Abstract: The longstanding challenge in powder metallurgy (PM) of producing fully dense and fine equiaxed-grained titanium (Ti) with superior mechanical properties via pressureless sintering is hereby addressed by optimizing the sintering kinetics and grain growth behavior. Results show that an obvious boost in grain boundary diffusion occurs with the decrease of activation enthalpy E_a by switching to fine powder. Meanwhile, the grain boundary migration activation enthalpy H(T) is significantly raised by limiting the sintering temperature below 1100 °C, suppressing the grain boundary motion. A potential sintering window is uncovered with the capacity of achieving synergy of full density and fine equiaxed-grain via co-regulating the grain boundary diffusivity and grain boundary mobility. Accordingly, a homogeneous equiaxed PM Ti6Al4V with an average grain size of 14.8 µm and relative density of 99.8 % ± 0.1 % is successfully obtained. During plastic deformation, the refined equiaxed-grain provides a high level of stress to activate the <c+a> dislocations, which facilitates the release of stress concentration at α/β interface and enhances the uniform deformation ability, avoiding the premature failure caused by crack propagation. Therefore, the fabricated near-fully dense and fine equiaxed-grained Ti6Al4V exhibits excellent room-temperature tensile properties with the ultimate tensile strength (UTS) of 1047 ± 9 MPa and elongation (EL) of 23.0 % ± 0.9 %. These values significantly exceed the ASTM B381 and other pressureless sintered PM Ti6Al4V reported thus far. This work offers a novel scheme to discover the sintering window for advantageous microstructure in PM Ti alloys with superior strength and ductility.

Key words: Titanium, Powder metallurgy, Sintering kinetics, Grain growth, Mechanical properties