晶界在钕铁硼无液相变形织构化中的作用及其调控机理

Strong-textured nanograin magnets have a promising future for their potential-high coercivity and high energy product. By breaking the strong texture’s dependence on liquid phase in Nd-Fe-B magnets, the slow deformation technology produces high-performance nanograin Nd-Fe-B magnets by realizing strong textures in conditions of low temperature deformation without liquid phase. However, the strong texture mechanism in the absence of liquid phase is still unclear, and the key factors that affect significantly the texture strength are also unknown. This project takes Nd-rich, Nd-near stoichiometric, and Nd-lean Nd-Fe-B magnets, which have different rare earth contents, as research objects. The magnets were subjected to low-temperature slow deformations in the absence of liquid phase, and the effects of boundary types and amount of grain boundaries on the texture strength of the deformed magnets were studied. By means of magnetic analysis, fracture morphology analysis, in-situ pressure analysis, phase melting behavior analysis, and microstructural change analysis of magnets before and after deformation, this project is focused on the intrinsic relationship among the texture strength, the relative strength of grain boundaries and grains, the bonding strength of grain boundaries, the types of grain boundaries activity, and activity depth of each-type grain boundaries. From the point of view of the grain boundary activity and the completion degree of each grain boundary motion, the mechanisms of strong texture formation and texture regulation in the absence of liquid phase will be revealed, the key microstructure parameters that affects significantly the texture strength will be found, and the reasons why Nd-stoichiometric and Nd-lean magnets are difficult to texture in conventional deformation will be explained. As a consequence, experimental and theoretical guidance will be provided for developing high-performance nanograin magnets by directional design of initial microstructures.

纳米晶强织构磁体因具有潜在的高矫顽力和高磁能积而颇具发展前途。低速变形技术克服了钕铁硼磁体强织构对液相的依赖，实现了低温无液相条件下的变形强织构化，制备出高性能纳米晶钕铁硼磁体，但无液相下的强织构机理尚不清楚，哪些微结构参数是影响织构强度高低的关键因素也不了解。本项目以具有不同稀土含量的富分、近正分、贫分钕铁硼磁体为研究对象，研究晶界类型和晶界数量对无液相条件下低速变形磁体织构强度的影响规律。利用磁性分析、断口形貌分析、原位压力分析、相熔化行为分析、以及变形前后微结构变化分析等手段，重点研究织构强度与界晶相对强度、晶界结合力、晶界活动类型、不同晶界活动进行深度的内在关系，从晶界活性和晶界活动完成程度角度揭示无液相条件下的强织构化机理和织构调控机理，找出影响织构强度的关键微结构参数，并阐明常规变形时正分、贫分磁体难织构化的原因，为磁体的定向结构设计和高性能纳米晶磁体的开发提供实验和理论指导。

纳米晶强织构磁体因具有潜在的高矫顽力和高磁能积而颇具发展前途。低速变形技术克服了钕铁硼磁体强织构对液相的依赖，实现了低温无液相条件下的变形强织构化，制备出高性能纳米晶钕铁硼磁体，但无液相下的强织构机理尚不清楚，哪些微结构参数是影响织构强度高低的关键因素也不了解。本项目着眼晶界这个新视角，以具有不同稀土含量的富分、近正分、贫分钕铁硼磁体为研究对象，一方面重点研究了晶界类型、晶界数量等晶界参数对无液相条件下低速变形磁体织构强度的影响规律，确立了富稀土硬磁晶界类型和大数量晶界数量等晶界条件对无液相条件下强织构形成和提升的关键调控作用。另一方面，重点分析了不同磁体的磁性特征、断口形貌、原位压力变化、相熔化行为、以及变形前后微结构变化等信息，发现了小于1的界晶相对强度是强织构形成的首要前提，且更小的晶界结合力和更小界的晶相对强度，利于获得更强织构，进而说明较高的晶界活性是强织构形成的前提，且晶界活性越高，越容易获得强织构；与此同时，较高的晶界活性下，晶界的运动类型不再是发生在晶粒集合周围的剪切位移式，而是转变为单个晶粒周围的定向界扩散和和定向滑移式，这种定向的晶界扩散和定向滑移，会引发晶粒的定向变形和定向转动，最终形成晶粒c轴沿压力方向的择优排列，这也是无液相下强织构形成的主要机理；另外，在晶界活性较高且恒定的条件下，以定向晶界扩散和定向滑移为主的晶界活动，其进行程度也随外界参数的变化出现差异，晶界活动进行程度越深，磁体的织构就越强，这也是晶界数量变多、变形速率减小、变形量增加等因素变化时织构增强的原因。本项目不仅确立了晶界在钕铁硼无液相变形织构化中的关键作用，而且阐明了无液相下强织构形成的基本条件和实现强织构增强的基本原理，同时也对常规变形时正分、贫分磁体难织构化传统难题给出了较为清晰的认识和解释，进而为未来纳米晶磁体的定向结构设计和高性能纳米晶磁体的开发提供了实验和理论指导。