【作者】 王晓军；

【导师】 陈学定；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2006， 博士

【摘要】 镁是现代工业重要的金属材料，大块非晶态合金又是一种具有独特结构和性能的新型材料，制备和研究镁基大块非晶态合金是物理和材料研究者的共同热点，也具有很好的应用前景。采用机械合金化法研究了球磨时间、转速、球料比和过程控制剂等工艺参数对Mg-Cu二元合金相图上的五个主要成分配方的球磨过程的影响。发现化合物成分易于形成非晶态结构，球磨时间、转速和球料比对合金转变过程有着重要影响，过程控制剂的使用对球磨过程也有一定的影响。随着球磨时间的增加，MgCu合金粉末在球磨过程中首先发生粉末颗粒度的逐步细化，然后Mg颗粒逐步扩散、固溶入Cu颗粒中，并形成Mg在Cu中的过饱和固溶体，当固溶体的变形能量积聚到很大时，会发生晶体结构的失稳，最终形成Mg和Cu分布均匀的非晶态合金粉末。采用真空甩带法制备了MgCuY非晶态合金薄带，DSC测试确定了表示非晶形成能力的T_g、T_x、T_l、T_rg和ΔT_x等温度参数，同时提出了用晶化放热值和熔化吸热值的比值（称作约化玻璃转变焓）这一新参数来表示非晶形成能力，新参数与其它参数可以相互验证，配合使用。在确定温度参数时，提出了微商极值法处理DSC曲线数据，确定特征温度的方法。在确定晶化起始温度时，采用向下的纵坐标表示放热的DSC图；在确定玻璃转变温度时，采用向上的纵坐标表示放热的DSC图，对转变局域的曲线微商，n次微商曲线上的极小值温度比n-1次微商曲线上的极小值温度更低，更接近玻璃转变起始温度。采用2次微商曲线上的极小值表示特征温度就有很好的精度，该微商极值法对同一组数据求解的特征温度完全一致，存在很好的重复性和再现性。该方法也较好地解决了国际热分析联合会推荐的切线法在求解DSC曲线上的温度参数时精度较差的问题。设计制造了新的薄带脆性测量实验装置，该装置测量准确，较好地解决了薄带脆性测量重复性差的问题。根据热传导理论，数学解析求解得到薄带的温度场和薄带自由面的冷却速度方程式。发现薄带自由面的凝固结束瞬间的冷却速度与薄带厚度的平方根成反比。根据该方程式计算得到的50μm厚镁基非晶薄带的自由侧在凝固结束时的冷却速度为5.84×10⁶K／s，与文献报道的冷却速度（10⁶K／s）相当。研究了添加Al、Ni、La、Tb和Gd等第四组元对MgCuY三元非晶态合金非晶形成能力的影响，并制备了相应的四元大块非晶态合金。用Al部分替代Mg对合金的玻璃形成能力影响较大，虽然可以形成非晶，但其玻璃形成能力降低。当少量的Ni部分替代Cu时，扩大了过冷液相区的宽度（当x=1时，ΔT_x的宽度可达43.30K）。当用较多的Ni部分替代Cu时，提高了非晶形成能力(当x=3时，T_rg可达0.5892)。用Ni部分替代Cu，提高了合金的玻璃形成能力，但是降低了合金的热稳定性。La部分替代Y对合金的玻璃形成能力和热稳定性影响较大。当1％、2％的La部分替代Y时，降低了其热稳定性，提高了其玻璃形成能力。当3％、4％的La部分替代Y时，此时La的加入超过了合金所能承受的最低限度，合金从而失去热稳定性，玻璃形成能力降低，并出现晶体相。随着合金元素Tb含量的增加，Mg₆₅Cu₂₅Y_10-xTb_x（x=0，2，4，6，8，10）块体非晶态合金的玻璃形成能力逐渐增强，当x=8时，合金的玻璃形成能力最强，随后当x=10时，有所下降。Gd含量的增加有助于增加Mg₆₅Cu₂₅Y_10-xGd_x（x=0，4，10）块体非晶态合金的玻璃形成能力。研究了非晶态合金Mg₆₅Cu₁₅Ag₁₀Y₁₀和Mg₆₅Cu₂₂Ni₃Y₁₀的晶化过程，其晶化过程存在显著的动力学特征，等温加热时，随着保温温度的增加，孕育时间缩短，同耐热流量增加；连续加热时，随着加热速度的提高，所有特征温度移向高温端，并且热流量显著增加。非晶态合金Mg₆₅Cu₁₅Ag₁₀Y₁₀的等温晶化激活能是192.92 KJ／mol；非等温晶化用Kissinger方程、Ozawa方程和FWO方程确定的激活能分别是186.12，184.40和180.86 KJ／mol（加热速度是5，10，15和20 K／min）以及107.52，109.95和110.15 KJ／mol（加热速度是20，40，60和80 K／min）。非晶态合金Mg₆₅Cu₂₂Ni₃Y₁₀的等温晶化激活能是218.97 KJ／mol；非等温晶化用Kissinger方程、Ozawa方程和FWO方程确定的激活能分别是117.48，125.47和114.29 KJ／mol。非晶态合金Mg₆₅Cu₁₅Ag₁₀Y₁₀和Mg₆₅Cu₂₂Ni₃Y₁₀的等温晶化过程的Avrami晶化指数n均位于1.5～2.5区间内，说明晶化过程属于扩散控制的晶核生长过程。在深入理解非晶态合金理论的基础上，本文最后提出了表示非晶态合金配方的混乱度的新参数——相对标准差，其物理意义清楚，和表示非晶形成能力的临界冷却速度有较好的对应关系，对合金组元的选择有一定的指导作用，理论计算的二元合金体系组元的成分含量对现有的有些实验结果比较符合。但由于没有考虑合金组元的原子半径随着核最外层电子重新分配导致的变化，仅仅使用单质的原子半径进行计算的结果并不适合三元合金体系中组元成分的选择。指出合金组元的原子半径和单质的原子半径在数值上存在差异，混合热，价电子浓度等都是影响核外层电子分布和原子半径的因素。更多还原

【Abstract】 Magnesium is very important metallic material for modern industry. However, bulk metallic glasses, a new material, have unique structure and novel properties. Integrating these two sides into a magnesium based bulk metallic glass is the focus for many physical scientist and material specialist. The bulk metallic glasses also possess promising application.At first, the effect of milling time, milling speed, ball/powder weight ratio and process control agents on the GFA （glass forming ability） of five powder blend with three eutectic compositions and two compound compositions based on the Mg-Cu binary phase diagram was studied under mechanical alloying conditions. X-ray diffraction results show that compound composition may be vitrified easily. Milling time, speed and ball/powder ratio have a key influence on the structure