Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys

doi:10.1016/j.jmst.2020.10.038

Abstract

Abstract:

We present a new class of metastable high-entropy alloys (HEAs), triggering deformation-induced martensitic transformation (DIMT) from face-centered-cubic (FCC) to body-centered-cubic (BCC), i.e., BCC-DIMT. Through the ab-initio calculation based on 1^st order axial interaction model and combined with the Gibbs free energy calculation, the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy (ISFE) in Si_xV_(9-x)Cr₁₀Mn₅Fe₄₆Co₃₀ (x = 2, 4, and 7 at.%) alloy system. The ISFE decreases from -30.4 to -35.5 mJ/m² as the Si content increases from 2 to 7 at.%, which well corresponds to the reduced phase stability of FCC against HCP. The BCC-DIMT occurs in all the alloys via intermediate HCP martensite, and the HCP martensite provides nucleation sites of BCC martensite. Therefore, the transformation rate enhances as the Si content increases in an earlier deformation range. However, the BCC-DIMT is also affected by the phase stability of FCC against BCC, and the stability is the highest at the Si content of 7 at.%. Thus, the 7Si alloy presents the moderate transformation rate in the later deformation range. Due to the well-controlled transformation rate and consequent strain-hardening rate, the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room temperature. Our results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.

Key words: High-entropy alloy (HEA), Deformation-induced martensitic transformation (DIMT), Phase stability, Stacking fault energy (SFE)

Yong Hee Jo, Junha Yang, Won-Mi Choi, Kyung-Yeon Doh, Donghwa Lee, Hyoung Seop Kim, Byeong-Joo Lee, Seok Su Sohn, Sunghak Lee. Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys[J]. J. Mater. Sci. Technol., 2021, 76: 222-230.

Figures/Tables 12

Table 1 Chemical compositions of the as-annealed SixV(9-x)Cr10Mn5Fe46Co30 HEAs measured from the inductively coupled plasma-optical emission spectrometry (ICP-OES). (unit: at.%).

Alloy	Si	V	Cr	Mn	Fe	Co
2Si	1.92	6.71	10.46	5.31	45.84	29.76
4Si	3.80	4.90	10.13	4.92	46.15	30.10
7Si	7.14	1.93	10.29	4.90	46.10	29.64

Fig. 1. (a) Equilibrium phase diagram of SixV(9-x)Cr10Mn5Fe46Co30 system obtained from thermodynamic calculations in the 900-1800 K range and (b) variation in intrinsic SFE (ISFE) obtained from Ab-initio calculations at 0 K as a function of Si content.

Fig. 2. EBSD phase maps of the as-annealed (a) 2Si, (b) 4Si, and (c) 7Si alloys. Average grains size (Dave) is shown below each phase map. (d) SEM-energy dispersive spectroscopy (SEM-EDS) mappings of Si, V, Cr, Mn, Fe, and Co of the selected area in (c).

Fig. 3. TEM bright- and dark-field (BF and DF) images and selected area diffraction (SAD) patterns of the as-annealed (a) 2Si, (b) 4Si, and (c) 7Si alloys. Stacking faults and HCP phase are identified by DF images and SAD patterns, and are indicated by arrows.

Fig. 4. XRD profiles of the (a) as-annealed and (b) fully-tensile-deformed states of the 2Si, 4Si, and 7Si alloys.

Fig. 5. Room-temperature (a) engineering tensile stress-strain and (b) strain-hardening rate-true strain curves of the three alloys. The tensile strength improves in the increasing order of the 2Si, 4Si, and 7Si alloys.

Fig. 6. Volume fractions of (a) FCC, (b) HCP, and (c) BCC phases (VFCC, VHCP, and VBCC) measured from the XRD data as a function of true strain for the three alloys. The VFCC decreases gradually as the true strain increases, which indicates the DIMT occurs steadily as the deformation proceeds.

Fig. 7. EBSD phase maps superimposed by each image quality (IQ) map of the half-sectioned area of the tensile-strained specimens in the (a-c) earlier and (d-f) later deformation stages (true strain ranges of 5.5-8 and 17-23 %, respectively) for the three alloys.

Fig. 8. EBSD (a) phase, (b) inverse pole figure (IPF), and (c) IQ maps for the 7Si alloy deformed to a true strain of 5.5 %. The detailed orientation relationship (OR) between each phase is revealed by the pole figure analyses of (d) FCC, (e) HCP, and (f) BCC phases.

Table 2 Difference in Gibbs free energies between HCP and FCC (ΔGFCC→HCP) and BCC and FCC (ΔGFCC→BCC) at 298 K was calculated by using the Thermo-Calc software [[18], [19], [20], [21]] and actual alloy compositions (Table 1).

Alloy	ΔG^FCC→HCP (Jmol^-1)	ΔG^FCC→BCC (Jmol^-1)
2Si	1030.4	-4924.6
4Si	850.8	-4975.9
7Si	480.5	-4790.3

Table 3 Magnitudes of kinetics parameters (α and β) and adjusted R-squared (R2) determined from the transformation data by using Eq. (2).

Alloy	α	β	R²
2Si	5.54	2.85	0.999
4Si	7.66	3.91	0.997
7Si	10.24	2.15	0.982

Fig. 9. Comparison data of room-temperature tensile strength and elongation of the present SixV(9-x)Cr10Mn5Fe46Co30 HEAs with multi-component alloys whose characteristic deformations are classified into slip, TWIP, HCP-DIMT, and BCC-DIMT. The present SixV(9-x)Cr10Mn5Fe46Co30 alloys show the much higher tensile strength than the well-known FCC-single-phase CrMnFeCoNi alloy, while their elongation is similar. In particular, the 7Si alloy has the superior tensile strength beyond 1 GPa [2,4,[6], [7], [8],10,12,31,[47], [48], [49]].

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