Effect of Bimodal Grain Size Distribution on the Strain Hardening Behavior of a Medium-Entropy Alloy

doi:10.1007/s40195-020-01173-2

Abstract

Abstract:

The evolution of strain hardening behavior of the Fe₅₀(CoCrMnNi)₅₀ medium-entropy alloy as a function of the fraction of recrystallized microstructure and the grain size was studied using the Hollomon and Ludwigson equations. The specimens under study were partially recrystallized, fully recrystallized with ultrafine-grained microstructure, and fully recrystallized with coarse grains. The yield strength decreases steadily as the fraction of recrystallized microstructure and grain size increases due to the recovery process and the Hall-Petch effect. Interestingly, the bimodal grain distribution was found to have a significant impact on strain hardening during plastic deformation. For instance, the highest ultimate tensile strength was exhibited by a 0.97 μm specimen, which was observed to contain a bimodal grain distribution. Furthermore, using the Ludwigson equation, the effect of the bimodal grain distribution was established from the behavior of K₂ and n₁ curves. These curves tend to show very high values in the specimens with a bimodal grain distribution compared to those that show a homogenous grain distribution. Additionally, the bimodal grain distribution contributes to the extensive Lüders strain observed in the 0.97 μm specimen, which induces a significant deviation of the Hollomon equation at lower strains.

Key words: Metals and alloy, Mechanical properties, Microstructure, High-entropy alloys, Plastic deformation

Ibrahim Ondicho, Bernard Alunda, Fredrick Madaraka, Melody Chepkoech. Effect of Bimodal Grain Size Distribution on the Strain Hardening Behavior of a Medium-Entropy Alloy[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(4): 465-475.

Figures/Tables 8

Fig. 1 Backscatter electron (BSE) images showing the microstructure of the specimens annealed at 600 °C for a 5 min b 7 min c 10 min, d 30 min, e 1 h, f 24 h, 800 °C for g 4 h, and 1100 °C for h 30 min. The white arrows in a-c represent the non-recrystallized microstructure

Fig. 2 EBSD images of specimens annealed for a 5 min, b 7 min, c 10 min, d 30 min, e 1 h, and f 24 h. The white arrows in a-d show the non-recrystallized part of the microstructure

Fig. 3 a Engineering stress-strain curves of all the annealing conditions, b Hall-Petch relationship of fully recrystallized specimens, c true stress-strain curves d strain hardening rate vs. the true strain of both partially recrystallized and fully recrystallized specimens

Table 1 A summary of the grain sizes, yield strength, ultimate tensile strength, and total elongation of both partially and fully recrystallized specimens. Partially recrystallized specimens are denoted as PR, and the fully recrystallized specimens are denoted as (FR)

Heat treatment condition	Grain size (μm)	Yield strength (MPa)	Ultimate tensile strength (MPa)	Total elongation (%)
600°C_5 min	PR (0.42	1020	1180	17
600°C_7 min	PR (0.45)	920	1070	20
600°C_10 min	PR(0.49)	640	915	26
600°C_30 min	PR (0.50)	580	940	31
600°C_1 h	0.97	550	990	41
600°C_ 24 h	1.83	440	960	45
800°C_ 4 h	7.40	220	830	49
1100°C_30 min	49.30	170	800	54

Fig. 4 a Stress-strain curve of both partially recrystallized and fully recrystallized specimens with a Hollomon fit (dotted Cyan color). b Strain hardening exponent, n. c Strength coefficient, K versus grain size curves

Table 2 A summary of the grain sizes and strength coefficient and strain hardening exponent values from the Hollomon equation

Grain size (μm)	Recrystallized fraction (%)	K	n
PR (0.42)	74	1274	0.04
PR (0.45)	83	1237	0.07
PR (0.49)	87	1183	0.17
PR(0.50)	96	1277	0.23
0.97	100	1358	0.31
1.83	100	1337	0.36
7.40	100	1217	0.44
49.30	100	1127	0.47

Fig. 5 a Stress-strain curve of both partially recrystallized and fully recrystallized specimens with a Ludwigson fit (dotted line with dark cyan color). b Strain hardening exponents n1 and n2. c Strength coefficients (K1, K2) vs. grain size curves

Table 3 A summary of the grain sizes and parameters from the fitting of the true stress-strain curves using the Ludwigson equation

Grain size (μm)	K₁	n₁	K₂	${e}^{{K}_{2}}$	n₂
PR (0.42)	1268	0.04	5.5	244.69	-156
PR (0.45)	1237	0.07	5.28	196.37	-103
PR (0.49)	1208	0.18	5.32	204.38	-54
PR(0.50)	1299	0.25	5.5	244.69	-44
0.97	1206	0.82	6.23	507.76	-0.09
1.83	1227	0.69	5.86	350.72	-0.22
7.40	1176	0.56	4.87	130.32	-0.36
49.30	1098	0.6	4.74	114.43	-0.3

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