Selective laser melting of dense and crack-free AlCoCrFeNi2.1 eutectic high entropy alloy: Synergizing strength and ductility

doi:10.1016/j.jmst.2021.11.049

Abstract

Abstract:

Additively manufactured high-entropy alloys generally suffer from low strength and/or poor ductility. In this work, by leveraging the good castability of eutectic high entropy alloys and high cooling rate of selective laser melting (SLM), we report a nearly fully dense and crack-free as-SLM AlCoCrFeNi_2.1 eutectic high entropy alloy with an exceptional strength-ductility synergy, showing an ultrahigh yield strength of 982.1 ± 35.2 MPa and an ultimate tensile strength of 1322.8 ± 54.9 MPa together with an elongation to fracture of 12.3 ± 0.5%. Such strength-ductility enhancement is owing to the heterogeneous eutectic microstructure consisting of the columnar, equiaxed, and “L-shape” cells with much refined sizes down to nanoscales. The morphology of cells in the transition zone is related to the misorientation between the growth direction of adjacent layers. This heterogeneous eutectic microstructure is the result of the grain-growth behavior dominated by the mechanisms of the epitaxial growth and growth of heterogeneous nuclei in SLM. Our current results provide a new methodology for the future design of ultrahigh-strength and ductile SLM-fabricated metallic materials including HEAs, and other printable alloys for various structural applications.

Key words: Additive manufacturing, High entropy alloy, Microstructure, Mechanical properties, Laser, Crack-free

Lin He, Shiwei Wu, Anping Dong, Haibin Tang, Dafan Du, Guoliang Zhu, Baode Sun, Wentao Yan. Selective laser melting of dense and crack-free AlCoCrFeNi_2.1 eutectic high entropy alloy: Synergizing strength and ductility[J]. J. Mater. Sci. Technol., 2022, 117: 133-145.

Figures/Tables 24

Fig. 1. AlCoCrFeNi2.1 EHEA powder. (a) Morphology, (b) powder size distribution.

Table 1. Nominal compositions (at.%) of the raw powder.

Al	Co	Cr	Fe	Ni
16.393	16.393	16.393	16.393	34.428

Table 2. Processing parameters for the tensile specimens.

Samples	Power (W)	Scanning speed (mm/s)	Hatch space (μm)	Layer thickness(μm)
as-SLM	243	1250	60	50
as-cast	Vacuum arc melting

Fig. 2. Sketch graph of the dog-bone tensile sample (unit: mm).

Fig. 3. Microstructures of (a) as-cast and (b) as-SLM EHEA.

Fig. 4. XRD patterns of the raw powder, as-cast and as-SLM samples. (a) Global graph, (b) the magnified view of the part marked in the blue rectangle in (a).

Fig. 5. Microstructure of the as-cast sample. (a) SEM graph, (b) EBSD phase map, (c) elemental distribution along the red line in (a).

Table 3. Compositions of FCC and BCC measured by EPMA (at.%).

Phase	Al	Co	Cr	Fe	Ni
FCC-SLM	14.669 ± 0.035	17.348 ± 0.221	17.365 ± 0.117	16.809 ± 0.091	34.811 ± 0.022
FCC-cast	10.956 ± 0.146	17.346 ± 0.261	20.893 ± 0.426	18.615 ± 0.213	32.191 ± 0.072
BCC-cast	25.881 ± 0.738	12.731 ± 0.106	10.228 ± 1.188	10.649 ± 0.234	40.511 ± 0.519

Fig. 6. Microstructure of the as-SLM sample in the XY plane. (a) SEM image, the magnified view of (b) the red circle and (c) the green circle areas in (a), (d) elemental distribution along the yellow line in (b).

Fig. 7. TKD characterization of the as-SLM sample in XY plane. (a) Band contrast graph, (b) phase graph, (c) IPF map.

Fig. 8. Microstructure of the as-SLM sample in the YZ plane: (a) SEM image, (b-e) magnified views of local areas.

Fig. 9. SEM images of the growth behavior of the columnar cells during the SLM process: (a) low misorientation, (b) medium misorientation, (c) high misorientation, (d) magnified view of the pink circle area in (c).

Fig. 10. (a) TEM bright-field (BF) graph of the as-cast sample. (b, c) SADP patterns of the FCC and BCC networks in (a), respectively. (d, e) TEM DF graphs of the FCC and BCC networks, respectively.

Fig. 11. (a) TEM BF graph of the as-SLM sample. (b, c) SAED patterns of the FCC and BCC phases in (a), respectively. (d) TEM DF graph of the FCC phase.

Fig. 12. (a) FCC/BCC interface structure, (b) SAED pattern of the interface.

Fig. 13. (a) True stress-strain curves of the as-cast and as-SLM samples. (b) Yield and ultimate strengths of the as-SLM sample in comparison with those of the previously reported AlCoCrFeNi2.1 EHEAs manufactured via casting and directional solidification [[36], [37], [38], [39], [40], [41], [42],[56],[58], [59], [60]].

Table 4. Mechanical properties of as-cast and as-SLM EHEAs.

Sample	Yield strength (MPa)	UTS (MPa)	Elongation (%)
As-cast	492.3 ± 17.3	1221.3 ± 23.1	15.2 ± 0.1
As-SLM	982.1 ± 35.2	1322.8 ± 54.9	12.3 ± 0.5

Fig. 14. Microhardness of the as-cast and as-SLM samples before and after fracture.

Fig. 15. Representative load-depth curves (a) and indentation (b).

Table 5. Elastic moduli of phases at the as-cast and as-SLM states (GPa).

BCC-cast	FCC-cast	As-SLM
150.89 ± 3.171	172.56 ± 3.603	181.67 ± 2.705

Table 6. Calculation results for the as-cast and as-SLM samples.

Empty Cell	Dislocation density (m^-2)	Grain size (μm)	σ_D (MPa)	σ_G (MPa)
As-cast	1.079 × 10¹²	17.12	12.7	89.6
As-SLM	9.251 × 10¹⁴	7.37	373.1	136.7

Fig. 16. Taylor factor distribution of the FCC phase in both samples, indicated by its number fraction within the ranges of 2-3 and 3-4.

Fig. 17. Dislocation distribution in the as-SLM sample after fracture: (a) TEM BF graph, (b) corresponding TEM DF graph, (c) stacking faults in the FCC cells.

Fig. 18. Fracture surface morphologies of the (a-c) as-cast and (d-f) as-SLM specimens.

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