Microstructure-related fatigue short crack growth behavior in LPBF TiC/Ti6Al4V: Grain deformation and predictive modeling

This study employed in-situ fatigue testing to investigate the microstructural mechanisms governing fatigue short crack growth (FSCG) in laser powder bed fused TiC/Ti6Al4V titanium matrix composites under two stress ratios at room temperature. Advanced microscopic analyses were performed to reveal grain deformation along the crack path. Results indicate that FSCG occurred predominantly in a transgranular manner, showing pronounced effects of crystallographic orientation. Low-angle grain boundaries induced deflection, whereas high-angle grain boundaries effectively impeded short crack propagation. Furthermore, notch effects facilitated activation of multiple slip systems, and short cracks within grains tended to grow following prismatic and pyramidal slip planes. Moreover, the arrest of short cracks at grain boundaries was evaluated using a combination of microstructural and crystallographic parameters, including the geometrical compatibility factor, Schmid factor, and twist angle. These investigations along the crack path provide valuable insights into the microstructural factors that govern resistance to short crack growth, as well as clarify the pronounced fluctuations observed in the crack growth rate. Finally, building on this mechanistic understanding an analytical model for predicting the crack growth rate is proposed, showing good consistency with experimental results.