Hierarchical grain size and nanotwin gradient microstructure for improved mechanical properties of a non-equiatomic CoCrFeMnNi high-entropy alloy

doi:10.1016/j.jmst.2021.02.059

Abstract

Abstract:

This study explored a multi-mechanism approach to improving the mechanical properties of a CoCrFeMnNi high-entropy alloy through non-equiatomic alloy design and processing. The alloy design ensures a single-phase face-centered cubic structure while lowering the stacking fault energy to encourage the formation of deformation twins and stacking faults by altering the equiatomic composition of the alloy. The processing strategy applied helped create a hierarchical grain size gradient microstructure with a high nanotwins population. This was achieved by means of rotationally accelerated shot peening (RASP). The non-equiatomic CoCrFeMnNi high-entropy alloy achieved a yield strength of 750 MPa, a tensile strength of 1050 MPa, and tensile uniform elongation of 27.5%. The toughness of the alloy was 2.53 × 10¹⁰ kJ/m³, which is about 2 times that of the same alloy without the RASP treatment. The strength increase is attributed to the effects of grain boundary strengthening, dislocation strengthening, twin strengthening, and hetero-deformation strengthening associated with the heterogeneous microstructure of the alloy. The concurrent occurrence of the multiple deformation mechanisms, i.e., dislocation deformation, twining deformation and microband deformation, contributes to achieving a suitable strain hardening of the alloy that helps to prevent early necking and to assure steady plastic deformation for high toughness.

Key words: High entropy alloy, Gradient and hierarchical structure, Mechanical property, Deformation mechanism

Zibing An, Shengcheng Mao, Yinong Liu, Hao Zhou, Yadi Zhai, Zhiyong Tian, Cuixiu Liu, Ze Zhang, Xiaodong Han. Hierarchical grain size and nanotwin gradient microstructure for improved mechanical properties of a non-equiatomic CoCrFeMnNi high-entropy alloy[J]. J. Mater. Sci. Technol., 2021, 92: 195-207.

Figures/Tables 15

Table 1 Atomic radius (R) and valence electron concentration (VEC) of the constituent elements of CoCrFeMnNi alloys.

Element	Co	Cr	Fe	Mn	Ni
R (pm)	125	125	125	124	125
VEC	9	6	8	7	10

Table 2 Compositions (at.%) of the CoCrFeMnNi alloy.

Sample	Co (at.%)	Cr (at.%)	Fe (at.%)	Mn (at.%)	Ni (at.%)
A-HEA	22.9	21.9	22.1	19.3	13.8

Fig. 1. Microstructures of the Co21.5Cr21.5Fe21.5Mn21.5Ni14 alloy under three different conditions. (a) XRD patterns of the —C-HEA, A-HEA and R-HEA samples. (b) Enlarged views of the (111) diffraction peaks of the three samples. (c) Tri-color EBSD grain orientation map of the —C-HEA. (d) Tri-color EBSD grain orientation map of the A-HEA. (e) The distribution of grain misorientation angles corresponding to the area of (d). The inset shows a magnified view of some twin boundaries (indicated by the arrows). (f) Grain size distributions of —C-HEA and A-HEA.

Fig. 2. Microstructural analysis of the —C-HEA and A-HEA samples. (a) A bright-field TEM image of the —C-HEA sample. The inset is a selected area diffraction pattern, indexed to an FCC single crystal in the [110] zone axis. (b) A bright-field TEM image of the A-HEA sample. The inset is a selected area diffraction pattern showing the twinned structure. (c) A HAADF-STEM image and the elemental distribution maps of the A-HEA sample.

Fig. 3. Mechanical properties of the Co21.5Cr21.5Fe21.5Mn21.5Ni14 HEA after different thermomechanical treatments. (a) Schematic of the RASP treatment. (b) Nano-hardness distribution profiles though the thickness for the as-cast sample, the cold worked and annealed sample, and the RASP treated sample. (c) True tensile stress-strain curves of the three samples. (d) Comparison of the relative changes of strength and ductility for “Cantor” HEAs strengthened by different mechanisms.

Table 3 Mechanical properties of the three HEAs.

