Abstract
FeCoCrNiZrx (x=0.5, 0.75, 1) high-entropy alloys with different Zr contents were prepared by vacuum arc melting. The effects of Zr content on the microstructure, magnetic properties, and electrochemical corrosion properties of the alloys were studied. X-ray diffraction, scanning electron microscope, vibration sample magnetometer, and electrochemical workstation were used to study the magnetic property and electrochemical corrosion ability of FeCoCrNiZrx alloys. Results show that FeCoCrNiZrx alloy has typical eutectic structure, consisting of face-centered cubic solid solution and C15 Laves phase. With the increase of Zr content, the hardness of the alloy increases at first and then decreases. However, according to the synthesized static hysteresis curves, FeCoCrNiZr0.5 alloy shows a mixture characteristic of paramagnetism and ferromagnetism, FeCoCrNiZr0.75 alloy has paramagnetic behavior, and FeCoCrNiZr1 alloy has typical ferromagnetism. Meanwhile, the FeCoCrNiZrx alloy undergoes activation-passivation transition in 3.5wt% NaCl solution. When the content of Zr is 0.75at%, the polarization resistance of the alloy has the maximum impedance capacitance radius, and the corrosion resistance of the passive film is the strongest.
Science Press
FeCoCrNi high-entropy alloy is one of the most stable alloys, which easily forms a solid solution with simple structures and has excellent plasticity and toughness. Chen[1] prepared AlCoCrFeNiZrx high-entropy alloys and analyzed their microstructure and mechanical properties. Results show that the alloys present two types of microstructures and Zr addition significantly improves the mechanical properties. When the Zr content is greater than 0.1at%, the fracture strength and plastic strain of the alloy are decreased signifi-cantly, but the yield strength is increased with further increa-sing the Zr content. CoCrFeNiZrx alloys with different Zr contents were prepared by Huo[2]. Typical eutectic structures are found in the alloys which are composed of face-centered cubic (fcc) solid solution and lamellar C15 Laves phase. The crystal orientation relationship between the two phases is determined. When the volume fraction of the hard C15 Laves phase increases, the strength of the alloy increases, but the alloy becomes more brittle at room temperature. The fracture process changes from ductile interlamellar fracture to brittle transduction fracture. Many studies have focused on the microstructure and mechanical properties of FeCoCrNiZrx alloys, but only a few studies have reported their functional properties (magnetization, magnetocaloric effect, resistance, etc)[3].
Zr is an important element in alloying with transition elements and has strong corrosion resistance. The crystal structure of Zr at room temperature is close-packed hexagonal. Under high-temperature conditions, the crystal structure transforms into body-centered cubic. Zr has a larger atomic radius and produces larger lattice distortion during the alloying procedure with other transition elements of Co, Cr, Fe, and Ni[4,5]. The mixing enthalpy of these elements is smaller than that of Al element, showing a strong tendency to form intermetallic compounds[6]. Zr is the key element to form Laves phase. The Laves phases, such as Co2Zr, Cr2Zr, and Fe2Zr, all have C15 structure, which have excellent high-temperature oxidation resistance, alloy strength, and hot corrosion resistance. The FeCoCrNiZrx (x=0.5, 0.75, 1) high-entropy alloys with different Zr contents were fabricated in this research to examine the effect of different Zr contents on the structure, magnetic properties, and electrochemical corrosion ability of the alloy system. The results also provide a good application basis for FeCoCrNiZrx (x=0.5, 0.75, 1) high-entropy alloys in the fields of magnetic components and microelectronic equipment.
Fe, Co, Cr, Ni, and Zr (purity>99.9wt%) were smelted by arc on water-cooled plates in titanium-aspirated Ar gas. Table 1 shows the atomic radius and binary mixing enthalpy of each element. FeCoCrNiZrx high-entropy alloys with x=0.5, 0.75, 1 (at%) were prepared. The arc-melted ingot was remelted at least five times to ensure the uniformity of composition. After natural cooling, the ingot was cut into the specimen of 10×10×10 mm3 in dimension by wire cutting.
