Abstract
A CoCrCuFeNi high-entropy alloy (HEA) was diffusion welded using a Co filler at 850, 950, 1050, and 1100 °C, and the microstructure and diffusion mechanism of the joints were examined. Results show that firm connections are achieved at each temperature, no intermetallic compound forms in the joints, and some Kirkendall voids are remained on the HEA side around the interface. The diffusion coefficients of Cr, Fe, Ni, and Cu in the Co filler at 850 and 950 °C are calculated, ranked as follows: Cu>Cr>Fe>Ni. The diffusion rates of all elements are at the same level. The diffusion between the CoCrCuFeNi HEA and Co filler occurs under the combined action of the vacancy and grain boundary diffusion mechanisms.
High-entropy alloys (HEAs) are novel materials composed of five or more principal elements in equal or near-equal molar ratio
Diffusion welding is a solid-phase joining method, which is advantageous for the welding of dissimilar materials, ceramics and intermetallics. The diffusion welding of an HEA to a dissimilar-material usually has poor compatibility and requires the insertion of a filler metal that has good compatibility with both the HEA and the other material. As HEAs are emerging materials, studies on their welding to dissimilar materials are insufficient, and the existing research mainly focuses on the joining of HEAs and superalloys, steels, Ti alloys, Al alloys, Cu, and a few nonmetals.
In investigations of the weldability between HEAs and superalloys, the Inconel 718 superalloy was welded to an Al0.1CoCrFeNi HEA by gas tungsten arc weldin
Studies on dissimilar-material welding between HEAs and steels are mainly about the welding of CoCrFeMnNi HEAs and stainless steels, such as 31
Some researchers have joined HEAs and aluminum alloys; a FeCoCrNiMn HEA and Al 1060 were friction stir lap welded by Yao et a
Some studies have reported the welding of HEAs with copper or titanium; Wang et a
A few studies on the joining of HEAs and nonmetals have been published; an Al0.85CoCrFeNi HEA and a TiAl intermetallic alloy were vacuum diffusion welded by Lei et a
HEAs are usually used as filler metals to join dissimilar materials; a CoNiCuNb0.5V1.5 HEA filler was used by Gu et a
Studies on the welding of CoCrCuFeNi HEAs are scarce despite their mature preparation process and excellent comprehensive performance. Moreover, the research on weldability between HEAs and dissimilar materials is insufficient. The performance of joints between CoCrFeNi-based HEAs and titanium alloys is usually deteriorated by the brittle intermetallic compounds Ti-Fe and Ti-Cr, and Cu as a filler metal can reduce brittle compound
With the development of HEA preparation and processing, the dissimilar-material welding between HEAs and other materials, such as metals, ceramics, and intermetallics, will increase. A filler metal is needed in case of poor weldability between HEA and other materials. Nevertheless, the research on the weldability and bonding characteristics of HEAs with other metals is insufficient.
With good heat and corrosion resistance, cobalt and its alloys can be used as matrices to prepare superalloy
CoCrCuFeNi ingots were fabricated using a vacuum induction furnace and cast in a graphite mold (200 mm×100 mm× 3 mm). The ingots were all melted at least five times to ensure their homogeneity, and they were cut into dimensions of 10 mm×10 mm×3 mm and 15 mm×10 mm×3 mm via electric spark cutting. The raw materials were pure Co, Cr, Cu, Fe, and Ni powders with a purity of 99.9%. The CoCrCuFeNi HEA was composed of an fcc solid solution (intragranular) and a small Cu-rich phase (intergranular), as shown in

Fig.1 Microstructure (a) and EDS results of intergranular and intragranular (b) of CoCrCuFeNi
Before welding, the workpieces were polished using abrasive paper and ultrasonically cleaned using alcohol. A schematic of the assemblies of workpieces to be welded is shown in

Fig.2 Schematic of assemblies
No. | Temperature/ °C | Interlayer | Holding time/ min | Pressure /MPa |
---|---|---|---|---|
1 | 850 | Co | 120 | 30 |
2 | 950 | |||
3 | 1050 | |||
4 | 1100 |

Fig.3 Interfacial morphologies of joints diffusion welded at different temperatures: (a) 850 °C, (b) 950 °C, (c) 1050 °C, and (d) 1100 °C
The XRD patterns of the CoCrCuFeNi HEA base metal and the joints welded at different temperatures are shown in

Fig.4 XRD patterns of CoCrCuFeNi alloy and joints achieved at different temperatures
The EPMA surface scanning test results of the joints welded at different temperatures are shown in

Fig.5 Morphologies and corresponding EPMA surface scanning results of joints achieved at different temperatures: (a) 850 °C, (b) 950 °C, (c) 1050 °C, and (d) 1100 °C
The diffusion layers of the joints welded at different temperatures were quantitatively analyzed by EPMA point scanning, and the results are shown in
Point | Co | Cr | Fe | Ni | Cu |
---|---|---|---|---|---|
A | 94.676 | 2.372 | 1.607 | 0.949 | 0.395 |
B | 73.872 | 8.276 | 7.086 | 7.015 | 3.751 |
C | 74.478 | 9.252 | 7.784 | 4.812 | 3.675 |
D | 67.114 | 8.962 | 8.153 | 7.647 | 8.124 |
At the welding temperature of 850 °C, the diffusion layer is thin and the interface is straight (
EPMA line scanning was conducted on the joints welded at different temperatures to further analyze the element diffusion, and the results are shown in

