Cyclic softening of the high-strength steel HT780 studied by positron annihilation spectroscopy

Cyclic softening mechanism of the high strength steel HT780 has been investigated using transmission electron microscopy (TEM) and positron annihilation lifetime spectroscopy. It has been found by the TEM observation that a high density of dislocation exists in the specimen before fatigue test, which moves from inside to the boundaries of ferrite grains in the rapid softening stage. The positron lifetime is found to increase rapidly in the rapid softening stage indicating that the positron lifetime of the mobile dislocation created by the fatigue test is longer than that of the dislocation which exists before the fatigue test. The positron lifetime becomes nearly constant as softening saturates. The positron lifetime at the saturation is longer as the strain amplitude is larger, showing that the density of the mobile dislocation depends on the strain amplitude while is kept constant during the cyclic deformation with a given strain amplitude.