Alloying, thermal stability and strengthening in spark plasma sintered AlxCoCrCuFeNi high entropy alloys

• Microstructure of AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5) is sensitive to fraction of Al.
• AlxCoCrCuFeNi (x = 0.45, 1) exhibit excellent thermal stability at high temperatures.
• Quantification of strengthening is presented for AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5).
• Better estimate of theoretical density for HEA using phase fractions is discussed.

AlxCoCrCuFeNi (x = 0.45, 1, 2.5 and 5 mol) multi-component high entropy alloys synthesized by mechanical alloying were spark plasma sintered to produce high dense compacts. X-ray diffraction and scanning electron microscopy studies reveal that these sintered alloys exhibit varying microstructures from single phase to three phases depending on Al content. The thermal stability studies carried out in the temperature range of 400–600 °C for duration of 2–10 h in Ar atmosphere suggest that these alloys exhibit excellent thermal stability in terms of phases and crystallite size. Highest specific hardness of 160 (HV/g cm−3) is achieved in the sintered Al5CoCrCuFeNi alloy and there is no significant change in the hardness after heat treatment of Al0.45CoCrCuFeNi and AlCoCrCuFeNi alloys. Hall–Petch analysis based on hardness measurements carried out on sintered samples reveals that solid solution strengthening seems to increase with increase in Al content.