Novel Co-rich high entropy alloys with superior tensile properties

ABSTRACT We developed a series of Co-rich CoxCr25(FeNi)75−x (x = 35, 45, 55, 65) high entropy alloys with improved strength and/or ductility, derived from lowering the stacking fault energy (SFE) and reducing the fcc phase stability of the equiatomic CoCrFeNi alloy. Thermodynamics and ab initio calculations demonstrated that increasing Co while decreasing Fe and Ni concentrations lower the SFE and reduce the fcc phase stability. The Co35Cr25Fe20Ni20 and Co45Cr25Fe15Ni15 alloys with single fcc phase, exhibit superior tensile properties, contributing to the twinning and fcc → hcp martensitic transformation. The present study offers a guideline for designing high-performance high entropy alloys. GRAPHICAL ABSTRACT IMPACT STATEMENT A series of novel Co-rich non-equiatomic high entropy alloys with enhanced tensile properties were developed by lowering the stacking fault energy and reducing the phase stability of equiatomic CoCrFeNi alloy.


Introduction
High entropy alloys (HEAs) comprise four or more multiple principle elements have attracted extensive academic interests, which was firstly proposed in the year 2004 [1,2]. Thereinto, the equiatomic FeMnCoCrNi alloy is one of the most thoroughly studied HEAs because of the exceptional fcc phase stability, good combinations of strength and ductility, and superior fracture resistance at cryogenic temperatures [2,3]. The interesting properties are considered as an outcome of heavy lattice distortions and sluggish diffusion by mixing the multiple principle elements [1,[4][5][6]. Dislocation slip is the sole plastic deformation mode of the alloy at room temperature, but deformation twinning occurs at cryogenic temperatures overcome the strength-ductility trade-off, owing to the assistance of deformation twinning and strain-induced martensitic transformation (SIMT), respectively. The low SFE of metastable fcc phase of the alloys gained from tuning the chemical composition and concentration, enables the formation of Shockley partial dislocations, which is a crucial requisite for the activation of twinning and SIMT when the applied stress exceeds a threshold value.
On the other hand, Co-based superalloys have been widely used in manufacturing vanes, gas turbines, and metallic orthopedic implants, owing to the superior mechanical properties and corrosion resistance [11][12][13]. The alloys generally exhibit two kinds of crystal structures including γ -fcc and -hcp, where the γ phase is thermodynamically stable at elevated temperatures whereas the phase is stable at ambient temperature [14]. Generally, the SFE of these alloys is extremely low, reaching negative values at room temperature for some compositions [15]. As a result, the γ → SIMT proceeds during plastic straining of the metastable γ phase, which partially contributes to the exceptional mechanical properties [16,17]. Furthermore, elements of Fe and Ni are γ phase stabilizers for the alloy while Cr is the phase stabilizer (i.e. alloying with Fe and Ni can stabilize the γ phase and increase the SFE, whereas alloying with Cr will stabilize the phase and lower the SFE of the alloys) [18]. In the present study, we aimed to design novel Co-rich HEAs with manipulated mechanical properties and deformation behaviors by regulating the SFE and phase stability, achieved by adjusting the elemental concentrations of the equiatomic CoCr-FeNi alloy based on the thermodynamics and ab initio calculations.

