Dislocation glide and mechanical twinning in a ductile VNbTi medium entropy alloy

Abstract

• {112}<111> mechanical twinning was activated in an equiatomic refractory VNbTi medium entropy alloy at room temperature. • Twinning nucleation mostly occurred at the beginning of plastic deformation, and a twinning-kinking mixed microstructure was formed. • Dislocation glide dominates the subsequent deformation, and kinking was helpful to relax the stress concentration. • The synergistic activation dislocation glide, twinning and kinking provide a new path to design ductile refractory medium-/high-entropy alloys. An equiatomic VNbTi medium-entropy alloy with outstanding tensile properties and unique deformation behavior is reported. The screw dislocation glide, deformation twinning, and dislocation accumulation induced kink bands are identified as three deformation mechanisms that contribute to a large elongation above 20%. The {112}<111> twins are activated at the beginning of the yield stage accompanied by sudden stress-drop and pronounced acoustic emission. Dislocations dominate subsequent tensile deformation, and the prevalent multiplanar dislocation slip promotes the formation of complex dislocation configurations (e.g., debris, dipoles, and loops) and dense dislocation networks. The twin bands and kink bands can further impede the dislocation motion meanwhile effectively alleviate stress concentration. The synergistic activation of these deformation mechanisms provides new opportunities to design ductile refractory medium- and high-entropy alloys.