Effects of Varying Copper Content on the Microstructure and Mechanical Properties of FeCoNiCux

YANG Yanru, ZHANG Yichen, LI Jiawen, ZHU Heguo

(College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China)

Abstract: Five sets of high entropy alloys (HEAs) FeCoNiCux (x =0.5, 1.0, 1.5, 2.0, 2.5) were produced by vacuum induction smelting. The effects of Cu content on the microstructure and mechanical properties of the alloys were interrogated by X-ray diffractometer (XRD), field scanning electron microscope (FESEM) and tensile mechanical test. The result shows that the HEAs form single FCC solid solution phase. With the increase of Cu content , the diffraction peak first deviated to the right and then shifted to the left. The alloys changed from equiaxed crystal structure to refined dendritic crystal structure, as Cu content increased. A large number of Cu atoms are isolated in the inter-crystalline region. The tensile mechanical tests show that with the increase of Cu content, the ultimate tensile strength first increased and then decreased. When x is 2.0, the ultimate tensile strength reaches a maximum of 473 MPa, the percent elongation is 43.0%, and the fracture presents ductile behaviour.

Key words: FeCoNiCu alloy; copper content; microstructure; elemental segregation; mechanical properties

1 Introduction

In recent years, high entropy alloys (HEAs)have become a hot research spot due to their excellent mechanical properties[1-3]. High entropy alloys usually have more than five constituent elements, and the mole fraction of each component is between 5% and 35%[4-10]. Yewet alfirst proposed the design concept of high entropy alloys and proved that AlxCoCrCuFeNi consists of a simple mixture of FCC and BCC solid solutions. The results also show that the high entropy alloy has a simple crystal structure and almost no intermetallic compounds[11]. Zhanget alsuccessfully predicted the microstructure of high entropy alloys by applying the Hume-Rothrey law of binary alloys to multi-element high entropy alloys, which also provided a theoretical basis for predicting the structure of high entropy alloys[12].

High entropy alloys are usually composed of a variety of elements with different properties[13]. High entropy alloys with specific properties by adding different alloy elements can be obtained[14-16]. For example,the addition of Cu in the alloy can improve the toughness and corrosion resistance of the material, while the addition of Co can improve the oxidation resistance of the material. The phenomena that the alloy may have different properties due to the addition of different elements is called the “cocktail characteristics” of high entropy alloys[17]. A number of studies show that the mechanical properties of high entropy alloy can be changed through appropriate composition design, for example the excellent plasticity can be maintained on the basis of improving its strength. Duanet alsuccessfully improved the electromagnetic properties of the material by changing the Cr element content of FeCo-NiAlCrx(x=0.1, 0.3, 0.5, 0.7, 0.9) in high entropy alloy[18]. Therefore, this paper studied the microstructure and mechanical properties of FeCoNiCux(x=0.5, 1.0,1.5, 2.0, 2.5) high entropy alloy with different Cu contents.

2 Experimental

Iron, cobalt, nickel and copper powders are used in the experiment and the purity of which is over 99.9%. Five kinds of high entropy alloys FeCoNiCux(x=0.5, 1.0, 1.5, 2.0, 2.5) were produced by vacuum induction smelting method. The raw materials are shown in Table 1.

Table 1 Molar ratio of the composite specimens fabricated under different conditions

The microstructure and morphology of the melted samples were analyzed by SEM, energy dispersive spectrometer (EDS) and XRD. Room-temperature tensile specimens were prepared by spark cutting from the ingots and the tensile tests were carried out on a UTM/CMT 5000 universal material test machine with a strain rate of 0.5 mm/min. The mechanical properties and strengthening mechanism of these five materials are also analyzed.

3 Results and discussion

3.1 Microstructure

In high entropy alloys, due to the differences in physical and chemical properties between elements,the formation of a single solid solution phase is often accompanied by the emergence of intermetallic compounds. The parameters for predicting the formation of solid solution in FeCoNiCuxhigh entropy alloys are given in Table 2.Ω≥1.1 andδ≤6.6% are usually selected as the criteria for determining whether a solid solution phase can be formed[19]. In addition, valence electron concentration (VEC) can be used to predict the phase composition of high-entropy alloys[20]. The formula is:

Fig.1 XRD results of five FeCoNiCux high-entropy alloys with different copper content ratios: (a) XRD result; (b) 42°-47°XRD result

where,cis the atomic percentage of each element, andiis the number of components. When VEC＞7.5, FCC solid solution is easy to form. When VEC＜7.5, BCC solid solution is easy to form.

