Abstract: Titanium alloys are of keen interest as lightweight structural materials for aerospace and automotive industries. However, a longstanding problem for these materials is their poor tribological performances. Herein, we designed and fabricated a multiphase Ti-Mo-Ag composite (TMA) with heterogeneous triple-phase precipitation (TPP) structure by spark plasma sintering. A lamellar α-phase (α_L) precipitates from the β-phase under furnace cooling conditions and maintains a Burgers orientation relationship (BOR) with β-matrix. An active eutectic transition also occurs in the titanium matrix, resulting in TiAg phase. The intersecting acicular TiAg and lamellar α_L cut β grains into fine blocks and the primary equiaxed α phase also provides many interfaces with β phase, which together effectively impede dislocation movement and increase strength. Compared with other titanium composites, TMA with TPP microstructure gets an excellent combination of strength (yield strength 1205 MPa) and toughness (fracture strain 27%). Furthermore, the TPP structure endows TMA with strong cracking resistance, which aids in reducing abrasive debris at high temperatures during sliding and obtaining a low wear rate. Simultaneously, Ag particles distributed at grain boundaries will easily diffuse to the wear surface, in situ forming the necessary lubricating phase Ag₂MoO₄ with Mo-rich matrix debris via oxidation. TMA possesses excellent tribological properties with especially low wear rate of 8.0 × 10^-6 mm³N^-1m^-1 and friction coefficient (CoF) of merely 0.20 at 600 °C. Unlike other self-lubricating composites with high volume fraction of soft ceramic lubricants, which inevitably sacrifice their mechanical strength and ductility, the composite TMA possesses well-balanced strength, toughness and self-lubricating properties. It holds important implications to design other metal matrix self-lubricating composites (MMSCs) used for load-bearing moving parts.

Key words: Titanium, High-temperature oxidation, Self-lubricating, Mechanical properties