【作者】 唐翠勇；

【导师】 肖志瑜；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2012， 博士

【摘要】 铁基非晶及纳米晶合金具有优异的软磁、力学性能，低廉的价格，在功能材料和结构工程材料领域具有广泛的应用前景。然而，非晶形成能力较低，两次晶化温度区间过窄，非晶相居里温度偏低以及非晶合金缺少室温塑性，严重限制铁基非晶合金的广泛应用。另外，非晶/纳米晶合金的导磁、矫顽力机理和材料脆性断裂机制也不明晰。因此，开发新型具有优异软磁、力学性能的大块铁基非晶合金及纳米晶复合材料成为材料学界研究的热点，具有重要的应用和学术意义。本文采用“Miedema”理论、“元素替代”、“共晶法则”和“模量判据”进行成分设计，通过单辊快淬甩带工艺和机械合金化工艺制备出具有优异软磁性能的非晶合金薄带和全金属组元的铁基非晶合金粉末，采用放电等离子烧结（SPS）成形高密度大块铁基非晶合金及纳米晶复合材料。研究不同球磨、烧结工艺以及退火工艺对材料的微观组织，软磁、力学性能的影响；并对非晶合金粉末的晶化动力学、致密化过程和导磁、脆断机制进行研究。通过半经验性热力学“Miedema”理论计算了Fe与常见过渡族金属元素和类金属元素形成中间化合物的形成焓，利用“元素替代”、“共晶法则”和“模量判据”优选合金成分，成功开发出具有优异软磁性能的Fe₈₁Cu₂Nb₃Si₁₄，Fe₆₉Co₈Nb_7-xV_xB₁₅Cu₁（x=0,2,5,7at.%）非晶合金薄带和全金属组元Fe-Nb-X （X=Al, Zr, Ti, T_a）非晶合金粉末及Fe_94-xZr₂Nb₄B_x（x=10,15,20at.%）过饱和固溶体纳米晶合金粉末。Fe₈₁Cu₂Nb₃Si₁₄非晶合金薄带具有优异的软磁性能，其饱和磁化强度高达142.15emu/g，矫顽力低至0.32Oe，居里温度为310.11℃。通过机械球磨方法将非晶薄带破碎成微米级粉末，然后运用SPS成形技术得到大块铁基非晶或纳米晶复合材料。粉末在烧结过程出现两级晶化模式，即amorphous→amorphous+α-Fe（Si）→α-Fe（Si）+Cu+Nb5Si3。烧结温度对烧结体的致密度，组织结构，微观硬度和软磁性能有显著影响。随着烧结温度的升高，烧结体的致密度、微观硬度和饱和磁化强度呈单调递增，而矫顽力随烧结温度升高表现出先减小后增大的趋势。在Fe₈₁Cu₂Nb₃Si₁₄非晶合金的基础上，根据Slater-Pauling曲线，采用“元素替代”方法开发出Fe₆₉Co₈Nb_7-xV_xB₁₅Cu₁（x=0,2,5,7at.%）非晶合金，研究表明：随着V含量的增加，合金系的非晶形成能力、热稳定和矫顽力逐渐降低，而饱和磁感应强度、居里温度逐渐增加。当x=2时非晶合金具有最宽两次晶化温度区间，x=7时非晶合金具有最佳软磁性能。对该体系的非晶合金进行退火热处理，当T_a<T_g时，由于结构弛豫，内应力的释放，材料的矫顽力降低；当Tx1<T_a<T_x2时，由于bcc结构α-Fe（Co）相的析出，饱和磁感应强度增大；当T_a>T_x2时，由于晶粒长大和第二相的析出，材料的软磁性能急剧恶化。Fe₆₉Co₈Nb₅V₂B₁₅Cu₁合金在580℃退火1h，合金表现出优异的软磁性能，其B_s=1.15T，H_c=0.9928A/m，μ_i=48460。采用机械合金化方法制备出Fe_94-xZr₂Nb₄B_x（x=10,15,20at.%） α-Fe过饱和固溶体纳米晶粉末。研究表明：类金属元素B的添加，没有提高合金系的非晶形成能力，饱和磁化强度由161.70emu/g （x=10）下降至152.74emu/g （x=20）。随着球磨时间的延长，Fe₈₄Zr₂Nb₄B₁₀合金的饱和磁化强度逐渐增加，矫顽力先增大后减小。通过纳米尺度效应和Herzer模型合理解析。将球磨130h后Fe₈₄Zr₂Nb₄B₁₀粉末在650K退火1h，矫顽力由39.58Oe降至11.74Oe，饱和磁化强度由161.70emu/g升至163.75emu/g。SPS烧结后无明显相变和晶粒长大现象，烧结体的致密度达到理论密度的92%，烧结体的饱和磁化强度和初始磁导率得到明显的提高，其饱和磁化强度达到182.53emu/g。采用机械合金化成功制备出全金属组元Fe-Nb-X （X=Al, Zr, Ti, T_a）非晶合金粉末，非晶合金粉末元素分布均匀，无明显的成分偏析，其非晶化机制可归结为粉末元素连续扩散固溶，晶格失稳而形成非晶。非晶合金为单阶段晶化特点，表现出明显的晶化动力学效应和较强抗晶化能力。晶化初期的晶化激活能较大，随着晶化温度的提高，激活能迅速降低，尔后又增大。SPS过程中，非晶合金粉末在过冷液相区具有黏流性和SPS特殊烧结机制是其致密化的主要原因。随着烧结温度的增加，样品的致密度和微观硬度逐渐增加。在室温单向压缩载荷作用下，烧结样品由于孔隙缺陷的存在触发样品脆性断裂。更多还原

【Abstract】 Iron-based amorphous/nanocrystalline alloys possess unique soft magnetic, mechanicalproperties and relatively low material cost, which is considered to be have potentialengineering applications as functional and structural materials. However, this alloy has somedisadvantages including low glass forming ability, low temperature difference between thefirst and second crystallization temperature, low Curie temperature of amorphous phase andvery poor ductility at room temperature. These disadvantages severely limit their wideengineering application. Moreover, the mechanism of the permeability, coercivity and brittlefracture is not well understood. For industrial applications and academic research, it is ofgreat interest to develop new iron-based bulk amorphous/nanocrystalline alloy with good softmagnetic property