DISLOCATION CONFIGURATIONS AND INTERNAL-STRESSES IN THE CREEP OF NIMONIC-91

The internal stress, sigma(i), developed during the creep of Nimonic 91 was determined as a function of applied streSS, sigma(a), using the strain transient dip technique Transmission electron microscope observations of thin films of crept specimens showed Orowan dislocation loops to exist around gamma' phase particles at low stresses with partial dislocation loops around faulted gamma' particles at high stresses. The numbers of loops per specimen volume were counted and the resulting internal stress calculated. The results indicate that a significant part of the mechanically measured internal stress can be attributed to Orowan loops around gamma' particles which are stabilized against climb by the superlattice fault resulting from partial penetration of the gamma' particle by the dislocation. The variation of internal stress with applied stress can be accounted for qualitatively by the variation of loop density with stress at low stresses and the initiation of relaxation processes involving partial or complete shearing Of gamma' particles by loops at high stresses. It is suggested that creep in Nimonic 91 is dependent on the magnitude of the effective (sigma(a) - sigma(i)) and that the internal stress is determined largely by the density of dislocation loops around gamma' particles.