Fig.6 EPMA line scanning of joints welded at different temperatures: (a) 850 °C, (b) 950 °C, (c) 1050 °C, and (d) 1100 °C
As shown in Fig.
Combined with the surface scanning results, the line scanning findings indicate the following results. First, the Cu atoms diffuse into the Co filler faster than the Cr, Fe, and Ni atoms at 1050 and 1100 °C. Second, the diffusion rates of Cr, Fe, and Ni are close in most areas at 1050 and 1100 °C. Third, the Ni atoms more easily penetrate the Co filler in local areas at 1050 or 1100 °C. Fourth, the Cr, Fe, and Ni atoms have the same diffusion path at all diffusion temperatures.
In order to discuss the properties of the joints welded at different temperatures, shear tests were carried out. All shear tests were unloaded manually when they were unable to continue after the joints suffer severe plastic deformation. The bearing capacity of the joint welded at 850
The diffusion coefficients of each element at 850 and 950 °C were calculated using the diffusion couple model in Fick's second law, where the compositions at both ends of the diffusion couples are not affected by diffusion. The relationship between the mass concentration of each element and the diffusion distance and time is described by the following error function:
(1) |
where ω(x,t) is the element mass concentration for diffusion time t and diffusion position x; ω1 is the element mass concen-tration in the Co filler; ω2 is the element mass concentration in the CoCrCuFeNi HEA base metal, which is the average values of 20 points in the intragranular; D is the diffusion coefficient of atoms; t is the diffusion time (7200 s in this study); and x is the position of the composition test point, that is, the distance from the composition test point to the original interface. The element concentration of the original interface is (ω1+ω2)/2.
Initial condition of equation (t=0):
(2) |
Boundary conditions (t > 0):
(3) |
The intermediate variable is set to . The value of β can be found according to the error function table.
(4) |
The diffusion coefficients of the Co atoms in the CoCrCuFeNi HEA at 850 and 950 °C are shown in
Temperature/°C | 850 | 950 |
---|---|---|
Diffusion coefficient/×1 | 0.3181 | 3.3115 |
Temperature/°C | Co | Cr | Fe | Cu |
---|---|---|---|---|
850 | 0.4450 | 0.3591 | 0.3052 | 0.4726 |
950 | 3.8580 | 3.6131 | 0.6173 | 4.1605 |
coefficients of Cr, Fe, Ni, and Cu from the HEA to the Co filler at 850 and 950 °C are ranked as follows: Cu > Cr > Fe > Ni. At 850 °C, the diffusion coefficients of all elements are similar. At 950 °C, the diffusion coefficient of Co, Cr, Fe, and Cu is the same, whereas that of Ni is slightly smaller than that of other elements.
The atom concentration distributions near the interface were calculated and then compared with the measured element distributions in

Fig.7 Measured and calculated composition distribution of each element produced at 950 °C: (a) Co, (b) Cr, (c) Fe, (d) Ni, and (e) Cu
The experimental and calculation results suggest that the diffusion rate of Cu is slightly higher than that of other elements at all diffusion temperatures. The reason may be that Cu is more active than other elements due to its lower melting point. The melting points of Co, Cr, Cu, Fe, and Ni are listed in
Element | Co | Cr | Cu | Fe | Ni |
---|---|---|---|---|---|
Melting point/°C | 1495 | 1907 | 1083 | 1538 | 1453 |
The diffusion rates of Ni atoms from the CoCrCuFeNi HEA to the Co filler are slower than that of Cr and Fe at 850 and 950 °C, which is consistent with the calculation results for the Cu/CoCrFeMnNi diffusion system of Ding et a
The diffusion between the CoCrCuFeNi HEA and the Co filler occurs under the combined action of the vacancy and grain boundary diffusion mechanisms. Atoms diffuse into a crystal through the vacancy mechanism. The higher the diffusion temperature, the greater the vacancy concentration in the crystal and the easier the atom migration. This is also the main reason for the increase in the diffusion coefficient of each element with temperature increasing from 850 °C to 950 °C. At grain boundaries and dislocation regions, the lattice distortion caused by crystal defects is conducive to atom migration, so the atom diffusion rate is higher than that inside the intact crystal. Atoms easily penetrate weld seams with long distances in these areas. This is why diffusion precursors form in the local areas, as seen in the EPMA surface scanning results (
1) Firm connections of CoCrCuFeNi HEAs diffusion welded using a Co filler are obtained at each temperature, no intermetallic compounds form in the joints, and some Kirkendall voids are remained on the HEA side around the interface.
2) The diffusion coefficients of each element at 850 and 950 °C are calculated. The diffusion coefficients of Cr, Fe, Ni, and Cu in the Co filler are ranked as follows: Cu > Cr > Fe > Ni. The Cu atoms diffuse faster than other elements at all temperatures. The Ni atoms diffuse slower at low temperatures, but their long-distance diffusion ability is stronger than other elements at high temperatures. The diffusion rates of all elements are at the same level.
3) The diffusion between the CoCrCuFeNi HEA and Co filler occurs under the combined action of the vacancy and grain boundary diffusion mechanisms.
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