Experiments involving the indentation of single crystals of both tungsten and an iron alloy show that the observed yield phenomena can be predicted using a superdislocation model driven by the change in shear stress between the elastically and fully plastic loading conditions. A low density of dislocation multiplication sites is required to support elastic loading which approaches applied shear stresses on the order of the theoretical shear strength of the material. Oxide film thickness and crystal orientation are examined as parameters in controlling the yield phenomena. A model based on activation of dislocation multiplication sources is suggested to explain the initiation of the yield point during indentation and the overall load-depth relationship during indentation.
Bibliographical noteFunding Information:
The authors wish to thank N. R. Moody of Sandia National Laboratories for helpful discussions and the use of the Nanoindenter II. This work was supported by the Office of Naval Research under grants N00014-95-10539 and N00014-91-J-1998. Additionally, the authors would like to gratefully acknowledge the discussions and information provided by J. B. Pethica of Oxford University and A. B. Mann of Johns Hopkins University.