Methodology
We designed quaternary HEAs described as Co x Cr 25 (FeNi) 75−x (x = 25, 35, 45, 55, 65, hereinafter denoted by Co 25 , Co 35 , Co 45 , Co 55 , and Co 65 alloys, respectively). The Gibbs free energy difference between hcp phase and fcc phase ( G fcc→hcp , which is defined as G fcc→hcp = G hcp − G fcc ) of the various alloys, were calculated using Thermo-Calc software with the TCFE2000 thermodynamic database and its upgraded version [19,20]. The generalized stacking fault energy (GSFE) of the alloys were calculated by ab initio calculations based on density functional theory (DFT) method [21], and the Kohn-Sham equations were solved using the exact muffin-tin orbitals method (EMTO) [22,23].
Ingots of the alloys were produced by arc melting in a water-cooled copper hearth, followed by homogenization at 1200°C for 5 h in an Ar atmosphere to ensure the chemical homogeneity. Then, the alloys hot forged at 1200°C to a 50% reduction in thickness, and finally room temperature rolled to 40% reduction in thickness again with subsequent annealing at 1000°C for 6 min. After that, dogbone-shaped tensile specimens with a gauge geometry of 10 mm × 2 mm × 1 mm were sliced by electrical discharge machining. The tensile specimens were grinded and mirror-finished. Then, uniaxial tensile tests were conducted at 25°C with a strain rate of 1.5 × 10 −4 s −1 . The microstructures of the specimens were observed by scanning electron microscope (SEM) equipped with an electron backscatter diffraction detector (EBSD), using an acceleration voltage of 15 kV. The phase identification was carried out by X-ray diffraction (XRD) with Co Kα radiation. The fractured samples were characterized by scanning transmission electron microscope (STEM) with operation voltage of 200 kV, and the thin-foil specimens for STEM observation were prepared by ion milling.

