Effect of Cu-Rich Phase Precipitation on the Microstructure and Mechanical Properties of CoCrNiCux Medium-Entropy Alloys Prepared via Laser Directed Energy Deposition

doi:10.1007/s40195-021-01316-z

Abstract

Abstract:

CoCrNiCu_x (x = 0.16, 0.33, 0.75, and 1) without macro-segregation medium-entropy alloys (MEAs) was prepared using laser directed energy deposition (LDED). The microstructure and mechanical properties of CoCrNiCu_x alloys with increasing Cu content were investigated. The results indicate that a single matrix phase changes into a dual-phase structure and the tensile fracture behaviors convert from brittle to plastic pattern with increasing Cu content in CoCrNiCu_x alloys. In addition, the tensile strength of CoCrNiCu_x alloys increased from 148 to 820 MPa, and the ductility increased from 1 to 11% with increasing Cu content. The nano-precipitated particles had a mean size of approximately 20 nm in the Cu-rich phase area, and a large number of neatly arranged misfit dislocations were observed at the interface between the two phases due to Cu-rich phase precipitation in the CoCrNiCu alloy. These misfit dislocations hinder the movement of dislocations during tensile deformation, as observed through transmission electron microscopy. This allows the CoCrNiCu alloy to reach the largest tensile strength and plasticity, and a new strengthening mechanism was achieved for the CoCrNiCu alloy. Moreover, twins were observed in the matrix phase after tensile fracture. Simultaneously, the dual-phase structure with different elastic moduli coordinated with each other during the deformation process, significantly improving the plasticity and strength of the CoCrNiCu alloy.

Key words: Medium-entropy alloys, Mechanical properties, Laser directed energy deposition, Misfit dislocations, Cu-rich phase

Yong Xie, Zhixin Xia, Jixin Hou, Jiachao Xu, Peng Chen, Le Wan. Effect of Cu-Rich Phase Precipitation on the Microstructure and Mechanical Properties of CoCrNiCu_x Medium-Entropy Alloys Prepared via Laser Directed Energy Deposition[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(11): 1591-1600.

Figures/Tables 13

Table 1 Feasibility analysis of CoCrNiCux

	$\delta $ (%)	${\Delta H}_{\mathrm{mix}} $(kJ/mol)	${\Delta S}_{\mathrm{mix}} $(K mol) J/	${T}_{\mathrm{m}}$ (K)	$\Omega $	VEC	Phase
CoCrNiCu_0.16	1.06	- 3.03	10.22	1851	6.2	8.5	FCC
CoCrNiCu_0.33	1.09	- 1.32	10.89	1824	15.0	8.6	FCC
CoCrNiCu_0.75	1.15	1.56	11.39	1774	13.0	8.9	FCC
CoCrNiCu	1.19	2.75	11.52	1746	7.3	9	FCC

Fig. 1 Schematic of the laser directed energy deposition setup and tensile sample size

Fig. 2 X-ray diffraction patterns of the CoCrNiCux alloys

Fig. 3 SEM images of a CoCrNiCu0.16, b CoCrNiCu0.33, c CoCrNiCu0.75, d CoCrNiCu. Insets: back-scattered electron (BSE) images of CoCrNiCux prior to etched

Fig. 4 SEM-EDS images of a CoCrNiCu0.16, b CoCrNiCu0.33, c CoCrNiCu0.75, d CoCrNiCu

Fig. 5 Stress-strain curves of the CoCrNiCux alloy

Fig. 6 Fracture surfaces in the SEM images of a CoCrNiCu0.16, b CoCrNiCu0.33, c CoCrNiCu0.75, d CoCrNiCu, and the EDS of e CoCrNiCu0.16, f CoCrNiCu0.33, g CoCrNiCu0.75, and h CoCrNiCu

Table 2 Content of each element in the second-phase particles

	Co (%)	Cr (%)	Ni (%)	Cu (%)
Sample 1	6.0	34.7	7.4	51.9
Sample 2	5.4	28.9	8.8	56.9
Sample 3	8.4	20.1	12.4	59.1
Sample 4	14.5	9.1	15.6	60.8

Fig. 7 Fitting relationship between load and pressure depth of CoCrNiCu alloys

Table 3 Indentation hardness and indentation modulus of elasticity of CoCrNiCu alloy

	Cu-rich	Matrix
Indentation hardness (H_IT, MPa)	777.9	1680.0
Indentation modulus of elasticity (E_IT, GPa)	128.3	213.6

Fig. 8 Stress-strain curves of Cu-rich and matrix alloy

Fig. 9 Bright-field TEM images for the CoCrNiCu alloys: a Cu-rich region, b matrix phase region, c interface of the Cu-rich and matrix phases. Insets: selected area electron diffraction patterns of FCC1 and FCC2, respectively

Fig. 10 Bright-field TEM images of stretched CoCrNiCu alloys: a interface of the Cu-rich and matrix phases, b Cu-rich region, c matrix phase region

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