Strengthening mechanism of additively manufactured Ti-6Al-4V titanium alloy by deformable-restoring granular α′

Abstract

Fatigue cracks often initiate from the small defects, leading to the degradation of fatigue property, which has been a bottleneck problem that restricts the application of additive titanium alloys. To improve this problem, we propose a new low stress cyclic strengthening method (LSCSM) to precisely induce the precipitation of new granular α′ around the internal small pore, and ultimately achieve a significant enhancement in the high cycle fatigue life (HCFL) of additive Ti-6Al-4V titanium alloys. By combining cross-scale microstructure characterization and molecular dynamics simulation, we reveal the strengthening mechanism of LSCSM based on the micro-plastic deformation evolution during phase transformation. We find that a new granular α′ (three different crystal structures with crystal band axes of $\left[\overline{\text{2}}\text{4}\overline{\text{2}}\text{3}\right]$, $\left[\text{01}\overline{\text{1}}\text{1}\right]$ and $\left[\text{01}\overline{\text{1}}\text{0}\right]$, respectively) can precipitate during LSCSM, which nucleates with dislocations as nucleation points and grows by dislocations annihilation. During this process, the number of dislocations and stress/strain field within the granular α′ are gradually reduced, which can significantly enhance HCFL (52 times) due to the recovery from fatigue damage. This work has important engineering application value in strengthening of key components of aircraft engines.