Table 2 Prediction of parameters of FeCoNiCux high entropy alloy

Fig.1(a) shows the XRD results of the five samples, and the XRD results of 42°-47° are presented in Fig.1(b). From the XRD patterns, the alloy has three obvious diffraction peaks, which indicates a FCC structure ((110), (200) and (220)). Combined with the data in Table 2, the VEC values of the five high entropy alloys are all greater than 7.5, which is consistent with the results obtained by XRD. From Fig.1(b), the diffraction peaks first deviate to the right and then to the left with the increase ofx. In order to analyze the reason, the lattice constants of FCC phase in the five high entropy alloys are given in Table 3. With the increase of x content from 0.5 to 2.0, the diffraction angle increases and the diffraction peak deviates to the right due to the decrease of lattice constant. When the copper content increases to 2.5, the lattice constant increases and the diffraction peak deviates to the left obviously. In addition, the lattice constant increases with the increase of copper content, which may be ascribed to the large solute Cu atoms. However, whenxcontent is 2.0, the lattice constant decreases obviously. In order to further study the reason, the microstructure of the composite is analyzed.

Fig.2(a) shows the SEM image of FeCoNiCu2.0high entropy alloy. The high entropy alloy has a dendritic structure and a large number of dispersed particle phases are distributed in the intergranular region. According to Fig.2(b), in the EDS of area A, a large number of Cu elements are concentrated in the intergranular region and some of them form copper-rich spherical granular materials, which is also proved by the surface scanning pattern of the FeCoNiCu2.0high entropy alloy after corrosion in Fig.3 and the microstruture may be related to diffusion. When the high entropy alloy is solidified, the diffusion rate of each element will slow down because of the lattice distortion. The phase separation in the alloy will be very slow at high temperature and even be restrained and delayed to low temperature due to the difficulty in coordinate diffusion among multiple elements, which causes nano-phase to appear in the as-cast high entropy alloy[21]. However, when the content of Cu changed, no obvious particulate matter was observed. In addition, the segregation rate of these five high entropy alloys is obtained from the calculation formula of the segregation rate[22].

Fig.2 SEM image and EDS spectrum of FeCoNiCu2.0 high entropy alloy: (a) SEM image; (b) EDS spectrum of area A

where,SRis the segregation rate.

Fig.3 Surface scanning pattern of FeCoNiCu2.0 high entropy alloy after corrosion

Table 3 Lattice constants of FCC phase in FeCoNiCux high-entropy alloy

Table 4 Segregation rate of different elements in alloy FeCoNiCux

The segregation rate gradually increases with the increase of Cu content (Table 4), and the maximum value is determined to be 2.94% when the content is 2.0.The segregation of Cu is more likely to occur in the intergranular region due to its positive mixing enthalpy with the other three elements. It can be seen from Table 2 that as the copper content increases, the ΔSmixalso decreases. Moreover, the high entropy effect is weakened due to the decrease in mixing enthalpy, so the precipitation of the copper-rich particle phase can be seen.

Fig.4 SEM images of five FeCoNiCux high entropy alloys with different copper content ratios after corrosion: (a) x=0.5; (b)x=1.0; (c) x=1.5; (d) x=2.0; (e) x=2.5

The microstructures of five high entropy alloys are observed by SEM. Fig.4 are the SEM images of five FeCoNiCuxhigh entropy alloys with differentxcontent after corrosion. It can be seen that when the copper contentx= 0.5, the alloy has an equiaxed structure.As the content ofxincreases, the structure of the alloy changes from equiaxed structure to dendritic structure.More dendritic structures can be observed on the interface, which indicates that the grains of the alloy are also refined. Therefore, the mechanical properties of the alloy are affected.

Fig.5 (a) The stress-strain curves of FeCoNiCux high entropy alloys; (b) Curves of ultimate tensile strengths and percent elongations of FeCoNiCux high entropy alloys

