高熵CoCrNiFeBSi合金非晶熔覆层脉冲激光制备原理及磨损机理

A large number of possibilities are the advantages of high-entropy alloy materials. In this project, the independently developed CoCrNiFeBSi high-entropy alloy materials with comparatively excellent glass forming ability are organically combined with pulsed high-energy beam processing method to develop a new technology of fabricating laser-cladded amorphous coatings with excellent properties. The microstructure and mechanical properties of the coatings with gradient microstructure are explored while the thermal stability of the amorphous phase is evaluated. The formation mechanism of the amorphous phase in the coatings under the condition of pulsed laser cladding is revealed based upon the thermodynamics and kinetics concerning the formation of amorphous phase. The mapping function from laser cladding process parameters to amorphous phase content is built together with that from amorphous phase content to wear weight loss. Then with the microstructure characteristics of the coatings as the core link, the theory system will be finally established about manipulating the mechanical properties through adjusting laser cladding parameters and the chemical composition of cladded materials. The acceleration of mechanical wear caused by the crystallization behavior of the amorphous layer is evaluated under the condition of high-temperature friction and wear tests. The high-temperature wear damage style in the coatings is proposed to be the coupling of crystallization and mechanical wear damage. The calculation model of high-temperature wear loss will be established and verified. The research results of the project can provide a theoretical basis for the research of failure behaviors of amorphous composites under the condition of high-temperature services, the design of surface engineering materials and the integrated control of forming and performance of coatings. The research results also offer researchers a novel idea about manufacturing or re-manufacturing the surface of key structures made of different alloys.

大量的可能性是高熵合金材料的优点。本项目将自主研发的非晶形成能力较高的CoCrNiFeBSi系高熵合金材料与脉冲高能束加工技术进行有机结合，开发一种制备高性能非晶激光熔覆层的新技术。研究梯度结构覆层的微观组织、力学性能特征及非晶相的热稳定性；根据非晶相形成的热力学及动力学理论，揭示脉冲激光熔覆条件下，覆层中非晶相的形成机理。基于覆层非晶相含量与激光熔覆工艺参数及覆层磨损失重之间的映射函数，构建以覆层微观组织特征为中心，通过改变激光熔覆工艺参数和熔覆材料成分调控覆层力学性能的理论体系。探索高温磨损条件下覆层非晶区的晶化行为对机械磨损的促进作用，提出覆层的高温磨损形式系晶化/磨损耦合损伤，建立并验证高温磨损失重计算模型。项目研究成果可为非晶复合材料高温服役失效行为的研究、表面工程材料的设计及覆层成形—成性一体化控制提供理论基础，并为不同合金材料关键结构表面的制造及再制造提供一种新思路。

非晶合金具有晶态金属材料所不具备的优异性能，如超高的强度和硬度、卓越的耐磨性及耐腐蚀性等，可显著提高发动机、舰船关键零部件、热作模具和采石采油结构件的耐磨、耐蚀及高温性能。项目将自主研发的高熵合金材料与高能束加工技术进行有机结合，在热作模具钢、钛合金等金属基体表面成功制备了不同体系的高熵合金非晶覆层，基于焊接过程数值模拟对激光熔覆过程进行工艺优化；研究了非晶覆层的显微织构和物相组成、非晶覆层的热稳定性、力学性能特征及非晶组织的形成机理，最后，揭示了熔覆粉末中主要合金元素含量对非晶覆层组织与性能的影响规律。研究结果表明：激光熔覆过程中，覆层中心区域温度随时间呈锯齿状变化，峰值温度随熔覆道次的增加而提高。由覆层表面向覆层中心，峰值温度和冷却速率均降低，最高峰值温度和最大冷却速率分别达到2100℃和104K/s。根据微观组织特征将覆层划分为三个特征区域：顶部非晶区、中部非晶/等轴晶混合区及底部等轴晶/柱状晶混合区；熔覆层微区的热循环特征为不同组织特征区域的形成提供了必要条件，与模拟所得的冷却条件相吻合，满足非晶形成能力临界冷却速率要求。温度场的模拟计算结果及检测结果为应力、应变场控制技术、裂纹产生机制及非晶组织形成机制的研究提供了理论依据。.CoCrFeNiSiB系非晶覆层的玻璃化温度、晶化温度和过冷液相区宽度分别为300oC、350oC和44oC。分析了非晶相结晶过程激活能与覆层成分的内在关联，非晶覆层的热稳定性取决于覆层的成分。非晶区的显微硬度最高约为1150HV0.2，500oC摩擦磨损条件下，非晶区的磨损机制为磨粒磨损+少量氧化磨损+微量脆性剥落。CoCrFeNiSiB系非晶覆层底部枝晶区中的基体相包括(Fe, Ni)固溶体相和(Co, Fe)固溶体相，析出相包括：Ni-Ni3B共晶组织、硼化物及碳化物。随Co原子含量减少，等轴晶尺寸由2-3μm减小至约1μm；覆层中部非晶/等轴晶混合区中CoFe15.7和γ(Fe, Ni)相含量减少，非晶相含量升高，显微硬度值明显升高，高温摩擦系数和磨损率均显著降低，高温磨损表面氧化物尺寸由约400μm减小至20μm，表面磨损机制表现为磨粒磨损+氧化磨损。常温冲击磨损条件下，非晶相先于晶化相从覆层表面脱落，并形成“雪花瓣”状二维形貌。项目研究成果可为非晶复合材料高温服役失效行为的研究、表面工程材料的设计及覆层成形—成性一体化控