Table 1 Atomic radius and binary mixing enthalpy of different elements
| Element | Atomic radius/pm | Binary mixing enthalpy/kJ·mol-1 |
|---|
| Co | Cr | Fe | Ni | Zr |
|---|
|
Co |
126 |
- |
-4 |
-1 |
0 |
-41 |
|
Cr |
127 |
-4 |
- |
-1 |
-7 |
-12 |
|
Fe |
127 |
-1 |
-1 |
- |
-2 |
-25 |
|
Ni |
124 |
0 |
-7 |
-2 |
- |
-49 |
|
Zr |
160 |
-41 |
-12 |
-25 |
-49 |
- |
The composition characteristic of specimens was analyzed by X-ray diffraction (XRD). The hardness was measured on the mirror surface of specimen by Wilson Hardness 401MVD microhardness tester. Four measured points were spaced 5 mm apart on the specimen surface, and the average hardness value was calculated. The surface morphology was observed using a scanning electron microscope (SEM). The magnetic properties of FeCoCrNiZrx high-entropy alloys were measured using a PPMSECII-97 integrated physical measurement system. The electrochemical corrosion properties of the alloys were measured using Autolab electrochemical workstation.
Fig.1 shows the XRD patterns of FeCoCrNiZrx (x=0.5, 0.75, 1) high-entropy alloys. The main phase is the fcc solid solution with γ-Fe structure. The fcc solid solution is an alloy consisting of Co, Cr, Fe, and Ni without Zr element. When the Zr content increases, the strongest peak moves to the left, indicating that the lattice constant of the cubic solid solution increases. In addition to the shift of diffraction peak of the cubic solid solution phase caused by the dissolution of Zr element into the FeCoCrNi system, some new corresponding diffraction peaks are observed[7]. When the Zr content is greater than 0.5at%, the reflection peak corresponding to the Laves phase of C15 structure appears, which is determined as ZrM2 (M=Co or Ni).
The binary mixing enthalpy of Zr and other elements is relatively small, compared to that of Ni element. For example, the binary mixing enthalpy of Co-Zr and Ni-Zr is -41 and -49 kJ/mol, respectively. Moreover, when the atomic radius difference in the alloy is small, the simple solid-solution phase is more likely to form in the alloy[8]. As shown in Fig.1, the increase of Zr content leads to a significant decrease in the mass fraction of fcc phase. In the FeCoCrNiZr0.5 alloy, the Laves phase has the same strength as the fcc solid-solution phase does, indicating that the main phase is still a mixture of cubic solid solution[9]. The further increase of Zr content causes a decrease in the relative strength of the diffraction peak of fcc solid solution, whereas the relative strength of diffraction peak of the Laves phase increases, indicating that the volume fraction of the Laves phases is increased with the increase of Zr content.
Fig.2 shows that FeCoCrNiZrx (x=0.5, 0.75, 1) alloys present a typical lamellar structure of eutectic alloy. Dark dendritic crystals of 10~100 μm in size are abundant in the FeCoCrNiZr0.5 alloy. Some dark dendrites remain in the FeCoCrNiZr0.75 alloy, but their amount significantly reduces. No dark dendritic crystals are found in the FeCoCrNiZr1 alloy. During an eutectic reaction, the Laves phase and fcc solid solution form a small coupled lamellar layer[10]. Because only the two phases (ignoring the trace phases) are detected by XRD, the dark dendritic crystals and dark lamellar layers in the eutectic tissues belong to the same phase[11].
Fig.2 SEM images of CoCrFeNiZrx alloys: (a, b) CoCrFeNiZr0.5, (c, d) CoCrFeNiZr0.75, and (e, f) CoCrFeNiZr1
For FeCoCrNiZrx alloys with Zr content greater than 0.5at%, the Laves phases are observed at the grain boundary of fcc phase. The morphology of fcc phase is dendritic, and the Laves phase is the main phase[12]. When the Zr content increases, the volume fraction of the Laves phase also increases obviously. For the areas without Laves phase, the structure consists of periodic light and dark phases. The width of the periodic tissue is significantly larger, especially near the interface[13]. The further increase in Zr content causes grain refinement of the surrounding tissues again. It is difficult to detect the Zr element in the region without Laves phase. The dark phase without Laves phase is rich in Ni, while the light phase is rich in Fe, Cr, and Co evenly[14]. For the regions with Laves phase, the structure shows a mixed characteristic of fcc and Laves phases. This is due to the large lattice strain caused by the huge difference in the atomic size[15].
