Center for Functional Nanomaterials Seminar

The role of nanoscale features as grain size, twin boundaries, and layer spacing are investigated primarily through the use of triboindentation test methods. The mechanical response of nanostructured materials and laminates are measured to high strain-rate conditions. It is well known that plastic deformation can affect significantly the strength behavior. For metals that exhibit strain hardening, the amount of plastic deformation varies with the indentation depth that, in turn, affects the strength as seen through a non-zero strain-hardening exponent. An objective is to separate the simultaneous effects of the strain-rate sensitivity exponent (m) from strain hardening. Independent assessment of these power-law relationships with strength is accomplished by: using scratch indents of a uniform minimum size to determine m; and comparing strength with increasing indent size to determine the hardening exponent. In this presentation, m is evaluated over a 10-5 to 10+5 s-1 strain-rate range using tensile tests and triboindentation tests at both the micro- and nanoscale. Materials examined include nickel foils, nanocrystalline coatings, and nanolaminates. An assessment is made of the change in the operative deformation mechanism that accompanies: a loss of the Hall-Petch strengthening effect from dislocation-based strengthening to grain boundary rotation-translation for nanoscale dimensions less than 10 nm; and the onset of strain wave interactions for rates above 10+2 s-1. It is found that nanocrystalline nickel has strain-rate sensitivity greater than microcrystalline counterparts for strain rates below 10-1 s-1. The strain rate sensitivity exponent m then increases above 10-1 - 10+3 s-1 to a smaller extent for the nanocrystalline than for the microcrystalline material, where it is proposed that the phonon drag effect couples better with structural features on the micron scale.