Titanium alloys are particularly well-suited for the fabrication of high-temperature components in aero-engines due to their low density, high specific strength, and excellent oxidation resistance and fatigue performance. However, the difficulty in sintering and insufficient high-temperature performance of Y₂O₃ ceramic cores are the main problems to restrict their application because of the high chemical activity of titanium. At present, the addition of mineralizers can reduce the sintering temperature to a certain extent. However, this reduction in temperature inevitably results in a decrease in porosity, thereby affecting the leachability of Y₂O₃ ceramic cores. To satisfy the criteria associated with investment casting, a dual-mineralizer system can be added to improve the porosity and sintering performance of Y₂O₃. This study was to investigate the synergistic strengthening mechanism of Y₂O₃-based ceramic cores modified with Al₂O₃ and TiO₂ as dual mineralizers.

Y₂O₃ powders with different particle sizes were used as a matrix material, and nano-sized alumina (Al₂O₃) and titanium dioxide (TiO₂) were used as mineralizers. Y₂O₃ ceramic cores with different mixed mineralizer ratios were prepared by a hot-press injection molding method. Key performance parameters of each core sample were obtained based on the aviation industry standard HB 5353. The fracture morphology and element distribution of the cores were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), so as to investigate the synergistic strengthening mechanism of the Y₂O₃-based ceramic cores modified by the dual mineralizers. Meanwhile, a pouring verification was conducted on each ceramic core sample, including wax pressing fracture, the ceramic/metal interface reaction in the alloy after pouring, and the surface quality of the castings.

The sintering process is typically categorized into three distinct stages. The early stage of sintering is marked by the occurrence of bond merging and rearrangement, as well as the predominance of point contact between particles. The middle stage of sintering is marked by the transition from point contact to face contact between the particles and the pores. This progression leads to a further narrowing of the contact area, while maintaining the connection of pores with the surroundings. The late sintering stage is characterized by persistent mass transfer, grain growth, and the pores changing to closed pores, culminating in increased density. The resulting Al₂Y₄O₉(YAM) phase exerts a partial inhibition effect on the particle interface migration during the late-stage sintering process due to the incorporation of Al₂O₃ into Y₂O₃. However, a minimal shrinkage alteration occurs under these dual counteracting mechanisms due to the nano-sizes of Al₂O₃ particles. The incorporation of TiO₂ into Y₂O₃ during the middle stage of sintering results in a significant enhancement of the rate of TiO₂ diffusion into Y₂O₃. This is attributed to the continuous promotion of the mass transfer process by the point defects generated during this stage. The degree of densification exhibits a substantial increase from the middle stage of sintering to the late stage of sintering, accompanied by an increase in the strength of the sample.

In addition, no obvious oxide layer or phase transformation appears in the titanium alloy cast for the sample A3T0.5. The X-ray diffraction (XRD) patterns demonstrate that the titanium alloy is composed of α-Ti and β-Ti phases-phases typically formed during the cooling process of TC4 castings. No characteristic peaks associated with residual Y₂O₃ or interfacial reaction products occur in the XRD patterns. The cross-sectional and internal compositions of the titanium alloy remain pure, without any signs of element infiltration from the ceramic core. This can be attributed to two critical factors, i.e., the Y₂O₃ ceramic core modified by dual mineralizers exhibits a high sintering density, efficiently suppressing the diffusion of core-derived elements into the molten alloy; and Y₂O₃ inherently possesses exceptional chemical inertness, resulting in a weak chemical reactivity with titanium alloy even under the elevated temperatures encounters during casting. Consequently, the inner cavity surface of the as-cast titanium alloy attains favorable quality, fully complying with the service requirements for aerospace castings.

Al₂O₃ reacted with Y₂O₃ to form YAM phase with a better high temperature performance, and formed a looser network structure as Al₂O₃ and TiO₂ contents increased. The addition of TiO₂ could promote the mass transfer process between Al₂O₃ and Y₂O₃, a stable network structure that resisted the sintering shrinkage of Y₂O₃, achieving sufficient high-temperature performance and porosity. The optimal composition of the Y₂O₃ ceramic cores obtained was the addition of 3% of nano-sized Al₂O₃ and 0.5% of nano-sized TiO₂. The shrinkage of the ceramic cores in the length, width, and height directions was 1.86%, 2.33%, and 2.27%, respectively. The high- and room-temperature flexural strengths were 26.09 MPa and 24.25 MPa, respectively. The deflection was 0.158 mm, and the open porosity was 37.1%. Titanium alloy castings of the sample A3T0.5 exhibited a surface roughness of < 2 μm within the inner cavity. These castings could lack a distinct hardening layer as well as the limited penetration of elements from the ceramic matrix into the titanium alloy surface. This could meet a demand for Y₂O₃ ceramic cores in titanium alloy casting for the production of qualified titanium alloy castings.