Defect-controlled dislocation evolution behavior and life assessment for fatigue subsurface cracking of LPBF titanium matrix composites at high temperature

The fatigue behavior of additive manufacturing metal matrix composites in service environments has attracted increasing attention. However, the failure mechanisms and life prediction methods associated with microstructural features, particularly dislocation evolution, remain insufficiently explored, and lack robust quantitative descriptions. Herein, high-temperature axial fatigue tests were conducted to investigate the failure mechanisms and life prediction method reflecting microstate characteristics of a laser powder bed fusion (LPBF) TiC-reinforced titanium matrix composite in high cycle fatigue regime, utilizing multiscale testing and microscale simulation techniques. The results indicate that the debonding of reinforcements, lack of fusion (LOF) defect induced cracking and pore cracking are three subsurface failure modes, corresponding to dislocation evolution mechanisms. Around the reinforcements, dislocations bypass the particles via the Orowan mechanism. In the presence of LOF defects, dislocations preferentially penetrate the defect area, while near pores, deformation is dominated by dislocation source activation and dislocation emission. These distinct dislocation evolution patterns reflect the regulatory effect of defect types on local deformation mechanism. Based on these observations, a defect-type-sensitive fatigue life prediction model was established using the Tanaka-Mura theory, incorporating key microstructural parameters such as dislocation density and inclusion structure. The consistency between experimental and predicted results demonstrates that the model can effectively reflect the differences in fatigue life under various defect conditions. This work provides a physically informed basis for defect control strategies and fatigue performance evaluation in additive manufacturing titanium matrix composites operating at elevated temperatures.