Breaking Geometry‑Performance Trade‑Off in B4C: Hot Pressing of Hemispherical Shell by Pressure‑Induced Phase Transition

Pressure-assisted sintering of high-performance boron carbide (B 4 C) into complex shapes remains a significant challenge. In this study, a near‑net‑shape B 4 C hemisphere was successfully prepared by hot pressing using a specially designed mold. The uniform dispersion of nano‑Al 2 O 3 within the B 4 C matrix, achieved via a hydrothermal process, enhances both sinterability and mechanical properties. A pressure-induced phase transformation from Al 2 O 3 to Al 18 B 4 O 33 is identified at applied pressures exceeding 40 MPa, which tailors the densification rate and determines the final phase assemblage. The as‑obtained hemisphere achieved full density, exhibited uniform thickness, and was free of internal defects, demonstrating a viable route to break the conventional performance–geometry trade‑off in B 4 C‑based ceramic components.