Homogeneous Ti/Al2O3 composites with different volume percentages of Ta2O5 addition were prepared at different temperatures via hot pressing sintering. Laminated Ti/Al2O3 composites with different volume percentages of Ta2O5 added were prepared. The effects of Ta2O5 on the composition, microstructure, mechanical properties and elements diffusion of the composites were characterized and investigated. Ta2O5 inhibited the production of TiAl and Ti3Al by forming solid solution with Ti or new reaction product of Al. This solid solution melted and filled the void of Al2O3 phase to increase the density of Ti/Al2O3 composites at high temperature. Mechanical properties had also been improved by this phenomenon. Because Al element couldn’t diffuse in Ta or react with it, Al couldn’t diffuse through the Ta-enriched area at the interface of Ti and Al2O3.

Al2O3 multi-phase composites with different volume fractions of SiC varying from 0 vol% to 30.0 vol% were fabricated by vacuum hot pressing sintering at 1600 °C under the pressure of 30 MPa for 2.0 h. The aim of this work was to investigate the effect of SiC content on the morphology and mechanical properties of the Al2O3 multi-phase composite. The results show that the addition of SiC and Ti can produce new strengthening and reinforcing phases include Ti3SiC2, TiC, Ti5Si3, which would hamper the migration of grain boundaries and promote sintering. The mechanical performances could reach the comprehensive optimal values for 20.0 vol% SiC, delamination and transgranular fracture being the major crack propagation energy dissipation mechanisms.

•Nd2O3 improved the oxidation and mechanical properties of Ti/Al2O3 composites.•Appropriate content of Nd2O3 was required for the optimal performances.•Excessive Nd2O3 weakened its positive effect.•Plate-like grains which improved the properties of the composites were found.A range of Ti/Al2O3 composites with different Nd2O3 content (0–5 vol.%) were fabricated by vacuum hot pressing sintering at 1420 °C for 1.5 h under the pressure of 30 MPa. The sintered samples with Nd2O3 additives exhibited more superior performances than those without additives. When 3.0 vol.% Nd2O3 was added, Ti/Al2O3 composite showed optimal density (relative density of 98.79%), hardness (Vickers hardness of 15.87 GPa), strength (flexural strength of 412.45 MPa), toughness (fracture toughness of 8.77 MPa m1/2) and oxidation resistance (oxidation depth of ∼244 μm). However, excessive additive (＞3.0 vol.%) would weaken these positive effects. SEM results revealed that the superior performances could be attributed to the grain refinement and microstructure densification. Moreover, plate-like grains were found at the interface and additional experiments demonstrated that their formation was caused by enrichment of Nd element, which is beneficial to the microstructure densification.

•Thickness of reaction layer is related to temperature and holding time.•The kinetic equation for the thickness is d=1.2864×103exp(−(153.1×103)/RT)t0.96d=1.2864×103exp(−(153.1×103)/RT)t0.96.•The forming process of the reaction layer can be separated into three stages.•Al ions dissolve in the Ti phase substitutionally.•Ti ions dissolve in the Al2O3 phase by interstitial mechanism.Ti/Al2O3 composites are prepared by vacuum hot-pressing sintering in order to study the elements diffusion and formation of the reaction layer at the interfaces. The results indicate that the forming process of the reaction layer can be separated into three stages, and the thickness of the interfacial layer is greatly related to the sintering temperature and holding time. By calculating, the kinetic equation for the growth of the thickness is expressed as d=1.2864×103exp(−(153.1×103)/RT)t0.96d=1.2864×103exp(−(153.1×103)/RT)t0.96. When temperature is above 1250 °C, Al element has more powerful diffusion ability than Ti, so reaction layer mainly composed of TiAl and Ti3Al is formed at the side of Ti zone. In the case of Al2O3 phase, Al ions dissolve in the Ti phase substitutionally and migrate by exchanging with vacancies, which is in contrast with the direct interstitial mechanism of Ti and O ions.

