Melting behavior and globular microstructure formation in semi-solid CoCrCuxFeNi high-entropy alloys

doi:10.1016/j.jmst.2020.04.009

Abstract

Abstract:

High-entropy alloys can be compelling raw materials for semi-solid applications. In the present study, the influence of the Cu content on the melting behavior and semi-solid microstructure of CoCrCu_xFeNi (x = 0, 1, 2, 3) alloys was investigated. Arc-melted samples were cross-rolled at room temperature and then isothermally treated at 1175 °C in the semi-solid state for 300 s. Microstructural characterization showed that the alloys containing Cu were formed by two fcc phases. Notably, the increase in Cu content also led to an increase in the volumetric fraction of the Cu-rich phase. During solidification, this phase, which is the last to form, nucleates and epitaxially grows on the Cu-lean phase. All the studied CoCrCuFeNi alloys exhibited the same melting behavior. The Cu-rich phase melts at approximately 1120 °C, whereas the Cu-lean phase melts at approximately 1350 °C, providing a suitable processing temperature range of more than 200 °C. The semi-solid microstructures were considerably refined and globular regardless of the alloy composition, being suitable for semi-solid processing. Furthermore, each fcc phase exhibited roughly the same composition under the different processing conditions. The Cu content in the Cu-lean phase was approximately 10 at.%, while Co, Cr, Fe, and Ni were in an approximately equiatomic ratio. Meanwhile, the Cu content was between 80 at.% and 86 at.% in the Cu-rich phase. The isothermal treatment of the CoCrCu₃FeNi alloy at a higher temperature (1300 °C) only caused the globules to coarsen. In conclusion, this work showed that these alloys can be potential candidates for semi-solid processing.

Key words: Semi-solid processing, Thixoforming, High-entropy alloys, Microstructure

Kaio Niitsu Campo, Caio Chaussê de Freitas, Leonardo Fanton, Rubens Caram. Melting behavior and globular microstructure formation in semi-solid CoCrCu_xFeNi high-entropy alloys[J]. J. Mater. Sci. Technol., 2020, 52: 207-217.

Figures/Tables 10

Table 1 Nominal and experimental compositions (at.%) of the produced alloys.

Alloy	CoCrCu₀FeNi		CoCrCu₁FeNi		CoCrCu₂FeNi		CoCrCu₃FeNi
Element (at.%)	Nom.	Exp.	Nom.	Exp.	Nom.	Exp.	Nom.	Exp.
Co	25	26.0 ± 0.1	20	20.5 ± 0.1	16.67	16.7 ± 0.1	14.29	13.9 ± 0.1
Cr	25	24.9 ± 0.1	20	19.4 ± 0.1	16.67	17.1 ± 0.1	14.29	14.4 ± 0.1
Cu	0	-	20	20.7 ± 0.1	33.32	33.4 ± 0.3	42.84	44.1 ± 0.3
Fe	25	23.8 ± 0.1	20	19.1 ± 0.1	16.67	15.8 ± 0.1	14.29	13.2 ± 0.1
Ni	25	25.3 ± 0.1	20	20.3 ± 0.1	16.67	17.0 ± 0.1	14.29	14.4 ± 0.1

Fig. 1. SEM backscattered images of the as-cast microstructures for the CoCrCuxFeNi alloys: (a) x = 0, (b) x = 1, (c) x = 2, and (d) x = 3. (e) Typical SEM-EDS elemental mapping of the as-cast microstructures for the alloys containing Cu.

Fig. 2. XRD patterns of the as-cast CoCrCuxFeNi alloys.

Fig. 3. SEM-EBSD results showing the inverse pole figure (IPF) maps of the as-cast (a) CoCrFeNi and (b) CoCrCu3FeNi alloys. (c) Combined maps (EDS-Cu and IPF) presenting a more detailed view of the highlighted region in (b). (d) Misorientation profile measured across the line ‘AB’ and simulated crystal orientation at points ‘1’ and ‘2’ of (c).

Fig. 4. DSC results obtained during heating at 20 °C min-1 for the as-cast CoCrCuxFeNi alloys: (a) thermograms; (b) integration areas of the DSC thermograms in (a), showing the liquid fraction as a function of temperature.

Fig. 5. EBSD results for the cross-rolled CrCuCu3FeNi alloy: (a) combined EDS-Cu and inverse pole figure (IPF) maps; (b) IPF map only for the Cu-lean phase (fcc2 phase); (c) IPF map only for the Cu-rich phase (fcc3 phase); (d) pole figures obtained for both fcc2 and fcc3 phases, (e) pole figures only for the fcc2 phase; (f) pole figures only for the fcc3 phase. Pole figure intensities are given in multiples of uniform density (m.u.d.).

Fig. 6. Quenched microstructures from the semi-solid state at 1175 °C after an isothermal treatment of 300 s. Typical SEM backscattered images for the CoCrCuxFeNi alloys: (a) x = 1, (b) x = 2, and (c) x = 3. (d) Typical SEM-EDS elemental mapping. Quantitative image analysis: (e) mean equivalent diameter (Deq) and circularity (shape factor), and (f) liquid fraction.

Fig. 7. EBSD results for the semi-solid CrCuCu3FeNi alloy: (a) combined EDS-Cu and inverse pole figure (IPF) maps; (b) misorientation profile measured across the line ‘AB’. (c) Pole figures obtained for both the fcc2 and fcc3 phases; (d) pole figures only for the fcc2 phase; (e) pole figures only for the fcc3 phase. Pole figure intensities are given in multiples of uniform density (m.u.d.).

Fig. 8. Chemical composition (at.%) determined by SEM-EDS for the CoCrCuxFeNi alloys under all conditions investigated here: (a) Cu-lean phase (fcc2); (b) Cu-rich phase (fcc3).

Fig. 9. Typical SEM backscattered image of the microstructure for the CoCrCu3FeNi alloy after quenching from the semi-solid state at 1300 °C (isothermal treatment of 300 s).

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