Tensile and compressive behavior of tungsten, molybdenum, tantalum and niobium at the nanoscale

In situ nanomechanical tests are carried out to investigate the tensile and compressive behavior of < 0 0 1 >-oriented body-centered cubic (bcc) metals W, Mo, Ta and Nb with nanometer dimensions. We find that the strength of these metals exhibits strong size dependence. The compressive size effect in Nb, as evaluated by the log log slope of strength vs. nanopillar diameter, is -0.93, a factor of 2.1 greater than that for the other three metals W, Mo and Ta (-0.44). In tension, however, Ta and Nb show higher size effect slopes (-0.80 and -0.77) as compared with W and Mo (-0.58 and -0.43). We also report that while the yield strength of these metals is a strong function of size, the strain-hardening behavior does not present any size-dependent trends. We further discuss the effects of strain-rate on deformation behavior and provide transmission electron microscopy analysis of microstructural evolution in the same Mo nanopillar before and after compression. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.