MICROMECHANISM OF ACCELERATED NEAR-THRESHOLD FATIGUE-CRACK GROWTH IN AL2O3 AL-CU COMPOSITE AT ELEVATED-TEMPERATURE/

Fatigue crack growth behavior of a peak-aged Al2O3/Al-Cu composite was examined at 150-degrees-C and compared to the behavior at room temper ature (RT). At 150-degrees-C, fatigue crack growth rates showed strong dependence on loading time. At short loading time, when stress-intens ity range was decreased to approach fatigue threshold, crack growth ra tes at 150-degrees-C were comparable to those measured at RT. Prolonge d fatigue testing at near-threshold crack growth rates resulted in osc illations of crack growth rate until the fatigue crack growth behavior was stabilized to become similar to that in an overaged composite. Me asurement of the matrix hardness at different distances from the crack plane and transmission electron microscopy examination of the fatigue specimen have shown that the matrix microstructure at the tip of the fatigue crack underwent overaging during prolonged testing in the near -threshold regime. Consequently, the fatigue fracture mechanism was mo dified, a lower crack closure developed, and the fatigue threshold red uced to that of the overaged composite.