Structures and mechanical properties of Ti-Nb-Cr-V-Ni-Al refractory high entropy alloys

Four refractory high entropy alloys with different chemical compositions, which can be calculated as Ti(501.5625x)Nb(30-0.9375x)Cr10V10Ni1.5xAlx (x = 0, 5, 7, 10), were prepared by arc melting to determine the effect of Ni and Al on the phase composition, structure and mechanical properties. Each alloy was studied in both the as-cast and annealed at 1000 degrees C for 24 h conditions; compression tests at room temperature or at 800 degrees C was used to examine mechanical behavior of the alloys and the effect of deformation on microstructure. The replacement of Ti and Nb with Al and Ni resulted in the formation of the Ti, Ni-rich sigma-phase and Ti2Ni phases in the bcc matrix in contrast to the ThermoCalc predictions which suggests the formation of the bcc solid solution, B2, and Laves phases. The Ti, Ni-rich s-phase can't also be expected from the analysis of the corresponding binary and ternary phase diagrams. Annealing did not result in noticeable changes in the microstructure. The formation of the sigma and Ti2Ni phases resulted in a considerable increase in strength (from 745 to 1600 MPa) and decrease in ductility (from a thickness reduction >= 50% to fracture in the elastic region) at room temperature. An increase in the testing temperature to 800 degrees C resulted in softening to and a substantial increase in ductility of all the alloys. Complex relationships between the fraction of the second phases and mechanical properties were discussed.