Microstructure features of high-entropy equiatomic cast AlCrFeCoNiCu alloys

Abstract

Abstract-The structural and phase transformations that take place in the cast high entropy equiatomic alloy AlCrFeCoNiCu after solidification, homogenizing heat treatment, and cooling have been studied. Analytical transmission microscopy, scanning electron microscopy, X ray energy dispersive spectroscopy, and X ray dif fraction analysis were used to conduct the studies. The elastic modulus, nano , and microhardness have been measured. The alloy decomposition has been found to occur with the precipitation of no less than six nanos cale phases with different morphologies, structures (A2, B2, L1 2 ), and chemical compositions. All the nanophases are multicomponent solid solutions enriched with several elements, which indicates the pro nounced elemental and phase nanomodulation over the alloy volume.

Specific features of cast high-entropy AlCrFeCoNiCu alloys produced by ultrarapid quenching from the melt

“…5). In this case, the substructure of the alloy is apparently most close to the structure of the as cast alloy [19]. The majority of precipitating particles, both globular and lamellar, are no more than 20-30 nm in size after annealing at 550°С; the lamellar particles are 200-300 nm long after annealing at 600°С for 2 h.…”

“…After annealing at 550°С for 2 h, a matrix somewhat enriched in Fe, Cr, and Ni is identified along with the B2 phase in the form of equiaxed 10-20 nm nanoparticles based on the Cu-Ni-Co-Al multicom ponent solid solution enriched in copper to 40 at % ( Table 2). After annealing at 550°С for 5 h, there is also present a matrix somewhat enriched in Fe, Cr, and Ni and the particles of two nanophases are identified including, first, a B2 phase based on the Cu-Ni-CoAl multicomponent solid solution enriched in copper to 40 at % and, second, apparently, a disordered A2 phase [19], which represents a solid solution enriched in Cr, Fe, and Co and depleted of Ni, Al, and Cu (Table 3). Finally, after annealing at 600°С for 2 h, the following three phases are identified in the alloy: first, a lamellar B2 phase based on the Cu-Ni-Al solid solution enriched in copper to 75 at %; second, apparently, an A2 phase that has a brighter contrast, the solid solution of which is even more enriched in Cr (up to 35 at %), Fe (up to 30 at %), and Co (up to 22 at %), but is noticeably depleted of Ni, Al, and Cu; and, third, a nanophase with 20-30 nm equiaxed particles based on a B2 Ni⎯Al-Co-Fe phase, whose solid solution also con tains Cu and Cr (Table 4).…”

“…When discussing the experimental results in the context of the above said, it is important to note that a highly nonequilibrium, but inhomogeneous structural and phase state is retained, even in the as cast high entropy alloy slowly cooled at a rate of 10 K/s [19]. As was mentioned above, when studying the as cast alloy of this composition [19], it was found that phase sepa ration occurred in the initially arising bcc dendritic and fcc interdendritic structures during solidification and subsequent cooling at a rate of 10 K/s.…”

“…In addition to characteristics typical of metal alloys, they demonstrate unique and unusual properties inherent in, e.g., metal ceramics, including high hardness and resistance to softening at elevated temperatures, a pos itive thermal strengthening coefficient, high strength characteristics at elevated temperatures, good corro sion resistance, etc. [14][15][16][17][18][19].…”

“…In our previous work [19], we studied the microstruc ture and the phase and chemical compositions of the multicomponent equiatomic alloy AlCrFeCoNiCu in the initial as cast state. It is known that, in the alloys of this system, bcc and fcc phases are formed with a dendrite microstructure, the morphology and phase composition of which depend on the chemical com position of an alloy, in particular, on the aluminum concentration [14][15][16].…”