Time-dependent characteristics of incipient plasticity in nanoindentation of a Ni3Al single crystal

Depth-sensing nanoindentation was used to trace the transition from elasticity to plasticity in single crystal Ni3Al. We found, as others have reported, that the incipient plasticity occurs spontaneously on increasing the load over a narrow load range. However, when the load is held at a value smaller than the spontaneous excursion value, excursion will also occur, albeit after a certain holding time. The holding time is found to be longer at lower holding loads. The magnitude of the displacement excursion is also found to increase with the holding load. After the excursion, all the tests showed similar elastoplastic deformation behavior in the load-displacement plot. A passive layer following electropolishing was found to be responsible for the pre-excursion plasticity observed in some samples. This passive layer, however, has no significant effect on the critical load for incipient plasticity. A model based on the homogeneous nucleation of a dislocation loop was proposed to explain the observations. The time-delay effect of the pop-in is thought to be due to the slow growth of an initially stable dislocation loop at the most highly stressed point under the indenter to an unstable loop by absorption of vacancies from the matrix. (Author)