O5 were obtained by reaction sintering of mixtures of the binary oxides at 1350–1600 °C using different precursor powders. For the composition Mg0.1Al1.8Ti1.1O5, ceramics sintered at 1400–1500 °C have high relative density (⩾90%), reduced grain size (2–6 μm), low thermal expansion (−0.8 to 0.3×10−6 K−1 in the range 200–1000 °C) and reproducible expansion behaviour. At higher temperature, grain size rapidly increases owing to anisotropic and exaggerated grain growth (EGG) resulting in severe microcracking. Microstructure evolution is affected by the nature of the starting oxides, in particular for what concerns the onset temperature of EGG, the size and the fraction of abnormal grains. For the composition Mg0.5AlTi1.5O5, EGG already takes place at 1350 °C and materials with grain size < 5 μm are difficult to obtain by conventional reaction sintering. Large grained samples (>10 μm) of both compositions show a reduced hysteresis and complex thermal expansion behaviour. In particular, heating to 1000 °C results in a significant increase in specimen size on return to room temperature. Repeated thermal cycling leads to an increase of the hysteresis.