In situ neutron diffraction unravels deformation mechanisms of a strong and ductile FeCrNi medium entropy alloy

doi:10.1016/j.jmst.2021.10.034

Abstract

Abstract:

We investigated the mechanical and microstructural responses of a high-strength equal-molar medium entropy FeCrNi alloy at 293 and 15 K by in situ neutron diffraction testing. At 293 K, the alloy had a very high yield strength of 651 ± 12 MPa, with a total elongation of 48% ± 5%. At 15 K, the yield strength increased to 1092 ± 22 MPa, but the total elongation dropped to 18% ± 1%. Via analyzing the neutron diffraction data, we determined the lattice strain evolution, single-crystal elastic constants, stacking fault probability, and estimated stacking fault energy of the alloy at both temperatures, which are the critical parameters to feed into and compare against our first-principles calculations and dislocation-based slip system modeling. The density functional theory calculations show that the alloy tends to form short-range order at room temperatures. However, atom probe tomography and atomic-resolution transmission electron microscopy did not clearly identify the short-range order. Additionally, at 293 K, experimental measured single-crystal elastic constants did not agree with those determined by first-principles calculations with short-range order but agreed well with the values from the calculation with the disordered configuration at 2000 K. This suggests that the alloy is at a metastable state resulted from the fabrication methods. In view of the high yield strength of the alloy, we calculated the strengthening contribution to the yield strength from grain boundaries, dislocations, and lattice distortion. The lattice distortion contribution was based on the Varenne-Luque-Curtine strengthening theory for multi-component alloys, which was found to be 316 MPa at 293 K and increased to 629 MPa at 15 K, making a significant contribution to the high yield strength. Regarding plastic deformation, dislocation movement and multiplication were found to be the dominant hardening mechanism at both temperatures, whereas twinning and phase transformation were not prevalent. This is mainly due to the high stacking fault energy of the alloy as estimated to be 63 mJ m^-2 at 293 K and 47 mJ m^-2 at 15 K. This work highlights the significance of lattice distortion and dislocations played in this alloy, providing insights into the design of new multi-component alloys with superb mechanical performance for cryogenic applications.

Key words: Medium entropy alloy, Multi-component alloy, Cryogenic temperature, Neutron diffraction

L. Tang, F.Q. Jiang, J.S. Wróbel, B. Liu, S. Kabra, R.X. Duan, J.H. Luan, Z.B. Jiao, M.M. Attallah, D. Nguyen-Manh, B. Cai. In situ neutron diffraction unravels deformation mechanisms of a strong and ductile FeCrNi medium entropy alloy[J]. J. Mater. Sci. Technol., 2022, 116: 103-120.

Figures/Tables 17

Fig. 1. Monte Carlo simulations of FCC supercell configurations for equiatomic FeCrNi alloys with different sizes and temperatures using DFT based CE Hamiltonian: (a) 9 × 9 × 9, 300 K; (b) 9 × 9 × 9, 2000 K; (c) 3 × 3 × 3, 300 K; (d) 3 × 3 × 3, 2000 K. The color code for Fe, Cr, and Ni is orange, blue and grey, respectively.

Fig. 2. Microstructure characterization of the virgin FeCrNi sample: (a) EBSD map (inverse pole figure) perpendicular to loading direction (LD); (b) band contrast map overlapped with boundaries map of (a), the red, black, and green lines in the boundary map represent the low-angle grain boundaries (LAGBs), high-angle grain boundaries (HAGBs), and annealing twin boundaries (TBs, Σ3 {111}), respectively; (c) SEM-BSE images and corresponding elemental mapping of Fe, Cr, and Ni obtained from EDS; (d) typical bright TEM image and SAED pattern taken along [110] zone axes; (e) three-dimension APT tip reconstructions of Fe, Cr, and Ni atoms; (f) frequency distribution analysis of the three constituent elements and corresponding parameters used for qualifying the fit; (g) and (h) HRSTEM images and the corresponding FFT images of the alloy taken from [110] and [112] zone axis, respectively.

Fig. 3. (a) Engineering and true stress-strain curves of the alloy deformed at 293 and 15 K; (b) yield strength-uniform elongation comparison among the current FeCrNi MEA, several high-strength steels, and single or multi-phase MEAs/HEAs [[48], [49], [50], [51], [52], [53], [54], [55]].

Table 1. Uniaxial tensile properties of different alloys at 293 K and cryogenic temperature.

Alloys	Temperature (K)	YS (MPa)	UTS (MPa)	Elongation (%)
FeCrNi	293	651 ± 12	1020 ± 3	48 ± 5
FeCrNi	15	1092 ± 22	1451 ± 2	18 ± 1
FeCoCrNiMo_0.2 [11]	15	710	1423	42
CrMnFeCoNi [12]	15	740	2500	62
CrCoNi [56]	15	670	2292	60
316LN stainless steel [57]	4.2	1045	1528	33

Fig. 4. In situ neutron diffraction spectra collected during deforming at (a) 293 K and (b) 15 K.

Fig. 5. Lattice strain evolution of crystallographic planes of {111}, {200}, {220}, {311}, and {222} from axial and radial directions during deforming at (a) 293 K and (b) 15 K.

Table 2. Experimentally measured elastic properties of the FeCrNi alloy at different temperatures.