Table 2 shows the hardness test results. For FeCoCrNiZrx (x=0.5, 0.75, 1) alloys, CoCrFeNiZr0.75 alloy has the highest hardness with an average value of 5401 MPa. When Zr content is less than 0.75at%, the hardness of FeCoCrNiZrx high-entropy alloys shows an upward trend with the increase of Zr content. When Zr content exceeds 0.75at%, the hardness decreases. When the Zr content increases, the lattice constant of fcc phase in the alloy increases significantly. Zr addition causes an obvious lattice distortion effect, which increases the strength and hardness of the alloys. Therefore, the hardness of FeCoCrNiZr0.75 alloy is higher than that of FeCoCrNiZr0.5 alloy[16]. When the Zr content is 1at%, the hardness decreases to some extent probably due to the precipitation of γ-Fe in the phase[17].
Table 2 Hardness HV0.2 of FeCoCrNiZrx (x=0.5, 0.75, 1) alloys (MPa)
| No. | FeCoCrNiZr0.5 | FeCoCrNiZr0.75 | FeCoCrNiZr1 |
|---|
|
1 |
3997 |
5342 |
4985 |
|
2 |
3786 |
5439 |
5303 |
|
3 |
3986 |
5285 |
5484 |
|
4 |
4118 |
5522 |
4956 |
|
5 |
4192 |
5419 |
5315 |
|
Average |
4016 |
5401 |
5209 |
Fig.3a and 3b show the hysteresis curves of FeCoCrNiZrx (x=0.5, 0.75, 1) alloys at room temperature. According to the static hysteresis curves, FeCoCrNiZr0.5 alloy exhibits a mixture characteristic of paramagnetism and ferromagnetism, and FeCoCrNiZr0.75 alloy exhibits the paramagnetic behavior. The saturation magnetization value of FeCoCrNiZr1 alloy increases significantly. FeCoCrNiZr1 shows the typical ferromagnetism characteristic[18] with a small coercive force Hc (1115.1986 A/m) and a large saturation magnetization Ms (8.5625 A·m2/kg), indicating the soft magnetic properties. As the hysteresis curves of FeCoCrNiZrx alloys shown in Fig.4a and 4b, only FeCoCrNiZr0.5 alloy shows a relatively large hysteresis. The hysteresis area of FeCoCrNiZr0.75 and FeCoCrNiZr1 is small, and is basically zero for FeCoCrNiZr0.75 alloy.
The existence of paramagnetic element Zr and antiferro-magnetic element Cr in FeCoCrNiZrx high-entropy alloys has a great influence on the ferromagnetic elements (Fe, Co, and Ni). When the content of the diamagnetic elements and paramagnetic elements is large, the saturation magnetization value decreases, and the behavior of the alloy changes from ferromagnetic to paramagnetic[19,20]. Combined with the XRD analysis, the content of Laves phase in the physical phase decreases when the Zr content is 1at%, and the diffraction peak intensity suggests that γ-Fe phase is the basic phase. When the γ-Fe phase in the alloy becomes the main phase, the hardness decreases, but the paramagnetism of the FeCoCrNiZr1 alloy is transformed into ferromagnetism[21,22]. When the Zr content is 0.5at%, the Laves phase is dominant in the alloy, and γ-Fe phase also exists in the alloy, so the com-posite alloy mainly shows a mixture characteristic of paramagnetism and ferromagnetism with a large hysteresis area. However, when the Zr content is 0.75at%, only the Laves phase is found in the physical phase, and the presence of the γ-Fe phase cannot be detected. The FeCoCrNiZr0.75 alloy shows the most significant paramagnetic phenomenon, and the hysteresis area is basically zero. The analysis shows that the relative volume fraction of the synthesized fcc phase changes significantly. When the Zr content changes, the lattice parameters of the fcc phase change slightly, indicating that the increase of saturation magnetization value and the improvement of magnetic properties of the high-entropy alloys are related to the change in the relative volume fraction of the fcc phase.
