【作者】 何世文；

【导师】 黄伯云； 刘咏；

【作者基本信息】 中南大学 ， 材料学， 2008， 博士

【摘要】 非晶合金以其优异性能而引起了广泛的关注和极大的兴趣。寻找非晶形成能力强的合金成分系列,一直是许多科研者的努力方向。由于缺乏一个完善的预测合金GFA理论,本文利用元素替代法以某一元素或多个元素替代A_xB_yC_100-x-y型合金的部分元素,而不改变其它元素的原子百分含量,以Co₄₅Fe₂₅Nb₁₀B₂₀和Y₅₆Al₂₄Co₂₀两个合金体系为研究对象,采用单辊快淬法和铜模吸铸法制备了多种非晶合金,并研究了其晶化动力学和力学行为,得到以下主要结论:（1）研究了Zr元素对Co₄₅Fe_25-xZr_xNb₁₀B₂₀（x=0,2,4,6 at.%）合金的母合金铸态组织、非晶形成能力和热稳定性的影响。其中Zr元素含量为4 at.%时,合金具有最均匀细小的铸态组织、最强的非晶形成能力和最高的热稳定性。采用Kissinger法对Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀合金的表观激活能进行了计算,得到其玻璃转变激活能E_g和初始晶化激活能E_x1分别为720.3kJ/mol和424.5kJ/mol。（2）研究表明适量添加Y元素可以提高Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀合金的非晶形成能力和热稳定性。其主要原因是Y原子对氧有很强的亲和力,大大减少了氧对合金非晶形成能力的不利影响。由Kissinger法计算得到(Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀)₉₆Y₄合金的初始晶化激活能E_x1和晶化峰激活能E_p1分别高达940.7 kJ/mol和825.0 kJ/mol,是目前所有非晶合金中的最高值。本工作研究的Co基合金体系不仅具有较高的非晶形成能力,而且具有较好的软磁性能,有望作为实用型软磁非晶材料。（3）基于相似元素替代法,利用单辊快淬法制备了Y_56-x（Ce,Nd,Pr）_xAl₂₄Co₂₀（x=0,5,10,15,20at.%）、Y₃₆Ce₁₀Nd₁₀Al₂₄Co₂₀、Y₃₆Ce₁₀Pr₁₀Al₂₄Co₂₀和Y₃₆Nd₁₀Pr₁₀Al₂₄Co₂₀非晶薄带,热分析表明这些合金均具有高的GFA和热稳定性,并且随着添加元素含量的增加,合金的GFA和热稳定性得到提高。该Y基非晶合金体系的过冷液相区ΔT_x与反映合金混合热变化的3个键参数之间存在如下关系:ΔT_x=-273.5+639.6Δx²+1074.5δ²+363.1Δn^2/3,相关系数为95.6%。（4）研究了元素Zr和Sc对Y₅₆Al₂₄Co₂₀合金的非晶形成能力和热稳定性的影响,发现元素Sc较Zr更能提高Y₅₆Al₂₄Co₂₀合金的GFA,其主要原因是Sc原子与合金其它元素相比,对氧具有更强的亲和力,大大减少了氧对合金非晶形成能力的不利影响。同时提出了一个修正理论计算参数ε^*(ε^*=-ΔAS^mix/ΔH^form),可以用来预测合金的GFA。（5）用铜模吸铸法制备了直径分别为3mm和5mm的Y₅₆Al₂₄Co₂₀和Y₄₁Sc₁₅Al₂₄Co₂₀大块非晶合金。基于不同冷却速率条件下合金液体熔点偏移的计算方法得到Y₄₁Sc₁₅Al₂₄Co₂₀大块非晶合金的临界实际冷却速率为36K/s。在玻璃转变温度T_g和晶化温度T_x之间的等温晶化过程中,该大块非晶合金的主要晶化过程Avrami指数n（x）＞3,为三维形核长大过程。（6）由VFT方程拟合得到Y₄₁Sc₁₅Al₂₄Co₂₀大块非晶合金的脆性参数m=24,小于典型非晶形成能力很强Zr基和Pd基合金的m值,表明该合金具有很好的非晶形成能力。该大块非晶合金是目前最脆的金属玻璃,其泊松比为0.254,是研究脆性问题的理想材料。（7）以Y基非晶合金为研究对象,研究表明,随着冷却速率的降低,非晶合金的硬度增大。同时在压痕周围伴有多重剪切带（pile-up）现象,这是由压痕形变区域内自由体积显著增加造成的。基于自由体积模型,高的冷却速率引起更高的缺陷浓度,导致原子的跃迁和扩散的可能性越大,从而增加了原子的通量和降低流变应力,最终导致非晶合金软化。更多还原

