| 摘要 | 玻璃形成能力(GFA)的研究一直是非晶态合金领域中一个热点问题。尽管目前有关GFA理论和判据较多，但往往难以普适于各种合金体系。“相选择理论”以凝固过程中抑制晶体竞争相长大为基础，通过研究竞争相的变化，可较为准确地确定出最佳GFA合金成分。基于“相选择理论”，本文研究了具有不同GFA的Zr、Ni和Al体系中三元或者伪三元基本合金系的GFA，并进一步总结和发掘每个体系中非晶形成的一些新特征和规律。采用铜模铸造和熔体急冷的方法制备非晶态合金样品，利用X-射线、扫描电镜、透射电镜、热分析等实验手段进行分析和表征。主要结论如下：
1. 在三元共晶合金Zr64Cu18Ni18附近，发现一新的块体金属玻璃形成区域，非晶形成的临界尺寸为1 mm，成分区域为56～64 at.% Zr，16～26 at. % Cu 和 16～24 at.% Ni。与非晶形成相竞争的晶体相为Zr2Cu和fcc-Zr2Ni。易于形成非晶的原因归因于共晶合金附近过冷液体相对稳定和Cu与Ni元素的非等同作用。以该体系为基础，发现添加少量的Nb元素可进一步增强了GFA，合金(Zr0.96Nb0.04) 64Cu18Ni18可以得到非晶形成临界尺寸可提高至1.5 mm的非晶样品。
2. 铸态Ni-(Ti,Zr)-Si合金的凝固组织非常敏感于合金的成分，通过显微结构的观察和合金成分的优化，在57.5～58.5 at.% Ni、 36.5～38.5 at.% (Ti+Zr)和5～5.5 at.% Si成分区域内可以制备出2 mm的非晶圆棒。非晶的主要竞争相是Ni10(Zr,Ti)7、Ni(Ti,Zr) 和未知结构的富Si相。
3. 基于非晶与初生晶体相分别为Al、Al11Ce3和Al3Ni复合结构的演变行为研究，确定出Al-Ni-Ce体系中GFA最佳区域位于合金Al85Ni10Ce5附近，在熔体急冷条件下的临界厚度均为80-95 m。通过用RE简单替代Ce元素，在Al-Ni-RE (RE = La, Pr, Nd, Mm) 体系中，非晶形成呈现出相似的特点，即相同的临界尺寸和相近的最佳合金成分。这些特征主要源于Al-Ni-RE体系相近的热力学性质和相似的竞争相。在楔型铜模铸造的条件下，合金Al85Ni10Mm5可以得到厚度为0.4 mm的完全非晶样品。
4. 确定出Al-TM-Ce(TM = Co, Fe, Cu)体系中最佳GFA合金分别为Al89Co6Ce5、Al90Fe5Ce5和Al85Cu9Ce6。研究表明，不同稀土元素RE(RE = La, Ce, Pr, Nd)对Al-(Co, Fe)-RE合金的GFA影响一致，临界尺寸相同；然而，不同过渡族元素TM (TM = Ni, Co, Fe, Cu)对Al-TM-Ce合金的GFA影响差别很大，临界尺寸的大小依次为：Ni > Co > Fe > Cu，这主要归结于其约化玻璃转变温度Trg和过冷温度区间TL-Tg等热力学参数的显著差异。
5. Al-RE-TM合金系非晶形成与Al-TM之间作用密切相关。Al与TM原子之间的混合热大小次序：Al-Ni > Al-Co > Al-Fe > Al-Cu，这与GFA大小次序一致，表明Al-TM之间结合力显著影响了Al-RE-TM体系的GFA。由于Al-TM之间键长明显缩短了7-9%，Al-TM之间存在强的类似共价键的相互作用。最佳GFA合金中溶质元素富TM元素的现象与这种强的相互作用密切相关。 |
| 其他摘要 | Glass-forming ability (GFA) of metallic glass has been a fundamental issue since the first metallic glass was discovered. So far, a variety of criteria have been proposed to evaluate the GFA. Nevertheless, none of them can realiy serve as a universal scheme to understand the nature of GFA in various alloys. Based on the concept of suppressing crystal growth during rapid solidification, phase selection criterion shows an advantage at pinpointing the best glass-forming composition. In this work, we have studied the glass formation of the Zr-, Ni- and Al-based alloys with different GFA by appling phase selection criterion. Amorphous samples were prepared by suction casting and melt-spinning method. These glassy systems were characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and differential scanning calorimetry. The main results are summaried as follow:
1. New Zr-based bulk metallic glasses have been found near the eutectic alloy Zr64Cu18Ni18. The critical size for glass formation was found to be 1 mm in a relatively wide composition range of 56 to 64 at.% Zr, 16 to 26 at.% Cu and 16 to 24 at.% Ni. The phases competing with glass formation were found to be Zr2Cu and Zr2Ni. More stable liquid with low liquidus temperature and more competing phases facilitate the glass formation and result in forming BMGs. Small amount of additions of Nb enhanced the GFA and bulk metallic glass with critical size of 1.5 mm was obtained for alloy (Zr96Nb4)0.64Cu18Ni18.
2. The solidification microstructures of Ni-(Ti, Zr)-Si alloys strongly depended on composition. By carefully adjusting the alloy compositions and studying the microstructure changes, the optimum glass-forming alloys with a 2-mm diameter are pinpointed in a narrow composition region of 57.5 to 58.5 at.% Ni, 36.5 to 38.5 at.% (Ti+Zr) and 5 to 5.5 at.% Si. The main competing crystalline phases, identified by XRD and SEM, were Ni10(Zr,Ti)7, Ni(Ti,Zr) and an unidentified Si-containing phase.
3. By the observation of phase changes in the composites with primary phase α-Al, Al11RE3 or Al3Ni, the best glass-forming regions with critical thickness of 80-95 m centered at the alloys around Al85Ni10Ce5 have been discovered in the Al-Ni-Ce system by melt spinning. The glass formation in Al-Ni-RE (RE = La, Ce, Pr, Nd and Mm) exhibits two similarities, i.e. similar critical size at the similar compositions. Such features are interpreted in terms of the competing crystalline phases and thermodynamic properties in Al-Ni-RE systems. Under wedge casting, a fully amorphous structure with critical thickness of 400 m can be obtained for Al85Ni10Mm5.
4. The best glass formers in Al-Co-Ce, Al-Fe-Ce and Al-Cu-Ce systems are pinpointed at alloys Al89Co6Ce5, Al90Fe5Ce5 and Al95Cu9Ce6, respectively. RE elements (La, Ce, Pr and Nd) have similar effects upon glass formation in Al89Co6RE5 and Al90Fe5RE5 alloys. The various TM (TM = Ni, Co, Fe, and Cu) elements show distinct effects on glass formation in Al-TM-RE systems. The sequence of critical thickness is: Ni > Co > Fe > Cu, which is dicussed and understood by considering the parameters of Trg and TL-Tg.
5. The interaction between Al-TM shows a good correlation with glass formation in the Al-RE-TM systems. The values of mixing enthalpy between Al-TM follow the sequence: Ni > Co > Fe > Cu, which is the same as that of the critical thickness of Al-TM-RE systems, indicating a close correlation between ΔHAl-TM and GFA. The solute elements of the best formers in Al-RE-TM contain more TM than RE. This interesting finding is suggested to be associated with the strong atomic interaction between Al and TM. |
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