Bulk nanocrystalline boron-doped VNbMoTaW high entropy alloys with ultrahigh strength, hardness, and resistivity

Abstract

Bulk nanocrystalline (NC) boron-doped refractory (VNbMoTaW)99B1 high entropy alloys (HEAs) were fabricated by mechanical alloying (MA) and high-pressure/high-temperature consolidation techniques. The consolidation was carried out between 800 and 1500 °C to examine the evolution of crystal phases, microstructure and mechanical performance. The bulk NC (VNbMoTaW)99B1 HEAs consolidated at 1350 °C are composed of a matrix with an average grain size of 43 nm and boride and carbide nanoprecipitates with an average particle size of 17 nm. The bulk NC (VNbMoTaW)99B1 HEAs exhibit ultrahigh hardness of 16.3 GPa (approx. triple that of their coarse-grained (CG) counterparts), an ultrahigh yield strength of 5.7 GPa (approx. five times that of their CG counterparts) under compression, and a total engineering strain of approx. 10%. The ultrahigh hardness and strength have four origins: solid solution, precipitation, grain boundary, and dislocation strengthening. The room-temperature resistivity of bulk NC (VNbMoTaW)99B1 is as high as ∼ 387 μΩ·cm, approx. five times that of their CG counterparts. This high resistivity can be related to the high volume fractions of grain boundaries and phase interfaces, the point defects and dislocations near grain boundaries, and the severe lattice distortions.