Temp. (K)	E (GPa)	E₁₁₁ (GPa)	E₂₀₀ (GPa)	E₂₂₀ (GPa)	E₃₁₁ (GPa)	v₁₁₁	v₂₀₀	v₂₂₀	v₃₁₁	v	G (GPa)
293	200.5	248.1	148.6	220.7	185.9	0.256	0.339	0.314	0.335	0.292	77.6
15	237.2	260.4	162.3	225.7	197.2	0.233	0.320	0.282	0.302	0.281	92.6

Fig. 6. The reciprocal diffraction elastic moduli (1/Ehkl and vhkl/Ehkl) plotted as a function of the elastic anisotropy factor, Ahkl, which were obtained from the experimental measured (Mea.) elastic lattice strain and fitted by Kroner, Voigt, and Reuss models at (a) 293 K and (b) 15 K, respectively.

Table 3. Elastic constants and moduli predicted by DFT calculations and compared with experimental data (More detailed comparison can be found in Table S2 in the Supplementary Materials).

FeCrNi	Symmetry	K-points	Empty Cell	Elastic constant (GPa)										Young's modulus (GPa)	Poisson's ratio	Polycrystalline moduli (GPa)
FeCrNi	Symmetry	K-points	Empty Cell	C₁₁	C₁₂	C₁₃	C₂₂	C₂₃	C₃₃	C₄₄	C₅₅	C₆₆	C’(average)	E_av	P_av	B_(VRH)	G_(VRH)	E_(VRH)
DFT, 2000 K	P1	4 × 4 × 4	Relaxed	210.79	141.19	136.06	200.47	127.72	206.05	123.42	122.36	122.19	35.4	180.83	0.3093	158.33	74.57	192.64
DFT, 2000 K	P1	4 × 4 × 4	Unrelaxed	222.45	134.8	142.6	213.16	132.09	218.31	125.36	138.84	131.67	40.7	199.81	0.2962	163.53	82.45	211.12
DFT, 300 K	Pnnm	4 × 4 × 4	Relaxed	239.41	117.01	136.48	231.37	109.34	257.46	163.14	143.22	141.19	60.9	248.02	0.2429	161.09	103.62	255.64
DFT, 300 K	Pnnm	6 × 6 × 6	Relaxed	237.44	116.4	137.24	229.99	108.92	259.19	163.46	142.18	140.99	60.7	247.12	0.2433	160.77	103.27	254.82
In situ Exp. 293 K				225	151	-	-	-	-	123	-	-	37.0	187.95	0.3216	175.65	76.17	199.06
In situ, Exp. 15 K				220	134	-	-	-	-	127	-	-	43.0	201.8	0.2932	162.67	82.35	210.9

Fig. 7. Elastic properties of the FeNiCr alloy at 293 K determined via in situ neutron diffraction measurements (a-h) and DFT calculations (i-l). The magnitude concerning the directions in three dimensions for (a) and (i) Young's modulus, (b) and (j) shear modulus, (c) and (k) Poisson's ratio, and (d) and (l) linear compressibility. The maximum and minimum values of shear modulus and Poisson's ratio are represented by two surfaces. The magnitude with directions of XY, XZ, and YZ plane for (e) Young's modulus, (f) shear modulus, (g) linear compressibility, and (h) Poisson's ratio. The maximum and minimum values of shear modulus and Poisson's ratio are represented by dashed and solid lines, respectively.

Fig. 8. Microstructure evolution during straining at 293 K and 15 K: (a) comparison of stacking fault probability (SFP) evolution between the FeNiCr alloy and FeCoCrNiMo0.2 alloy at different temperatures; (b) dislocation density evolution of the FeNiCr alloy during deformation; the normalized peak intensity evolution of {111}, {200}, {220}, {311}, and {222} of the FeNiCr alloy during deforming at (c) 293 K and (d) 15 K.

Fig. 9. Deformed microstructure of the alloy at different strain levels and different temperatures, 293 K and 15 K: (a) typical bright-field TEM image (at the strain of ～0.1, 293 K) and corresponding SAED pattern of the area marked by the yellow dashed circle; (b) and (c) bright-field TEM images and corresponding SAED patterns at the strain of ȣC0.1 and near the fracture surface at 293 K, respectively; (d) bright-field TEM image and corresponding SAED pattern at the strain of ～0.1, 15 K; (e) and (f) HAADF-STEM images of the deformed structure at the strain of ～0.1 and near the fracture surface at 15 K, respectively; (g) HRTEM image of the dislocation tangling zone in (e); (h) the geometric phase analysis (GPA) conducted on (g).

Table 4. Multiple phase formation parameters, δ, ΔHmix, ΔSmix, and Ω of the FeCrNi alloy.

Alloy	ΔS_mix (J K^-1 mol^-1)	ΔH_mix (KJ mol^-1)	T_m (K)	Ω	δ	VEC
FeCrNi	9.132	-4.3972	1906.6	3.9596	0.002	7.97

Fig. 10. Dependence of SRO parameters as a function of temperature between the three different pairs in equiatomic FeCrNi alloys.

Table 5. The modeled and measured strength of the alloy at different temperatures.

Temperature (K)	σ_dis (MPa)	σ_gb (MPa)	σ_fr (MPa)	Modeled σ_YS(MPa)	Measured σ_YS (MPa)
293	164	200	316	680	651 ± 12
15	196	218	629	1043	1092 ± 22

Fig. 11. Schematic illustration of the deformation procedure at 293 and 15 K.

Fig. 12. Comparison between the calculated strength originating from different strengthening resources and the measured total flow stress at (a) 293 K and (b) 15 K.

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