超细晶FeCoMoNi系高熵合金的力学性能及磨损机理研究

MECHANICAL PROPERTIES AND WEAR MECHANISM OF ULTRAFINE-GRAINED FeCoMoNi HIGH-ENTROPY ALLOYS

成卓

11849131

硕士

材料学

任富增

2020-05-27

2020-07-01

哈尔滨工业大学

深圳

近年来，高熵合金因在力学性能上的优异表现而备受瞩目。单相面心立方（fcc）结构的高熵合金表现出优越的延展性，但往往在强度上表现不足，从而严重制约了其作为结构材料的发展和应用。为了突破这个发展瓶颈，本课题在以 fcc 结构为基体的高熵合金中充分发挥第二相强化和细晶强化的优势，得到了兼具高压缩强度和断裂应变的高熵合金。具体而言，基于计算相图法辅助设计了 FeCoMoNi 系高熵合金，通过改变合金中 Fe 元素含量，来调控嵌入 fcc 基体中硬质金属间化合物 μ 相的相对含量，并以此优化力学性能。块体合金均采用机械合金化和放电等离子体烧结（SPS）的一次成型工艺制备。制备的 Fe40Co20Mo20Ni20（若无特殊说明合金成分均指原子百分数）高熵合金具有超细晶组织，其中 fcc 基体相和 μ 相的晶粒尺寸分别为 266 nm 和 252 nm。室温压缩测试表明，该合金的屈服强度高达1877 MPa，同时保持了近 40%的断裂应变，该压缩性能优于大多数报道的高熵合金体系，使其在结构材料领域有潜在的应用价值。本课题借助 X 射线衍射（XRD）、扫描电子显微镜（SEM）及透射电镜（TEM）等先进表征技术系统地研究了 Fe40Co20Mo20Ni20 高熵合金压缩变形前后的微观组织演变，揭示了fcc 基体相通过位错滑移开展塑性变形，而高密度的 μ 相通过阻碍位错的运动来增强基体。针对 Fe40Co20Mo20Ni20 高熵合金强化机制的定量分析结果表明，第二相强化和细晶强化占据了主导地位，其次依次为位错强化、孪晶强化和固溶强化。其次，本课题结合材料的服役环境，着重研究了 Fe40Co20Mo20Ni20 高熵合金的在室温下与氧化铝球对磨时的干滑动磨损行为。球-盘式摩擦磨损试验结果表明，Fe40Co20Mo20Ni20 高熵合金的摩擦系数和磨损率随变量（载荷、滑动速度）的增加而增大，在不同载荷和滑行速度下均保持 10-6 ~ 10-5 mm3/(N·m)级的低磨损率，展现出优异的耐磨性能。结合 SEM、EDX 和 3D 轮廓仪分析了磨损表面、磨屑的形貌和成分，用 TEM 深入分析了亚表面的组织演变和变形机理，以及借助 XPS 分析了表面氧化层的价态分布。结果表明，合金的磨损机理包括磨粒磨损、氧化磨损及疲劳磨损，而优异耐磨性能则主要归因于大量 μ 相颗粒在磨损过程中对位错运动进行了有效的阻碍，从而抑制了塑性变形。综上所述，本研究对新型高强及耐磨高熵合金系统地设计、制备、表征和分析有重要的参考价值。

High-entropy alloys (HEAs) have aroused extensive attention in recent years, due to their excellent mechanical performance. Single-phase face-centered-cubic (fcc) HEAs display outstanding plasticity but low strength, which severely hinders their structural applications. In this dissertation, we demonstrate a strategy that takes advantages of second phase strengthening and grain refinement to obtain an exceptional compressive property. Specifically, we firstly designed Fex(CoMoNi)100-x HEAs using the calculated phase diagram method, which indicates that the relative content of ductile fcc matrix and μ phase can be adjusted with varying Fe contents. The combination of mechanical alloying and spark plasma sintering was selected to fabricate a series of bulk HEAs. The as-fabricated Fe40Co20Mo20Ni20 HEA exhibits an ultrafine-grained microstructure, where the average grain sizes of fcc matrix and μ phase are 266 nm and 252 nm respectively. Compression tests at ambient temperature show that the HEA achieve an optimized performance with a yield strength of 1877 MPa and an appreciable fracture strain of ~40%, which outperforms most existing HEAs and shows promise in structural materials. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) studies were carried out to unveil the microstructure evolution of Fe40Co20Mo20Ni20 HEA. The systematic characterization revealed that the fcc matrix provides the plastic accommodation via dislocation slip and the hard μ phase functions as strengthening particles to impede dislocation movements. Quantitative analysis of strengthening contributions in the HEA indicates that the high strength is mainly governed by dispersed μ phase and grain refinement, followed by dislocation strengthening, twinning strengthening and solid solution strengthening. On the other hand, we also focus on the sliding wear behavior of Fe40Co20Mo20Ni20 HEA at room temperature. Ball-on-disk wear tests showed that the HEA exhibits extremely low wear rate of the order of 10-6 ~ 10-5 mm3 /(N·m), upon sliding against alumina ball under different loads and sliding speeds. Systematic studies on morphology, composition of the worn surface and generated debris were performed using SEM, energy dispersive X-ray spectroscopy (EDX) and 3D optical microscopy. Besides, subsurface microstructure evolution was characterized by TEM in detail, while the chemical state of the oxide layer was analyzed using X-ray photoelectron spectroscopy (XPS). The results show that the wear mechanisms remained almost unchanged as primarily abrasive, oxidation and fatigue wear from low to high load and sliding velocity. The massive μ particles are mainly responsible for the excellent wear resistance as they serve as obstacles to both dislocation movements as well as plastic deformation. In summary, this study can provide deep insights into the systematic design, fabrication, characterization and analysis of novel HEAs with ultra -high strength and outstanding wear performance.

高熵合金 力学性能 微观组织 强化机制 磨损行为 磨损机理

high-entropy alloys mechanical properties microstructure strengthening mechanisms wear behavior wear mechanisms

中文

联合培养

南方科技大学

成卓. 超细晶FeCoMoNi系高熵合金的力学性能及磨损机理研究[D]. 深圳. 哈尔滨工业大学,2020.