2.5 Electrochemical corrosion behavior
Fig.5 shows the potentiodynamic polarization curves of FeCoCrNiZrx high-entropy alloys in 3.5wt% NaCl solution at room temperature. Table 3 shows the electrochemical parameters of the alloy in the 3.5wt% NaCl solution. The corrosion current density (Icorr) was obtained from the intersection of the corrosion potential (Ecorr) and the tangent line in the Tafel region of the cathode[23]. Obvious passivation zones are observed in FeCoCrNiZrx alloys, and the transition zone between the activation and passivation regions occurs during the transition from the Tafel region to the passivation region. During the electrochemical experiment, the alloys undergo an activation-passivation transition. FeCoCrNiZr0.75 high-entropy alloy has the highest corrosion potential and the lowest corrosion current density, indicating the lowest corrosion rate. The wider the passivation area, the stronger the ability to inhibit the corrosion reaction. Moreover, the FeCoCrNiZr0.75 alloy is mainly composed of Laves phase, and Zr compounds present a higher tendency to form a passivating film[24,25].
Table 3 Electrochemical parameters of FeCoCrNiZrx alloys in 3.5wt% NaCl solution
| Alloy | Corrosion potential, Ecorr/V | Corrosion current density, Icorr/A·cm-2 |
|---|
|
FeCoCrNiZr0.5 |
0.139 |
-9.633 |
|
FeCoCrNiZr0.75 |
0.181 |
-10.059 |
|
FeCoCrNiZr1 |
0.089 |
-8.909 |
Fig.6 shows the semicircular Nyquist plots of FeCoCrNiZrx high-entropy alloys, suggesting the charge transfer me-chanism. The larger the radius of the impedance capacitance, the stronger the corrosion resistance of high-entropy alloys[26,27]. The impedance capacitor of FeCoCrNiZr0.75 alloy has the largest radius, i.e., the passivation film of FeCoCrNiZr0.75 alloy has the strongest corrosion resistance, which is consistent with the aforementioned results[28].
Fig.7 shows the Bode diagrams of FeCoCrNiZrx alloys. Within the lower frequency range, FeCoCrNiZr0.75 alloy has the highest complex impedance modulus, indicating the highest corrosion resistance of the passivation film. The phase angle diagram shows a significant difference in the performance of passivation film of FeCoCrNiZrx alloy[29]. The FeCoCrNiZr0.75 high-entropy alloy has the largest phase angle of about 23° and two peaks, showing the best corrosion resistance. The peak phase angles of FeCoCrNiZr0.5 and FeCoCrNiZr1 alloys are 17° and 13°, respectively. The lower phase angle indicates a weakened protective ability of the passivation film.
1) FeCoCrNiZrx alloys mainly consist of face-centered cubic (fcc) solid solution, γ-Fe phase, and ZrM2 (M=Co, Ni) with C15 Laves phase. The Zr addition increases the lattice constant of fcc solid solution. The microstructure of the eutectic alloy is the typical fine layer structure. When the Zr content and the relative volume fraction of the Laves phase increase, the hardness of the alloy firstly increases and then decreases.
2) With the increase of Zr content, the magnetic properties of FeCoCrNiZrx (x=0.5, 0.75, 1) alloys are gradually enhanced. FeCoCrNiZr1 alloy has the best magnetism, and the saturation magnetization Ms and coercive force Hc are 8.5625 A·m2/kg and 1115.1986 A/m, respectively.
3) FeCoCrNiZr0.75 alloy has the highest corrosion potential and the lowest corrosion current density, i.e., FeCoCrNiZr0.75 alloy has the best corrosion resistance. The Zr compounds form an excellent passivation film during the electrochemical corrosion and improve the corrosion resistance of the alloys.
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