Sample	σ_y (MPa)	σ_UTS (MPa)	δ_E (%)	K (J/m³)
C-HEA	200	560	34.0	1.28×10¹⁰
—A-HEA	300	765	33.2	1.91×10¹⁰
R-HEA	750	1050	27.5	2.53×10¹⁰

Fig. 4. Gradient microstructure of the RASP treated Co21.5Cr21.5Fe21.5Mn21.5Ni14 plate. (a) A schematic of the grain size gradient through the plate thickness. (b)-(d) EBSD micrographs of the alloy at different depths from the surface. (e)-(g) TEM micrographs of twin structures in regions (I), (II) and (III). The insets in (e)-(g) are selected area electron diffraction patterns from each region. (h) A high-resolution HAADF-STEM image of the deformation twins in region (I). (i) Variation of the average grain size through the plate thickness. (j) Variation of the average thickness of deformation twins through the plate thickness.

Fig. 5. TEM analysis of the micro-mechanisms of deformation of R-HEA. The sample was pre-deformed in tension to 7.5% of strain. (a) and (b) TEM bright-field images of region (I). The region contained annealing twins and deformation twins. (c) A HRTEM image showing details of twin boundaries and stacking faults. (d) A HRTEM image revealing several dislocations, twin boundaries and stacking faults. (e) A HAADF-STEM image identifying the Burgers vector of a $\frac{1}{2}\left[ 0\bar{1}1 \right]$ full dislocation.

Fig. 6. TEM analysis of the micro-mechanisms of deformation of R-HEA in region (III) after 7.5% tensile deformation. (a) Bright field image showing annealing twins and massive networks of dislocations. (b) A SAED pattern in [011] zone axis showing the twin structure.

Fig. 7. TEM analysis of micro-mechanisms of plastic deformation the R-HEA at the tensile strain of 20%. (a) Dark field image revealing deformation twins in region (I). (b) A [011] SAED pattern of the twin structure in (a). (c) Dark field image showing microbands in region (I). (d) Dark field image showing deformation twins in region (III). (e) A [011] SAED pattern of the twin structure in (d). (f) Bright field image revealing massive dislocations in cell structure in region (III).

Fig. 8. TEM micrographs of the —C-HEA at different tensile strains. (a) TEM image of the alloy after 7.5% tensile deformation. (b) TEM image of the alloy after 25% tensile deformation. (c) TEM image of the alloy after 30% deformation, revealing a high density and networked stacking faults. (d) HRTEM image of stacking faults and partial dislocations.

Fig. 9. Schematic illustrations of the deformation mechanisms in —C-HEA and R-HEA. (a) Microstructure of the —C-HEA. (b)-(d) Microstructures of the —C-HEA at different deformation levels, expressing the interactions among grain boundaries, deformation twins, stacking fault networks and dislocations. The long and parallel lines represent deformation twins. The grids in (d) represent a stacking fault network. “T” represents dislocations. (e) Microstructure of the A-HEA with annealing twins (yellow regions). (f)-(h) Microstructure of the R-HEA in region (Ⅰ) at different levels of deformation, including annealing twins (yellow regions), deformation twins (pink lines), stacking faults (red lines) and geometrically necessary dislocations. (i)-(k) Microstructure of the R-HEA in region (Ⅲ) at different deformation levels, including annealing twins, dislocations and deformation twins.

Fig. 10. Schematic illustrations of the strengthening mechanisms in R-HEA. (a) Lattice friction strength. (b) Solid solution strengthening. (c) The grain size strengthening. (d) The twin strengthening. (e) Dislocation strengthening. (f) The hetero-deformation induced strengthening.

Fig. 11. Effect of grain size on tensile strength of Co21.5Cr21.5Fe21.5Mn21.5Ni14 alloy. (a)-(c) EBSD IPF maps of the alloy annealed at three different temperatures, as indicated in the figures. (d)-(f) Grain size distributions in the three samples corresponding to (a)-(c). (g) Engineering stress-strain curves of the three samples with different average grain sizes. (c) Dependence of the yield strength on the average grain size.

Fig. 12. Contributions of different strengthening mechanisms to thew yield strength of the RASP treated Co21.5Cr21.5Fe21.5Mn21.5Ni14 alloy.

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