transforming process and process control agents have a definite influence. The particle size of powder becomes smaller and smaller firstly after milling. Then magnesium atoms diffuse and melt into crystal lattice of copper. At last all the magnesium atoms melt into the crystal lattice and supersaturation solid solute is formed. If the solid solute gets more energy at this time, the crystal lattice will crash and metastable amorphous state will replace.MgCuY amorphous ribbons were prepared by melt spinning process and the amorphous structure was testified by X-ray diffraction analysis and differential scanning calorimeter analysis. The values of T_g、 T_x、 T₁、T_rg and ΔT_x, which are the parameters to describe the GFA for bulk metallic glasses, are determined. The ratio of the crystallization heat and melting heat was presented as a new parameter—reduced glass transition enthalpy. DSC quantification analysis showed that this new parameter and old parameters gave consistent results when described the GFA of MgCuY amorphous ribbons. A new differential and extremum method was proposed to analysis DSC data and to determine characteristic temperature. Negative direction of y axis expresses exothermic in the DSC graph when determining onset crystallization temperature. While position direction of y axis expresses exothermic when determining T_g. If differentiating local data of DSC, the minimum value of the n multiple derivative is smaller than that of n-1 multiple derivative and it is closer to T_g. The minimum value of two times derivative is determined as T_g. The temperature is only and has enough precision. It is better than tangent method recommended byICTA （international confederation for thermal analysis）.A new experimental instrument for measuring the brittleness of ribbons was designed and fabricated. It solved the measurement of brittleness successfully. Heat transfer during rapid solidification processing of a ribbon prepared by melt spinning can be approximately modeled by a one dimensional heat conduction equation. The temperature distribution and the cooling rate within the ribbon are determined by integration of the equation based on heat transfer principle. According to the integration function, the cooling rate is in inverse proportion to the square of the thickness of the ribbon. When molten liquid of a 50 μm thick magnesium ribbon solidifies, the calculated cooling rate at the free surface of the ribbon is up to 5.85×10⁶K/s. This result agrees very well with other estimated values reported previously.The influence of fourth constituent, Al, Ni, La, Tb and Gd, addition on the GFA of MgCuY bulk metallic glasses was studied and corresponding quaternary alloys were prepared. The GFA of Mg_60-xAl_xCu₃₀Y₁₀ （x=0.3, 0.9, 2.1） bulk amorphous alloy will decrease while Al substituting Mg and the thermal stability and GFA of the amorphous alloy decreased at the same time. It is identified that the width of super cooled region ΔT_x of Mg₆₅Cu_25-xNi_xY₁₀ （x=1, 2, 3） will be enlarged with little Ni addition and the width will reach as much as 43.30K when x=1. The GFA will be increased with