【作者】 马斌；

【导师】 高学绪；

【作者基本信息】 北京科技大学 ， 材料科学与工程， 2021， 博士

【摘要】 烧结Nd-Fe-B磁体在新能源汽车、风力发电、消费电子等领域应用日益广泛,对磁体性能的要求也越来越高。目前普遍采用速凝铸片(SC)+氢破(HD)+气流磨(JM)的方法制备烧结Nd-Fe-B用磁粉,但该法制备的磁粉存在表面富Nd相包覆不均匀、尺寸较大且分布不均匀、颗粒形状不规则等缺陷,制约了烧结后磁体的细晶化及富Nd相的均匀分布,从而限制了磁体性能特别是矫顽力的提升。针对这一现状,本文设计了一种HDDR-G(HDDR+晶体生长)制备Nd-Fe-B磁粉的技术,期望获得平均尺寸2.0-3.0 μm且粒度分布窄、颗粒形状规则、颗粒表面富Nd相包覆好的Nd-Fe-B磁粉,为制备高性能烧结Nd-Fe-B磁体奠定基础。研究了三元 Nd-Fe-B 和多元 Nd-Fe-B-M(M=Cu、Al、Ga、Co、Zr)速凝铸片的HDDR反应及反应过程的组织演变。研究表明,吸氢过程中氢气首先与晶间富Nd相发生氢化反应,并沿着富Nd相通道进入速凝铸片内部,歧化反应从速凝铸片表面和内部富Nd相通道同时开始,并扩展到整个速凝铸片。HDDR过程中,速凝铸片柱状晶被细化成Nd2Fe14B等轴晶,富Nd相经过扩散和迁移,重新分布在Nd2Fe14B等轴晶的角隅处和晶界处。研究了 HDDR处理后Nd2Fe14B晶粒的生长行为及影响晶粒长大的因素。复合形核后,Nd2Fe14B晶粒先是均匀长大,然后出现晶粒异常长大现象。Nd-Fe-B合金中Nd含量的增加,有利于富Nd润湿晶界的形成,使晶界迁移由固态原子跃迁逐渐向液态原子扩散转变,提高晶界迁移率,促进晶粒均匀长大;温度升高一方面能大大提高晶粒生长速率,促进晶粒均匀长大,另一方面也会缩短晶粒异常长大孕育期,使异常晶粒提早出现。通过延长晶粒异常长大孕育期(高温短时+低温长时热处理技术)和提高晶粒生长速率获得了平均尺寸2.0-3.0μm且粒度分布窄、形状规则、表面富Nd相包覆均匀的Nd2Fe14B等轴晶组织。揭示了 HDDR Nd2Fe14B晶粒生长机制,阐明了抑制晶粒异常长大的机理。HDDR Nd-Fe-B合金中与大体积角隅富Nd相接触的Nd2Fe14B晶粒会形成facet光滑界面,并发展成异常晶粒。Faceted Nd2Fe14B晶粒具有最大晶粒生长驱动力Δgmax,当△gmax小于晶粒异常长大临界驱动力△gc时,不会出现晶粒异常长大现象,当Δgmax大于Δgc时,faceted Nd2Fe14B晶粒会快速长大成异常晶粒。抑制Nd2Fe14B晶粒异常长大的方法是调控Δgmax和Δgc的值,使Δgmax低于Δgc。温度升高会降低Δgc,高温热处理Δgc值较低,晶粒异常长大孕育期较短,在△gmax即将增至Δgc时进行降温处理使△gmax重新低于Δgc,从而延长了 Nd2Fe14B晶粒异常长大的孕育期,抑制了晶粒的异常长大。采用氢破、球磨的方法制备了 HDDR-GNd-Fe-B磁粉,磁粉具有颗粒形状规则、粒度分布窄、表面富Nd相包覆良好的优点。三元Nd-Fe-B磁粉d50、粒度分布d90/d10值分别为2.26 μm、4.07,多元Nd-Fe-B-M磁粉的d50、粒度分布 d90/d10值分别为 1.66 μm、3.52。更多还原

