Effect of niobium alloying on the microstructure, phase stability and mechanical properties of CoCrFeNi_(2.1)Nb_x high entropy alloys: Experimentation and thermodynamic modeling

摘要

A systematic study was carried out to understand the effect of Nb addition on the microstructure, phase stability, and mechanical properties in a series of cast and aged CoCrFeNi_(2.1)Nb_x (x = 0, 0.2, 0.4, 0.74) high entropy alloys (HEAs). The base alloy without Nb addition showed a fully face-centered cubic (FCC) phase, whereas Nb addition was found to promote the formation of the hexagonal Laves phase. The cast HEAs were further aged at temperatures varying from 800°C to 1200°C. Aging of the different Nb alloyed HEAs at 800°C and 1000°C resulted in the precipitation of D0_(19) structured E-phase having fine acicular morphology. In contrast, the HEAs aged at 1200°C did not reveal the presence of e precipitates. These experimental observations were critically assessed with the thermodynamic predictions. The fine precipitate network in the CoCrFeNi_(2.1)Nb_(0.2) and CoCrFeNi_(2.1)Nb_(0.4) HEAs improved the hardness and tensile strength considerably. However, the CoCrFeNi_(2.1)Nb_(0.74) HEA showed considerable embrittlement due to the presence of a high fraction of the Laves phase. Present results indicated considerable potential of achieving superior mechanical properties in these cast HEAs even without involving thermo-mechanical processing.