Abstract

William D. Nix
Department of Materials Science and Engineering
Stanford University

March 26, 2007
3:30- 4:30pm
MEC Rm. 341

 

"Plastic Deformation of Crystals at the Micrometer Scale: Strain Gradients and Dislocation Source-Controlled Plasticity"

 

Crystalline materials are almost always stronger when plastic deformation is confined to smaller volumes. Here we review some of the effects that appear to be responsible for this.

The size dependence of the hardness of crystals is indicated by the indentation size effect (ISE) wherein the hardness increases with decreasing indentation size. This has been described in terms of the gradients of strain and the related geometrically necessary dislocations (GNDs) that accompany indentations in small volumes. The strain gradients and GNDs also produce lattice curvatures, which can be directly measured using x-ray diffraction and the streaking of Laue spots. Using a scanning x-ray microdiffraction (µ-SXRD) technique, with a white x-ray source available at the Advanced Light Source in Berkeley, we have recently measured the lattice rotations and curvatures in small indentations in copper single crystals. These results show that the curvatures and computed GNDs in indentations are consistent with those expected from the observed indentation size effect for copper. This provides support for the idea that size effects on strength can be related to strain gradients.

But recent experiments have shown that single crystal pillars in the micrometer size range show significant size effects, with smaller being stronger, even when the crystals are tested in uniaxial compression. Plastic deformation in these tests is unconstrained in the sense that dislocations are free to leave the crystal during deformation and no significant gradients of strain are imposed. These size effects suggest that plasticity is dislocation source-controlled, wherein smaller volumes are stronger because fewer sources of dislocations are available. The evidence for source-controlled plasticity is reviewed and recent attempts to describe the corresponding size effects on strength are described. In particular we show, using both TEM and scanning x-ray microdiffraction (µ-SXRD), that even after extensive plastic deformation very few dislocations are left in the crystal. The results suggest that source-controlled plasticity can dominate the strength of crystals in small volumes. .