Anisotropy study of the microstructure and properties of AlCoCrFeNi2.1 eutectic high entropy alloy additively manufactured by selective laser melting

doi:10.1016/j.jmst.2022.04.020

Abstract

Abstract:

The AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) was prepared by selective laser melting (SLM), focusing on the anisotropy of microstructure and mechanical performance. The AlCoCrFeNi2.1 EHEA printed by SLM has face-centered cubic (FCC) and body-centered cubic (BCC)/B2 phases, and the FCC phase accumulates at the boundary of the molten pool. With the increase in volumetric energy density (VED), the content of the FCC phase parallel to the surface in the direction of the building increases from 30.9% to 41%. The X-Z plane mainly produces columnar crystals along the temperature gradient, while a large number of equiaxed crystals are produced in the X-Y direction. When the volume energy density is 157 kJ/mol (VED 157), the average grain size is 1.33 µm for BCC and 1.14 µm for FCC, which is significantly smaller than other preparation methods. Due to defect distribution and size, the microhardness of the X-Y plane is generally higher than that of the X-Z plane, with maximum hardnesses of 660 HV and 655 HV, respectively, which is much higher than those of traditionally manufactured counterparts. The compression performance of SLM process AlCoCrFeNi2.1 EHEAs shows maximum yield strength of 1567 MPa, and the ultimate compressive strength can reach 3276 MPa.

Key words: Selective laser melting, High-entropy alloys, Anisotropy, Metallurgical defects, Microstructure evolution

Liwei Lan, Wenxian Wang, Zeqin Cui, Xiaohu Hao, Dong Qiu. Anisotropy study of the microstructure and properties of AlCoCrFeNi2.1 eutectic high entropy alloy additively manufactured by selective laser melting[J]. J. Mater. Sci. Technol., 2022, 129: 228-239.

Figures/Tables 19

Fig. 1. Raw AlCoCrFeNi2.1 alloy powders: (a) and (b) SEM morphology; (c) XRD pattern; (d) distribution of powder size; (e) and (f) macro morphology and microstructure inside the powder.

Table 1. Requested and tested chemical compositions of AlCoCrFeNi2.1 powder (wt.%).

Element	Al	Co	Cr	Fe	Ni
Requested	8.51	18.59	16.40	17.62	38.88
Tested	8.15	18.75	16.43	18.21	38.45

Fig. 2. (a) Schematic illustration of SLM scanning strategy; (b) the samples produced by different SLM processing parameters.

Table 2. SLM processing parameters for preparing the samples.

Parameters	Value
Laser power, P (W)	200-400
Scanning speed, v (mm/s)	600-1000
Layer thickness, t (µm)	50
Hatching space, h (mm)	0.07

Fig. 3. Effect of VED on the density of AlCoCeFeNi2.1 HEAs prepared by SLM.

Fig. 4. Macro morphology of (a) VED 106, (b) VED 133, (c) VED 134, (d) VED 142, (e) VED 157, and (f) VED 166.

Fig. 5. XRD patterns of SLM-processed AlCoCrFeNi2.1 HEAs samples taken from the plane (a) perpendicular and (b) parallel to the build direction.

Fig. 6. Phase distribution of different VED in different planes: (a) VED 106, X-Z; (b) VED 142, X-Z; (c) VED 157, X-Z; (d) VED 166, X-Z; (e) phase content of raw powder and different planes as a function of VED; (f) VED 157, X-Y.

Fig. 7. Microstructure and EDS mapping of VED 157 in (a) X-Z plane and (b) X-Y plane.

Fig. 8. EBSD images pole figure (IPF) of SLM printed samples: (a) VED 157 (X-Y plane), (b) VED 157 (X-Z plane). (c) Average grain size as a function of VED.

Fig. 9. Pole figure and Z-inverse pole figure corresponding to different VED and phase of SLM samples.

Fig. 10. Distribution of grain boundaries for AlCoCrFeNi2.1 HEAs samples with different VED: (a) VED 106, (b) VED 142, (c) and (e) VED 157, (d) VED 166. (f) Variation trend of the distribution content of LAGBs and HAGBs in different VED.

Fig. 11. EBSD-KAM map and bar graphs of the VED157 samples: (a) and (c) Perpendicular to the building direction (X-Y); (b) and (d) parallel to the building direction (X-Z).

Table 3. Summary of KAM values degree of recrystallization of two phases in different samples.

Plane	VED	KAM		Recrystallization (%)
Plane	VED	BCC	FCC	BCC	FCC
X-Z	VED 106	0.1842	0.2047	35.5	38.5
	VED 142	0.1792	0.2046	36.8	39.5
	VED 157	0.1629	0.1969	41.9	42.4
	VED 166	0.1568	0.1944	41.6	42.1
X-Y	VED 157	0.2434	0.2579	28.6	32.4

Fig. 12. Micro-hardness of SLM AlCoCrFeNi2.1 HEAs X-Y and X-Z plane corresponding to different VED.

Fig. 13. Compressive stress-strain curves of the SLM printed AlCoCrFeNi2.1 HEAs simples at various VEDs: (a) perpendicular to build direction; (b) parallel to build direction; (c) compression strength and plastic strain of some typical HEAs [45], [46], [47], [48], [49], [50], [51], [52].

Table 4. Compressive mechanical properties of the SLM specimens with different VEDs and loading directions.

		σ_0.2 (MPa)	σ_max (MPa)	ε_f (%)
VED 106	⊥BD	944	1802	5.66
VED 106	∥BD	1095	1939	4.88
VED 133	⊥BD	1567	2872	6.81
VED 133	∥BD	1321	2889	9.70
VED 134	⊥BD	1029	1647	3.66
VED 134	//BD	1084	1947	4.26
VED 142	⊥BD	1204	2090	5.7
VED 142	//BD	1145	2074	4.04
VED 157	⊥BD	1334	3276	10.53
VED 157	//BD	1464	3175	9.51
VED 166	⊥BD	1436	2805	9.60
VED 166	//BD	1317	2705	7.56

Table 5. Schmid factor on different surfaces of VED 157, and the percentage of crystal grains with Schmid factor values ranging from 0.44 to 0.5.

Schmid factor	BCC	FCC	0.44-0.5
X-Y plane (⊥BD)	0.458	0.449	74.98%
X-Z plane (//BD)	0.428	0.458	60.72%

Fig. 14. Surface defects (X-Y plane) corresponding to different VEDs.

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