Achieving a High-Strength CoCrFeNiCu High-Entropy Alloy with an Ultrafine-Grained Structure via Friction Stir Processing

doi:10.1007/s40195-020-01037-9

Abstract

Abstract:

High-entropy alloys (HEAs) are a new class of materials with a potential engineering application, but how to obtain ultrafine or nano-sized crystal structures of HEAs has been a challenge. Here, we first presented an equiatomic CoCrFeNiCu HEA with excellent mechanical properties obtained via friction stir processing (FSP). After FSP, the Cu element segregation in the cast CoCrFeNiCu HEA was almost eliminated, and the cast coarse two-phase structure (several micrometers) was changed into an ultrafine-grained single-phase structure (150 nm) with a large fraction of high-angle grain boundaries and nanoscale deformation twins. This unique microstructure was mainly attributed to the severe plastic deformation during FSP, and the sluggish diffusion effect in dynamics and the lattice distortion effect in crystallography for HEAs. Furthermore, FSP largely improved the hardness and yield strength of the CoCrFeNiCu HEA with a value of 380 HV and more than 1150 MPa, respectively, which were > 1.5 times higher than those of the base material. The great strengthening after FSP was mainly attributed to the significant grain refinement with large lattice distortion and nano-twins. This study provides a new method to largely refine the microstructure and improve the strength of cast CoCrFeNiCu HEAs.

Key words: Friction stir processing, High entropy alloys, Strengthening, Ultrafine grains, Microstructure

Ning Li, Cun-Lei Jia, Zhi-Wei Wang, Li-Hui Wu, Ding-Rui Ni, Zheng-Kun Li, Hua-Meng Fu, Peng Xue, Bo-Lv Xiao, Zong-Yi Ma, Yi Shao, Yun-Long Chang. Achieving a High-Strength CoCrFeNiCu High-Entropy Alloy with an Ultrafine-Grained Structure via Friction Stir Processing[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(7): 947-956.

Figures/Tables 11

Fig. 1 Schematic diagram of processing area and tensile sample

Fig. 2 a Typical processing surface with defects under a high heat input of 500 rpm, 50 mm/min using W-Re alloy tool, b typical processing surface with defects under an insufficient heat input of 300 rpm, 50 mm/min using TiC-reinforced Ni-based ceramic welding tool, c surface picture of the FSP plate without defects at 500 rpm, 50 mm/min using TiC-reinforced Ni-based ceramic welding tool, d cross-sectional structure of defect-free FSP zone of c

Fig. 3 XRD results of as-cast and FSP CoCrFeNiCu alloys

Fig. 4 SEM and EDS line scanning graphs of Cu element for CoCrFeNiCu alloy with different states: a as-cast and b FSP

Table 1 Chemical mixing enthalpy (6Hchem) of a pair of atoms [37]

Element	Co	Cr	Fe	Ni
Cu	6	12	13	4
Co	-	- 4	- 1	0
Cr	-	-	- 1	- 7
Fe	-	-	-	- 2

Fig. 5 a Hardness distribution along depth direction of FSP CoCrFeNiCu alloy, and its corresponding microstructure at different zones: b PZ, c, d transition zones, e BM

Fig. 6 Engineering stress-strain curves for BM and whole FSP samples

Fig. 7 a, b Macrographs of fracture surfaces of BM and FSP samples, c-e magnified images of fracture surfaces corresponding to positions C, D and E in a and b

Fig. 8 a, c EBSD maps of BM and PZ of FSP CoCrFeNiCu alloy, b, d their corresponding distributions of misorientation angle

Fig. 9 TEM microstructures of FSP CoCrFeNiCu alloy: a nano-grains, b nano-twins within grains, c detailed nano-twins, d SAD pattern of nano-twins

Table 2 Grain sizes of different HEAs after FSW/FSP

Materials	Grain size (μm)	References
Al_0.1CoCrFeNi	0.35	[30]
Al_0.3CoCrFeNi	2.66	[32]
Co₁₆Fe₂₈Ni₂₈Cr₂₈	1.29	[33]
Fe₅₀Mn₃₀Co₁₀Cr₁₀	6.5	[29]
Fe₄₂Mn₂₈Co₁₀Cr₁₅Si₅	1.3	[41]
CoCrFeNiMn	2	[42]
CoCrFeNiCu	0.15	This study

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