【作者】 郭晶；

【导师】 边秀房；

【作者基本信息】 山东大学 ， 材料加工工程， 2008， 博士

【摘要】 本文主要选择稀土基大块非晶合金为研究对象，系统地研究了它们的玻璃形成能力（GFA）、热膨胀行为及液体脆性，并对相关机理进行了探讨。利用高温X射线衍射仪研究了金属液体中团簇随温度变化的行为，提出了液体结构稳定性参数的概念，并就其与玻璃形成能力的相关性进行了讨论。利用X射线衍射（XRD）、差示扫描量热（DSC）以及热力学计算等方法，分析了RE₅₅Al₂₅Co₂₀（RE=Ce，Pr，Nd，Sm，Gd）合金系的玻璃形成能力，发现利用铜模吸铸方法制备成直径为2mm的合金棒时，只有Sm₅₅Al₂₅Co₂₀和Gd₅₅Al₂₅Co₂₀合金能够得到完全的非晶结构，Pr₅₅Al₂₅Co₂₀和Nd₅₅Al₂₅Co₂₀合金得到非晶与晶体共存的结构，而Ce₅₅Al₂₅Co₂₀只能得到晶体结构。因此，在五种合金中，Sm₅₅Al₂₅Co₂₀和Gd₅₅Al₂₅Co₂₀合金的玻璃形成能力最强，而Ce₅₅Al₂₅Co₂₀最差。在此基础上深入研究了Sm₅₅Al₂₅Co₂₀与Gd₅₅Al₂₅Co₂₀合金的玻璃形成能力，利用铜模吸铸方法首次成功制备了直径达4mm的Sm₅₅Al₂₅Co₂₀大块非晶合金棒和直径为2mm的Gd₅₅Al₂₅Co₂₀大块非晶合金棒。并发现，与Gd₅₅Al₂₅Co₂₀合金相比，玻璃形成能力强的Sm₅₅Al₂₅Co₂₀合金具有更小的过冷液体脆性参数和更小的玻璃转变温度处的吉布斯自由能差(△G^1-x（T_g）)。采用热扫描方法计算了一系列Gd基大块非晶合金的过冷液体脆性参数（m），从小到大依次为：Gd₅₅Al₂₅Cu₁₀Co₅Ni₅（m=37）<Gd₅₅Al₂₅Co₁₀Cu₁₀（m=45）<Gd₅₅Al₂₅Co₁₀Ni₁₀（m=58）<Gd₅₅Al₂₅Co₂₀（m=74）。在这些合金中，过冷液体脆性参数越大，合金的玻璃形成能力越强。对Gd基非晶合金的热膨胀行为研究发现，过冷液体脆性参数与非晶固体平均热膨胀系数成正的线性关系，与最近邻原子距离和有效势阱深度成负的线性关系，这些相关性为从过冷液体脆性角度研究玻璃形成能力以及理解非晶形成本质提供了有益信息。另外，发现在Gd基非晶合金的热膨胀曲线上，并没有因为从非晶态向过冷液态转变而发生热膨胀系数的增大，经分析认为这种反常的现象与合金在过冷液相区内的黏性流动能力有关。利用回转振动式高温熔体黏度仪测量了一系列La基和Sm基大块非晶合金的过热液体黏度，发现用Cu替代La₅₅Al₂₅Ni₂₀合金中的部分Ni以后，合金的黏度降低，黏度值从小到大依次为：La₅₅Al₂₅Ni₅Cu₁₅<La₅₅Al₂₅Ni₁₅Cu₅<La₅₅Al₂₅Ni₂₀。利用黏度数据计算了三种合金的过热液体脆性参数，从小到大分别为：La₅₅Al₂₅Ni₅Cu₁₅（M=1．4509）<La₅₅Al₂₅Ni₁₅Cu₅（M=1．6428）<La₅₅Al₂₅Ni₂₀（M=1．7747）。过热液体脆性参数最大的La₅₅Al₂₅Ni₂₀合金玻璃形成能力最差。Sm基大块非晶合金的过热液体脆性参数值从小到大依次为：Sm₅₅Al₂₅Co₁₀Ni₁₀（M=1．8260）<Sm₅₀Al₃₀Co₂₀（M=1．9588）<Sm₅₅Al₂₅Co₁₀Cu₁₀（M=2．1005）。在这几种Sm基合金中，过热液体脆性参数与玻璃形成能力之间有负的相关性，即过热液体脆性参数越小，合金的玻璃形成能力越强。通过公式推导和实验验证发现，在La基和Sm基大块非晶合金中过冷液体脆性参数（m）与约化玻璃转变温度和过热液体脆性参数倒数的乘积(（T_g/T₁）×（1/M）成正比关系。利用液态X射线衍射技术研究了Au-Si合金系的液体微观结构特征，发现在Au-Si合金系液体双体分布函数上主峰的右侧存在次峰，它是局域原子有序结构的反映。在Au₈₅Si₁₅，Au₈₁Si₁₉和Au₇₅Si₂₅三种合金中，随着Si含量的提高次峰越来越明显，次峰强度最高的Au₇₅Si₂₅合金玻璃形成能力最强。根据经典形核理论和热力学理论提出了液体结构稳定性参数B的概念，定义如下，B=dr_c/dT其中，r_c是液体中团簇的相关半径，T为绝对温度。参数B反映了液体中团簇尺寸随温度变化的快慢。|B|越小表明液体结构越稳定，越有利于非晶的形成。因此，可以用|B|来预测不同液体的玻璃形成能力，即|B|越小玻璃形成能力越强。对Al-Cu合金系的液体结构稳定性与过热液体脆性进行了研究，发现随着Cu元素含量的提高，|B|增大，表明Cu元素的加入使液体中团簇尺寸随温度的变化增快，在宏观上表现为液相线温度附近黏度随温度的变化增快，即M值的增大。在Al-Cu合金中，|B|和M具体表现为正的线性关系，这种相关性将液体的宏观动力学性质与微观动力学性质联系起来。更多还原

【Abstract】 In the present thesis, the rare earth-based bulk metallic glasses were mainly chosen for the study of glass-forming ability （GFA）, behaviors of thermal expansion and fragility of liquids. The cluster behaviors of liquid metals with temperature were investigated by means of X-ray diffraction measurement. A concept of the structural stabilization factor of liquids has been proposed, and its correlation with the glass-forming ability has also been discussed.The GFA of RE₅₅Al₂₅Co₂₀ （RE = Ce, Pr, Nd, Sm, Gd） alloys was studied by X-ray diffraction （XRD） and differential scanning calorimetry （DSC）. When the alloys are cast into rods up to 2 mm in diameter, only the Sm₅₅Al₂₅Co₂₀ and Gd₅₅Al₂₅Co₂₀ alloys show entirely amorphous structures; the Pr₅₅Al₂₅Co₂₀ and Nd₅₅Al₂₅Co₂₀ alloys are mixtures of amorphous and crystal structures; and the Ce₅₅Al₂₅Co₂₀ alloy shows mostly crystal structure. As a result, among the five alloys, the Sm₅₅Al₂₅Co₂₀ and Gd₅₅Al₂₅Co₂₀ alloys exhibit the highest GFA and the Ce₅₅Al₂₅Co₂₀ alloy shows the lowest GFA. Based on the results, the GFA of the Sm₅₅Al₂₅Co₂₀ and Gd₅₅Al₂₅Co₂₀ alloys was further investigated. By sucking-cast into a Cu-mold, the Sm₅₅Al₂₅Co₂₀ glassy rod with a diameter of 4mm and the Gd₅₅Al₂₅Co₂₀ glassy rod with a diameter of 2mm have been prepared for the first time. Compared with the Gd₅₅Al₂₅Co₂₀ alloy, the Sm₅₅Al₂₅Co₂₀ alloy with a higher GFA shows a smaller fragility parameter of supercooled liquids and a smaller Gibbs free energy difference at glass transition temperature (ΔG^1-x（T_g）).The fragility parameters of supercooled liquids, m, of Gd-based glass-forming alloys were calculated by thermal scanning method and the values of m rank as follows: Gd₅₅Al₂₅Cu₁₀Co₅Ni₅（m=37） < Gd₅₅Al₂₅Co₁₀Cu₁₀（m=45） < Gd₅₅Al₂₅Co₁₀Ni₁₀ （m=58） < Gd₅₅Al₂₅Co₂₀（w=74）. Among the Gd-based alloys, the alloy with a larger m exhibits a higher GFA. Based on the dilatometric measurements, a positive linear correlation is found between the fragility parameter of supercooled liquids and the average thermal expansion coefficient of glassy solids. Negative linear correlations are obtained between m and the nearest-neighbor atom distances （r_l）as well as the depths of the effective pair potentials （V₀） in the Gd-based bulk metallic glasses （BMGs）. These relationships help to study GFA and understand the essence of glass formation from the view of supercooled liquid fragility. In addition, an abnormal thermal expansion phenomenon can be observed in the Gd-based BMGs. There is no increase of thermal expansion coefficient derived from glassy state to supercooled liquid state. This behavior has a close correlation with the viscous flow ability of the sample in the supercooled liquid region.The viscosities of superheated liquids for a series of La- and Sm-based BMGs were measured by an oscillating vessel method. After Cu element is substituted partly by Ni element in the La₅₅Al₂₅Ni₂₀ alloy, the viscosities of the liquid alloys decrease, from low to high as follows: La₅₅Al₂₅Ni₅Cu₁₅< La₅₅Al₂₅Ni₁₅Cu₅< La₅₅Al₂₅Ni₂₀. According to the fragility concept of superheated liquids, the