Microstructure evolution and formation mechanism of CoCrCu1.2FeNi high entropy alloy during the whole process of semi‐solid billet preparation

doi:10.1016/j.jmst.2021.12.044

Abstract

Abstract:

Dual face-centered cubic (FCC) CoCrCu_1.2FeNi semi-solid billets were prepared by semisolid isothermal treatment of wrought high entropy alloy (HEA) (SSITWH) method, and the microstructure evolution in the whole process of billets preparation was systematically investigated by optical microscopy, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The hot deformed feedstock was mainly composed of deformation structure with preferred orientations and a small number of dynamically recrystallized grains of FCC₁ phase. In the semi-solid stage, the effect of temperature and soaking time on semi-solid microstructure was studied in the range of 1130-1250 °C and 5-120 min. The semi-solid microstructure was evaluated quantitatively. The average grain size and average shape factor increased with the increase of soaking time and isothermal temperature. After isothermal heat treatment, the segregation of Cu in FCC₁ phase reduced to a certain extent. Semi-solid coarsening kinetics analysis showed that the alloy had low coarsening coefficients. When the temperatures were 1130 °C, 1175 °C, 1200 °C, 1225 °C and 1250 °C, the coarsening coefficients were 1.08 μm³/s, 5.95 μm³/s, 6.17 μm³/s, 17.58 μm³/s, 38.67 μm³/s, respectively. A coarsening kinetic equation describing solid grain growth was established. During heating up, FCC₁ and FCC₂ phase recrystallized successively. At higher temperature, FCC₂ phase was spheroidized to a certain extent. When temperature was raised to semi-solid range, the grains of FCC₂ phase coalesced, grew up and spheroidized and the preferred orientations basically disappeared. The types of semi-solid melting characteristics of HEAs were summarized in this paper. The semi-solid melting behavior of alloys is essentially affected by phase structure, phase number, phase volume content and composition.

Key words: High entropy alloy, Semi-solid billets, Microstructure, Grain coarsening, Formation mechanism

Jufu Jiang, Minjie Huang, Ying Wang, Yingze Liu, Ying Zhang. Microstructure evolution and formation mechanism of CoCrCu_1.2FeNi high entropy alloy during the whole process of semi‐solid billet preparation[J]. J. Mater. Sci. Technol., 2022, 120: 172-185.

Figures/Tables 24

Fig. 1. Preparation process of CoCrCu1.2FeNi semi-solid billets.

Fig. 2. Characterization of as-cast CoCrCu1.2FeNi ingot: (a) low magnification BSE image; (b) high magnification BSE image; (c) XRD pattern.

Table 1. EDS composition point analysis results of as-cast CoCrCu1.2FeNi alloy.

Region	Empty Cell	Empty Cell	Element (at.%)	Empty Cell	Empty Cell
Empty Cell	Co	Cr	Cu	Fe	Ni
A	23.56	24.13	11.36	20.22	21.89
B	3.13	1.85	84.29	2.80	7.92

Fig. 3. BSE image of hot deformed microstructure.

Fig. 4. Relationship between heat flow and temperature and relationship between liquid fraction and temperature.

Table 2. Liquid fraction of CoCrCuxFeNi alloys at 1175 °C (steady rising stage).

x=1 [27]	x=1.2	x=2 [27]	x=3 [27]
∼12%	∼15%	∼25%	∼38%

Fig. 5. Microstructure of CoCrCu1.2FeNi semi-solid billets: (1)-(6), (7)-(12), (13)-(18), (19)-(24) and (25)-(30) are microstructure at 1130 °C, 1175 °C, 1200 °C, 1225 °C, 1250 °C for 5 min, 15 min, 30 min, 60 min, 90 min and 120 min, respectively.

Fig. 6. Quantitative analysis of CoCrCu1.2FeNi semi-solid microstructure: (a) average grain size; (b) average shape factor.

Fig. 7. SEM-EDS element maps of CoCrCu1.2FeNi semi-solid billet (1175 °C/15 min).