3.2 Mechanical properties

In order to ensure the accuracy and reproducibility, we tested each group of samples three times.Fig.5(a) shows the most representative stress-strain curves of FeCoNiCuxhigh entropy alloys with differentxcontents. The average value of ultimate tensile strength and percent elongation from three experiments were calculated as shown in Fig.5(b). It shows the relationship between elongation and ultimate tensile strength of five FeCoNiCuxhigh entropy alloys with different copper contents under the change ofx. It can be seen that with the increase ofxthe ultimate tensile strength of the alloy increases first and then decreases.It shows that the precipitation of Cu in the intercrystalline region can improve the strength of the alloy because the precipitation of Cu in the intercrystalline region hinders the movement of dislocation slip to a certain extent, resulting in the improvement of the strength of the alloy[23]. In addition, FeCoNiCu2.0high entropy alloy has a large number of spherical Cu-rich nanoparticles precipitated, and the segregation rate of Cu is also the highest, which may also be the reason for its highest strength. The elongation gradually increases with the increase of Cu content due to the fact that the crystal structure of the alloy is FCC, and the crystal structure of Cu is also FCC. The FCC structure of both phases makes the phase boundary of the copper-rich phase and the matrix have a good coherent interface, so the plasticity of the high entropy alloy increases with the increase of Cu content.

In this experiment, FeCoNiCu2.0high entropy alloy has the best comprehensive performance, and its strengthening mechanism is mainly Orowan strengthening and solution strengthening[24,25]. The strengthening effect of Orowan strengthening mechanism is mainly related to the volume fraction, size and spacing of particles. In this paper, the ultimate tensile strength is 473 MPa when the copper contentxis 2.0 (curve 4),which may be related to the microstructure and lattice distortion of high entropy alloy. The grain structure of the FeCoNiCu2.0high entropy alloy has been significantly refined. At this time, the Orowan strengthening mechanism dominates to improve the material properties. With the further increase of copper content, the degree of increase in grain refinement decreases. Whenxis 2.0, the segregation rate reaches the highest, and the dispersively distributed copper-rich nanoparticles appear in the intercrystalline region, which is the reason why the FeCoNiCu2.0high entropy alloy has the highest strength.

Fig.6 Fracture morphology of five FeCoNiCux high entropy alloys with different copper content ratios: (a) x=0.5; (b) x=1.0; (c)x=1.5; (d) x=2.0; (e) x=2.5

In order to further analyze the effect of differentxcontents on the mechanical properties of high entropy alloys, the tensile fracture morphology of five alloys was analyzed. Fig.6(a) shows the tensile fracture morphology of FeCoNiCu0.5. It can be seen from Fig.6(a)that the material after induction melting may have many pores due to the low relative density. There are some circular or elliptical pits of different sizes on the interface, namely dimples, and no crystalline particles on the surface. The appearance of river-like patterns can also be observed, which is a typical cleavage fracture. When the copper content increases to 1.0, it can be seen from Fig.6(b) that there are a large number of uniformly distributed dimples on the interface and the size of dimples is relatively uniform and the depth is large. There are river patterns on the fracture surface,but there are also tear ridges generated by large plastic deformation. The appearance of a large number of dimples shows obvious plastic deformation characteristics,indicating that the fracture is ductile fracture. When the copper content is further increased, it can be seen from Fig.6(c) that there are a large number of dimples on the interface and no river-like pattern, indicating that the fracture is ductile fracture. This is because when the copper content is 1.5, there is a large number of Cu precipitation in the intercrystalline region, which significantly improves the elongation of the alloy. Therefore,many dimples with different sizes can be observed on the interface. Then with the increase ofxcontent to 2.0 and 2.5, it can be seen from Fig.6(d) and 6(e) that the size of the dimples also gradually becomes larger and deeper, which is because the increase of copper content can further increase the ability of plastic deformation of the material, resulting in the gradual increase of the elongation of the material, resulting in changes in the size and depth of the dimples.

4 Conclusions

a) The change of copper content does not affect the formation of FCC phase in the FeCoNiCuxalloys,however, varying copper content would change the lattice constant of the FCC structure.

b) As copper content increased in the alloy, the equiaxed grain structure changed into dendritic structure, and the dendrite was also refined. In addition, due to the difference in mixing enthalpy between Cu and other four elements, the segregation rate of Cu is the highest, and Cu diffused to the inter-dendritic region to form copper-rich phase during solidification.

c) When the copper content reaches 2.0, the maximum ultimate tensile strength is 473 MPa, and the percent elongation increased with the increase of copper content. FeCoNiCu2.0high entropy alloy has the optimum overall properties. Numerous small and uniform dimples are observed in the tensile fracture surface,which represents a typical ductile response.

d) The strengthening mechanisms are mainly Orowan strengthening mechanism and solid solution strengthening. On one hand, the precipitation strengthening was enabled by a large number of dispersed copper-rich nanoparticles in the intercrystalline region.On the other hand, the segregation of a large amount of copper elements in the intercrystalline region led to the increase of material strength.