Synergy effect of multi-strengthening mechanisms in FeMnCoCrN HEA at cryogenic temperature

doi:10.1016/j.jmst.2020.12.079

Abstract

Abstract:

High-entropy alloys (HEAs) have attracted great research interest owing to their good combination of high strength and ductility at both room and cryogenic temperatures. However, expensive raw materials are always added to overcome the strength-ductility trade-off at low temperatures, leading to an increased production cost for the cryogenically used alloys. In this work, a series of nitrogen-doped FeMnCoCr HEAs have been processed by homogenization annealing, cold rolling and recrystallization annealing followed by water quenching. The microstructural evolution and mechanical properties of the alloys are studied systematically. The Fe₄₉Mn₃₀Co₁₀Cr₁₀N₁ alloy shows excellent mechanical properties at both 293 K and 77 K. Particularly, the yield and ultimate tensile strength of 1078 and 1630 MPa are achieved at the cryogenic temperature, respectively, while a satisfactory uniform elongation of 33.5 % is maintained. The ultrahigh yield strength results from the microstructure refinement caused by the activation of athermal martensitic transformation and mechanical twinning that occur in the elastic regime together with the increased lattice friction due to the cryogenic environment. In the plastic regime, the dynamic Hall-Petch effect caused by twinning, martensitic transformation, and reverse transformation together with the high barrier to dislocation motion jointly contribute to the ultrahigh tensile strength. Simultaneously, the transformation induced plasticity (TRIP) and the twinning induced plasticity (TWIP) effects jointly contribute to the ductility. The design strategy for attaining superior mechanical properties at low temperatures, i.e. by adjusting stacking fault energy in the interstitial metastable HEAs, guides the development of high-performance and low-cost alloys for cryogenic applications.

Key words: High-entropy alloy, Cryogenic deformation, Nitrogen doping, Phase transformation, Twinning

Zhufeng He, Nan Jia, Hongwei Wang, Haile Yan, Yongfeng Shen. Synergy effect of multi-strengthening mechanisms in FeMnCoCrN HEA at cryogenic temperature[J]. J. Mater. Sci. Technol., 2021, 86: 158-170.

Figures/Tables 10

Table 1 Comparison of crystal structure, average grain size, yield strength, ultimate tensile strength, uniform elongation and raw-materials cost of the studied FeMnCoCrN HEA and other cryogenic alloys under uniaxial tensile testing. The average grain size in the heterostructured alloys denotes that in the recrystallized region. Mechanical properties are obtained at both 293 and 77 K.

Alloys	Crystal structure	Average grain size (μm)	Yield strength at 293 K/77 K (MPa)	Ultimate tensile strength at 293 K/77 K (MPa)	Uniform elongation at 293 K/77 K (%)	Raw-materials cost (USD/kg)	Ref.
Fe₄₉Mn₃₀Co₁₀Cr₁₀N₁	fcc	5.8	463 / 1078	845 / 1630	53 / 33.5	5.3	This work
equiatomic FeCoMnCrNi	fcc	6	375 / 728	733 / 1250	46 / 71	11.9	[18]
equiatomic FeCoMnCrNi	fcc	0.65	798 / 1240	887 / 1460	26 / 41	11.9	[19]
FeCoNiCrTi_0.2	fcc	51	700 / 860	1240 / 1580	36 / 46	14.4	[20]
V₁₀Cr₁₅Mn₅Fe₃₅Co₁₀Ni₂₅	fcc	1.5 (heterostructured)	761 / 970	936 / 1314	28.3 / 36.3	42.3	[25]
equiatomic CoCrNi	fcc	16	362 / 530	860 / 1236	46 / 55	18.9	[26]
Fe₄₀Mn₄₀Co₁₀Cr₁₀	fcc	8	272 / 481	567 / 1003	47 / 65	5.3	[27]
(Fe₄₀Mn₄₀Co₁₀Cr₁₀)B	fcc	3.3	418 / 1098	850 / 1370	67 / 22	5.3	[28]
FeCoMnCrNiMo_0.2	fcc	35	376 / 637	767 / 1212	52.5 / 71.2	13.1	[50]
Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₃C₂	fcc	3.7	617 / 1057	1010 / 1978	39.8 / 53.3	10.2	[51]
Fe_22.5Co_23.5Mn₂₂Ni₂₄Cr₆C₂	fcc	150	320 / 605	685 / 1060	48 / 47	13	[52]
Fe₄₀Mn₂₀Cr₂₀Ni₂₀	fcc	3 (heterostructured)	1000/ 1185	- / 1320	- / 22	4.9	[53]
equiatomic FeCoCrNi	fcc	35	260 / 480	624 / 997	57 / 73	14.5	[54]
Al_0.3CoCrFeNi	fcc	50	220 / 515	620 / 1010	58.4 / 68	14.1	[55]
316 LN	fcc	-	284 / 683	642 / 1508	57 / 61	4.6	[56]
HfNbTaTiZr	bcc	60	984 / 1638	1125 / 1710	4 / 2	152	[22]
Al_0.6CoCrFeNi	fcc + bcc	0.6 (heterostructured)	786 / 964	1101 / 1422	28 / 22	14.1	[23]
V₁₀Cr₁₀Co₃₀Fe₅₀	fcc + bcc	2.3	409 / 520	1026 / 1990	20 / 33	45.5	[24]

