Study on strength and toughness of FeCoCrNiAlx high entropy alloy

This experiment analyzes the effects of Al content on the microstructure and mechanical properties of FeCoCrNiAlx high entropy alloy. We can observe that the high entropy alloy is FCC single-phase structure when the Al element content is low[1]. With the Al content increases, the alloy structure gradually changed from the FCC phase to a new phase composed of the FCC phase and BCC/B2 phase, and finally into the BBC single phase. The difficult element diffusion phenomenon and high entropy effect make the alloy form a simple solid solution structure. With the precipitation of the second phase, the properties of the alloy are strengthened[2]. After the Al element is gradually added to the alloy, the grain structure is refined, and fine crystal strengthening occurs. The experiment results show that with the increase of Al content, the mechanical properties of the alloy are significantly improved, and the tensile strength is increased from 358 MPa to 1265 MPa. Yield strength increased from 166 MPa to 878 MPa. Hardness increased from 1.4 Gpa to 4.22 Gpa. However, the fracture elongation of the alloy decreased from 51.3% to 12.2%.


Introduction
With the vigorous development of emerging technologies such as aerospace, electronic communications, new energy, and traditional industries such as machinery and equipment manufacturing, transportation, and building materials, people expect to get more excellent performance alloys.Traditional alloys comprise one or two metal elements, adding trace alloying elements and then regulating alloy properties.Still, it is gradually difficult to meet the requirements of some harsh environments for metal properties, so people began to devote themselves to studying new metal materials [3] .As the concept of high entropy alloy has been proposed, high entropy alloy has attracted the attention and research of researchers because of its unique setting concept and excellent comprehensive properties.The high entropy alloy comprises more than five main elements according to the equal atomic ratio or close to the equal atomic ratio, which breaks the principle of traditional alloy principal element design concept.High entropy alloys also exhibit a unique structure different from traditional alloys.Due to the high mixing entropy present in the alloy after solidification, the structure of the alloy is usually a face-centered cubic structure, body-centered cubic structure, or closepacked hexagonal structure [4] .The unique atomic structure of the high entropy alloy let it perform excellent performance in extreme environments such as irradiation or high and low temperature.The high strength and good ductility of the alloy make it have broad engineering application prospects.
Although FCC single-phase alloys usually have good flexibility, the strength is very low, while the single BCC phase alloy has high yield strength but poor ductility.In this study, Al was added into the FeCoCrNi matrix to obtain AlxCoCrFeNi high-entropy alloy with high strength and good shape.The change of strength and shape of the alloy after adding the Al element was tested, and a conclusion was drawn [5] .

Sample preparation
With purity of 99.9%Al, Co, Cr, Fe, and Ni particles raw materials and under the protection of argon, AlxCoCrFeNi(x=0, 0.15, 0.3, 0.5, 1) alloy ingot with different Al content is prepared by vacuum arc melting method.Each alloy cast ingot melted about 5 times, and the electromagnetic stirring function is used to ensure the internal composition of the alloy is uniform.

Experimental method
Japan Shimadzu XRD-6100 X-ray diffractometer was used with a scanning range of 20°~90°, scanning speed of 2°/min, and scanning step length of 0.02°.We use MDI Jade 6.0 software to analyze the high entropy alloy's phase structure.And we compare the alloy diffraction pattern with the standard PDF database card.The crystal plane index corresponding to each diffraction peak was determined, and the lattice constant was calculated.Finally, the data was imported into the origin software to draw the XRD diffraction pattern [6] .
Using FEI QUANTA 400FEG thermal field emission scanning electron microscopy, combined with energy dispersion spectroscopy (EDS), it was used to analyze the distribution of elements in the alloy by point, line, and plane scanning.Main parameters of scanning electron microscopy are that the acceleration voltage range is 0.2~30 KV, and the current is not less than 100 mA.
A WAW-6000G electro-hydraulic servo universal testing machine conducted compression and tensile tests at room temperature.The cylindrical and tensile standard samples Φ5×10 mm were extracted from alloy ingot by an electric spark wire cutting machine.The test strain rate was 1×10 -3 s -1, and the loading speed was 0.5 mm/min.Three tests were conducted for each alloy component to obtain the quasi-static compression performance data at room temperature.Three sets of experiments measured the tensile properties of metal materials at normal temperatures.