Results and discussion
The Gibbs free energy difference curves of G fcc−hcp in Figure 1(a) illustrates that an increase in Co content leads to a decrease in fcc phase (γ phase) stability in the Co x Cr 25 (FeNi) 75−x alloys. Moreover, γ phase is more stable at elevated temperatures whereas the hcp phase ( phase) tends to be more stable at lower temperatures, which is in accordance with traditional Co-based alloys. The curves of GSFE of the alloys at 0 K in Figure 1 show the initiation of one whole stacking fault by shifting along 1/6 < 112 > γ at close-packed {111} γ planes. The unstable SFE of γ usf , which represents the lowest energy barrier for dislocation nucleation [24], increases gradually from 343.2 to 412.5 mJ/m 2 accompanied by the increase of Co content. It indicates that the Corich alloys will exhibit higher resistance for dislocation nucleation (i.e. higher yield strength than the Co-lean alloys). On the other hand, the values of intrinsic and extrinsic SFEs marked by γ isf and γ esf are compared in Figure 1(c). It indicates that both of the γ isf and γ esf are lowered by increasing Co content, and the values of γ isf are smaller than γ esf . It has been found that the Fe and Ni stabilize the γ phase in Co-based superalloys. In addition, independent reduction of Ni concentration or an increase in the Co concentration does significantly lower the SFE [18]. Therefore, the coordination lowers the SFE and reduces the γ phase stability, which will affect the deformation behaviors.  increase of Co content. Because of the tiny difference in grain sizes of the Co 25∼45 samples, the effect of grain size on the tensile properties of those samples can be neglected. On the other hand, the corresponding phase maps in Figure 2(f-j) show the phase composition of the samples. The Co 25∼45 samples are single γ phase containing some annealing twin bands, while the Co 55∼65 samples are γ + dual phase with lath morphologies. The amount of phase in the Co 55 and Co 65 samples are 62.8 and 91.3%, respectively. The phase composition would affect the tensile properties dramatically, because the γ phase is generally soft and ductile but the phase is hard and brittle.
The engineering stress-strain curves in Figure 3(a) and the strengths and elongations in Figure 3(b) demonstrate the tensile properties of the alloys at room temperature. The Co 25∼45 samples comprising single γ phase show superior fracture elongation lager than 76%, but the Co 55∼65 samples consisting of a large amount of phase have very limited elongations less than 21%. On the other hand, the yield strength and ultimate tensile strength (UTS) increase co-instantaneously accompanied by the increase of Co content, with the exception of Co 55 sample. The enhanced strength is probably due to the increase of γ usf and the different amount of phase. It is noteworthy that the fracture elongation firstly increases and then decreases with a maximum value of 96.1% (Co 35 ), and the strength enhances without the expense of ductility by changing the Co content from 25 to 45%. The Co 35 and Co 45 samples exhibit UTS of 721 and 841 MPa with fracture elongations of 96.1% and 76.2% respectively, which are superior to the Co 25 sample. The strain hardening curves in Figure 3(c) indicate that the Co-rich samples show relatively higher strain hardening rates, despite all the Co 25∼45 samples are single γ phase. Moreover, the γ + dual phase samples show much higher hardening rates than single γ phase samples, due to the lack of operative slip systems of the -hcp phase.  Figure 4 shows the XRD patterns of the samples both before (Figure 4(a)) and after (Figure 4(b)) tensile fracture. The Co 25∼45 samples comprise single γ phase meanwhile the Co 55∼65 samples consist of γ + dual phases before tensile in accordance with the EBSD results in Figure 2. However, the peaks of the phase detected in the fractured Co 45 sample and the enhancement of the phase peaks in the fractured Co 55∼65 samples indicate that the γ → phase transformation proceeded in those samples during plastic deformation. In contrast, the transformation did not occur in the Co 25∼35 samples, because of no phase peak is found in those fractured samples. It can be concluded that the room temperature plastic deformation behaviors of the samples are significantly influenced by the Co content, i.e. the SFEs and phase stabilities of the new designed HEAs alloys. Figure 5(a-c) shows the bright filed image (Figure 5(a)), high-magnification HADDF image ( Figure 5(b)), and diffraction pattern (Figure 5(c)) of the fractured Co 25 sample. A high density of dislocations and deformationinduced nanotwins less than 10 nm thick indicated by arrowheads are observed. The twinning elements are K 1 = (111) γ , η 1 = [112] γ , K 2 = (111) γ , η 2 = [112] γ as indicated in Figure 5(b,c), which is the common operative twinning system in fcc metals formed by the motion of Shockley partial dislocations on every closepacked {111} planes. Figure 5(d-f) is the bright field image ( Figure 5(d)), high-magnification HADDF image ( Figure 5(e)), and diffraction pattern ( Figure 5(f)) of the fractured Co 45 sample. A lamellar structure consisting of strip-shaped γ and phases is observed. The atomic stacking sequences are ABCABC γ and ABABAB along the < 111 > γ and [0001] orientations, respectively. These two phases follow an orientation relationship described as: {111} γ // (0001) , < 110 > γ // [1120] , which is analogous the lamellar structure consisting of γ and α 2 phase in lamellar Ti-Al alloys [31,32]. The phase was formed in the γ matrix during plastic deformation by SIMT, which is achieved by the nucleation and propagation of Shockley partial dislocations on every second {111} γ planes. On the other hand, it can be deduced that twinning is relatively easier to be activated in the Co 35 sample, because the SFE is lower than Co 25 sample meanwhile no phase was detected in the fractured Co 35 sample by XRD. The higher probability of activation of deformation twinning partially contributes to the improved strength and elongation.
The SFE determines the plastic deformation mechanisms of fcc metals, which switches the deformation mode from dislocation slip to twinning and finally The SFE and γ phase stability are lowered by increasing the Co content in the present study, which then manipulate the plastic deformation mechanisms of the alloys. In Co 25∼65 alloys, dislocations and deformation twinning are formed during plastic deformation. However, the fcc → hcp martensitic transformations   occurred in the Co 55∼65 alloys due to the lowered SFE. As a result, the Co 55∼65 alloys strengthened by TRIP effect exhibit much higher hardening rate and strength than the Co 25∼35 alloys with TWIP effect during plastic deformation at room temperature.

Conclusions
A series of novel Co-rich Co x Cr 25 (FeNi) 75−x (x = 25-65) HEAs with enhanced tensile properties and manipulated deformation behaviors, were designed by lowering the SFE and fcc phase stability of the equiatomic CoCrFeNi alloy. The increase of Co concentration at the expense of decreasing Fe and Ni concentrations yields a lower SFE and reduced fcc phase stability, accompanied by the deformation mechanism switches from deformation twinning to fcc → hcp martensitic transformation. Hereinto, the Co 35 Cr 25 Fe 20 Ni 20 and Co 45 Cr 25 Fe 15 Ni 15 alloys with metastable single fcc phase possess superior combination of strength and ductility compared to other single fcc phase counterparts. The findings here provide a feasible guideline for developing high-performance high entropy alloys.

Disclosure statement
No potential conflict of interest was reported by the authors.