Depth-sensing nano-indentation was used to trace the transition from elasticity to plasticity in single crystal 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 still occurred; albeit after a certain holding time. This 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 revealed a similar elastoplastic deformation behavior in the load-displacement plot. A passive layer, following electropolishing, was found to be responsible for the pre-excursion plasticity which was observed in some samples. However, this passive layer had no significant effect upon the critical load for incipient plasticity. A model which was based upon the homogeneous nucleation of a dislocation loop was proposed in order to explain the observations. The time-delay effect of the pop-in was suggested 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 via absorption of vacancies from the matrix.

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