【作者】 袁子洲；

【导师】 陈学定；

【作者基本信息】 兰州理工大学 ， 材料科学与工程， 2008， 博士

【摘要】 以X-射线衍射、差示扫描量热分析（DSC）、扫描电镜及振动样品磁强计等为主要研究手段，以Co₄₃Fe₂₀Ta_5．5B_31．5非晶合金为对象，首次系统研究了该合金在不同条件下的晶化行为、等温退火对玻璃转变和晶化的影响以及晶化对磁性能的影响。然后，又以Co₄₃Fe₂₀Ta_5．5B_31.5、Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂和Co_74．4B_25．6为对象，首次系统研究了熔体冷速或制备工艺对它们的晶化行为及磁性能的影响。此外，对目前已有的合金玻璃形成能力判据进行了详细研究，新提出了一个能更好地表征合金玻璃形成能力的判据。最后研究了稀土元素Y的微量添加对Co₄₃Fe₂₀Ta_5．5B_31．5合金的玻璃形成能力的影响，开发出玻璃形成能力更强的新型(Co₄₃Fe₂₀Ta_5．5B_31．5)₉₇Y₃块体非晶合金。以上研究发现：升温速度对Co₄₃Fe₂₀Ta_5．5B_31．5非晶合金的非等温晶化激活能存在显著影响，其晶化过程可以分成前后2个阶段。前一阶段符合形核与核长大模型；后一阶段符合晶粒正常长大模型。这两个阶段在整个晶化过程中所占的比重与升温速度有关，但是，非等温晶化的大部分阶段是靠晶粒的正常长大来完成的。等温晶化温度对Co₄₃Fe₂₀Ta_5．5B_31．5非晶合金的形核与核长大行为有显著影响。随着温度的升高，晶核长大由受扩散控制转变为受界面控制。局域晶化激活能计算表明非等温晶化阻力越来越小，而等温晶化阻力越来越大。在玻璃转变温度附近进行退火处理可使Co₄₃Fe₂₀Ta_5．5B_31．5非晶合金产生结构弛豫。结构弛豫对合金的晶化动力学效应没有显著影响，但可以减弱玻璃转变的动力学效应。另外，结构弛豫会阻滞合金发生玻璃转变，但对晶化的影响却具有双重性，即有促进的一面，又有阻滞的一面。晶化对Co₄₃Fe₂₀Ta_5．5B_31．5非晶合金的磁性能存在显著影响。在928K等温条件下，饱和磁感应强度M_s和矫顽力H_c都随保温时间的延长而增大。但是，保温时间小于30min时，M_s和H_c的变化幅度很小。当保温时间达到30min时，二者突然增高。保温时间大于30min之后，M_s的变化不大而H_c有明显的增高。在非等温晶化条件下，晶化试样的M_s和H_c也都比制备态试样大许多。就不同终止温度来说，M_s随终止温度的提高而单调增大，但增加幅度很小。H_c随着晶化终止温度的提高而变化的幅度较大，并且变化是非单调的。当晶化终止温度位于第一个晶化峰温度范围内时，H_c随晶化温度而提高，当晶化终止温度高于第二个晶化峰结束温度时，矫顽力又急剧下降。熔体冷速影响Co基非晶合金内部短程序原子团簇的大小，并且这种影响可以在合金的等温晶化动力学及磁性能方面得到充分体现。对于冷速相差不大的Co₄₃Fe₂₀Ta_5．5SB_31．5非晶条带来说，冷速对合金的非等温晶化动力学及晶化产物没有影响，但对其等温晶化动力学存在少许影响。主要表现在冷速改变形核速率随时间增加的晶化阶段占整个晶化过程的比例。磁滞回线的测量表明，非晶条带的饱和磁感应强度和矫顽力都随冷速降低而增大。对于冷速相差较大的Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂非晶条带和圆棒来说，冷速对合金的非等温和等温晶化动力学都有显著影响。对非等温晶化来说，随着冷速降低，试样的玻璃转变温度下降，玻璃转变激活能减小。虽然晶化峰值温度不受冷速的影响，但是晶化激活能同样随冷速降低而降低。对等温晶化来说，虽然不同冷速条件下制备的试样都表现出扩散控制的晶核三维长大过程，但是形核速率与冷速有关。冷速越快，形核速率越低，同时晶化孕育时间越短。熔体冷速对Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂非晶合金的磁性能影响很大。冷速高时，合金表现出顺磁性；冷速低时，合金表现出铁磁性。饱和磁感应强度随冷速的降低而增大。制备工艺对Co-B二元非晶的晶化和析出相存在显著影响。虽然甩带法制备的非晶条带和化学还原法制备的非晶态粉末成分十分相近，但在晶化产物方面却相差很大。对于同样用化学还原法制备的成分几乎相同的非晶态粉末来说，由于具体的制备条件不同，它们在晶化行为方面也表现很大的差异。向Co₄₃Fe₂₀Ta_5．5B_31．5合金中添加微量的稀土元素Y可以增大其玻璃形成能力。新型块体非晶合金(Co₄₃Fe₉20)Ta_5．5B_31．5)₉₇Y₃可以被容易地制备成Φ2mm的非晶圆棒。但是Y的添加导致Co₄₃Fe₂₀Ta_5．5B_31．5非晶合金的软磁性能下降。新提出了一个基于合金特征温度的玻璃形成能力表征参数β，统计分析表明，参数β对合金玻璃形成能力的表征更加可靠，而且对不同合金的玻璃形成能力鉴别能力更强，可以作为开发新型块体非晶合金时的一个有用和有效的工具。更多还原

