Microstructure and phase stability analysis of laser cladding CoCrCu0.4FeNi high entropy alloy coatings
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摘要:
为提高零部件的硬度和耐磨性，采用Co、Cr、Cu和Ni单质金属粉末在Q235钢基体上激光熔覆CoCrCu0.4FeNi高熵合金涂层，利用扫描电镜、能谱仪和X射线衍射仪分析了涂层的微观组织，测试了涂层的显微硬度，并利用第一性原理计算了涂层中各相的晶格常数和弹性常数。结果表明，涂层与基体形成了良好的冶金结合，且无宏观裂纹和气孔等缺陷；涂层微观组织主要由树枝晶和枝晶间组成，其中树枝晶为一种面心立方相（FCC1），富Cu贫Cr，枝晶间为另一种面心立方相（FCC2），富Cr贫Cu。涂层厚度约为1.50～1.98 mm，涂层枝晶大小约为7.9～10.4 μm。涂层的显微硬度约为HV0.2 170～230，约为基体1.7倍，随着与涂层表面距离的增加，涂层的硬度逐渐降低。另外，激光功率越低，扫描速度越大，树枝晶越细小，细晶强化的作用越强，涂层的硬度越高。涂层中面心立方（FCC）相的晶格常数计算值与实验值误差为1.33%～2.60%，FCC相的生成热均为负值，且弹性常数C11、C12和C44满足立方结构高熵合金的力学稳定性限制条件，可知FCC相是稳定的。由剪切模量与体积模量之比（G/B）＜0.57、泊松比（ν）＞0.26可知，树枝晶和枝晶间处的FCC相总体呈现韧性特征。从涂层下部到上部，计算的弹性模量逐渐增加，硬度增大，与实验硬度变化规律相符合。
Abstract:To improve the hardness and wear resistance of parts, the Co, Cr, Cu, and Ni elemental metal powders were used to prepare the CoCrCu0.4FeNi high entropy alloy coatings on Q235 steel substrate by laser cladding. The microstructure of the high entropy alloy coatings was analyzed by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD), and the microhardness of the coatings was measured. The lattice and elastic constants of each phase in the coatings were calculated by the first principle. The results show that, the coatings show the good metallurgical bond with the substrate without the macro-cracks and pores. The microstructure of the coatings is mainly composed of dendrite and interdendrite. The dendrite is one kind of face-centered cubic phase (FCC1), rich Cu and poor Cr, the interdendrite is another kind of face-centered cubic phase (FCC2), rich Cr and poor Cu. The thickness of the coatings is about 1.50~1.98 mm, and the size of the coating dendrite is about 7.9~10.4 μm. The microhardness of the coatings is about HV0.2 170~230, about 1.7 times that of the substrate, and the hardness of the coatings gradually decreases with the increase of the distance from the coating surface. In addition, the lower the laser power, the higher the scanning speed, the finer the dendrite, the stronger the effect of fine grain strengthening, and the higher the hardness of the coating. The error between the calculated lattice constant and the experimental value of face-centered cubic (FCC) phases in the coatings is 1.33%~2.60%. The formation heat of FCC phases is negative, and the elastic constants C11, C12, and C44 meet the mechanical stability constraints of the cubic high entropy alloy, showing the FCC phases are stable. The FCC phases at the dendrite and interdendrite are generally present as the characteristics of toughness according to the ratio of shear modulus to bulk modulus (G/B)＜0.57 and Poisson's ratio (ν)＞0.26. From the lower part to the upper part of the coatings, the calculated elastic modulus and hardness increase gradually, which is consistent with the variation law of the experimental data.
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Keywords:
- laser cladding /
- high-entropy alloys /
- microhardness /
- coatings /
- first-principles
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图 1 未球磨和球磨Co、Cr、Cu和Ni混合金属粉X射线衍射图谱
Figure 1. XRD patterns of the Co, Cr, Cu, and Ni mixed powders with and without ball milling
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图 3 CoCrCu0.4FeNi涂层宏观形貌：（a）试样A；（b）试样B；（c）试样C；（d）试样D
Figure 3. Macro morphology of the CoCrCu0.4FeNi coatings: (a) sample A; (b) sample B; (c) sample C; (d) sample D
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图 4 CoCrCu0.4FeNi涂层线扫描图谱：（a）试样A；（b）试样B；（c）试样C；（d）试样D
Figure 4. Line scanning spectrum of the CoCrCu0.4FeNi coatings: (a) sample A; (b) sample B; (c) sample C; (d) sample D
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图 5 CoCrCu0.4FeNi涂层与基体结合区显微形貌：（a）试样A；（b）试样B；（c）试样C；（d）试样D
Figure 5. SEM images in the bonding area between the CoCrCu0.4FeNi coatings and the substrate: (a) sample A; (b) sample B; (c) sample C; (d) sample D
