Room temperature creep mechanisms of Ti–6Al–4V ELI alloy with equiaxed microstructure under different applied stresses

Titanium alloys are often used to make deep-sea pressure hulls and the creep strain produced during service is nonnegligible. The variation of dislocation structures with room temperature creep stresses in equiaxed α grains of Ti-6Al-4V ELI alloy was examined using transmission electron microscope. The plastic deformation mechanism varied with the applied stress, mainly manifested by the change of dislocation patterns and the sequential activation of slip systems. At the stress below the creep threshold, even though almost no macroscopic creep was generated, a small number of immobile dislocations and dislocation networks occurred in the sample. The networks were composed of dislocations with different Burgers vectors, and part of them would evolve into low angle grain boundaries (LAGBs). At the stresses above the threshold, the density of dislocations in α grains increased with the stress level, and the basal , pyramidal and slips were activated successively which also contributed to creep and participated in the formation of LAGBs. Prismatic slip was always the predominant creep deformation mechanism for the studied stress range. The activation of difficult slip systems was attributed to the stress concentration and stress redistribution between soft and hard phases/grains. In general, the amount of LAGBs increased with the applied creep stress, but the LAGBs formed by dislocation networks decreased.