【Abstract】 By employing the X-ray diffraction, differential scanning calorimetry （DSC）, scanning electron microcopy and vibrating sample magnetometer, the crystallization behaviors of the amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy under different conditions, the effect of annealing on the kinetics of glass transition and crystallization for this alloy and the effect of the crystallization on the magnetic properties of Co-（43）Fe₂₀Ta_5.5B_31.5 amorphous alloy have been systematically investigated. The effect of cooling rate on crystallization and/or magnetic properties for the amorphous alloys Co₄₃Fe₂₀Ta_5.5B_31.5, Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ and Co_74.4B_25.6 has also been investigated. Besides these investigations, the currently used criteria and newly proposed criteria for evaluating the glass-forming ability （GFA） of alloys have been studied detailed, and a new dimensionless criterion has been proposed, which is statistically better than other criteria in reflecting the GFA. At last, the effect of minor additions of the rare earth element Y on the glass-forming ability of Co₄₃Fe₂₀Ta_5.5B_31.5 alloy has been investigated, and A bulk metallic glass (Co₄₃Fe₂₀Ta_5.5B_31.5)₉₇Y₃ with higher GFA has been developed. Based on above-mentioned investigations, the following results can be obtained:The heating rate has an important influence on the activation energy for crystallization of the metallic Co₄₃Fe₂₀Ta_5.5B_31.5 glass, and its non-isothermal crystallization mechanism is composed of the processes of two types, namely the Johnson-Mehl-Avrami （JMA） nucleation-and-growth mode and the mode which has been well described by the normal grain growth kinetic law （NGG）. Though the proportion between the JMA-like and the NGG-like modes is related to the heating rate, the main crystallization process is completed through NGG-like kinetics. The phase transformation mechanism depends on temperature during isothermal annealing. Along with increasing temperature, the nucleus growth mechanism changes from the diffusion-controlled growth to the interface-controlled growth. The calculation of local activation energy for crystallization indicates that the non-isothermal crystallization becomes easier, whereas the isothermal crystallization becomes harder, along with the development of crystallization.The structural relaxation caused by annealing near glass transition temperature diminishes the kinetic effect of glass transition, whereas the kinetics effect of crystallization was not significantly affected by the structural relaxation. The glass transition is delayed by the structural relaxation, but the effect of the structural relaxation on crystallization is twofold, namely promotion and delay.Crystallization has significant influence on the magnetic properties of the amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy. The saturation magnetization M_s and the coercivity H_c y increase with the annealing time when isothermal annealing at 928K is applied to the alloy. When the annealing time is smaller than 30min, the values of H_c and M_s gradually increases with the annealing time. The values of H_c and M_s increase sharply when annealing time is equal to 30min. After then, the variety of M_s is small, whereas the variety of H_c is large. Under the non-isothermal crystallization conditions, the value of M_s increases monotonously with the stopping temperature, but the increase is very small. The value of H_c varies substantially with the stopping temperature, and the varying is non-monotonous. Within the temperature range of the first crystallization peak, the value of H_c increases monotonously with the stopping temperature. When the stopping temperature is higher then the end temperature of the second crystallization peak, the value of H_c decreases drastically.In the case of non-isothermal crystallization of the amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy, the thermodynamic parameters and the precipitation phases are not affected by the cooling rate. During the isothermal annealing, the cooling rate also does not change the crystallization kinetics of metallic Co₄₃Fe₂₀Ta_5.5B_31.5 glass, but the nucleation rate in the whole crystallization process. The magnetic hysteresis loops measuring indicates that the values of saturation magnetization and coercivity increase with decreasing of cooling rate.The cooling rate had significant influence on the crystallization kinetics of metallic Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ glass. In case of non-isothermal crystallization process, the glass transition temperature and the activation energy for glass transition decreased with decreasing of cooling rate. The activation energy for crystallization also decreased with decreasing of cooling rate, in despite of the peak temperature of crystallization was independent of cooling rate. Although the isothermal crystallization was governed by diffusion-controlled three-dimensional growth with decreasing nucleation rate for Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ glasses under different cooling rates, the nucleation rate decreased, while the incubation time decreased, with increasing cooling rate. The magnetic properties of Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ glass were significantly affected by cooling rate. The sample obtained at high cooling rate was paramagnetic, whereas the sample obtained at low cooling rate was ferromagnetic. The saturation magnetization increased with decreasing cooling rate. The preparation conditions affect the crystallization kinetics and the precipitation phases of the binary Co-B amorphous alloy. Though the amorphous melt-spun ribbon has the almost identical composition with the amorphous powder prepared by chemical reduction, their precipitation phases are very different. The amorphous powders obtained by chemical reduction with different preparation condition also present different crystallization kinetics.The glass forming ability of Co₄₃Fe₂₀Ta_5.5B_31.5 alloy can be improved by minor addition of Y. The glassy rod with diameter of 2 mm can be easily prepared with (Co₄₃Fe₂₀Ta_5.5B_31.5)₉₇Y₃ alloy. But compared with the amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy, the magnetic property of amorphous (Co₄₃Fe₂₀Ta_5.5B_31.5)₉₇Y₃ alloy is worsened.A new dimensionless criterionβhas been proposed, which is statistically better than some previous criteria in reflecting the GFA and more sensitive to the GFA of alloy. It can be anticipated that theβcriterion will be used as a useful and efficient guideline for exploring new BMG formers.更多还原