【作者】 王欢；

【导师】 孙剑飞； 邢大伟；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 熔体抽拉成形技术利用高速旋转的辊轮从熔体中蘸取液态薄层，依靠自身粘度、表面张力和重力共同作用实现低维材料尺寸和形状的精密控制。熔体抽拉成形技术适应性强，冷却速率高，可实现多种金属、陶瓷纤维材料及薄带的非平衡凝固甚至非晶化，所制备材料的高度几何对称性使其具有优异的磁、电、力学性能。本文以熔体抽拉法成形微米尺度金属非晶纤维及其性能为研究对象，主要进行了熔体抽拉金属纤维成形工艺与几何尺寸、微结构及其性能间依赖性、非晶态纤维拉伸行为、CoFe基非晶纤维多道次冷拔处理及其微结构、力学、电磁学性能演变基本规律与相关机理研究。熔体抽拉过程中纤维直径随辊轮转速增加呈先增加后减小趋势，高转速、高导热能力辊轮有利于获得非晶态金属玻璃纤维，Cu轮转速30m/s时制备的CoFe基非晶纤维具有高达360%的巨磁阻抗(GMI)效应；母合金进给速度主要影响纤维截面圆度，进给速度低于90μm/s时非晶丝获得最大圆整度98%；熔体过热度通过影响合金熔体的粘度和表面张力控制纤维的成形过程，熔体温度过高容易导致辊轮轮缘热蚀，造成纤维化学成分的改变；辊轮预热处理有利于改善合金熔体与辊轮之间的润湿，降低纤维内部热应力，提高纤维冶金质量。不同工艺条件下熔潭形貌研究表明，随着辊轮转速的增加，作用于熔潭表面的切向应力克服表面张力的作用在熔潭下方形成舌形凸起，其特征参量边界层厚度δ和与辊轮接触长度Lc随着辊轮转速的增加而增大，液态金属沿辊轮运动方向铺展、细化成丝。熔体抽拉过程中瑞利波形成区间与纤维直径、辊轮直径、界面润湿情况以及固液界面推进速度和熔体层离开熔潭所需时间等参数决定，获得了无瑞利波缺陷所对用的辊轮转速表达式。熔体抽拉纤维的传热行为分为熔潭内、随轮运动和空冷三个阶段。熔潭内部形成的边界层受到周围剩余未加速熔体的热传导冷速较低；当其从熔潭被抽出后沿辊轮前进方向运动时，由于辊轮的热传导和周围空气的辐射换热，冷却速率较大，此时的冷速均在10~6K/s以上，纤维横截面出现的沟槽缺陷主要在该阶段形成；当加速后的液态纤维飞入保护气氛中空冷，表面张力开始占据主导地位，纤维热量传输以对流辐射为主，冷速降低。微尺寸CoFe基非晶纤维表现出较高的断裂强度和非线性变形特征，随着直径的增大纤维非晶化程度降低同时沟槽、孔洞等冶金缺陷造成应力集中导致其强度减小。FeSiB非晶纤维拉伸曲线经历了弹性、非线性变形阶段后表现出0.6%的拉伸塑性应变能力。断面上的“韧窝”状断口形貌显示其断裂过程在微纳米尺度呈现出塑性断裂特征。裂纹的扩展中裂纹前端塑性变形区内微孔聚集、长大及其相互连接是造成其具有一定拉伸塑性的原因。CoFe基非晶纤维拉伸强度和塑性显现出明显的加载速率依赖性，随着加载速率的增加，纤维强度降低，塑性增大，并在低应变速率条件下表现出屈服特性。临界剪切台阶尺寸代表非晶合金中剪切带稳态扩展的能力，可以用来定量衡量纤维塑性变形能力的高低。CoFe基非晶纤维相比于其它成分非晶态金属纤维具有较高的Weibull模数和门槛应力值，表现出优良的断裂可靠性和使用安全性。CoFe基非晶纤维经过多道次冷拔处理可以实现高达75%的截面变形，拉拔后纤维表面光洁，宏观偏析和缺陷消除。HRTEM表明随着拉拔量的增加，非晶纤维基体出现纳米尺度的球形富Co颗粒，并随着拉拔过程的进行迅速增大。纤维拉伸强度和塑性随着拉拔率的增加均呈现先增加后减小的趋势，51%拉拔率纤维表现出最大的4250MPa断裂强度和1.64%的拉伸塑性。CoFe基非晶微丝的GMI效应在51%变形量下达到了最大值，10MHz条件下达到160%。各向异性场Hk在经历过一个由1Oe到5Oe的较快增长后增幅减小，最后在7Oe达到稳定。非晶纤维冷拔过程分析表明，拉拔后纤维内部同时存在径向和轴向残余应力，径向残余压应力有闭合裂尖的作用，裂尖扩展到该应力区时会减小裂纹扩展速率，抑制裂尖的扩展。拉拔过程中产生的应力降低非晶-纳米晶形核能障，从热力学角度解释了纳米晶形成的可能性。应力诱发纳米晶作为钉扎点会耗散了裂纹尖端的断裂能量，阻碍剪切带的快速扩展，进而提高其力学性能。拉拔后纤维产生的轴向拉应力和径向压应力导致纤维外部的环向磁畴扩大，芯部磁畴减小，进而改善了其GMI性能。当变形量超过51%后应力诱发纳米晶尺寸开始急剧增大，引起磁晶各向异性和硬磁增加，导致纤维GMI性能恶化。更多还原

【Abstract】 The basic principle of melt extraction technique (MET) is to use a high-speedwheel to extract a thin liquid layer from the molten alloy and then rapidly solidify tobe wires under its viscosity, surface tension and gravity. Several nonequilibrium evenamorphous metallic, ceramic wires and ribbons can be fabricated by MET, andexhibit excellent magnetic, electric and mechanical properties due to their highcooling rate and geometric symmetry. In the present thesis, the fabrication processand characterization of melt-extracted amorphous metallic microwires wasinvestigated and the correlation between process parameters and microstructure wasrelated. Uniaxial tensile deformation and fracture reliability analysis ofmelt-extracted amorphous microwires was performed. The effect of cold drawing onthe structural evolution, mechanical and magnetic properties was investigated aswell.It was found that the variation of wire diameters vs. wheel velocity passesthrough a maximum and then decreases with the increase of wheel velocity. UniformCoFe-based amorphous microwires with larger GMI value of360%was fabricatedusing Cu wheel with speed of30m/s.The roundness of the wires was influencedmainly by the molten alloy feed rate. Circular wires with roundness exceed98%wasfabricated when the feed rate is lower than90μm. While viscosity and surfacetension of the molten varied considerably by temperature and determining the wireformation process. Thermal pretreatment of the extracted wheel was identified tofacilitate the wettability between the molten and wheel, thus improved the geometryof the wire.A high-resolution CCD video camera recorder was used to monitor the changingof the surface shape of molten alloy contacting the wheel tip under differentconditions. Liquid metal spread along the wheel rotary direction and a sharpenfrontier was formed under the shear forcce applied to the puddle. The thickness δ andthe length of the extracted boundary layer Lcincreased with increasing wheel velocity.The formation region of Rayleigh waves correlated with the fiber diameter, wheelradius, wettability between the molten layer and extracted wheel. Heat transfer duringextraction process can be devided to three regions: in the puddle, move along thewheel and in the air. It was found that the mechanism of the wire formation duringmelt extraction was controlled by the