Microstructure-dependent predictive modeling of small crack growth and plastic zone size of LPBF-fabricated TiC/Ti-6Al-4V composite

In-situ fatigue tests were conducted at 25°C to explore the influence of microstructural characteristics on small-crack growth behavior and to predict the crack-growth rate and plastic-zone size in laser powder bed fused-fabricated TiC/Ti-6Al-4V composites at the stress ratios ( R ) of 0 & 0.3. The microscopic examinations were conducted to characterize the deformation of grains along the crack propagation route. Results revealed that fatigue small cracks advanced predominantly in a transgranular manner, with their progression strongly controlled by grain-level lattice orientation and slip-system activation. Grain boundaries intermittently arrested cracks, after which propagation resumed through coalescence with secondary cracks or reorientation onto alternative slip planes. Microstructure-induced crack-growth fluctuations were pronounced below 200 μm but gradually transitioned to long-crack behavior near 500 μm. Below this threshold, crack-growth rates were discontinuous but stabilized with increasing crack size. Furthermore, crack-opening measurements were incorporated into a closure-corrected stress-intensity-factor approach, which predicted maximum forward plastic-zone sizes of approximately 31.2 μm at R = 0 and 32.9 μm at R = 0.3, while DIC qualitatively captured the corresponding evolution of crack-tip strain localization. Finally, following these mechanistic insights, a modified semi-empirical model was developed that incorporates the material yield strength, average grain size, and crack-tip plastic-zone evolution, which accurately predicts fatigue small-crack growth rate under the examined conditions.