Effects of Nb additions on structure and mechanical properties evolution of CoCrNi medium-entropy alloy

A serious medium-entropy alloys (MEAs), CrCoNiNbx (x = 0-0.5 mole ratio), was designed by adding Nb element into CrCoNi matrix to investigate alloying effects of large atom Nb on the phase evolution, microstructurel and their impacts on the subsequent mechanical properties. It was found that Nb element addition into the CoCrNi MEAs changed the original phase constitution and led to the formation of an ordered Nb-riched Laves phase with HCP structure embedded in the FCC matrix. The current alloy system structure evolved from hypoeutectic (x < 0.4), to eutectic (x = 0.4), then to hypereutectic (x > 0.4) with Nb content increases. Laves phase volume fraction increase is mainly responsible for the increment in the hardness and compression yield strength, and the decrement in the fracture strain. This strategy to obtain the balance of strength and ductility by adding a certain element will provide a new design idea for FCC type MEAs engineering applications.