Heterogeneous microstructure in nonequiatomic MoNbTaVW refractory high entropy alloy after high pressure torsion: Evolution mechanisms and mechanical properties

Abstract

The non-equiatomic MoNbTaVW refractory high entropy alloy (HEA) has been systematically investigated with the intent of developing a heterogeneous microstructure using high pressure torsion (HPT). The heterogeneous microstructure is successfully developed and is characterized by coarse grains, kink bands and ultrafine grains (UFGs) at strain levels (ε) ≈ 0–2.4, 2.4–6.2, and 8.7-higher, respectively. In the initial stage (ε ≈ 2.4–6.2) of deformation, the microbands (slip bands and kink bands) developed within the grain depending on the orientation. The slip bands are developed by slip activity along {110} <11‾1 > slip system, and kink bands are formed due to slip-induced lattice rotation about the <110> axis with activation of {112}<111‾ > slip system. At higher strain (ε > 8.7), high density of accumulated low angle grain boundaries (LAGBs) dynamically recovered into polygonized subgrain structures that later transform into high angle grain boundaries (HAGBs) due to gradual lattice rotation. The synchrotron diffraction pattern confirms that the single-phase bcc structure is stable during the deformation, and no phase transformation takes place. Further, the microhardness values for coarse grained, kink banded and UFG microstructure are determined to be 8.1, 10.3, and 12.5 GPa, respectively. The mechanical behaviour of the HPT microstructure indicates that the UFG have a good impact resistance and elastic recovery index, while the coarse grains has inherent plasticity and better fracture resistance. The kink band microstructure remarkably combines strong impact loading and inherent plasticity. Therefore, the hierarchical distribution of these microstructures can achieve a superior combination of strength and ductility