Structural, Magnetic, and Exchange Bias Behavior of Nickel-Based Ni2CuCrFeAlx (x = 0.5, 1.0, 1.5, and 2.5) High-Entropy Alloys

Abstract

The magnetic properties of powdered and sintered samples of Ni2CuCrFeAlx (x = 0.5, 1.0, 1.5, and 2.5) high-entropy alloys were investigated in detail by observing its relation to morphology by SEM, the presence of different phases by XRD, magnetization at the different operating field, and exchange bias with different training of the hysteresis loops. Two structural phases were observed in the sintered Ni2CuCrFeAlx HEAs, Al-free BCC solid solution in FCC matrix, and Al-containing C15 laves phase intermetallic compounds. In both cases, Ni, Cu, Cr, and Fe substituted randomly into the lattice that would result in the formation of martensitic structure with ferromagnetic characteristics beyond Tc. These results showed the corresponding state of Ni2CuCrFeAlx HEAs in which Cr- and Fe-rich particles (BCC structure) were noticed with higher Al content, and moreover, Ni-rich particle crystallizes in B2 structure dispersed in BCC phase. The substitution of Al in Ni2CuCrFeAlx composition increases the saturation magnetization with increasing applied field, regardless the alloy showing FCC, BCC and B2 phase structure during sintering. In spite of antiferromagnetic constituents of Cr in Ni2CuCrFeAlx alloy, polycrystalline HEAs exhibit higher ferromagnetic interaction at the interface of Ni-rich phase, which induces the exchange bias effect from the negative shift of magnetic hysteresis loop, observed in FC and ZFC modes. Likewise, sintered alloys are hard materials, indicated by their high values of exchange bias along with their coercivity. The addition of Al in Ni2CuCrFeAlx alloy improves the magnetic properties due to the existence of order–disorder phase transition from L21 to B2 or B2 to A2 phase.