larger Ni addition and will reach up to 0.5892 when x=3. It indicates that a proper amount of Ni substituting Cu in the alloy will increase the GFA but decrease the thermal stability. La substituting Y have a significant influence on the thermal stability and GFA of the Mg₆₅Cu₂₅Y_10-xLa_x （x=0, 0.35, 1, 2, 3, 4） bulk amorphous alloys. Bulk amorphous alloy has a largest glass-forming ability (T_rg=0.5872) when x=2. It is found that some crystalline phases appear in the matrix of the amorphous alloys when x=3 and x=4, indicating that the GFA of these alloys decreases markedly. The GFA of Mg₆₅Cu₂₅Y_10-xTb_x （x=0, 2, 4, 6, 8, 10） amorphous alloys become more and more strong with the increment of Tb. The GFA of the Mg₆₅Cu₂₅Y_10-xTb_x amorphous alloy is the strongest when x=8, and then drops when x=10. It is helpful to the GFA of Mg₆₅Cu₂₅Y_10-xGd_x（ x=0, 4, 10） alloy with larger Gd addition.The crystallization kinetics of amorphous alloy Mg₆₅Cu₁₅Ag₁₀Y₁₀ and Mg₆₅Cu₂₂Ni₃Y₁₀ were studied. The remarkable dynamics character exists in the crystallization process revealed by DSC curves. The incubation time is becoming shorter and heat flux is obviously enhancing with increasing temperature whenisothermal heating. And the exothermal peaks are significantly shifted to higher temperatures and heat flux is also obviously enhancing with increasing heating rate when continuous heating. The overall activation energy for crystallization of amorphous alloy Mg₆₅Cu₁₅Ag₁₀Y₁₀ are determined as 186.12, 184.40, and 180.86 KJ/mol for the heating rates used being 5, 10, 15, and 20 K/min, and 107.52, 109.95, and 110.15 KJ/mol for the heating rates used being 20, 40, 60, and 80 K/min, when using the Kissinger peak method, Ozawa peak method, and Ozawa’s isoconversional method, respectively. The overall activation energy for crystallization of amorphous alloy Mg₆₅Cu₂₂Ni₃Y₁₀ are determined as 117.48, 125.47, and 114.29 KJ/mol when using the Kissinger peak method, Ozawa peak method, and Ozawa’s isoconversional method, respectively. The nucleation-and-growth kinetics is manifested as a rule in the early stages of the crystallization. The Avrami exponent, n, is larger than 1.5 and less than 2.5 for amorphous alloy Mg₆₅Cu₁₅Ag₁₀Y₁₀ and Mg₆₅Cu₂₂Ni₃Y₁₀ during isothermal heating; suggesting that the diffusion controlled three-dimensional growth dominates in the growth of the nucleation.A statistical term, standard deviation of radii, is used for describing the confusion degree of dense random packing of hard spheres with different radii in multicomponent amorphous alloys. The parameter has clear physical meaning and coincides with our common understanding of confusion degree. The greater the RD （r） value of a glass is, the greater its confusion degree and the lower its R_c is. The parameter can describe the packing confusion exactly and may be used for constituent selection and composition selection for binary alloy after further investigation. But it is not fit for ternary alloy. For further explaining the deviation of radii caused by mixture enthalpy, redistribution of valence electron and other factors should be considered.更多还原