Microstructure and mechanical properties of SPD processed nanocrystalline tantalum

In this study, the refractory metal, Tantalum (Ta), having a Body Centered Cubic (BCC) structure is selected as a model material. Bulk nanocrystalline Ta (NC-Ta) is used to study the process-microstructure-property relationship for a nano-structured BCC metal.;For the first time, high pressure torsion (HPT) technique was applied to produce NC-Ta. Microstructural features of HPT NC-Ta were characterized by X-ray Diffraction (XRD) and Transmission Electron Microcopy (TEM). The resulting microstructures show nano-size grains (grain size smaller than 100 nm) together with high angle grain boundaries. The grain size was estimated to be around 35 nm based on the broadening of the (110) peak of XRD, while a few tens of nanometers based on TEM observation. A high density of atomic steps and ledges were observed along the grain boundaries while a large population of edge dislocations within the grains by high resolution TEM. It shows the non-equilibrium and high-energy nature of the grain boundaries. The mechanical properties of HPT NC-Ta were investigated using instrumented nanoindentation and micro-compression techniques, respectively.;Instrumented nanoindentation intends to study the uniformity of the distribution of mechanical properties, the strain rate effect, and the thermally activated processes associated with plastic deformation. The results show that the elastic modulus of HPT NC-Ta is considerably reduced compared to the coarse-grain counterparts. The results are explained based on the hypothesis that in the HPT NC-Ta greatly increased populations of grain boundaries (GBs) and triple junctions (TJs) have been induced along with a high concentration of lattice vacancies in the severely deformed metal. The microhardness measurement shows a peak across the radial direction. It is believed this indicates an inverse Hall-Petch effect. Also, the nanoindentation hardness measurement, independent of the indentation depth, indicates the absence of indentation size effect in the HPT NC-Ta. The strain rate sensitivity (SRS) of HPT NC-Ta derived from the instrumented nanoindentation at different loading rates is 0.08∼0.10, which is more than double the SRS of coarse-grained tantalum (∼0.04). The accompanied activation volume associated with plastic deformation is only in the order of ∼b3. It implies that the double-kink mechanism, which operates in the plastic deformation of coarse-grained Ta, no longer controls the rate of plastic deformation in NC-Ta. This observation is in agreement with MD simulations performed on computer generated NC Ta with grain size from 3.5 nm to 13 nm. In addition, the grain size based on XRD and TEM investigations is in accordance with the nanoindentation results based on the Hall-Petch estimation.;True strain-true stress curves of micro-compression test for HPT NC-Ta exhibit elastic perfect plastic behavior. Plastic instability in the form of plastic buckling is the primary mechanism for any unstable plastic deformation of the compressed HPT NC-Ta pillars. The buckling is due to the diminished strain hardening and slight miss-alignment between the pillar and the loading axis. No shear banding is observed on the surfaces of compressed HPT NC-Ta pillars. The strong resistance of HPT NC-Ta to shear banding instability is considered to be a possible consequence of the much increased strain rate sensitivity of HPT NC-Ta.