Effect of hot rolling and annealing on microstructure and mechanical properties of the Fe-Co-Cr-Ni-V-Zrx(x =0–5) high entropy alloys

Abstract

The effect of hot rolling and annealing on the microstructural evolution and mechanical properties of the Fe25−xCo25Cr20Ni25V5Zrx (x=0, 2.5, 5) HEA with Zr variation has been investigated. The Fe25−xCo25Cr20Ni25V5Zrx (x = 0, 2.5, 5) HEA was designed using the CALPHAD approach with varying Zr content from 0 to 5 at%. The structural and microstructural characterization has been carried out by X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM) analysis. The microstructure of the investigated samples is found to be composed of FCC solid solution + lamellae of FCC solid solution and Ni7Zr2 intermetallic phase, which is consistent with CALPHAD prediction. Differential scanning calorimetry (DSC) analysis has also been carried out to confirm the evolution of different phases. The room temperature mechanical response of the alloys in as-cast and hot rolled annealed (HRA) conditions are evaluated by hardness and tensile testing. The hardness of the alloy increases by 70% for an increment of Zr content from 0 to 5%, respectively, which further increases after HRA. The tensile test performed on the as-cast samples shows a significant reduction in uniform elongation to 13.5% in the case of Zr5, whereas the yield strength increases by more than 125% for those compositions. After HRA treatment, the grain size and uniform elongation of the investigated samples reduced, and the yield strength further improved. The extent of grain size reduction depends on the distribution of the intermetallic Ni7Zr2 phase. The strain hardening exponent ‘n′ is found to be dependent on the grain size, and it reduces with the reduction in grain size. Also, the variation of n is dependent on the presence of the intermetallic Ni7Zr2 phase, and it is found to increase initially if the second phase is present inside the grain interior, whereas it fails to improve if the second phase is present at the grain boundary.