Stabilized sub-grain and nano carbides-driven 1.2 GPa grade ultra-strong CrMnFeCoNi high-entropy alloy additively manufactured by laser powder bed fusion

doi:10.1016/j.jmst.2021.12.010

Abstract

Abstract:

High-entropy alloys (HEAs) with interstitial atoms that are produced by additive manufacturing have gained intensive interest in the materials science community because of their suitability for constructing high-strength net-shape components. Here, a strategy to additionally enhance the strength of selective laser melted carbon-containing HEAs was investigated. The as-built carbon-containing HEAs (C_x(Cr₂₀Mn₂₀Fe₂₀Co₂₀Ni₂₀)_100-_x (x = 0.5 at.%, 1.0 at.%, and 1.5 at.%)) contain supersaturated carbon, and the extent of supersaturation increases as the carbon content increases. When subjected to aging treatment at 650 °C for 1 h, the microstructure of the three alloys did not change at the grain scale. However, the microstructure at the sub-grain scale changed markedly, and these changes influenced the tensile properties and deformation mechanism. In particular, the tensile strength of aged 1.5C-HEA at 650 °C was ∼1.2 GPa at room temperature, which is higher than those reported for CrMnFeCoNi HEAs. Furthermore, the main deformation mechanism changed from deformation twinning to dislocation-mediated slip, resulting in much higher strain hardening capacity after the aging treatment. This work led to the development of an alternative promising method that involves tailoring the microstructure, to enhance the mechanical properties of additively manufactured metallic materials that contain interstitial atoms.

Key words: Selective laser melting, High-entropy alloy, Carbon contents, Aging, Microstructure evolution, Tensile property, Deformation mechanism

Young-Kyun Kim, Kee-Ahn Lee. Stabilized sub-grain and nano carbides-driven 1.2 GPa grade ultra-strong CrMnFeCoNi high-entropy alloy additively manufactured by laser powder bed fusion[J]. J. Mater. Sci. Technol., 2022, 117: 8-22.

Figures/Tables 14

Fig. 1. (a) Heat-treatment strategies for the SLM-built carbon-containing CrMnFeCoNi HEAs and (b) corresponding Vickers hardness values of the heat-treated alloys.

Fig. 2. Inverse pole figure maps showing the grain structures of the as-built and aged alloys with different carbon contents (0.5 at% C, 1.0 at% C, and 1.5 at% C).

Fig. 3. Electron channeling contrast images showing the microstructure evolution at sub-grain scale upon undergoing aging treatment.

Fig. 4. Backscattered electron images showing the evolution of nano-sized precipitates during aging treatment.

Fig. 5. (a) STEM-DF image showing the substructures consisting of dislocations and precipitates, and (b) TEM-EELS elemental mapping results.

Fig. 6. Typical tensile stress-strain curves of the SLM-built C-HEAs recorded by varying the nominal carbon content and aging treatment conditions: (a) 0.5C—HEA, (b) 1.0C—HEA, and (c) 1.5C—HEA; (d) alloys aged at 650 °C for 1 h.

Table 1. Tensile test data of the SLM-built C-HEAs in the as-built state and aged states.

Sample ID	Condition	Yield strength, MPa	Ultimate tensile strength, MPa	Elongation,%
0.5C-HEA	As-built	653.0±8.3	766.3±18.5	28.9±1.3
	H800	557.2±13.6	824.2±15.3	37.4±2.0
	H650	650.8±10.5	884.9±12.2	31.7±1.9
1.0C-HEA	As-built	751.6±13.2	895.0±22.3	31.6±2.5
	H800	722.6±8.1	1014.2±19.5	28.3±2.3
	H650	856.2±10.6	1101.2±25.8	19.1±2.6
1.5C-HEA	As-built	753.7±15.8	911.1±25.1	38.1±2.8
	H800	747.8±8.2	1054.4±18.6	25.9±3.2
	H650	920.6±18.8	1178.1±15.3	20.1±2.4
Annealed CrMnFeCoNi HEA [6]		293.1	625.6	63.1

Fig. 7. (a) Yield strength as a function of real carbon content in SLM-built 1.5C-HEA under different conditions and (b) comparison of the ultimate tensile strength vs. the elongation-to-failure of SLM-built carbon-containing CrMnFeCoNi HEAs with other CrMnFeCoNi HEAs.

Fig. 8. Tensile fractographies of SLM-built C-HEAs with different nominal carbon content and aging treatment conditions.

Fig. 9. High magnification tensile fractographies showing the magnitude of the dimples and precipitates in the (a1-a3) as-built alloys and (b1-b3) H650 alloys.

Fig. 10. Tensile deformation microstructure of the 0.5C-HEAs (a1, a2) as-built alloy and (b1, b2) H650 alloy, and the 1.5C-HEA (c1, c2) as-built alloy and (d1, d2) H650 alloy.

Fig. 11. (a1, a2) Grain boundary detachment observed at tensile fractured surface and (b) HAADF-STEM image and corresponding EDS elemental mapping results of the SLM-built 1.5C-HEA H650 alloy.

Fig. 12. Electron channeling contrast images showing the deformation microstructure of the 1.5C-HEA (a) as-built alloy, (b) H650 alloy, and (c) magnified image of the area demarcated in (b).

Fig. 13. High magnification ECC images of the SLM-built 1.5C-HEAs: (a) as-built and (b) H650. (c) Schematic illustration showing the microstructure evolution at the sub-grain scale.

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