*2.1. Sample Preparation and Characterization*

High-purity α-Al2O<sup>3</sup> (Condea HPA 0.5, USA; *d*<sup>50</sup> = 0.35 µm) and TiO<sup>2</sup> (anatase; Merck, 808, Darmstadt, Germany; *d*<sup>50</sup> = 0.35 µm) powders were used as starting powders. Monoliths of Al2O3–Al2TiO<sup>5</sup> (A–AT) composites with 10 and 40 vol.% of AT were manufactured by the reaction sintering of green compacts prepared from mixtures of Al2O<sup>3</sup> and TiO<sup>2</sup> with relative TiO<sup>2</sup> contents of 5 and 20 wt.%, respectively. Following colloidal filtration techniques [24], the powder mixtures were dispersed in deionized water by adding 0.5 wt.% (on a dry solids basis) of a carbonic acid-based polyelectrolyte (Dolapix CE64, Zschimmer-Schwarz, Lahnstein, Germany). Stable suspensions of a mixture of undoped alumina and titania with a solid loading of 80 wt.% were prepared by 4 h ball milling, using an alumina jar and balls. Green compact blocks, 70 mm × 70 mm with 12 mm thickness, were slip cast in plaster of Paris molds. The cast bodies were carefully removed from the molds and dried in air at room temperature for a minimum of 24 h. The dried blocks were sintered in an electrical box furnace (Termiber, Madrid, Spain) with heating and cooling rates of 2 ◦C/min. The first step involved sintering at 1200 ◦C with a 4 h dwell time during heating. In the second step, two different treatments with different maximum sintering temperatures, i.e., 2 h dwell times at 1450 and 1550 ◦C, were implemented. The materials were denoted as A-10AT(1450), A-10AT(1550), A-40AT(1450), and A-40AT(1550) to describe compositions and sintering temperatures.

The phase identification of the sintered specimens was performed using an X-ray diffractometer (XRD, DX-2700, Dandong, China) using CuKα radiation at room temperature over a 2*θ* range of 10 to 80◦ , with a step size of 0.03◦ and a counting time of 1.0 s at each step. Quantitative phase analysis was undertaken using the Rietveld pattern fitting method. Microstructural characterization of polished and chemically etched (HF 10 vol.%, 1 min) samples was performed by scanning electron microscopy (SEM). The average grain sizes of Al2O<sup>3</sup> and AT particles were determined using the linear intercept method, considering at least 200 grains for each phase. The densities of samples were determined using Archimedes' method (European Standard EN 1389:2003). The relative densities of α-Al2O<sup>3</sup> (ASTM 42-1468) and AT (ASTM 26-0040) were calculated as percentages of their theoretical densities, i.e., 3.99 and 3.70 g/cm<sup>3</sup> , respectively. The flexural strength was determined by a three-point bending test on the rectangular bar samples (geometry of 28 <sup>×</sup> <sup>10</sup> <sup>×</sup> 2 mm<sup>3</sup> ), under conditions of a span length of 20 mm and a constant loading speed of 0.5 mm/min. For each material, three samples were prepared for the bending test to obtain an average value and the standard deviation. The fracture surfaces of the fractured samples were characterized by SEM after the three-point bending test. diffractometer (XRD, DX-2700, Dandong, China) using CuKα radiation at room temperature over a 2 range of 10 to 80°, with a step size of 0.03° and a counting time of 1.0 s at each step. Quantitative phase analysis was undertaken using the Rietveld pattern fitting method. Microstructural characterization of polished and chemically etched (HF 10 vol.%, 1 min) samples was performed by scanning electron microscopy (SEM). The average grain sizes of Al2O3 and AT particles were determined using the linear intercept method, considering at least 200 grains for each phase. The densities of samples were determined using Archimedes' method (European Standard EN 1389:2003). The relative densities of α-Al2O3 (ASTM 42-1468) and AT (ASTM 26-0040) were calculated as percentages of their theoretical densities, i.e., 3.99 and 3.70 g/cm3, respectively. The flexural strength was determined by a three-point bending test on the rectangular bar samples (geometry of 28×10×2 mm3), under conditions of a span length of 20 mm and a constant loading speed of 0.5 mm/min. For each material, three samples were prepared for the bending test to obtain an average value and the standard deviation. The fracture surfaces of the fractured samples were characterized by SEM after the three-point bending test. *2.2. Residual Stress Measurement* 

during heating. In the second step, two different treatments with different maximum sintering temperatures, i.e., 2 h dwell times at 1450 and 1550 °C, were implemented. The materials were denoted as A-10AT(1450), A-10AT(1550), A-40AT(1450), and A-

The phase identification of the sintered specimens was performed using an X-ray

*Materials* **2021**, *14*, 7624 4 of 24

40AT(1550) to describe compositions and sintering temperatures.

### *2.2. Residual Stress Measurement* For neutron diffraction strain scanning, rectangular parallelepiped samples (30 × 30

For neutron diffraction strain scanning, rectangular parallelepiped samples (30 × 30 <sup>×</sup> 10 mm<sup>3</sup> ) were employed. Residual strain measurements were performed by neutron diffraction on the ENGIN-X TOF instrument [25] at the ISIS neutron and muon source in Rutherford Laboratory, UK. Two detector banks were set at Bragg angles of 2*θ<sup>B</sup>* = ±90◦ for simultaneous strain measurement in two directions: the in-plane direction (parallel to the major plane of 30 mm × 30 mm samples) and normal direction (perpendicular to the major plane of 30 mm × 30 mm samples). The experimental setup is illustrated in Figure 1. × 10 mm3) were employed. Residual strain measurements were performed by neutron diffraction on the ENGIN-X TOF instrument [25] at the ISIS neutron and muon source in Rutherford Laboratory, UK. Two detector banks were set at Bragg angles of 2 = ±90° for simultaneous strain measurement in two directions: the in-plane direction (parallel to the major plane of 30 mm × 30 mm samples) and normal direction (perpendicular to the major plane of 30 mm × 30 mm samples). The experimental setup is illustrated in Figure 1.

**Figure 1.** Experimental setup on TOF neutron strain scanner ENGIN-X at ISIS. (**a**) Residual strains were collected both in in-plane and normal directions. (**b**) Strain scanning was implemented along sample thickness with interior gauge volume. **Figure 1.** Experimental setup on TOF neutron strain scanner ENGIN-X at ISIS. (**a**) Residual strains were collected both in in-plane and normal directions. (**b**) Strain scanning was implemented along sample thickness with interior gauge volume.

The set measurement gauge volume was 16 <sup>×</sup> <sup>3</sup> <sup>×</sup> 1 mm<sup>3</sup> , with the centroid defined as the location of measurement. Through-thickness strain scanning was implemented along the sample thickness (10 mm) with a 1.5 mm measurement step. To avoid anomalous strain due to the effects of partial neutron gauge volume filling near the surface [26], the scanning points were no closer than 2 mm from the sample surfaces. The stress-free reference lattice parameters of Al2O<sup>3</sup> and AT phases were obtained by measuring Al2O<sup>3</sup> and AT powders with the same gauge volume. As the experimental standard, CeO<sup>2</sup> powder was measured to calibrate the peak profile parameters to be employed in Rietveld refinement. All the measurements were performed at room temperature.
