*Article* **Effect of Al2TiO<sup>5</sup> Content and Sintering Temperature on the Microstructure and Residual Stress of Al2O3–Al2TiO<sup>5</sup> Ceramic Composites**

**Kunyang Fan 1,2,\*, Wenhuang Jiang 1,2 , Jesús Ruiz-Hervias <sup>3</sup> , Carmen Baudín 4 , Wei Feng 1,2, Haibin Zhou <sup>5</sup> , Salvador Bueno <sup>6</sup> and Pingping Yao <sup>5</sup>**


**Abstract:** A series of Al2O3–Al2TiO<sup>5</sup> ceramic composites with different Al2TiO<sup>5</sup> contents (10 and 40 vol.%) fabricated at different sintering temperatures (1450 and 1550 ◦C) was studied in the present work. The microstructure, crystallite structure, and through-thickness residual stress of these composites were investigated by scanning electron microscopy, X-ray diffraction, time-of-flight neutron diffraction, and Rietveld analysis. Lattice parameter variations and individual peak shifts were analyzed to calculate the mean phase stresses in the Al2O<sup>3</sup> matrix and Al2TiO<sup>5</sup> particulates as well as the peak-specific residual stresses for different *hkl* reflections of each phase. The results showed that the microstructure of the composites was affected by the Al2TiO<sup>5</sup> content and sintering temperature. Moreover, as the Al2TiO<sup>5</sup> grain size increased, microcracking occurred, resulting in decreased flexure strength. The sintering temperatures at 1450 and 1550 ◦C ensured the complete formation of Al2TiO<sup>5</sup> during the reaction sintering and the subsequent cooling of Al2O3–Al2TiO<sup>5</sup> composites. Some decomposition of AT occurred at the sintering temperature of 1550 ◦C. The mean phase residual stresses in Al2TiO<sup>5</sup> particulates are tensile, and those in the Al2O<sup>3</sup> matrix are compressive, with virtually flat through-thickness residual stress profiles in bulk samples. Owing to the thermal expansion anisotropy in the individual phase, the sign and magnitude of peak-specific residual stress values highly depend on individual *hkl* reflection. Both mean phase and peak-specific residual stresses were found to be dependent on the Al2TiO<sup>5</sup> content and sintering temperature of Al2O3–Al2TiO<sup>5</sup> composites, since the different developed microstructures can produce stressrelief microcracks. The present work is beneficial for developing Al2O3–Al2TiO<sup>5</sup> composites with controlled microstructure and residual stress, which are crucial for achieving the desired thermal and mechanical properties.

**Keywords:** Al2TiO<sup>5</sup> ; ceramics; crystal structure; residual stresses; neutron diffraction
