Mechanical Alloying Behavior and Thermal Stability of CoCrCuFeMnNix High-Entropy Alloy Powders Prepared via MA

Abstract

CoCrCuFeMnNix (x = 0, 0.5, 1.0, 1.5, 2.0 mol, named as Ni0, Ni0.5, Ni1.0, Ni1.5, and Ni2.0, respectively) high-entropy alloy powders (HEAPs) were prepared via mechanical alloying (MA), and XRD, SEM, EDS, and vacuum annealing were used to study the alloying behavior, phase transition, and thermal stability. The results indicated that the Ni0, Ni0.5, and Ni1.0 HEAPs were alloyed at the initial stage (5–15 h), the metastable BCC + FCC two-phase solid solution structure was formed, and the BCC phase disappeared gradually with the prolonging of ball milling time. Finally, a single FCC structure was formed. Both Ni1.5 and Ni2.0 alloys with high nickel content formed a single FCC structure during the whole mechanical alloying process. The five kinds of HEAPs showed equiaxed particles in dry milling, and the particle size increased with an increase in milling time. After wet milling, they changed into lamellar morphology with thickness less than 1 μm and maximum size less than 20 μm. The composition of each component was close to its nominal composition, and the alloying sequence during ball milling was Cu→Mn→Co→Ni→Fe→Cr. After vacuum annealing at 700~900 °C, the FCC phase in the HEAPs with low Ni content transformed into FCC2 secondary phase, FCC1 primary phase, and a minor σ phase. The thermal stability of HEAPs can be improved by increasing Ni content.

Mechanical Alloying as a Way to Produce Metastable Single-Phase High-Entropy Alloys beyond the Stability Criteria

“…On the other hand, in general, those compositions for which the microstructure agrees with the predicted one remain in that stable microstructure after annealing. Exceptions to this are found in the Co x CrCuFe(NiMn) series [ 52 , 63 , 66 ] with > 20. Although these alloys exhibit fcc microstructure in the as-milled state, after annealing, two fcc phases are detected.…”

“…Table 3 provides a comprehensive overview of stability criteria (Ω, Λ, ϕ Ye , and ) for the two studied alloys and a wide range of other alloys produced via mechanical alloying, drawn from various sources in the literature [ 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 ]. The table shows the reported microstructure in the as-milled state as well as in the annealed state.…”