in addition to excellent mechanical property. This issue has been actuallythe subject of intense research in recent years.The chemical composition of amorphous alloy was designed by “Miedema Model”,“element substitution”,“binary eutectic rules” and “elastic module criterion”. The amorphousmelt-spun ribbons with superior soft magnetic property and glassy powders with pure metalelements were prepared by melt-spinning and mechanical alloying method, respectively. Thehigh density of bulk iron-based amorphous and its nanocrystalline composite was formed byspark plasma sintering technique. Effects of milling time, sintering temperature and annealingtemperature on the evolution of microstructure and related properties were systematicallyinvestigated. Meanwhile, crystallization kinetics, densification behaviors, and the mechanismof the permeability, coercivity and brittle fracture of the amorphous alloys were alsoinvestigated.Formation enthalpy for iron with common transition metal and metalloid elements to formbinary intermetallic compounds was calculated on the basis of semi-empirical thermodynamic“Miedema theory”. The compositions were selected through “element substitution”,“binaryeutectic rules” and “elastic module criterion”. We successfully developed a series ofFe₈₁Cu₂Nb₃Si₁₄, Fe₆₉Co₈Nb_7-xV_xB₁₅Cu₁（x=0,2,5,7at.%） amorphous ribbons, Fe-Nb-X（X=Al, Zr, Ti, T_a） glassy powders and Fe_94-xZr₂Nb₄B_x（x=10,15,20） supersaturated solidsolution nanostructured powders.The Fe₈₁Cu₂Nb₃Si₁₄amorphous ribbons with excellent soft magnetic properties, such assaturation magnetization up to142.15emu/g, coercivity as low as0.32Oe, curie temperature310.11℃. Fe₈₁Cu₂Nb₃Si₁₄amorphous powders were prepared by ball milling of melt-spun ribbons, then bulk Fe₈₁Cu₂Nb₃Si₁₄compacts were consolidated by spark plasma sintering.The crystallization of Fe₈₁Cu₂Nb₃Si₁₄amorphous powders proceeds through two reactionsduring SPS. Namely, amorphous→amorphous+α-Fe（Si）→α-Fe（Si）+Cu+Nb5Si3.Sintering temperature has significant effects on the densification, microhardness and magneticproperties of the compacts. With an increase in the sintering temperature, the relative density,microhardness and saturation magnetization of the sintered samples improved obviously, butthe coercive force decreased at the beginning and then increased with the increase of sinteringtemperature.On the basis of Fe₈₁Cu₂Nb₃Si₁₄composition, a new multi-element Fe₆₉Co₈Nb_7-xV_xB₁₅Cu₁（x=0,2,5,7at.%） amorphous alloys were developed through a method of “elementsubstitution”. It is found that increasing V content can reduce glass forming ability, thermalstability and coercivity, but increase saturation magnetic flux density and Curie temperature ofamorphous phase. The desirable soft magnetic property in these amorphous alloys is x=7, andV content of x=2amorphous alloy has the widest heat treatment temperature range. Thesystem of amorphous alloys