Stress-based fatigue behavior of Ti-6Al-4V alloy with a discontinuous lamellar microstructure fabricated by thermomechanical powder consolidation

A powder metallurgy (PM) Ti-6Al-4V alloy with a novel discontinuous lamellar microstructure and a high relative density was fabricated by thermomechanical powder consolidation (TMC) of TiH2-Al60V40 powder and subsequent heat treatment. It was found that the PM Ti-6Al-4V alloy exhibited a high fatigue performance relative to that of the Ti-6Al-4V alloy fabricated using the traditional powder metallurgy process of pressing and sintering. The fatigue cracks initiated at grain boundary alpha phase along the near-basal planes instead of pores due to their small sizes. The TMC defects enlarged and induced secondary cracks under low-stress cyclic loading, leading to stress redistribution at the crack tip. The discontinuous lamellar microstructure caused an increase of fatigue crack deflection frequency and easier crack closure relative to the fully-lamellar microstructure, owing to the increase of the number density of the alpha/beta interfaces and the random distribution of the crystallographic orientations of the fine domains of alpha/beta lamellar structure. The high plasticity of the crack tip makes the crack blunt due to the strain mismatch between the alpha and beta phases of the discontinuous lamellar microstructure. Consequently, the factors associated with microstructures and defects for limiting the fatigue performance of powder metallurgy titanium alloys were identified, and a meaningful route to enhance the fatigue properties was proposed for fabrication of low cost and high performance PM titanium alloys.