The Ti/Al2O3 composites are fabricated by hot pressing sintering, and the effect of Y2O3 on the microstructure and mechanical property of Ti/Al2O3 composites has been studied by means of SEM, EDS and XRD. The results show that the Al2O3 decomposition to Al and O first, and then Al diffusion to Ti formed the intermetallics such as Ti3Al and TiAl phases in the composites. Thermodynamics calculation shows us the Gibbs free energy are −120.23 kJ/mol and −73.44 kJ/mol for Ti3Al phase and TiAl phase, respectively. The addition of yttria to the composites makes enthalpy changes, reduces the amount of Ti3Al phase. The effect of Y2O3 addition on the microstructures and mechanical properties of the Ti/Al2O3 composite was characterized. When the Y2O3 content is 1.0 vol%, the flexural strength, fracture toughness, Vickers hardness and relative density of the composite reach the maximum values of 302.73 MPa, 6.45 MPa m−1/2, 16.78 GPa and 98.63%, respectively, and the fracture mode changes from intergranular to transgranular.

Laminated Ti/Al2O3 composites with Nb addition were fabricated by tape casting and vacuum hot pressing sintering. Effects of Nb addition on the elements diffusion (Al, O and Ti) and the mechanical properties of the laminated composites were investigated. When the amount of layers was 40, the flexural strength and the fracture toughness of the laminated composite with 1.0 vol% Nb addition in both Ti and Al2O3 layers reached the maximum values of 387.93 MPa and 9.56 MPa m1/2. Based on the results of the SEM, EDS and XRD, the improvement of the mechanical properties could be attributed to the generation of AlNb2 which formed “barrier” between the Ti and Al2O3 layers. As a result, the diffusion of Al and O was hindered and the thicknesses of the reaction layers were reduced eventually. Moreover, the strengthening and toughening mechanism of Nb addition was illustrated and discussed.

Ti/Al2O3 composites with different volume percentages of Pr6O11 added (0–12.0  vol.%) were prepared by pressureless sintering at 1600 °C for 1.5 h. The influences of Pr6O11 on the composition, microstructure and mechanical properties of the composites were characterized and investigated. The results showed that Pr6O11 could promote the sintering of the composites by generating some new interfacial reaction products, such as AlTiO2, Pr2Ti2O7 and PrAlO3. Pr6O11 could also inhibit the production of TiAl and Ti3Al by the same mechanism. Additionally, Pr6O11 changed hexagonal alumina to tetragonal alumina. The latter could improve the mechanical properties of the composites by the effects of crack deflection and particle pullout when it was present in proper amounts. Composites showed satisfactory comprehensive properties when the content of Pr6O11 was no more than 3.0 vol.%.

Graphene has been successfully fabricated by a novel method, using graphite powder and NMP (N-Methyl Pyrrolidone) as the raw materials based on the principles of liquidoid exfoliation and mechanical milling. SEM, TEM and Raman spectrum were utilized to characterize the morphology of the homemade graphene, illustrating the few defects and rare layers were endowed in this study. Afterwards, the homemade and commercial graphene were doped into Al2O3 powder with the mass ratio of 0%, 1%, 2%, and 3% to reinforce the mechanical properties of the matrix. The composites were processed at 1600 °C, pressure of 30 MPa and soaking time of 1 h by vacuum hot pressing. The test results illustrated the bending strength and fracture toughness tended to be intensive at first and subdued afterwards, achieving the optimal performance of 625.4±18.2 MPa and 6.07±0.22 MPa m1/2 at 2 wt% prepared graphene additive, and the commercial grapheme owned the best heighten effect in 3 wt% graphene/Al2O3 composites. Compared to the blank Al2O3 sintered samples, the graphene/Al2O3 specimens (both prepared and commercial additive) behaved evident increase in mechanical properties, even upon 30% enhanced in fracture toughness and bending strength generally by the prepared grapheme. Moreover, the prepared graphene had better improvement effect than commercial graphene in enhancing mechanical properties of Al2O3 ceramic.