类Mo型高熔点高熵合金的成分设计及其激光熔覆合成

High-entropy alloys (HEAs) with high melting point elements possess high hardness, high wear resistance as well as high temperature strength, which allow them to be of great application in some fields such as high speed cutting tools. Laser cladding provides a rapid heating and cooling method that easily reaches high temperature and facilitates thick coating. The combination of high melting point HEAs with laser cladding technique is expected to produce a new generation coating technique for heat-resistant coating. In the present proposal, a series of Mo-like HEAs with high melting point are designed, the average Valence Electron Concentration (VEC) and atomic radius of such alloys are close to that of Mo. The Mo-like heat-resistant coating could be expected to be fabricated by laser cladding. The effect of laser cladding technological parameters on the microstructure and properties of coating can be discussed systematically to optimize the technological parameters. Secondly, the influence of Fe and Nb on the microstructure and properties of coating is concerned especially to obtain the dependent relationship between the variation of Fe, Nb and the microstructure and properties of coating, which located in the offside and left side of Mo in the periodic table of the elements, the composition of coating will be optimized. Finally, the evolutional rules of microstructure and properties of coatings under high temperature are investigated, the high-temperature strength mechanism of HEAs coatings could be clarified and a new-type HEAs coating with high melting point can be developed to provide fundamental data for its application for high speed cutting tools.

含有高熔点元素的高熵合金具有高硬度、高耐磨性及抗高温软化等特性，在高速切削用刀具等领域有广阔的应用前景。激光熔覆具有快速加热和冷却、温度高以及易于实现厚层涂覆等特点，高熔点高熵合金与激光熔覆的结合有望成为新一代耐高温涂层技术。本项目拟设计出系列类Mo型高熔点高熵合金，即合金的平均价电子浓度和原子半径接近Mo元素。期望采用激光熔覆技术制备出类似Mo的耐高温涂层。系统探讨激光熔覆工艺参数对涂层组织与性能的影响规律，获得组织与性能最佳的涂层，实现工艺参数的优化；其次，重点关注在元素周期表中位于Mo右侧的Fe和左侧的Nb的含量对涂层组织及性能影响，获得Fe、Nb含量变化与组织及性能之间的相互依赖关系，实现涂层的成分优化。最后，研究高温下的涂层组织结构的演化规律与涂层高温性能，揭示涂层的高温强化机制，发展一种新型高熔点高熵合金涂层，为在高速切削刀具上的应用提供借鉴。

为了获得高耐磨、优良高温抗软化和抗氧化的高速切削刀具涂层材料，从平均原子半径和平均价电子浓度的角度，并结合高熵合金固溶体热力学经验判据和Miedema金属间化合物形成焓计算规则，设计了类Mo型高熔点高熵合金成分。采用激光熔覆技术在M2工具钢表面制备MoFexCrTiWAlNby高熵合金涂层。首先，研究了激光工艺参数对涂层组织及性能的影响规律，揭示涂层各相形成机理及高硬度高耐磨性增强机制。研究发现高熵合金涂层最优熔覆工艺参数为：P=3.0 kw，v=4mm/s。其主要相结构为BCC + C14 - Laves + MC +未熔W颗粒相。高熵合金涂层具有较高的室温硬度(~820 HV)。涂层室温下也表现出优良的耐磨性，其磨损机制主要为磨粒磨损。其次，通过同时改变Fe和Nb含量，研究了Fe和Nb对MoFexCrTiWAlNby高熵合金涂层的组织结构及性能的影响规律，优化了类Mo型高熵合金涂层成分，并研究MoFe1.5CrTiWAlNb1高熵合金涂层在相同时间不同退火温度及相同退火温度不同退火时间条件下组织结构及性能演化规律，考察了其高温热稳定性和抗氧化性能。涂层的相结构并不随Fe和Nb含量的改变而发生变化，但随着Nb含量增加，涂层中C14相明显上升，且显微组织随着Nb含量增加逐渐由胞状树枝晶向共晶和过共晶组织转变。同时随着C14相的增多，涂层裂纹数量显著升高。其中，Fe1.5Nb高熵合金涂层具有均匀的显微组织、低的开裂敏感性以及极高的平均显微硬度 (913.5 HV)。Fe1.5Nb1涂层在600 ℃退火开始，其BCC相基体中不断析出C14相。其中在650℃退火时，涂层内部C14相颗粒的尺寸和体积分数达到最优的分配，因此表现出最高的硬度和耐磨性。总体来说，Fe1.5Nb1涂层在800℃具有优良的抗高温软化性能。Fe1.5Nb1高熵合金涂层在800 ℃高温下，其表面会形成致密且稳定性好的氧化膜，表现出优异的抗高温氧化特性。另外，随着保温时间的延长，涂层晶间BCC相中脱溶析出的Laves相不断粗化，内应力不断减小，第二相强化效果基本消失，导致显微硬度和耐磨性能发生明显下降，且磨损机制由磨粒磨损和氧化磨损向黏着磨损转变。