Research on the microscopic characteristics of low-cycle-fatigue and creep coupling in cast aluminum alloys for cylinder heads

As one of the most structurally complex components in diesel engines, the cylinder head is subjected to increasing thermal loads. This paper investigates the microscopic characteristics of fracture morphology in cast aluminum alloy materials for cylinder heads under high-temperature low-cycle-fatigue-creep (LCFC) coupling conditions. The study explores the effect of creep on the microscopic evolution of the material's microstructure, clarifies the fracture failure modes, and identifies the underlying causes. The results show that at 250 °C, the fracture surfaces in low-cycle-fatigue (LCF) tests mainly exhibit brittle fracture characteristics. As the temperature increases, the fracture gradually transitions from brittle to ductile. Due to the influence of creep, the fracture surfaces in LCFC tests already show ductile fracture characteristics at 250 °C. Microscopic observations reveal that the reasons for the more pronounced cyclic softening phenomenon in LCFC conditions are attributed to creep's effects on dislocation annihilation, dislocation rearrangement, and the coarsening and dissolution of precipitates. Two fracture modes, defect-induced fracture and Si-phase fracture, are proposed. Observations indicate that high-temperature creep can cause the eutectic Si structures within the material to fracture more easily, thereby altering the fracture mode.