Yield points were detected in monocrystals upon contacting the surface with sharp diamond tips. The results demonstrated that a unique point in the load-displacement curve could be associated with the first dislocation that was nucleated. This occurred at loads of the order of 100N, for a 66nm radius tip. This then led to avalanches of between 15 and 74 dislocations; depending upon the magnitude of the yield-point load. A model which was based upon discretized dislocations was proposed for the initiation of yielding at an upper yield point, and for arrest of the indenter at a lower yield point. The upper yield point was explained in terms of Rice’s unstable stacking energy concept. The lower yield point was explained in terms of the back-forces which were produced by previously emitted shielding dislocations. These 2 approaches provided first-order predictions, of the upper and lower yield points, which varied by as much as a factor of 4 from test to test. Large variations in nucleation load were attributed to oxides whose thicknesses ranged from 4.5 to 8.5nm.

W.W.Gerberich, J.C.Nelson, E.T.Lilleodden, P.Anderson, J.T.Wyrobek: Acta Materialia, 1996, 44[9], 3585-98