Phase struct of FeCoCrNiAlx HEAs
The XRD of cast FeCoCrNiAlx alloys is shown in Figure 1.radius of Fe, Co, Cr and Ni, and the Al atom is dissolved in the FeCoCrNi matrix so that the lattice distortion degree of FeCoCrNi matrix becomes larger, and the lattice constant also increases.When x=0.15, the crystals of the high entropy alloy have a single phase FCC structure.x>0.5, the alloy is composed of BCC+FCC phase.When Al0.75~Al1.0, the biphase structure +B2 phase (100) peak appears, and the alloy consists of single-phase BCC when Al1.0.

Microstructure of FeCoCrNiAlx HEAs
Figure 2(a)~(e) shows the microstructure of FeCoCrNiAlx (x= 0, 0.15, 0.3, 0.5, 1) high entropy alloy.By observing the following crystal phase diagram, it can be found that with the addition of Al element to the alloy matrix, the microstructure of the high-entropy alloy is refined, and the fine crystal strengthening phenomenon appears in the alloy.Grain thinning can significantly increase the grain boundary density, strengthen the alloy well, and improve the flexibility and the alloy strength.This is also why FeCoCrNiAlx high entropy alloy has high yield strength, tensile strength, and hardness.Because the Al atoms' atomic radius is larger than that of other solutes atomic radius, Al atoms can be solidly dissolved in the FCC matrix phase in large quantities, resulting in interstitial solid solutions, lattice deformation, lattice strain energy increase, and solid solution strengthening, which is manifested as an increase in alloy hardness on a macro level.
It is generally believed that a variety of factors cause solid solution strengthening.Elastic interaction, chemical interaction, or electrostatic interaction between solute atoms and dislocation can strengthen solid solutions.After plastic deformation of solid solution, dislocation motion changes the distribution state of solute atoms, which appears in short distance order or segregation.5 shows the tensile strength changes after adding different Al content to FeCoCrNiAlx high entropy alloy matrix.By observing the line chart, it can be found that with the gradual addition of Al element to the alloy matrix, the tensile strength of the alloy presents a rising trend, which is because the phenomenon of solid solution strengthening occurs after the addition of Al element to the alloy matrix, which increases the number of lattice distortions, and the high-entropy alloy obtains a higher tensile strength after the addition of Al element.The tensile strength of the FeCoCrNi alloy matrix without the Al element is only 358 MPa.In comparison, the yield strength of FeCoCrNiAl high-entropy alloy with Al element of equal atomic proportion can reach 1265 MPa, and the tensile strength is 3.5 times that of FecocrNi alloy without Al element.It is concluded that adding Al element can greatly increase the yield strength of high entropy alloy.

Tensile strength analysis of FeCoCrNiAlx high entropy alloy.
Figure 6 shows the linear relationship between the hardness of FeCoCrNiAlx alloys with different Al content and the addition of Aluminum content.By observing the line chart, it is found that with the continuous addition of Aluminum content, the hardness of the alloy first increases greatly after the addition of Al element and then gradually becomes stable.After 0.2, the BCC phase gradually appears in the alloy, and the BCC phase is hard and brittle, and the higher the proportion is, the greater the hardness of the alloy is.The hardness of FeCoCrNi alloy without Al element is 1.4 Gpa.In comparison, the hardness of FeCoCrNiAl high-entropy alloy with equal Al content is 4.22 Gpa, and the tensile strength is 3 times that of FeCoCrNiAl without Aluminum content.