fragility parameters of superheated liquids, M, for the La-based alloys were calculated as follows: La₅₅Al₂₅Ni₅Cu₁₅（M=1.4509）<La₅₅Al₂₅Ni₁₅Cu₅（M=1.6428）<La₅₅Al₂₅Ni₂₀（M=1.7747）. Among the three alloys, the La₅₅Al₂₅Ni₂₀ alloy with the highest value of M shows the lowest GFA. Also, the fragility of superheated liquids for the Sm-based glass-forming alloys was investigated. The fragility parameters M of the Sm-based alloys rank as follows:Sm₅₅Al₂₅Co₁₀Ni₁₀（M=1.8260）< Sm₅₀Al₃₀Co₂₀（M=1.9588）< Sm₅₅Al₂₅Co₁₀Cu₁₀ （M=2.1005）. A negative correlation between GFA and M is found in the Sm-based alloys. According to the results of formula deduction and experiments, it is found that the fragility parameter of supercooled liquids, m, is proportional to the product of the reduced glass transition temperature and the reciprocal of fragility parameter of superheated liquids （（T_g/T_l）×（1/M））.The microstructure characteristics of the Au-Si alloys were studied by X-ray diffraction measurement. A sub-peak reflecting the local atomic structural ordering is observed on the right side of the main peak in the pair correlation function curves. In the Au-Si alloys, the height of the sub-peak increases with the Si content increasing and the Au₇₅Si₂₅ alloy with the most obvious sub-peak has the highest GFA. In addition, a concept of structural stabilization factor of liquids, B, has been proposed based on the theories of classical homogeneous nucleation and thermodynamics. It is defined as,B=dr_c/dTwhere r_c is the correlation radius of clusters in liquids and T is the absolute temperature. The structural stabilization factor of liquids reflects the change in cluster size with temperature in different liquids. A liquid with a low absolute value of B should more likely keep its liquid structure, which favors the formation of glass, so it can be used to predict GFA. A liquid with a low absolute value of B tends to have a high GFA. The structural stabilization of liquids and the fragility of superheated liquids in the Al-Cu alloys were also studied. It is found that |B| increases with the increase of Cu content, indicating that the change in cluster size with temperature in liquids accelerates with the addition of Cu element. From a macroscopical point of view, the increase of |B| can be reflected by the fast change in the viscosity with temperature around liquidus temperature （the increase of M）. Among the Al-Cu alloys, there is a positive linear correlation between |B| and M, which correlates macrokinetics properties with microkinetics properties of liquids.更多还原