Tailoring nanostructures and mechanical properties of AlCoCrFeNi2.1 eutectic high entropy alloy using thermo-mechanical processing

Abstract

The effect of thermo-mechanical processing on the evolution of microstructure and mechanical properties was investigated in an AlCoCrFeNi2.1 high entropy alloy. For this purpose, the alloy was cold-rolled to 90% reduction in thickness and annealed at temperatures ranging from 800°C to 1200°C. The as-cast alloy revealed eutectic lamellar mixture of (Ni, Al) rich but Cr depleted B2 phase and Al-depleted L12 phases, having volume fractions of ~35% and 65%, respectively. Nanosized precipitates enriched in Cr and having disordered BCC structure were found dispersed inside the B2 phase. Cold-rolling resulted in progressive disordering of the L12 phase but the B2 phase maintained the ordered structure. The disordering of the L12 phase was accompanied by the evolution of ultrafine lamellar structure and profuse shear band formation. Annealing of the 90% cold-rolled material at 800°C resulted in the formation of a duplex microstructure composed of two different phases with equiaxed morphologies, having significant resistance to grain growth up to 1200°C. The annealed materials showed disordered FCC and precipitate-free B2 phases. This indicated that quenching of the annealed specimens to room temperature was sufficient to prevent the ordering of the L12 phase and the formation of the Cr-rich nano-precipitates which were dissolved in the B2 phase during annealing. Significant improvement in tensile properties compared to the as-cast alloy could be achieved by thermo-mechanical processing. All the specimens annealed at 800°C to 1200°C were having good tensile ductility over 10% as well as high tensile strength greater than 1000MPa. These indicated that the properties of the EHEA could be successfully tailored using thermo-mechanical processing for a wide range of engineering applications.