[en] Highlights: • The microstructural correlated damage mechanisms were studied in in-situ TiB₂/Al composite. • A microstructural-based multistage damage in HCF was identified. • Effects of GBs, grains orientations and TiB₂ particles on damage mechanisms were discussed. • An energy model for dislocation slip in nano or sub-micron particles reinforced metal composites was proposed. The damage mechanisms during high-cycle fatigue (HCF) process were systematically investigated in the in-situ TiB₂/2024 Al-composite. It is found the HCF endurance limit of in-situ TiB₂/2024 Al-composite is ~ 360 MPa, which is much higher than the reported ex-situ particle-reinforced composites (~ 180–300 MPa). A microstructural-based multistage damage in HCF is identified from fracture surface: Stage I (crack initiation), Stage II (stable crack propagation), and Stage III (ultimate fracture). In Stage I, the (S/θ + TiB₂) particles generally act as initiation sites in most cases. The nano or sub-micron TiB₂ particles can homogenize stress and reduce dislocations piling-up at grain boundaries (GBs), impeding the crack nucleation from GBs. In Stage II, the GBs, grain orientations and TiB₂ particles are the major factors for the damage behaviors. The GB effects depend on their misorientations, geometries and nearby particles. The crack propagation shows crystallographic characteristics of {100}〈001〉, {111}〈110〉 and {111}〈112〉, which have different propagation rates. For TiB₂ particles, the complex effects on the HCF damage behavior depend on their size and distribution. Considering the microstructural factors, the HCF damage mechanisms was discussed in detail and an energy model of dislocation slipping for nano or sub-micron particle-reinforced metal composites was proposed.