was taken on heat treatment in vacuum quartz tube. When T_a<T_g,The decrease of H_cfor annealed amorphous ribbons as result of relaxation of the internalstress of the as-quenched amorphous alloy. As Tx1<T_a<T_x2, the saturation magnetic fluxdensity increased due to the partial crystallization of amorphous precursors to create ahomogeneous distribution of bcc α-Fe nanocrystals within a residual amorphous matrix. Themagnetic properties drop rapidly when T_a>T_x2. This may be caused by grain growth andparamagnetic phase formation which hinder the magnetic coupling between the ferromagnetica-Fe grains. Fe₆₉Co₈Nb₅V₂B₁₅Cu₁amorphous after annealed at580℃for1h has the bestsoft magnetic properties, such as B_s=1.15T, H_c=0.9928A/m, μ_i=48460.Fe_94-xZr₂Nb₄B_x（x=10,15,20at.%） supersaturated solid solution nanostructured powderswere produced by mechanical alloying. Results show that the addition of metalloid element B,did not improve the glass forming ability of the alloy system. By adding B to substitute Fe,the saturation magnetization （Ms） decreased from the161.70emu/g （x=10） to152.74emu/g（x=20）, The saturation magnetization increased with increasing milling time and becameconstant at130h, but the coercivity （H_c） increased firstly and then decreased. The variation ofmagnetic parameters can be explained by Nano-scale effect and Herzer model. Fe₈₄Zr₂Nb₄B₁₀alloy after milled for130h and then annealed at650K for1h, which the coercivity decreasedfrom39.58Oe to11.74Oe and the saturation magnetization increased from161.70emu/g to163.75emu/g. The consolidated bulk sample exhibited a high relative density which reaches92%of the theoretical density and there was no phase change during SPS process, the saturation magnetization and susceptibility of the SPSed bulk sample improved in comparisonwith the annealed powders. Its saturation magnetization is182.53emu/g.Formation of Fe-Nb-X （X=Al, Zr, Ti, T_a） amorphous alloys from pure metal elements bymechanical alloying. Amorphous powders have a homogeneous distribution of elements andno obvious contaminants coming from MA. The mechanism of amorphization can beattributed to reaction within solid state. The crystallization of Fe-Nb-X （X=Al, Zr, Ti, T_a）amorphous powders proceeds through single reactions, having an obvious dynamic effect andstrong resistance to crystallization. With an increase in temperature, the effective activationenergy for crystallization increased firstly and then decreased. This achievement of the fullydensified bulk compacts is ascribable to the viscous flow of amorphous powders and thespecial sintering mechanism of SPS. With the increase of sintering temperature, the densityand microhardness of the SPSed compacts increased obviously. The sintered samplesexhibited typical brittle fracture characteristics under single-axis compressive test at roomtemperature due to the presence of porosity in sintered compacts.更多还原