Microstructure and mechanical properties of super-strong nanocrystalline tungsten processed by high-pressure torsion

Abstract

Fully dense nanocrystalline tungsten (nc-W) with extremely high strength (∼3.0 GPa under quasi-static compression and ∼4.0 GPa under dynamic compression) has been obtained by high-pressure torsion (HPT) at low temperature (500 °C). The nanocrystalline microstructure is revealed by transmission electron microscopy (TEM). The grain boundaries (GBs) are mostly of the large-angle type. High-resolution TEM (lattice images) suggests that the GBs are clean and well defined (atomically sharp). GBs are non-equilibrium and of a high-energy nature. Edge dislocations are present within the grains. The authors hypothesize that these edge dislocations, combined with a depleted impurity concentrations along pre-existing GBs, contribute to enhance the ductility of nc-W. Under dynamic compression, the specimens exhibit localized shearing followed by cracking and subsequent failure, similar to their ultrafine-grain (UFG) counterparts processed by equal-channel angular pressing plus cold rolling, and to many other body-centered cubic metals with UFG/nanocrystalline microstructures. The shear band width in the HPT-processed nc-W is much smaller (shear band width <5 μm) than that observed in the UFG counterparts (shear band width ∼40 μm).