Strong layer thickness dependent strengthening effect in the nanolayered NbMoTaW/TiVZrNb refractory high entropy alloys

The size effect of hardness in nanolayered high entropy alloys differs from that of the conventional bimetallic materials owing to their distinctive interfacial characteristics. In this work, we have investigated the individual layer thickness (h) dependent mechanical behavior of nanolayered NbMoTaW/TiVZrNb refractory high entropy alloys. The results suggest that the TiVZrNb layer in the multilayers gradually evolves from an amorphous phase to a crystalline phase and the interfacial structure transforms from an incoherent interface to a coherent interface as the h decreases. The hardness of the multilayers monotonously increases from 7.6 GPa to the maximum value of 13 GPa as the h decreases from 100 nm to 5 nm, exhibiting a strong size effect. The softening behavior occurs as h further decreases to 2.5 nm, and hardness decreases to 12.4 GPa. The deformation mechanism shifts from a synergistic action of amorphous layer shear and dislocation nucleation at incoherent interface to the slip of single dislocation across the coherent interface as the h decreases.