The microstructures of high entropy alloys of the system CoCrCuFe xNi and CoCrCuFeNi x (where x indicates the molar ratio, which, where not specified, is 1) have been investigated. Many Cu rich spheres were evident in the microstructure of CoCrCuFe0.5Ni and CoCrCuFeNi0.5 alloys, which indicates that liquid phase separation had occurred before solidification. During liquid phase separation, the original liquids separated into two liquids: Cu rich and Cu depleted. In contrast, in other alloys ( x = 1.0, 1.5 and 2.0), typical dendritic and interdendritic structures are obtained. Cu and/or Cr rich precipitates, with various morphologies, can be seen in the interdendritic region. Additionally, Cu rich nanoparticles and Cr rich bird shaped structures can be observed in the Cu rich spheres. Sluggish cooperative diffusion causes the element segregation and formation of nanoprecipitates in the microstructures. The calculated positive mixing enthalpies of CoCrCuFe0.5Ni and CoCrCuFeNi0.5 alloys are likely reasons for their liquid phase separation.
With the increasing titanium, the volume fraction of face-centred cubic-structured dendrites decreased, and ordered B2 phase, σ, and Laves phase appeared in CoCrCu0.5FeNiTi x alloy when x > 0.5. With the increase of Ti content, Δ Smix, Δ Hmix, valence electron concentration (VEC), and Λ (=Δ Smix/ δ2) of CoCrCu0.5FeNiTi x alloys decreased, while atomic size difference ( δ) and electronegativity difference (Δ χ) increased. Topologically close-packed (TCP) structures are favoured when Δ Smix > 14.53 J K−1 mol−1. TCP phases can be found when δ > 5.05 and Δ χ ≥ 0.124. Simple solid solution phase is favoured when VEC ≥ 8.26 but multi-phases including TCP phase coexist when VEC < 8.26. Single-disordered solid solution forms only when Λ > 0.637, solid solution and TCP phases coexist when 0.531 < Λ < 0.637, and TCP phases are favoured when Λ < 0.531.
Manganese was added to face-centered-cubic (fcc) high-entropy CoCrCuFeNi alloy to investigate the effects of Mn content on the microstructures, phase selection, and properties of CoCrCuFeMnxNi. CoCrCuFeMnxNi showed typical dendrite and interdendritic structures, and nanoprecipitates including Cu-rich cubic and Cr-rich acicular ones, were obtained in the interdendritic region. Calculated phase-selection-related parameters indicated that the formation of fcc phase was favored. Although two fcc structures were detected by X-ray diffraction, the interdendritic region showed body-centered-cubic, Cr-rich nanoprecipitates when x < 0.5. It is worth noting that the CoCrCuFeMn2.0Ni interdendrites showed an amorphous phase. CoCrCuFeMnxNi showed considerable ductility and increasing compressive strengths with increasing Mn content.
In this work (CoCrNi)100−x
Nb
x
(x = 0–23.08 at.%) medium-entropy alloys were designed to investigate the solidification process and principle. The solid solubility of Nb in CoCrNi equiatomic MEA was discovered to be less than 0.46 at.%, and a fully eutectic structure was obtained at Co28.9Cr28.9Ni28.9Nb13.3 alloy. Fcc and Laves phases are the main component phases in the (CoCrNi)100−x
Nb
x
alloys. With the increasing Nb content, the volume fraction of fcc decreased, accompanied by the increment of Laves phase. Based on CALPHAD and experimental results, the top-left corner of the (CoCrNi)Nb pseudo binary phase diagram was achieved. Hypoeutectic (CoCrNi)100−x
Nb
x
(x < 13.3 at.%) alloys undergo the following solidification process: at first, primary dendrites with fcc structure nucleate and grow in the melt, then, eutectic reaction occurs and a mixture of Laves and fcc eutectic structure is obtained. Conversely, for hypereutectic (CoCrNi)100−x
Nb
x
(x > 13.3 at.%) alloys, the primary dendrite of Laves phase form, followed by the eutectic reaction of L → fcc + Laves.
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