【Abstract】 Bulk metallic glasses（BMGs）have attracted increasing interests due to their outstanding properties.The major goal is to design new alloy systems with superior glass forming ability（GFA）.Due to lack of complete theory predicting GFA for new metallic glasses,Co-based and Y-based amorphous alloys were fabricated by the element substitution method in this wprk.The conclusions are drawn as below:（1）The effects of Zr on the microstructure,GFA and thermal stability of Co₄₅Fe_25-xZr_xNb₁₀B₂₀（x=0,2,4 and 6at.%）alloys have been studied.Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀alloy shows the most homogeneous microstructure,the best GFA and the highest thermal stability.The apparent activation energies of Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀amorphous alloy from Kissinger equation are 720.3kJ/mol for T_g and 424.5kJ/mol for T_x1, respectively.（2）The addition of Y can improve greatly GFA and thermal stability of Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀alloy.The main reason is that the reaction between yttrium and oxygen is thermodynamically favored compared to the reaction between oxygen and the other elements in the system.Therefore, the stability of the remaining liquid could be enhanced by the alleviation of the harmful effect of oxygen,and the GFA of Co-based alloys can be improved.The apparent activation energies of(Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀)₉₆Y₄ amorphous alloy are 940.7kJ/mol for T_x1and 825.0kJ/mol for T_p1 respectively,which are the highest for all known amorphous alloys.The studied Co-based amorphous alloys not only have high GFA,but also have preferable soft magnetic properties,which are potential functional materials.（3）Based on equiatomic substitution,the additions of Ce,Pr and Nd element（5～20at.%）improve GFA of Y₅₆Al₂₄Co₂₀alloy,GFA of Y_56-xM_xAl₂₄Co₂₀（M=Ce,Pr,Nd;x=5,10,15,20）alloys increase with the increasing of M content.It seems that for the supercooled liquid region （ΔT_x）of Y-based amorphous alloy has a close correlation to bond parameters.It increases with the electronegativity difference（Δx）,atomic size parameter（δ）and electron concentration（Δn）of Y-based alloys.An equation was deduced for the first time by a linear regression analysis,i.e.,ΔT_x=-273.5+639.6Δx²+1074.5δ²+363.1Δn^2,3.The regression coefficient （R）for this fit is computed to be 0.956.（4）The effects of Zr and Sc elements on GFA of Y₅₆Al₂₄Co₂₀alloy have been studied.The results show that Sc element improves better the GFA than Zr element.The reason is that the heat of formation of the Sc₂O₃ is about -1908.8 kJ/mol,which is lower than that of the oxides of other constituent elements（e.g.Y₂O₃,-1903.6 kJ/mol;ZrO₂, -1100.5kJ/mol;Al₂O₃,-1653.5kJ/mol;CoO,-237.7 kJ/mol）.Therefore, the chemical affinity between scandium and oxygen is higher than that between oxygen and other component elements,and the formation of scandium oxide is thermodynamically favored compared to other oxidation reactions.At the same time,a modified calculable parameterε^*,defined as a negative ratio of mixing entropy to formation enthalpy, is proposed,which can predict the GFA of amorphous alloys prepared by the element substitution.（5）Y₅₆Al₂₄Co₂₀and Y₄₁Sc₁₅Al₂₄Co₂₀bulk cylindrical specimens with diameters of 3 and 5 mm were prepared by the method of copper mold suction casting,respectively.The actual cooling rate for Y₄₁Sc₁₅Al₂₄Co₂₀ BMG is 36K/s according to the offset temperature of fusion and the onset temperature of solidification.On the other hand,the isothermal kinetics was modeled by the Johnson-Mehl-Avrami（JMA）equation,the Avrami exponents were calculated to be larger than 3.This implies that the main crystallization of Y₄₁Sc₁₅Al₂₄Co₂₀BMG is governed by diffusion-controlled three-dimensional growth.（6）The fragility parameter m for Y₄₁Sc₁₅Al₂₄Co₂₀BMG from the Vogel-Fulcher-Tammann（VFT）equation is 24,which is smaller than those values for typical Zr_46.75Ti_8.25Cu_7.4Ni₁₀Be_27.5and Pd₄₀Ni₁₀Cu₃₀P₂₀ BMGs.This confirms that the Y₄₁Sc₁₅Al₂₄Co₂₀alloy can be classified into one of the best metallic glass formers.The Poisson’s ratio of Y₄₁Sc₁₅Al₂₄Co₂₀BMG is 0.254,which is the smallest for all known BMGs,which implies it is ideal material for studying the fragility of the BMGs.（7）For Y-based amorphous alloys,the hardness increases with the decrease of the cooling rate.There are multiple shear bands or pile-ups around indentations.The reason is that the increase of free volume in the deformation area of indentation.Based on the free volume mode,the higher cooling rate is,the defect concentration is expected to be higher. Higher defect concentrations enhance the probability that an atom will jump and diffuse,which in turn is expected to increase the flux of atoms and decrease the flow stress,thereby induces the softening of amorphous alloys.更多还原