Structure-property relationships in hot forged AlxCoCrFeNi high entropy alloys

Abstract

AlxCoCrFeNi is one of the most extensively studied high entropy alloy systems, as it has shown excellent mechanical properties suitable for structural applications. In this work, AlxCoCrFeNi(x = 0.3, 0.5, 0.7) alloys were successfully hot-forged without any visible cracks. Contributions from different strengthening mechanisms were estimated and compared with the experiment. Optical microscopy, x-ray diffraction (XRD), electron backscatter diffraction (EBSD), energy dispersive x-ray spectroscopy (EDS) and hardness tests were used for structural and mechanical property analysis. A single-phase face centered cubic (FCC) structured was observed in the as-cast Al0.3 alloy, whereas, dual-phase FCC and body centered cubic (BCC) structures were observed in the as-cast Al0.5 and Al0.7 alloys. An increase in the FCC phase fraction was observed in all the alloy compositions after forging. Microstructure analysis revealed that phase fraction, the formation of twins, low angle grain boundaries, and grain refinement played a vital role in strengthening the alloy systems. After forging, the hardness of the Al0.3 and Al0.5 alloys was increased by 22% and 48%, respectively. In contrast, a reduction of 8% in hardness was observed in Al0.7 alloy. Reduction in the hardness is mainly due to higher FCC phase fraction and interface stress in forged Al0.7 alloy. A new modified rule of mixture is proposed to estimate the total strength of dual-phase high entropy alloys.