Strength of Surface Nano-Crystalline Layers

H. Yu*, K.-S. Kim*, and Y. T. Cheng**

*Division of Engineering, Brown University, Providence, RI 02912

**General Motors R&D Center, 30500 Mound Rd, Warren, MI 48090-9055

A dislocation-based model on the strength of a surface nano-crystalline layer is presented. The strength of a surface layer is assessed as the strength under multiple-asperity indentation loading. When the crystallite size is large, the hardening and the plastic slip depend on obstacle strength and dislocation pile-up mechanisms. In this size range the cooperative slip processes are mainly internal and the Hall-Patch relation is satisfied.

On the other hand, as the crystallite size becomes small in nanometer scale, a dislocation glide generated by a source is limited by the size of the crystallite and the cooperative slip processes are mainly external. The scale-dependent mechanism transition of plastic deformation from obstacle-limited plasticity to single-glide-limited plasticity is analyzed for the strength of surface nano-crystalline layers. In addition, another possible mechanism transition from single-dislocation-limited plasticity to source-limited plasticity is also discussed.