Unraveling the role of particle size distribution in the cyclic hardening and softening mechanisms of alloy 718Plus during LCF at room temperature and 650 °C under plastic strain control

The effect of particle size distribution on the cyclic deformation mechanisms in alloy 718Plus has been examined in the unimodal (U_STD) and the bimodal (B_SD) γʹ distributions under fully reversed low cycle fatigue at room temperature (RT) and 650 °C. The U_STD specimens depicted a short initial hardening followed by an extensive but gradual cyclic softening, while the B_SD specimens manifested prolonged hardening followed by a brief yet steep softening response. This contrasting stress response behavior has been explained on the basis of the dislocation–precipitate interaction mechanisms through interrupted LCF specimens, deformed till 25 cycles at 0.15 % plastic strain amplitude at both temperatures. The extended softening in U_STD specimens occurred due to the weakly coupled dislocation shearing, which required lower shear stress, leading to faster fragmentation and dissolution of the unimodal precipitates, generating γ′-free bands. However, a higher CRSS was needed for the strongly coupled dislocations to shear the bimodal distribution, resulting in delayed softening. Although the extent of overall softening was reduced, the cyclic softening rate (per cycle) increased as a function of the plastic strain amplitude and temperature for both U_STD and B_SD specimens.