Nanocrystalline grains are prepared on the surface layer of CoCrFeMnNi high-entropy alloy by mechanical polishing to address the diffusion hysteresis effect of CoCrFeMnNi high-entropy alloy. By creating a nanostructured layer on the surface, a "short-circuit diffusion" strategy is employed, which increases grain boundaries and utilizes the high diffusion coefficient of nanocrystalline grains to enhance effective bonding at the interface. This approach achieves a highly reliable connection between the high-entropy alloy and stainless steel. The surface micro-morphology, composition, thickness, and grain size distribution of the nanostructured high-entropy alloy are analyzed using SEM, XRD, and EBSD techniques. The hardness distribution, elemental distribution in the joint area, microstructure and phase composition of the joint, mechanical properties, and fracture morphology of the CoCrFeMnNi high-entropy alloy and 304 stainless steel diffusion bonding area are examined before and after surface nanostructuring, with a Vickers hardness tester, SEM, TEM, and a universal tensile testing machine. A nanocrystalline layer is formed on the surface of the CoCrFeMnNi high-entropy alloy through surface mechanical polishing. The surface grain size of the high-entropy alloy is reduced from 10-40 μm to approximately 11-19 nm. After 2 hours of mechanical polishing, a nanocrystalline region with a thickness of about 66 μm is created, and after 4 hours, this thickness increases to approximately 154 μm. Although the diffraction peak positions of the CoCrFeMnNi high-entropy alloy does not change post-polishing, the peaks broadened, indicating that the crystal structure remains unchanged, maintaining a single face-centered cubic structure. Surface nanochemical processing effectively increases the diffusion distance of Fe atoms. Under the diffusion connection process with welding temperature of 900 ℃, insulation time of 2 h and pressure of 20 MPa, the diffusion distance of Fe atoms increases from 5.6 μm to 11.2 μm after 2 hours, and to 19.4 μm after 4 hours of surface mechanical polishing, demonstrating that this technique successfully overcomes the diffusion sluggishness effect of the high-entropy alloy. Microhardness results indicate that no detrimental hard phases or voids caused by the Kirkendall effect are present at the interface of the CoCrFeMnNi/304SS joint following surface mechanical polishing. Under the diffusion connection process with welding temperature of 900 ℃, insulation time of 2 h and pressure of 20 MPa, the shear strength of the diffusion welded joint after 2 hours of mechanical polishing reaches 304 MPa, and after 4 hours, it increases to 357 MPa, reflecting an enhancement of 18.9% and 38.9%, respectively, compared with the unpolished joint (257 MPa). This signifies a substantial improvement in joint strength. The fracture morphology of the joint without surface mechanical polishing at 900 ℃ displays a mixture of a few dimples and cleavage facets, with the fracture primarily exhibiting brittle characteristics. After 2 hours of surface mechanical polishing, some tearing-type fracture features are observed, although a full transition to ductile fracture does not occur. In contrast, the fracture morphology after 4 hours of polishing exhibits typical ductile fracture characteristics. In conclusion, this study demonstrates that the surface nanostructuring treatment mechanism can effectively mitigate the diffusion sluggishness effect in high-entropy alloys, and that nanostructuring significantly enhances the diffusion rate of these alloys, thereby improving the mechanical properties of joints between high-entropy alloys and stainless steel.
Key words
CoCrFeMnNi high-entropy alloy /
diffusion welding /
surface nanostructuring of HEAs /
304 stainless steel /
shear strength /
diffusion distance
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Funding
GDAS' Project of Science and Technology Development (2020GDASYL-20200301001, 2022GDASZH-2022010109); National Key Research and Development Program of China (2020YFE0205300)
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