Fig. 8. Element contents of the two phases in different states: (a) FCC1 phase; (b) FCC2 phase.

Fig. 9. Comparison of XRD patterns of hot deformed feedstock and semi-solid billet.

Fig. 10. Coarsening kinetic model calculation of CoCrCu1.2FeNi alloy: (a) coarsening coefficients of different temperatures; (b) relationship between ln K and 1/T.

Fig. 11. Relationship between coarsening coefficient and liquid fraction of different semi-solid alloys.

Fig. 12. EBSD analysis of microstructure during heating up process: BSE images, IPF maps, RF maps and GB maps of CoCrCu1.2FeNi hot deformed feedstock and feedstocks heated to 1000 °C and 1100 °C, respectively.

Fig. 13. Misorientation angle distribution of hot deformed feedstock and heated feedstocks: (a) hot deformed; (b) heated to 1000 °C; (c) heated to 1100 °C.

Fig. 14. EBSD analysis of microstructure heated to semi-solid temperature (1200 °C): (a) BSE image; (b) IPF map; (c) misorientation analysis of adjacent grains; (d) misorientation analysis of single grain; (e) polar figures.

Fig. 15. TEM analysis of hot deformed feedstock: (a) BF image of the two phases; (b) SAED pattern of FCC1 phase; (c) SAED pattern of FCC2 phase; (d) BF image of FCC2 phase; (e) BF image of Fine dynamic recrystallization grain in FCC1 phase; (f) BF image of dislocation walls in FCC1 phase.

Fig. 16. TEM analysis of semi solid billet (1130 °C / 5 min): (a) BF image of the two phases; (b) High magnification image of (a); (c) SAED pattern of FCC1 phase; (d) SAED pattern of FCC2 phase; (e) BF image of FCC2 phase; (f) BF image of FCC1 phase.

Fig. 17. Mechanical properties of hot deformed and heated feedstocks: (a) compression engineering stress-strain curve at room temperature; (b) yield strength.

Fig. 18. TF maps of hot deformed and heated feedstocks: (a) hot deformed; (b) heated to 1000 °C; (c) heated to 1100 °C; (d) heated to 1200 °C.

Fig. 19. Hardness of hot deformed feedstock and semi-solid billets under different parameters.

Fig. 20. Evolution model of semi-solid microstructure formation process: (a) as-cast stage; (b) hot deformed stage; (c) and (d) heating up stage; (e) semi-solid stage.

Fig. 21. Types of semi-solid range and liquid fraction control: (a) narrow type; (b) intermediate type; (c) wide type; (d) liquid fraction control.

Table 3. HEAs with wide semi-solid range.

HEAs	Phase structure	Semi-solid range
CoCrCuFeNi (This work)	FCC₁+FCC₂	1108-1386 °C
CoCrCu_0.5FeNi [27]	FCC₁+FCC₂	∼1100-1380 °C
CoCrCu_1.5FeNi [27]	FCC₁+FCC₂	∼1100-1380 °C
AlCoCrCuNi [25]	FCC+BCC	1085-1385 °C
FeCoNiCuAl [77]	FCC+BCC	∼1200-1350 °C
AlCoCrCuFeNi [78]	FCC₁+ FCC₂+BCC	∼1250-1400 °C
CrCu_0.5FeMnNi [28]	FCC₁+ FCC₂+BCC	1030-1331 °C
CrCuFeMnNi [28]	FCC₁+ FCC₂+BCC	1019-1344 °C
CrCu_1.5FeMnNi [28]	FCC₁+ FCC₂+BCC	1010-1357 °C
CrCu₂FeMnNi [28]	FCC₁+ FCC₂+BCC	1013-1370 °C
CrCu_2.5FeMnNi [28]	FCC₁+ FCC₂+BCC	1015-1379 °C
Sn_0.5CoCrFeMnNi [30]	FCC+MnNiSn rich L2₁	1066-1254 °C

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