Localized brittle intergranular cracking and recrystallization-induced blunting in fatigue crack growth of ductile tantalum

Laser powder bed fusion (L-PBF) induces cellular structures that are considered significant contributors to the enhancement of strength and plasticity. However, after conducting fatigue crack growth (FCG) rate tests on L-PBF fabricated tantalum (LPBF-Ta), we found that cellular structures with specific growth directions can abnormally induce local brittle intergranular cracking, indicating that cellular structures are not always a reinforcing factor for fatigue crack resistance. Multiscale microstructural characterization reveals that when cellular structures within grains are simultaneously perpendicular to the primary thermal gradient, loading direction, and the cellular structures in adjacent grains, residual stresses and stress concentrations in cell walls lead to inhomogeneous deformation at grain boundaries, triggering intergranular cracking. Additionally, these cellular structures are more likely to form dislocation networks, which inhibit the cross-slip of screw dislocations, preventing the formation of stable dislocation sources at crack tips and resulting in local embrittlement. Moreover, recrystallization at room temperature leads to inhomogeneous Schmid factors across grains, hindering the formation of persistent slip bands. This promotes fatigue crack blunting and effectively enhances resistance to FCG. The findings of this study may provide insights for researchers focused on grain boundary engineering and computational modeling of FCG.