Fig. 1. EBSD maps of the (a) N0, (b) N1.0, (c) N1.8 and (d) N2.6 alloys before uniaxial tensile loading: (a1-d1) phase distribution maps and (a2-d2) orientation maps showing grain orientation distributions relative to the rolling direction. In the phase distribution maps, high-angle boundaries with misorientation angles larger than 10° are indicated by thick black lines, low-angle boundaries with misorientation angles between 3° and 10° are indicated by thin black lines, and Σ3 twin boundaries in the fcc phase are indicated by white solid lines, respectively.

Fig. 2. (a) Engineering stress-strain curves, (b) true stress-strain curves, (c) strain hardening rates to true strain of the N0, N1.0, N1.8 and N2.6 alloys under uniaxial tensile loading at 293 K and 77 K. The inset of (c) shows the N1.0 specimen after tensile testing at 77 K. (d) XRD patterns of the alloys before and after tensile testing at different temperatures.

Table 2 Average grain sizes and mechanical properties of the studied alloys.

Alloys	Average grain size (μm)	Yield strength at 293 K/77 K (MPa)	Ultimate tensile strength at 293 K/77 K (MPa)	Uniform elongation at 293 K/77 K (%)
N₀	6.2	288/398	697/1323	68/53
N_1.0	5.8	463/1078	845/1630	53/33.5
N_1.8	5.3	517/1206	909/1620	49.5/11
N_2.6	4.5	596/-	977/-	47/-

Fig. 3. Microstructures of the specimens with 11 % reduction in the cross-section area of the (a) N0, (b) N1.0 and (c) N1.8 alloys deformed at 77 K. (d) and (e) Microstructures of the region close to the fracture surface of the N0 and N1.0 alloys deformed at 77 K, respectively. (a1-e1) Phase distribution maps, (a2-e2) orientation maps showing grain orientation distributions relative to the tensile axis. In the phase distribution maps, high-angle boundaries with misorientation angles larger than 10° are indicated by thick black lines, low-angle boundaries with misorientation angles between 3° and 10° are indicated by thin black lines, and Σ3 twin boundaries in the fcc phase are indicated by white solid lines, respectively.

Fig. 4. TEM micrographs of a region close to the fracture surface of the N1.0 alloy deformed at 293 K. (a) Dense deformation twins and the diffraction pattern taken from the selected area. (b) Multiple twin systems and the diffraction pattern for the selected area. (c) The magnified view of deformation twins. (c2, c3) Diffraction patterns and HRTEM image taken from the selected area in (c1), respectively.

Fig. 5. TEM micrographs of the N1.0 alloy with 11 % reduction in the cross-section area deformed at 77 K. (a) The refined microstructure consisting of ε-martensite laths and the diffraction pattern taken from the selected area. (b1) Dense ε-martensite laths indicated by green arrows and a small number of deformation twins indicated by red arrows, (b2, b3) diffraction patterns taken from the selected area in (b1), respectively.

Fig. 6. TEM micrographs of a region close to the fracture surface of the N1.0 alloy deformed at 77 K. (a) The refined microstructure consisting of dense ε-martensite laths and deformation twins and the diffraction patterns taken from the selected area. (b) The bright-field and the corresponding dark-field TEM micrographs taken from the red dotted rectangle in (a). (c) HRTEM image taken from the orange dotted rectangle in (a) along the [011]γ axis, showing the alternately distributed ε-martensite laths and deformation twins. GB: grain boundary.

Table 3 Material parameters used for calculating critical stresses of twin and ε-martensite formation.

b_twin (m)	L_twin (m)	b_tr (m)	L_tr (m)	G (GPa)	h (m)
1.2 × 10^-10	2.89 × 10^-7	1.47 × 10^-10	2.89 × 10^-7	76	5 × 10^-6

Fig. 7. (a) Various strength contributions to the flow stress of the N1.0 alloy during deformation at 77 K. (b) Schematic sketches showing the deformation mechanisms in the N-free and the N-doped FeMnCoCr HEAs under cryogenic loading. Note: Before deformation grains in the N-doped alloy are finer than those in the base alloy.

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