INDENTATION INDUCED DISLOCATION NUCLEATION - THE INITIAL YIELD-POINT

Yield points have been detected in both GaAs and Fe-3wt%Si single crys tals when contacting the surface with sharp diamond tips. The present study concentrates mostly on Fe-3wt%Si and demonstrates that a unique point in the load-displacement curve can be associated with the first dislocation nucleated. This occurs at loads in the vicinity of 100 mu N for a 66 nm radius tip. Subsequently, this produces an avalanche of dislocations estimated to range from about 15 to 74 in number dependin g on the magnitude of the yield point load. A model, based upon discre tized dislocations is proposed for both the initiation of yielding at an upper yield point (UYP) and the arrest of the indenter at a lower y ield point (LYP). The UYP is interpreted in terms of Rice's unstable s tacking energy concept, previously applied to crack lips, and accounts for tip radius, oxide film thickness and image force effects. The LYP is interpreted in terms of the back forces provided by previously emi tted shielding dislocations. These two approaches provide first order solutions of the upper and lower yield points which both vary from tes t to test by as much as a factor of four. The large variation in nucle ation load is proposed to be due to point to point differences in oxid e thickness which might range from 4.5 to 8.5 nm. Copyright (C) 1996 A cta Metallurgica Inc.