Strain Rate Sensitivity and Deformation Mechanisms of Nanocrystalline Pd Alloys
MRS Fall Meeting, Boston (2010)
Observations of size-dependent deformation are becoming more common as materials and structures are engineered to smaller and smaller dimensions. In general, a reduction in grain size leads to higher strength in metallic materials, with different mechanisms dominating deformation on smaller length scales. In the grain size regime below 30 nm, dislocation activity is reduced and it is well accepted that the motion of full dislocations is not the predominant deformation mechanism. Suggested mechanisms include nucleation and motion of partial dislocations, grain boundary sliding as well as grain rotation and growth. This mechanism change is supported by the observation that nanocrystalline metals exhibit much stronger strain rate sensitivity at low temperatures, with small activation volumes, compared to their coarse-grained counterparts. In this presentation, the mechanical behavior of nanocrystalline Pd, Pd-Ag and Pd-Au with grain sizes between a few and about 150 nm will be discussed. The activation volume at very small grain size has been observed to be of the order of a few atomic volumes and increases with increasing grain size. First results indicate that the addition of Au or Ag reduces the strain rate sensitivity at room temperature. Our findings will be discussed in the light of suggested models for the deformation of nanocrystalline materials.