Strengthening mechanisms of pure tungsten subjected to high pressure torsion: Deviation from classic Hall‐Petch relation

Abstract

AbstractIn this work, the ultrafine grained (UFG) tungsten has been fabricated via high pressure torsion (HPT) of various turning numbers at a temperature of 823 K and a pressure of 1.5 GPa. The microstructure characteristics and mechanical properties of initial and high pressure torsion processed tungsten have been comparatively investigated by means of x‐ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and microhardness tests. It is shown that high pressure torsion leads to microstructure refinement with an average grain size of ∼0.92 μm and the fraction of high angle grain boundaries (HAGB) increasing to 62.9 %. Moreover, the dislocation density increases from initial 1.17×1014 m−2 to 3.89×1014 m−2. The microhardness tests revealed that hardness increased gradually and its distribution became more homogeneous with torsion strain increasing. The strengthening model was established considering the grain boundary strengthening and dislocation strengthening mechanisms. Resultantly, deviation of Hall‐Petch slope was found from the classic values, which is attributed to the easy movement of the extrinsic dislocations in the high angle grain boundaries with high distortion energy and high density of defects.