【Abstract】 Sintered Nd-Fe-B magnets are widely used in the fields of hybrid/electric vehicles,wind turbines and consumer electronics.The improvement of performance,especially the coercivity,is still the research emphasis of Nd-Fe-B magnets.The Nd-Fe-B powders for sintered Nd-Fe-B magnets are generally prepared by strip casting,hydrogen decrepitation and jet milling processes.However,the powders exhibit characteristics of large particle size,wide size distribution,irregular particle shape and poor Nd-rich phase coating on particle surface,which result in grain coarsening and uneven distribution of Nd-rich phases in sintered magnets,and thereby restrict the improvement of magnet performance.In this study,a new HDDR-G technology(HDDR and Growth)is designed and utilized to prepare Nd-Fe-B single-crystal powders with 2.0-3.0 μm,narrow size distribution,regular particle shape and good Nd-rich phase coating on particle surface.The HDDR reactions and microstructure evolution of ternary Nd-Fe-B and multi-element Nd-Fe-B-M(M=Cu,Al,Ga,Co,Zr)strip cast alloys were studied.During hydrogen absorption process,hydrogen firstly reacts with the Nd-rich phases and enters into alloy interior along Nd-rich phase channels.Then disproportionation reaction starts from the surface of Nd-Fe-B strip cast alloy and the internal Nd-rich phase channels,finally propagates the entire Nd-Fe-B strip cast alloy.In HDDR process,not only the columnar grains are refined into Nd2Fe14B equiaxed grains,the Nd-rich phases are also refined and redistributed at triple junctions or along the grain boundaries.The growth behavior of HDDR Nd2Fe14B grains and the influence factors for grain growth were investigated.After recombination reaction,Nd2Fe14B grains grow up normally and then abnormal grain growth occurs.The increase of Nd content in HDDR Nd-Fe-B alloy is conducive to the formation of Nd-rich wetting grain boundaries,which promotes the grain growth due to the change of grain boundary migration from solid atom transition to liquid atom diffusion.The increase of temperature can greatly improve the grain growth rate,but it also shortens the incubation period for abnormal grain growth and make abnormal grains appear earlier.The Nd2Fe14B equiaxed grains with 2.0-3.0 μm,narrow size distribution,regular shape and uniform distribution of Nd-rich phases were obtained by prolonging the incubation period for abnormal grain growth and increasing the grain growth rate.The growth mechanism of HDDR Nd2Fe14B grains was revealed and the control mechanism for abnormal grain growth was also clarified.In HDDR Nd-Fe-B alloy,the Nd2Fe14B grain that directly contacts with the large liquid triple junction forms smooth facet interface and develops into abnormal grain.The faceted Nd2Fe14B grain presents the maximum driving force △gmax.When △gmax is less than the critical driving force for abnormal grain growth △gc,the faceted Nd2Fe14B grain will not grow abnormally.As △gmax is greater than △gc,the faceted Nd2Fe14B grain will grow into abnormal grain.The strategy for suppressing abnormal grain growth is adjusting Agmax and △gc to make △gmax lower than △gc.Increasing temperature will reduce △gc and shorten the incubation period for abnormal grain growth.The faceted Nd2Fe14B grains at higher temperature exhibits lower △gc,when △gmax is about to increase to △gc,the rapid cooling treatment reduces △gmax to below △gc again,thereby prolongs the incubation period for abnormal grain growth and suppresses the abnormal grain growth.The HDDR-G Nd-Fe-B powders were prepared by hydrogen decrepitation and ball milling treatments.The Nd-Fe-B powders exhibit narrow size distribution,regular particle shape and good Nd-rich phase coating on particle surface.The d50 size,size distribution of d90/d10 for ternary Nd-Fe-B powders are 2.26μm,4.07 while the d50 size,size distribution of d90/d10 for multi-element Nd-Fe-B-M powders are 1.66 μm,3.52.更多还原