Exceptional strength-ductility synergy of additively manufactured CoCrNi medium-entropy alloy achieved by lattice defects in heterogeneous microstructures

doi:10.1016/j.jmst.2022.03.024

Abstract

Abstract:

The selective laser melting (SLM) with subsequent cold rolling and annealing is used to produce high-density lattice defects and grain refinement in the CoCrNi medium-entropy alloys (MEAs). The superior comprehensive mechanical properties have been achieved in the as-SLMed CoCrNi alloy after rolling and annealing. The as-SLMed alloys delivered the yield strength of 693.4 MPa, the ultimate tensile strength of 912.7 MPa and the fracture strain of 54.4%. After rolling with 70% reduction in thickness and annealing at 700 °C for 2 h. the yield strength, ultimate tensile strength and fracture strain reached 1161.6 MPa, 1390.8 MPa and 31.5%, respectively. The exceptional strength-ductility synergy is mainly attributed to the refined hierarchical microstructures with coarsening grains at a level of 30 µm and ultrafine grains at a level of 1 µm, and the heritage of dislocation-formed sub-grains and other lattice defects. This investigation demonstrates that the SLM with subsequent rolling and annealing is beneficial to fabricate high strength and ductile MEAs with single face-centered cubic (fcc) structure.

Key words: Medium-entropy alloys, Selective laser melting, Strength-ductility synergy, Defects, Microstructure

Jianying Wang, Jianpeng Zou, Hailin Yang, Lijun Zhang, Zhilin Liu, Xixi Dong, Shouxun Ji. Exceptional strength-ductility synergy of additively manufactured CoCrNi medium-entropy alloy achieved by lattice defects in heterogeneous microstructures[J]. J. Mater. Sci. Technol., 2022, 127: 61-70.

Figures/Tables 12

Fig. 1. Characterization of pre-alloyed CoCrNi MEA powders: (a) powder particle morphology; (b) XRD spectrum; (c-e) EDS mappings for elemental distribution.

Table 1. Chemical Compositions of the CoCrNi SLMed alloy (wt.%).

Co	Cr	Ni	Fe	C	O	N	P	S
34.9	30.4	34.5	0.063	0.0036	0.0015	0.0021	0.0012	0.00034

Fig. 2. (a) Engineering tensile stress-strain curves of SLMed CoCrNi alloys without and with the 70% reduction in thickness and annealing at 700 °C for 2 h (RA treatment); (b) Comparison of tensile strength of CoCrNi alloys fabricated by different techniques without subsequent deformation and heat-treatment, including MA + SPS, casting, SLM in other literatures and in this study [8,16,18,28,27,40,41]; (c) Comparison of tensile properties of the as-SLMed CoCrNi alloys with RA treatment obtained from this study and CoCrNi alloys fabricated by casting + rolling, casting + HPT, SPS + rolling, and Mo/(Ti,Al)-doped CoCrNi alloys [14,24,28,45,46].

Fig. 3. SEM/BSE images showing the microstructure of SLMed CoCrNi alloys: (a) overall microstructure; (b) hierarchical microstructure consisted of columnar and equiaxed sub-grains; (c) detailed microstructure of equiaxed sub-grains; (d) detailed microstructure of columnar sub-grains; (e) heat-affected zone (HAZ); (f) prism-like sub-grains.

Fig. 4. SEM/BSE images showing the microstructure features of SLMed CoCrNi alloys with RA treatment: (a) overall microstructure; (b) refined grain structure with annealing twins at nano-sizes; (c) refined hierarchical microstructure composed of refined columnar and equiaxed sub-grains; (d) detailed microstructure showing refined hierarchical microstructure after deep inch; (e) detailed microstructure of refined columnar sub-grains; (f) detailed microstructure of equiaxed sub-grains. The yellow arrows indicate the formation of annealing twins.

Fig. 5. XRD spectra of SLMed CoCrNi alloys without and with RA treatment.

Fig. 6. (a) Inverse Pole Figure (IPF) map, (b) Image Quality (IQ) map, (c) Kernel Average Misorientation (KAM) map of SLMed CoCrNi alloys; (d, e) detailed IPF maps, (f) IQ map, (g) KAM map of SLMed CoCrNi alloys with RA treatment.

Table 2. Grain size, step size selected in EBSD testing, length of the Burgers vector, KAM value and ρGND determined for SLMed CoCrNi alloys without and with RA treatment.

Conditions	Grain size (µm)	Step size (µm)	\|b\| (nm)	KAM value	ρ_GND (m⁻²)
As-SLMed	47.83	1.20	0.258	0.839	5.42 × 10¹⁵
RA treatment	5.08	0.50	0.258	0.908	1.44 × 10¹⁶

Fig. 7. TEM micrographs showing the microstructure of SLMed CoCrNi alloys with RA treatment: (a) TEM image and (b) STEM image of high-density dislocation-formed equiaxed sub-grains; (c) TEM image of the refined columnar sub-grains; (d) dislocation structure of dislocation walls; (e) STEM and (f) SAED pattern showing the formation of annealing twins with nano-sizes.

Fig. 8. Corresponding EDS mapping of equiaxed sub-grains (a, a1-a3) including elements (a1) Co, (a2) Cr, (a3) Ni, and columnar sub-grains (b, b1-b3) including elements (b1) Co, (b2) Cr, (b3) Ni.

Fig. 9. TEM micrographs after tensile testing of as-SLMed CoCrNi alloys with RA treatment showing (a) the formation of stacking faults, (b) the interaction between stacking faults and dislocations; (c) STEM image, (d) high-resolution TEM image and SAED (L1) showing the formation of Lomer-Cottrell locks; (e) TEM image, (f) high-resolution TEM image and SAED (L2) showing the formation of deformation twins.

Fig. 10. Schematic diagram showing (a) the coarsening hierarchical microstructure of SLMed CoCrNi alloys and (b) the refined hierarchical microstructure of the SLMed CoCrNi alloy after RA treatment.

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