双相高熵合金熔覆层超声冲击塑性变形机制及强化机理研究

High entropy alloy possesses unique structure and excellent properties and laser cladding can achieve the high strength metallurgical bonding has become an important research direction in the field of high entropy alloy coating preparation. The ultrasonic impact treatment is carried on the surface of AlxCoCrFeMnNi dual-phase high entropy alloy coating to clarify the plastic deformation mechanism of laser cladding high entropy alloy coating with high entropy effect and lattice distortion effect, and improve the surface quality and properties. In this study, the forming and controlling mechanism of FCC+BCC dual-phase solid solution are proposed based on the calculation of thermodynamic parameters for FCC+BCC dual-phase high entropy alloy and the fast solidification kinetics analysis of laser cladding process. The stacking fault energy of FCC phase is analyzed by the first principle on the base of the theory of density functional theory, and the microstructure evolutions and synergetic deformation behavior of FCC phase and BCC phase under ultrasonic impact treatment are studied, and then the plastic deformation mechanism of dual-phase high entropy alloy coating with strong lattice distortion effect. In addition, combined with the strain induced phase transition behavior analysis, the grain refinement mechanism and strain hardening mechanism of dual-phase high entropy alloy coating under strong lattice distortion effect induced by ultrasonic impact treatment with high strain rate are revealed. The relationship between the performance of the strengthening layer, microstructural evolution and plastic deformation mechanism is established. Finally, the strengthening mechanism of laser cladding dual-phase high entropy alloy coating by ultrasonic impact treatment is clarified. This research will bring a breakthrough in the plastic deformation theory of the dual-phase high entropy alloy coating and provide a new way to optimize the performance of high entropy alloy coatings.

高熵合金具有独特的结构及优异的性能，激光熔覆能够实现高强度冶金结合成为高熵合金涂层制备领域重要的研究方向。本项目提出采用超声冲击处理AlxCoCrFeMnNi双相高熵合金熔覆层表面，拟厘清具有高熵效应和晶格畸变效应的高熵合金层塑性变形机制，并提高熔覆层表面质量和性能。基于FCC+BCC双相高熵合金热力学参数计算，从凝固动力学角度提出快速凝固高熵合金熔覆层双相固溶体形成与控制机制；采用第一性原理方法计算高熵合金熔覆层FCC相层错能，对比分析FCC相和BCC相的微结构演变及协同变形行为，揭示强烈晶格畸变效应下双相高熵合金层超声冲击塑性变形机制；结合应变诱导相变行为分析，探讨超声冲击双相高熵合金层晶粒细化及加工硬化机制；建立强化层性能与微观结构及塑性变形机制之间的相关性，阐明双相高熵合金熔覆层超声冲击强化机理。本研究拟实现双相高熵合金层塑性变形理论的突破，并为优化高熵合金涂层性能提供新思路。

本项目采用激光熔覆制备AlxCoCrFeMnNi、AlxCoCrFeCuNi、CoCrFeNiMox高熵合金层，系统研究相结构形成规律、超声冲击前后熔覆层组织及性能，阐明双相高熵合金层塑性变形机制及强化机理。Al含量增加导致AlxCoCrFeMnNi和AlxCoCrFeCuNi高熵合金层由单相FCC向双相FCC+BCC转变。单相FCC结构Al0.5CoCrFeMnNi高熵合金层超声冲击后硬度和耐磨性提高，抗晶间腐蚀能力提升。FCC+BCC双相Al1.5CoCrFeMnNi熔覆层超声冲击后相组成未改变，表面粗糙度降至0.28μm；纳米压痕硬度由4.59GPa提高到6.40GPa，弹性模量下降约39.4%；超声冲击后晶粒细化有利于钝化膜形成，耐蚀性提高6倍。AlxCoCrCuFeNi（x=0.5-1.25）高熵合金层均由FCC+BCC组成，随Al含量增加BCC相增多，强烈的晶格畸变效应导致XRD衍射峰向左移。当x=1.0时高熵合金层硬度约530HV0.2，冲击层硬度提高至715HV0.2，深度约15μm，冲击强化层内晶粒尺寸约18.6μm，表面粗糙度低至0.157μm，耐蚀性能明显提升。对于CoCrFeNiMox（x=0-0.75）高熵合金熔覆层，当x=0、0.25时，涂层为单相FCC结构，Mo0.5及Mo0.75为FCC+σ双相结构，且随Mo含量增加晶格常数增大。Mo0熔覆层内均为柱状晶，而Mo0.25、Mo0.5及Mo0.75熔覆层中上部为等轴晶，底部为树枝晶，Mo元素在晶界偏析。随Mo元素含量增加熔覆层硬度由178HV0.1增大到474HV0.1，耐磨性和耐腐蚀性能明显提高。超声冲击后表面强化层内显微组织更加细小、致密，晶间网状分布的σ相发生破碎并产生条带状褶皱，表面硬度最大提升119.61%，耐磨性显著提高，磨损机制由磨粒磨损与疲劳磨损转变为单一磨粒磨损。EBSD和TEM分析表明，双相高熵合金层超声冲击过程中表面发生严重晶粒变形，位错增殖并不断缠结转变为小角度晶界，位错在小角度晶界处堆积最终转变为大角度晶界、分割、细化晶粒，因此超声冲击高熵合金层强化机理主要为位错强化和细晶强化。通过本项目研究，明确了高熵合金熔覆层相形成机制、超声冲击塑性变形机制及强化机理，为高熵合金涂层的研究提供了理论依据，为石油钻采装备表面强化提供了重要指导。