process parameters. Momentum transferdetermined the thickness of the extracted layer in the low wheel speed region; whilein the low wheel speed region, heat transfer turned out to be a dominant factor.Micro-sized CoFe-based amorphous microwires exhibit high tensile fracture strength and nonlinear tensile behavior while decrease with the increase of thediameter due to the decrease of amorphous nature and the formation of concavedtrack and cavity, which will cause the stress concentration during tensile process. AFeSiB amorphous microwire shows a pronounced tensile plasticity of0.6%after alinear and nonlinear deformation during tensile test. The submicro-sized dimplesformed on the fracture surface reveal the plastic fracture behavior in the range ofmicro-or nano scale for these amorphous microwires. The plasticity origin of thesemelt-extracted FeSiB amorphous microwires can be elucidated by the formation,growth and coalescence of nanovoids in the plastic zone during crack propagation.The strength and plasticity of CoFe-based amorphous microwires shows adependence of strain rate upon dynamic loading. The tensile strength decreases withthe increase of strain rate while plastic property shows an opposite effect. Criticalshear offset formed during tensile process represent the stable shear deformationability and can be used as an important standard to measure the level of plasticity ofamorphous alloy. The higher Weibull modulus and threshold stress of CoFe-basedamorphous microwires compared with other metallic microwires indicats its excellentfracture reliability and failure safety.The CoFe-based amorphous microwires can be successfully cold-drawn with upto75%cross-sectional area reduction. All of the drawing wires exhibit smoothsurfaces without any visible scratches, while the grooves and fluctuations in theas-quenched wire can be seen occasionally on the surface. Cold-drawing can alsoeffectively reduce the composition segregation and improve macroscale chemistryhomogenization in the wires. HRTEM results confirm the presence of nano-sizedcrystallites precipitated in the amorphous matrix during drawing and growth rapidlywith the increase of drawing degree. The tensile ductility and tensile strengthincreased gradually with cross-sectional area reduction ratio until51%, and decreasedwith further deformation. The microwire with51%drawn exhibits the highest tensileductility of1.64%and tensile strength of4250MPa. Interestingly, the GMI effectalso attains the maximum value of160%at10MHz when drawn to51%(30%largerthan that of the as-cast wires), before decreasing with further cold deformation. Theanisotropy field Hkundergoes a rapid increase from1Oe to5Oe at10MHz after thefirst drawing step before a relatively small increase of2Oe with further drawing.Afterwards the anisotropy field levels off at7Oe. The generation of radial andunaxial residual stress can alter the mechanical and magnetic property of cold-drawnmicrowires. The compressive residual stress can close the crack tip and decrease thecrack propagation rate, leading to an improved mechanical property. The temperatureincrease during the drawing process cannot target the change of microstructure, whilethe generation of residual stress during drawing decreases the energy barrier fornanocrystallites nucleation and promote the phase transformation. Nano-sized crystallites can also alter the number and distribution of free volume generated duringwire fabrication process and will dissipate the fracture energy in front of the crack tip,block its quick extend, leading to the improved mechanical property. The complexdependence of GMI characteristics on cold drawing has been elucidated by a fullconsideration of residual stress and nanocrystalline structure, as well as geometricaldefects. The axial residual tensile stress and circumferential compressive stressinduced by the cold-drawing process together increase the volume of the outer shelland hence the circumferential permeability, giving rise to an improved GMI ratio.The existence of large-size nanocrystals after51%drawing causes an increase ofmagnetocrystalline anisotropy and magnetic hardness, resulting in a deterioration ofthe soft magnetic property and hence the reduction of the GMI ratio.更多还原