Effects of microstructure on fatigue crack propagation behavior in a bi-modal TC11 titanium alloy fabricated via laser additive manufacturing.

Abstract In this study, the crack propagation behaviors in the equiaxed and equiaxed-columnar grain regions of a heat-treated laser additive manufacturing (LAM) TC11 alloy with a special bi-modal microstructure are investigated. The results indicate that the alloy presents a special bi-modal microstructure that comprises a fork-like primary α (α p) phase surrounded by a secondary α colony (α s) in the β phase matrix after the heat treatment is completed. The samples demonstrate a fast crack growth rate with larger d a /d N values through the equiaxed grain sample versus across the equiaxed-columnar grain sample at low Δ K values (<13.8). The differences that are observed between the crack propagation behaviors (in the crack initiation stage) of the samples can be mostly attributed to the different size and morphology of the α p lamellae and α s colony within the grains in the equiaxed and columnar grain regions rather than the grain boundaries. The cracks prefer to grow along the α/β boundary with a smooth propagation route and a fast propagation rate in the equiaxed grain region, where the α p and α clusters have a large size. However, in the columnar grain region, small and randomly distributed α p lamellae generate a zigzag-shaped propagation path with a reduction in the d a /d N value. Additionally, the change in the size of the α p lamellae in the equiaxed grains (heat affected bands, HAB) is also observed to influence the propagation behavior of the crack during the crack initiation stage. [ABSTRACT FROM AUTHOR]