THE EFFECT OF NIOBIUM ON THE PHASE STABILITY AND MECHANICAL PROPERTIES OF NICKEL-BASED HIGH ENTROPY ALLOYS

Abstract

A fundamental understanding of the mechanisms of phase stability of high entropy alloys (HEAs) and the influence of refractory elements is a recent topic of active research. This study investigates the effect of Niobium (Nb), as a refractory element, on the phase stability and mechanical properties of NiCoCrAlNb HEAs. In this thesis, Ni0.55CoCrAlNbx (x= 0, 0.01, 0.03, 0.05) high entropy alloys are prepared by using a vacuum arc melting technique. X-ray diffraction results demonstrate a single solid solution phase FCC in all samples. The addition of Nb content is studied through thermomechanical assessments and microstructure processing. Remarkable tensile properties were achieved for the HEA with 5% Nb content. The thermomechanical treatments were optimized, and the best combination of mechanical properties was obtained at an aging temperature of 950°C for one hour. Significant improvements are detected in tensile properties as the Nb content is increased. The yield and ultimate strength values of the optimized sample were found to be 1050 MPa and 1534 MPa, respectively. All the specimens aged at 950°C had a good tensile ductility of 34%. This thesis indicates that the properties of HEAs could be well tailored using thermomechanical and microstructure management for a high-performance HEA in industrial engineering applications.