Effect of CeO2 Doping on Phase Structure and Microstructure of AlCoCuFeMnNi Alloy Coating

Materials Research. 2019; 22(1): e20180327 DOI: http://dx.doi.org/10.1590/1980-5373-MR-2018-0327 Effect of CeO2 Doping on Phase Structure and Microstructure of AlCoCuFeMnNi Alloy Coating Mingxing Maa , Zhixin Wanga*, Jiachen Zhoua, Cun Lianga, Deliang Zhangb, Dachuan Zhuc School of Materials and Chemical Engineering, Zhongyuan University of Technology, Zhengzhou 450007, China b School of Mechanical Engineering, Northeastern University, Shenyang 110819, China c School of Materials Science and Engineering, Sichuan University, Chengdu 610065, China a Received: May 05, 2018; Revised: September 29, 2018; Accepted: November 07, 2018 AlCoCuFeMnNi high-entropy alloy coating was prepared by plasma cladding method. The phase structure and microstructure of AlCoCuFeMnNi coating was investigated by XRD, SEM and EDS respectively. The results show that AlCoCuFeMnNi caotings have two BCC phase structure and typical dendrite structure and form good metallurgical bonding with substrate. The dendrite is the typical spinodal decomposition structure. After CeO2 doping, the change of peak intensity and FWHM is obvious due to the effect of Ce on the improvement of grain growth, microstructure and crystallinity. The addition of CeO2 is beneficial to reduce the cladding defect, make dendrite arm spacing enlarged and spinodal decomposition structure refined, and improve element segregation owing to the melioration effect in the temperature gradient, solidification rate, fluidity, wettability, and surface tension. Keywords: high-entropy alloy, AlCoCuFeMnNi, phase structure, microstructure. 1. Introduction As a new idea of alloy design, high entropy alloys (HEAs) break through convention alloy design method in which one or two elements are used as principal element and have more than five elements as principal element and the concentration of each element in the range of 5at%-35at%1. HEAs have a simple solid-solution phase structure, such as face-centered cubic (FCC), body centered cubic (BCC), FCC+BCC, hexagonal close-packed (HCP) lattice, rather than complex intermetallic compounds2,3. At the same time, HEAs have many excellent properties1-6, such as high strength, high hardness, high thermal stability, good wear resistance, high corrosion resistance, etc. As a new frontier in the field of metal materials, HEAs may exceed the performance limits of convention alloys and have broad application prospects. In 2004, AlxCoCrCuFeNi high entropy alloy was prepared and the concept of high entropy alloy was first proposed by Yeh1. Up to now, HEAs research has mainly focused on the aspects of composition design, mechanical properties, phase structure and so on 6-14. The composition design for HEAs is primarily based on CoCrFeNi series to add some other alloy elements to synthesize more than five multiprincipal alloys6-11. The mechanical properties of HEAs are mainly concentrated on the study of high hardness, high strength, compression and tensile properties and so forth 8-13. The phase structure of HEAs is chiefly in the terms of phase formation and phase composition 6-14. It is well known that CeO2 can play a role in the purifying to molten alloy, the improvement of alloy casting properties, the refinement of the microstructure, e-mail: * and the increasement of alloy hardness and wear resistance for convention alloy 15-17. However, there are few literatures about AlCoCuFeMnNi high entropy alloy and the effect of CeO2 on the phase structure and microstructure of high entropy alloys. In this paper, AlCoCuFeMnNi high-entropy alloy coating (HEAC) was prepared by plasma cladding, and the effect of CeO2 doping on its phase structure and microstructure were discussed in detail. 2. Experiment AlCoCuFeMnNi HEAC was fabricated by plasma cladding method. The pure metals of Al, Co, Cu, Fe, Ni and Mn with the particle size of 74 µm and higher purity than 99.5wt% were used as raw materials. The above metal powders having equal molar ratio were put in 304 stainless steel vials with GCr15 balls. Ball-to-powder weight ratio was selected as 10:1. After 2 h ball milling (50rpm), the powder was mixed into gel by organic glue ((97wt% turpentine transdermal alcohol + 3wt% ethyl cellulose). The gel was coated on 45 carbon steel substrates and dried at 120ºC. AlCoCuFeMnNi alloy coating was prepared by the LHD-300 plasma cladding apparatus (137A, 34V, 150mm/min). The preparation process of doped CeO2 alloy sample is exactly the same as that of undoped samples. The purity of CeO2 is 99.9wt%, and its doping ratio is 1 wt%. The sample was cut into 10mm× 10mm ×5mm block by DK7716 electrical discharge machining (EDM). The crystal structure and phase purity of the synthesized samples were identified by X-ray diffraction (XRD) analysis using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation 2 Ma et al. operated at 40kV and 200mA in the range of 2θ= 30º -90º. The scanning speed was 8º/min. Metallographic photos were observed by a ZEISS DMM-150C optical microscope. The morphology of the samples was observed in a JSM6360LV scanning electron microscope (SEM). The chemical compositions of samples were analyzed by Aztec X-Max 90 energy dispersive spectrometry (EDS). All the measurements were performed at room temperature. 3. Results and Discussion 3.1 XRD analysis Figure 1 shows the XRD patterns of AlCoCuFeMnNi without and with 1wt% CeO2 HEACs. As can be seen from Figure 1, the phase structures of the two HEACs are composed of BCC1 main phase and BCC2 mixed phase. The diffraction peaks of BCC1 and BCC2 show their peak positions at about 2θ=38.27º, 44.48º, 64.77º, 78.69º, 82.92º, and 42.98º, 49.95º, 73.53º, 89.11º, respectively. As shown in Fig. 1, the two-phase structures of AlCoCuFeMnNi coatings are composed of BCC1 and BCC2 phases. The lattice constants are calculated to be 4.0567 Å and 3.6460 Å by linear extrapolation method respectively. Table 1 shows the characteristic parameters of HEAC elements. According to Table 1, the atomic radii of Co, Fe and Mn are basically the same as that of Ni, which is obviously smaller than the atomic radius of Al, Cu and Ce. Besides, the contents of all elements except for Ce are equal. Therefore, the lattice expansion causes the diffraction peaks to shift toward a small angular direction 18,19. Figure 2 shows the diffraction peak area and full width at half maximum (FWHM) of BCC1 and BCC2 for AlCoCuFeMnNi without and with 1wt% CeO2 HEACs. The area of diffraction peak is normalized based on that of the strongest peak at 44.48º. Compared with the diffraction peak data of AlCoCuFeMnNi HEACs, the diffraction peak intensity of BCC1 phase decreases significantly, while the Figure 1. XRD patterns of AlCoCuFeMnNi without and with 1wt% CeO2 HEACs Materials Research intensity of BCC2 phase increases obviously, and the two BCC FWHM increases obviously for 1wt% CeO2 doping AlCoCuFeMnNi HEACs from Figure 1 and Figure 2. This is because CeO2 addition is helpful to improve temperature gr (...truncated)