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图 6 CoCrCu0.4FeNi涂层下部、中部和上部显微形貌：（a）试样A；（b）试样B；（c）试样C；（d）试样D
Figure 6. SEM images of the CoCrCu0.4FeNi coatings in the lower, middle, and upper part: (a) sample A; (b) sample B; (c) sample C; (d) sample D
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图 7 CoCrCu0.4FeNi涂层下部、中部和上部高倍显微形貌：（a）试样A；（b）试样B；（c）试样C；（d）试样D
Figure 7. Enlarged SEM images of the CoCrCu0.4FeNi coatings in the lower, middle, and upper part: (a) sample A; (b) sample B; (c) sample C; (d) sample D
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图 8 显微硬度压痕：（a）涂层压痕；（b）基体压痕
Figure 8. Indentation of the microhardness test: (a) coating indentation; (b) substrate indentation
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图 9 试样CoCrCu0.4FeNi涂层显微硬度
Figure 9. Microhardness of the CoCrCu0.4FeNi coatings on the samples A~D
表 1 Q235钢基体化学成分（质量分数）
Table 1 Chemical composition of the Q235 steel substrate
% C Si Mn S P Fe 0.160 0.080 0.021 0.012 0.015 余量 
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表 2 涂层下部、中部和上部能谱分析（原子分数）
Table 2 EDS analysis of the CoCrCu0.4FeNi coatings in the lower, middle, and upper part
% 元素 Co Cr Cu Fe Ni A1 19.1 17.7 5.4 41.0 16.8 A2 16.9 27.1 4.8 38.0 13.2 A3 28.4 21.6 7.2 20.6 22.2 A4 24.3 36.9 5.9 16.8 16.1 A5 27.3 23.8 7.3 19.4 22.2 A6 24.4 30.0 7.1 19.1 19.4 B1 24.3 23.7 7.7 22.0 22.2 B2 22.8 34.3 4.6 19.4 18.9 B3 28.2 24.9 7.6 16.0 23.3 B4 26.7 37.6 3.8 15.0 16.9 B5 29.3 21.5 8.0 16.5 24.8 B6 26.2 32.7 4.8 16.3 20.0 C1 17.3 15.2 4.2 50.8 12.5 C2 19.4 25.9 3.4 36.6 14.8 C3 26.7 21.0 8.2 21.9 22.3 C4 25.4 30.2 6.0 19.5 18.9 C5 27.6 22.6 7.8 18.8 23.2 C6 27.9 35.2 4.8 14.4 17.7 D1 25.8 19.8 7.8 28.5 18.0 D2 23.6 34.5 5.3 22.7 13.9 D3 26.8 22.8 7.3 20.5 22.6 D4 25.5 30.9 6.2 16.7 20.7 D5 29.3 24.0 8.5 16.3 21.9 D6 27.4 35.8 4.5 14.6 17.7
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表 3 FCC和BCC结构优化后的单原子基态能量
Table 3 Monatomic ground state energy after the FCC and BCC structure optimization
eV 位置 FCC BCC D1 − 3249.29 − 3249.01 D2 − 3106.14 − 3105.87 D3 − 3287.32 − 3287.06 D4 − 3201.61 − 3201.34 D5 − 3247.98 − 3247.73 D6 − 3172.63 − 3172.42
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表 4 D1～D6处相的晶格常数
Table 4 Lattice constants of the phases at D1~D6
位置 晶格常数计算值 / Å 计算值与实验值误差 / % D1 3.492 1.47～2.54 D2 3.497 1.33～2.40 D3 3.490 1.52～2.60 D4 3.492 1.47～2.54 D5 3.494 1.43～2.48 D6 3.490 1.52～2.60 注：误差=|实验平均值−计算值|/实验平均值
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表 5 D1～D6处相的基态总能量和生成热
Table 5 Total ground state energy and formation heat of the phases at D1~D6
位置 基态总能量，Etotal / eV 生成热，Eform / eV D1 − 12997.16 − 9263.95 D2 − 12424.56 − 9215.07 D3 − 13149.28 − 9740.94 D4 − 12806.44 − 9486.65 D5 − 12991.92 − 9612.07 D6 − 12690.52 − 9411.57
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表 6 D1～D6处相的弹性常数
Table 6 Elastic constants of the phases at D1~D6
位置 C11 / GPa C12 / GPa C44 / GPa (C11−C12) / GPa C11+2C12 / GPa D1 332.92 180.68 189.50 152.24 694.28 D2 353.45 190.64 187.36 162.81 734.73 D3 339.79 179.85 197.75 159.94 699.49 D4 341.07 189.36 200.93 151.71 719.79 D5 328.61 178.22 184.21 150.39 685.05 D6 357.18 193.07 207.43 164.11 743.32
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表 7 D1～D6处相的体积模量、剪切模量、杨氏模量、泊松比和G/B
Table 7 Bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and G/B of the phases at D1~D6
位置 体积模量，B / GPa 剪切模量，G / GPa 杨氏模量，E / GPa 泊松比，ν G/B BV BR BH GV GR GH EV ER EH νV νR νH D1 231.42 231.42 231.42 144.15 118.75 131.45 358.10 304.22 331.57 0.242 0.281 0.261 0.568 D2 244.91 244.91 244.91 144.98 123.21 134.10 363.26 316.56 340.20 0.253 0.285 0.268 0.548 D3 233.16 233.16 233.16 150.64 124.44 137.54 371.83 316.94 344.82 0.234 0.273 0.254 0.589 D4 239.93 239.93 239.93 150.90 121.08 135.99 374.24 310.93 343.14 0.240 0.284 0.262 0.557 D5 228.35 228.35 228.35 140.61 116.60 128.60 349.99 298.92 324.83 0.245 0.282 0.263 0.563 D6 247.77 247.77 247.77 157.28 128.74 143.01 389.44 329.21 359.81 0.238 0.279 0.258 0.577
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