An excellent synergy in yield strength and plasticity of NbTiZrTa0.25Cr0.4 refractory high entropy alloy through the regulation of cooling rates

Abstract

The solidification process often determines the microstructure of alloys from the atomic scale to the macroscopic scale, which distinguishes the performance of one alloy from another. Among all modification measures, the cooling rate plays a vital role in the solidification process. Hereinto, to thoroughly investigate the cooling rate effects on phase components, microstructures and mechanical properties, a series of NbTiZrTa0.25Cr0.4 refractory high entropy alloys were prepared under a wide range of cooling rates. The results show that the microstructures with combined dendrites and interdendrites gradually coarsened as the cooling rate decreased, coupled with the transition from a single BCC phase to BCC + Laves dual-phase. During the microstructural evolution, the prominent strengthening mechanism (i.e., solution strengthening) of the alloy prepared under high cooling rates became weak, while the secondary strengthening mechanism (i.e., second phase strengthening associated with Laves phase) was gradually reinforced. The reinforced second phase strengthening compensated for the weakened solution strengthening; as a result, the yield strength first decreased and then increased as the cooling rate decreased without sacrificing the plasticity of the alloys. It was found that the alloy prepared under an appropriate cooling rate of 62.5 K/s exhibited the maximum yield strength of 1452 MPa and maintained a competitive plasticity of approximately 23.2%, which exceeded the NbTiZrTa0.25Crx system and other reported refractory high entropy alloys. This study provides insight into the development of high-temperature alloys and a promising avenue to enhance the performance of an alloy by regulating the cooling rates.