Grain size and temperature effect on the tensile behavior and deformation mechanisms of non-equiatomic Fe41Mn25Ni24Co8Cr2 high entropy alloy

doi:10.1016/j.jmst.2019.09.034

Abstract

Abstract:

The effect of the grain size on the tensile properties and deformation mechanisms of a nonequiatomic Fe₄₁Mn₂₅Ni₂₄Co₈Cr₂ high-entropy alloy was studied in the temperature range between 298 and 1173 K by preparing the samples with three different grain sizes through severe plastic deformation and subsequent annealing: ultrafine (sub)grain size (≤0.5 μm), 8.1 μm and 590.2 μm. In the temperature between 298 and 773 K, the material with the large grain size of 590.2 μm exhibited the largest tensile ductility (57 %-82 %) due to its high strain hardening associated with mechanical twinning, but it exhibited the lowest strength due to its large grain size. The material with the ultrafine (sub)grain size exhibited the lowest tensile ductility (3 %-7 %) due to a greatly reduced strain hardening ability after severe plastic deformation, but it exhibited the highest strength due to the dislocation strengthening and grain refinement strengthening. At tensile testing at temperatures above 973 K, recrystallization occurred in the material with the ultrafine (sub)grains during the sample heating and holding stage, leading to the formation of fine and equiaxed grains with the sizes of 6.8-13.5 μm. The deformation behavior of the Fe₄₁Mn₂₅Ni₂₄Co₈Cr₂ with different grain sizes in the high temperature range between 973 and 1173 K, where pseudosteady-state flow was attained in the stress-strain curves, could be explained by considering the simultaneous contribution of grain boundary sliding and dislocation-climb creep to total plastic flow. The activation energies for plastic flow for the materials with different grain sizes were similar as ～199 kJ/mol. In predicting the deformation mechanism, it was important to consider the change in grain size by rapid grain growth or recrystallization during the sample heating and holding stage because grain boundary sliding is a grain-size-dependent deformation mechanism. The sample with the ultrafine (sub)grains exhibited the large tensile elongations of 30 %-85 % due to its high strain rate sensitivity, m (0.1-0.5) at temperatures of 973-1173 K. The material with the large grain size of 590.2 μm exhibited the very small elongations of 0.2 %-8 % due to its small m values (0.1-0.2) and occurrence of brittle intergranular fracture at the early stage of plastic deformation.

Key words: High entropy alloys, Grain size, High temperature deformation mechanism, Tensile behavior, Grain boundary sliding

H.T. Jeong, W.J. Kim. Grain size and temperature effect on the tensile behavior and deformation mechanisms of non-equiatomic Fe₄₁Mn₂₅Ni₂₄Co₈Cr₂ high entropy alloy[J]. J. Mater. Sci. Technol., 2020, 42: 190-202.

Figures/Tables 17

Fig. 1. (a) TEM micrograph of the as-HRDSRed sample. The EBSD inverse pole figure (IPF) maps of (b) ANN973 and (c) ANN1473.

Fig. 2. True stress-true strain curves of the (a) ANN1473, (b) ANN973 and (c) as-HRDSRed samples tested in the temperature range of 298-1173 K at a given strain rate of 10-3 s-1.

Fig. 3. Plots for (a) YS, (b) UTS, and (c) UTS-YS and (d) the tensile elongation as a function of temperature.

Fig. 4. Plots of true stress-true strain curves of the (a) ANN1473, (b) ANN973 and (c) the as-HRDSRed samples at 973, 1073 and 1173 K at various strain rates.

Fig. 5. Plots of log strain rate vs. log σ/G for (a) the ANN1473, (b) ANN973 and (c) as-HRDSRed samples. (d) The shear modulus G measured as a function of temperature. The solid lines in (a)-(c) represent the curve fitting by Eq. (11). The dashed lines in (a)-(c) represent the exponential curve fitting [18] used for the measurement of m values.

Fig. 6. (a) Plot of m measured from the curves for the as-HRDSRed, ANN973 and ANN1473 samples in Fig. 5(a)-(c). (b) The tensile elongations of the as-HRDSRed, ANN973 and ANN1473 samples as a function of strain rate at different temperatures.

Fig. 7. Plots of log $\dot{ε}$ - log sinh?(α$\frac{σ}{G}$) and log sinh?(α$\frac{σ}{G}$) - 1/T for the (a, b) ANN1473, (c, d) ANN973 and (e, f) as-HRDSRed samples.

Fig. 8. (a) Plots of log Z vs. log sinh?(ασ/G), where the α and Qc values determined for each material are used (b) Plots of log Z vs. log sinh?(α$\frac{σ}{G}$), where averages of the α and Qc values of the three materials are used. (c) Plots of log Z vs. log σ/G, where the avearge of the Qc values of the three materials is used. (d) Plots of m values measured from the fitting curves shown in (c), presented as a function of Z.

Fig. 9. Processing maps for the (a) ANN1473, (b) ANN973 and (c) as-HRDSRed samples.

Fig. 10. EBSD IPF maps of (a) the as-HRDSRed samples heated and held (for 20 min) at (a) 973, (b) 1073 and (c) 1173 K and (d) the ANN973 sample heated and held (for 20 min) at 1173 K.

Table 1 Grain sizes measured by EBSD after sample heating and holding (for 20 min) at various temperatures.

T(K)	Grain size (μm)	Grain size (μm) at 973K	Grain size (μm) at 1073K	Grain size (μm) at 1173K
As-HRDSRed	<0.5	6.8±0.53	8.6±0.36	13.5±1.80
ANN973	8.1±0.52	8.5±0.35	9.7±1.06	14.6±1.57
ANN1473	590.2±9.3		-	-

Fig. 11. IPF and GB maps (on the gauge regions of the tensile specimens) of the (a) ANN1473, (b) ANN973 and (c) as-HRDSRed samples after tensile elongation tests at 1173 K - 10-3 s-1. The IPF and GB maps of (d) the as-HRDSRed sample after the tensile elongation test at 1073 K - 10-3 s-1.

Fig. 12. XRD spectra for the ANN973 sample measured on the grip and gauge regions after the tensile elongation tests at different temperatures at a given strain rate of 10-3 s-1.

Fig. 13. Fractured surfaces of the tensile samples of the (a) ANN1473 (b) ANN973 and (c) as-HRDSRed samples tested at RT, 673, 973 and 1173 K at a given strain rate of 10-3 s-1.

Fig. 14. Plot of work hardening rates as a function of strain for ANN973 and ANN1473 at temperatures below 773 K.

Fig. 15. Calculated twinning stress as a function of grain size (solid curve) and the data of the maximum flow stresses upon necking at different temperatures below 773 K.

Fig. 16. Curve fitting with Eq. (11) to the data of other investigators’ data [11,14].

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Grain size and temperature effect on the tensile behavior and deformation mechanisms of non-equiatomic Fe₄₁Mn₂₅Ni₂₄Co₈Cr₂ high entropy alloy

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