Conclusion
(1) The addition of Al elements to the alloy continues to increase.The yield strength of FeCoCrNiAlx high entropy alloy increased from 164 MPa to 883 MPa.Tensile strength increased from 367 MPa to 1288 MPa.Hardness increased from 1.4 Gpa to 4.5 Gpa.
(2) The addition of Al elements to the alloy continues to increase, and the phase structure of FeCoCrNiAlx is transformed from single-phase FCC to FCC+BCC phase.Finally, when the Al element and other atomic ratios are added, the alloy phase structure is transformed into the BCC phase, and the fracture elongation decreases from 51.4% to 12.3%.
(3) The FCC phase is a soft and tough phase, which provides good plasticity for the material, and the BCC phase is a hard and brittle phase, which can provide higher strength for the material in the case of the presence of both phases in FeCoCrNiAlx alloy.The alloy has good mechanical properties.The tensile strength of the alloy is as high as 1054 MPa, the plasticity is 23.7%, and the hardness is 4.2 Gpa.The relationship between alloy strength and alloy plasticity is better balanced.
(4) The microstructure of FeCoCrNiAlx alloy was changed with the addition of Al element.Al0 alloy is an equiaxed crystal structure.With the content of Al added to the alloy matrix, the alloy structure gradually changes into a dendrite structure, and the dendrites become dense.Local position arrangement direction tends to be consistent.By EDS scanning analysis, it was found that the dendrites were enriched with Fe, Co, and Ni elements.Cr and Al elements are enriched at grain boundaries.

Figure 1 .
Figure 1.XRD of FeCoCrNiAlx HEAs.Figure1shows the situation after adding the Al element to the high entropy alloy.The stability of FCC structure in the alloy decreased, and the alloy gradually changed from single-phase FCC structure to BCC phase structure.This is because the atomic radius of the Al atom is larger than the atomic

Figure 1
Figure 1.XRD of FeCoCrNiAlx HEAs.Figure1shows the situation after adding the Al element to the high entropy alloy.The stability of FCC structure in the alloy decreased, and the alloy gradually changed from single-phase FCC structure to BCC phase structure.This is because the atomic radius of the Al atom is larger than the atomic

Figure 3
Figure 3 shows the microstructure of FeCoCrNiAlx high-entropy alloy under the backscattered electron characterization technique, and it is found that FeCoCrNiAlx high-entropy alloy has a typical microstructure.The observation of the crystal phase diagram shows that adding an Al element refines the grain of the high entropy alloy.The backscattered electron characterization technique shows that the addition of Al not only changes the microstructure of the alloy but also changes the distribution of solute in the alloy.It is found that Fe, Co, and Ni are enriched in the dendrites, and Cr and Al are enriched in the dendrites.Because the Al atoms' atomic radius is larger than that of other solutes atomic radius, Al atoms can be solidly dissolved in the FCC matrix phase in large quantities, resulting in interstitial solid solutions, lattice deformation, lattice strain energy increase, and solid solution strengthening, which is manifested as an increase in alloy hardness on a macro level.It is generally believed that a variety of factors cause solid solution strengthening.Elastic interaction, chemical interaction, or electrostatic interaction between solute atoms and dislocation can strengthen solid solutions.After plastic deformation of solid solution, dislocation motion changes the distribution state of solute atoms, which appears in short distance order or segregation.

Figure 4
shows the yield strength changes of FeCoCrNiAlx high-entropy alloy after adding different Al elements.It can be observed that the yield strength of FeCoCrNiAlx alloy increases linearly with the gradual addition of Al element to the alloy matrix.The yield strength of the FeCoCrNi alloy matrix without the Al element is only 166 MPa.The yield strength of FeCoCrNiAl high entropy alloy with equal atomic ratio Al can reach 878 MPa, 5.3 times that without Al.It is concluded that adding Al element can greatly increase the yield strength of the alloy.

Figure 5 .
Figure 5. Line chart of tensile strength of FeCoCrNiAlx alloys.

Figure 6 .
Figure 6.Line chart of the hardness of FeCoCrNiAlx alloys with different Al content.