Depth-sensing nano-indentation was used to trace the transition from elasticity to plasticity in monocrystalline Ni3Al. It was found that incipient plasticity occurred spontaneously upon increasing the load over a narrow load range. However, when the load was held at a value smaller than the spontaneous excursion value, excursions would also occur; albeit after a certain holding time. The holding time was found to be longer at lower holding loads. The magnitude of the displacement excursion was also found to increase with holding load. After the excursion, all of the tests exhibited a similar elastoplastic deformation behavior in load-displacement plots. A passive layer following electropolishing was found to be responsible for the pre-excursion plasticity observed in some samples. This passive layer had no significant effect upon the critical load for incipient plasticity. A model based upon homogeneous nucleation of a dislocation loop was proposed in order to explain the observations. The time-delay effect of the pop-in was thought to be due to the slow growth of an initially stable dislocation loop, at the most highly stressed point under the indenter, into an unstable loop by absorption of vacancies from the matrix.

Time-Dependent Characteristics of Incipient Plasticity in Nanoindentation of a Ni3Al Single Crystal. Chiu, Y.L., Ngan, A.H.W.: Acta Materialia, 2002, 50[6], 1599-611